FIG. 130.—Construction Joints for Concrete Sewers.

]

If the sewer is to be cast in two sections the invert is poured to the

springing line or higher. A triangular or rectangular timber is set in

the top of the wet concrete as shown in Fig. 130. When the concrete has

set the timber is removed and the groove thus left forms a working joint

with the arch. After the invert concrete has set, the arch centering is

placed and the arch is completed. This is the most common method for the

construction of medium-sized circular sewers.

Large sewers with relatively flat bottoms are poured in two or three

sections. First the invert is poured without forms and is shaped with a

screed. About 6 inches of vertical wall is poured at the same time. This

acts as a support for the side-wall forms. The side walls reach to the

springing line of the arch and are poured after the invert has set. At

the third pouring the arch is completed. The sewer shown in Fig. 1 is

being poured in two steps, as the side walls are so low that they are

poured at the same time as the invert. A transverse working joint

similar to one of the types used in Fig. 130 is set between each day’s

work.

The length of the form used and the capacity of the plant should be

adjusted so that one complete unit of invert, side wall, or arch can be

poured in one operation. The forms are left in place until the concrete

has set. Invert and side-wall forms are generally left in position for

at least two days, and in cold weather longer. The arch forms are left

in place for double this time. For example if 20 feet of invert and arch

can be poured in a day, 60 feet of invert form and 100 feet of arch form

will be required. As the forms are released they must be moved forward

through those in place. For this reason collapsible or demountable forms

are necessary and steel forms are advantageous. Wooden arch forms are

sometimes dismantled and carried forward in sections, but are preferably

designed to collapse as shown in Fig. 131, so that they can be pulled

through on rollers or a carriage.

=189. Construction in Tunnels.=—In tunnels the invert and side walls are

constructed in the same manner as for open cut work. The tunneling,

which acts as the outside form, is concreted permanently in place. The

concreting of a tunnel by hand is shown in Fig. 132. If the work is to

be done by hand the concrete is thrown in between the ribs of the arch

centering and behind the plates or lagging, which are set in advance of

the rising concrete. The lagging plates are 5 feet long which makes it

possible to throw the concrete in place at the arch, and to tamp it in

place from the end. A bulkhead and a well-greased joint timber are

placed in position as the concrete rises.

[Illustration:

FIG. 131.—Section through a Collapsible Wood Form.

]

Pneumatic transmission of concrete is also used for filling the arch

forms as well as the side walls and invert forms. In using this method

the mixer may be placed at the surface or at the bottom of the shaft or

other convenient permanent location which may be some distance from the

form. The mixture is discharged into a pipe line through which it is

blown by air to the forms. The starting pressure of about 80 pounds per

square inch can be reduced after flow has commenced. In constructing the

St. Louis Water Works tunnel the compressor equipment for moving the

concrete had a capacity of 1,600 cubic feet per minute at a pressure of

110 pounds. The tunnel is horseshoe shaped, 8 feet in height and with

walls varying from 9 to 20 inches in thickness. The extreme travel of

the concrete was 1,100 feet in an 8 inch pipe. The amount of air

consumed at 110 pounds varied from 1.2 to 1.7 cubic feet of free air per

linear foot of pipe. By the time the batch had been discharged the

pressure had reduced to 25 to 40 pounds, depending on the length of the

pipe. It is reported that a 6–inch pipe line would probably have given

better results.

[Illustration:

FIG. 132.—Ogier’s Run Intercepting Storm-Water Drain, Baltimore,

Maryland.

Placing concrete in Arch. The steel lagging of the forms is carried up

in sections as the concrete is deposited. The drain is horseshoe

shaped, and is 12 feet 3 inches high and 12 feet 3 inches wide.

]

The end of the concrete conveying pipe is provided with a flexible joint

the simplest form of which can be made by slipping a section of pipe of

larger diameter over the end of the transmission line. The concrete is

deposited directly on the invert or into the side-wall forms and can be

blown into the arch forms for 20 to 25 feet.

=190. Materials for Forms.=—The materials used in forms for concrete

sewers are: wood, wood with steel lining, and steel alone. The first

cost of wood forms is lower than that of steel but their life is

relatively short. If the forms are to be used a number of times steel is

more economical. With proper care and repairs steel forms will outlast

any other material. Because of the increasing price of lumber and

improvements in steel forms, wood forms are not frequently used. A

common type of specification under which forms are used is:

The material of the forms shall be of sufficient thickness and the

frames holding the forms shall be of sufficient strength so that

the forms shall be unyielding during the process of filling. The

face of the form next to the concrete shall be smooth. If wooden

forms are used the planking forming the lining shall invariably be

fastened to the studding in horizontal lines, the ends of these

planks shall be neatly butted against each other, and the inner

surface of the form shall be as nearly as possible perfectly

smooth, without crevices or offsets between the ends of adjacent

planks. Where forms are used a second time, they shall be freshly

jointed so as to make a perfectly smooth finish to the concrete.

All forms shall be water-tight and shall be wetted before using.

Any material in contact with wet concrete should be oiled or greased

beforehand in order to prevent adherence to the concrete.

=191. Design of Forms.=—The design of forms for reinforced concrete work

requires some knowledge of the strength of materials and the theories of

beams, columns, and arches. Forms can be constructed without such

knowledge but that they will be both economical and adequate is an

improbability. The ordinary beam and column formulas are applicable to

the design of forms. The maximum bending moment for sheeting and ribs is

taken as (_wl_^2)⁄8, where _w_ is the load per unit length, and _l_ is

the length between supports. Sanford Thompson recommends that the

deflection be calculated as (_wl_^3)⁄(128_EI_), in which _E_ is the

modulus of elasticity of the material, and _I_ is the moment of inertia

of the cross-section referred to the neutral axis. The horizontal

pressure of the concrete against the forms has been expressed

empirically by E. B. Smith,[101] as

_P_ = _H_^{0.2}_R_^{0.3} + 120_C_ − 0.3_S_

in which _P_ = lateral pressure in pounds per square inch;

_R_ = rate of filling forms in feet per hour;

_H_ = head of fill. Ordinarily taken as ½_R_, but in cold

weather or when continuously agitated it may be as

high as ¾_R_;

_C_ = ratio, by volume, of cement to aggregate;

_S_ = consistency in inches of slump.

Earlier investigators have usually concluded that the pressures were

equal to those caused by a liquid weighing 144 pounds per cubic foot,

but the tests of the United States Bureau of Public Roads, from which

the above formula was devised, show the pressures to be decidedly below

this amount under certain conditions.

[Illustration:

FIG. 133.—Centering for Large Forms.

]

With these units and formulas the design of the lagging becomes a matter

of substitution in, and the solution of, the equations produced.[102]

The forces acting on the ribs are indeterminate. No more satisfactory

design can be made for the ribs than to follow successful practice, or

what is seldom done, to determine the stresses in the forms by the

application of one of the theories for the solution of arch stresses.

The sizes of the lumber used in the ribs varies from 1½ × 6 inches to 2

× 10 inches, depending on the size of the sewer. If vertical posts are

used at the ends to support the arch forms they are computed as columns

taking the full weight of the arch. If the span is so wide that radial

supports are used as shown in Fig. 133 the load at the center is assumed

as one-fourth of the weight of the arch.

=192. Wooden Forms.=—Norway and Southern pine, spruce, and fir are

satisfactory for form construction. White pine is satisfactory but is

generally too expensive. The hard woods are too difficult to work. The

lumber should be only partly dried as kiln-dried lumber swells too much

when it is moistened, warping the forms out of shape or crushing the

lagging at the joints. Green lumber must be kept moist constantly to

prevent warping before use and when it is used it does not swell enough

to close the cracks. The lumber should be dressed on the face next to

the concrete and at the ends. Either beveled or matched lumber may be

used for lagging. The joint made by beveled lumber shown in Fig. 134 is

cheaper but less satisfactory than a tongued and grooved joint.

[Illustration:

FIG. 134.—Beveled Joint for Wood Fords.

]

[Illustration:

FIG. 135.—Collapsible Wooden Invert Form for Concrete Sewers.

]

[Illustration:

FIG. 136.—Support for Arch Centering.

]

[Illustration:

FIG. 137.—Wooden Forms Used in Tunnel, North Shore Sewer, Sanitary

District of Chicago.

Journal Western Society of Engineers, Vol. 22, p. 385.

]

Types of wooden forms are shown in Figs. 135 and 136 for use in sewers

to be built as monoliths or in two portions. Fig. 137 shows the details

of a built-up wooden form used in tunnel work for a 42½ inch egg-shaped

sewer.

=193. Steel-lined Wooden Forms.=—Sheet metal linings are sometimes used

on wooden forms. They permit the use of cheaper undressed lumber, demand

less care in the joining of the lagging, and when in good condition give

a smooth surface to the finished concrete. Their use has frequently been

found unsatisfactory and more expensive than well-constructed wooden

forms because of the difficulty of preventing warping and crinkling of

the metal lining and in keeping the ends fastened down so that they will

not curl. Sheet steel or iron of No. 18 or 20 gage (0.05 to 0.0375 of an

inch) weighing 2 to 1½ pounds per square foot is ordinarily used for the

lining.

[Illustration:

FIG. 138.—Blaw Standard Full Round Telescopic Sewer Forms, Showing

Knocked-Down Sections Loaded on a Truck.

Courtesy, Blaw Steel Form Co.

]

=194. Steel Forms.=—These are simple, light, durable, and easy to

handle. The engineer is seldom called upon to design these forms as the

types most frequently used are manufactured by the patentees and are

furnished to the contractor at a fixed rental per foot of form,

exclusive of freight and hauling from the point of manufacture. The

forms can be made in any shape desired, the ordinary stock shapes such

as the circular forms being the least expensive. The smaller circular

forms are adjustable within about 3 inches to different diameters so

that the same form can be used for two sizes of sewers. The same form

can be used for arch and invert in circular sewers. Fig. 138 shows the

collapsible circular forms and the manner in which they are pulled

through those still in position. Fig. 129 shows a half round steel form

swung in position by chains and turnbuckles from the trench bracing, and

Fig. 139 shows the free unobstructed working space in the interior of

some large steel forms.

[Illustration:

FIG. 139.—Interior of Steel Forms for Calumet Sewer, Chicago.

Sewer is 16 feet wide. Note absence of obstructions. Courtesy,

Hydraulic Steelcraft Co.

]

=195. Reinforcement.=—It is essential that the reinforcement be held

firmly in place during the pouring of the concrete. A section of

reinforcement misplaced during construction may serve no useful purpose

and result in the collapse of the sewer. In sewer construction a few

longitudinal bars may be laid in order that the transverse bars may be

wired to them and held in position by notches in the centering and in

fastenings to bars protruding from the finished work. This construction

is shown in Fig. 1. The network of reinforcement is held up from the

bottom of the trench by notched boards which are removed as the concrete

reaches them, or better by stones or concrete blocks which are concreted

in. Sometimes the reinforcement is laid on top of the freshly poured

portion of the concrete the surface of which is at the proper distance

from the finished face of the work. This method has the advantage of not

requiring any special support for the reinforcement, but it is

undesirable because of the resulting irregularity in the reinforcement

spacing and position.

In the side walls the position of the reinforcement is fixed by wires or

metal strips which are fastened to the outside forms or to stakes driven

into the ground. Wires are then fastened to the reinforcement bars and

are drawn through holes in the forms and twisted tight. When the forms

are removed the wires or strips are cut leaving a short portion

protruding from the face of the wall. The reinforcing steel from the

invert should protrude into the arch or the side walls for a distance of

about 40 diameters in order to provide good bond between the sections.

The protruding ends are used as fastenings for the new reinforcement.

The arch steel may be supported above the forms by specially designed

metal supports, by small stones or concrete blocks which are concreted

into the finished work; or by notched strips of wood which are removed

as the concrete approaches them. Strips of wood are not satisfactory

because they are sometimes carelessly left in place in the concrete

resulting in a line of weakness in the structure. Metal chairs are the

most secure supports. They are fastened to the forms and the bars are

wired to the chairs. In some instances the entire reinforcement has been

formed of one or two bars which are fastened into position as a complete

ring. This results in a better bond in the reinforcement, requires less

fastening and trouble in handling, but is in the way during the pouring

of the concrete and interferes with the handling of the forms.

=196. Costs of Concrete Sewers.=—Under present day conditions a general

statement of the costs of an engineering structure can not be given with

accuracy. Only the items of labor, materials, and transportation that go

to make up the cost can be estimated quantitively, and the total cost

computed by multiplying the amount of each item by its proper unit cost

obtained from the market quotations.

A summary of some of the items that go to make up the cost of a concrete

sewer and the relative amount of these items on different jobs is given

in Tables 69 and 70.

TABLE 69

DIVISION OF LABOR COSTS FOR THE CONSTRUCTION OF 96–INCH CIRCULAR

CONCRETE SEWER

─────────────────────────────────────────╥─────────────────────────────

Classification of Labor ║ Classification of Work

─────────────────┬───────────┬───────────╫─────────────────┬───────────

Task or Title │ │ Total ║ Type of Work │

│ Number of │dollars per║ │Dollars per

│ men │ day ║ │ foot

─────────────────┼───────────┼───────────╫─────────────────┼───────────

Superintendent │ 1│ 6.00║Excavation │ 1.80

Engineman │ │ ║Sheeting and │

│ 1│ 3.50║ bracing │ 0.58

Hoister │ │ ║Bottom plank │

(engineman) │ 1│ 2.00║ │ 0.17

Tag-men │ 2│ 3.30║Pulling sheeting │ 0.45

Earth diggers │ 10│ 16.50║Backfilling │ 0.33

On dump cars │ │ ║Making and │

│ 2│ 3.30║ placing invert │ 1.17

Carpenter on │ │ ║Making and │

bracing │ 2│ 3.00║ placing arch │ 1.54

Carpenters’ │ │ ║Laying brick in │

helpers │ 2│ 3.30║ invert │ 0.29

Laying bottom │ 2│ 3.30║ │

Moving pumps, │ │ ║Bending and │

etc. │ │ ║ placing steel │

│ 2│ 3.30║ in arch │ 0.20

Pulling sheeting │ 3│ 5.25║ │

Mixing and │ │ ║Bending and │

placing │ │ ║ placing steel │

concrete │ 16│ 26.40║ in invert │ 0.09

On steel forms │ │ ║Moving forms and │

│ 3│ 5.25║ centers │ 0.62

Water boy │ │ ║Watchmen, water │

│ 1│ 1.00║ boy, etc. │ 0.62

Coal and oil │ │ 5.00║ │

│ │ —————║ │ —————

Total │ │ 90.40║ Total │ 7.86

─────────────────┴───────────┴───────────╨─────────────────┴───────────

NOTES.—Trench was 12½ feet wide and of various depths. At depth of 12

feet the cost of excavation was $1.61 per foot. From Engineering and

Contracting, Vol. 47, p. 157.

BACKFILLING

=197. Methods.=—Careful backfilling is necessary to prevent the

displacement of the newly laid pipe and to avoid subsequent settlement

at the surface resulting in uneven street surfaces and dangers to

foundations and other structures.

The backfilling should commence as soon as the cement in the joints or

in the sewer has obtained its initial set. Clay, sand, rock dust, or

other fine compactible material is then packed by hand under and around

the pipe and rammed with a shovel and light tamper. This method of

filling is continued up to the top of the pipe. The backfill should rise

evenly on both sides of the pipe and tamping should be continuous during

the placing of the backfill. For the next 2 feet of depth the backfill

should be placed with a shovel so as not to disturb the pipe, and should

be tamped while being placed, but no tamping should be done within 6

inches of the crown of the sewer. The tamping should become

progressively heavier as the depth of the backfill increases. Generally

one man tamping is provided for each man shoveling.

TABLE 70

DIVISION OF COSTS FOR THE CONSTRUCTION OF CONCRETE SEWERS

Gillette’s Handbook of Cost Data.

──────────────┬────────────────────────────────────────────────────────

Item │ Location

──────────────┼─────────┬────────┬──────────┬──────────────────────────

│ Fond du │ South │ │

│ Lac │ Bend │Wilmington│ Richmond, Indiana

──────────────┼─────────┼────────┼──────────┼────────┬────────┬────────

Diameter in │ │ │ │ │ │

inches │ 30 │ 66 │ 53 │ 54 │ 48 │ 42

Shape │circular │circular│horseshoe │circular│circular│circular

Plain or │ │ │ │ │ │

reinforced │ plain │ rein. │ rein. │ rein. │ rein. │ rein.

Cubic yards │ │ │ │ │ │

per foot │ 0.11 │ 0.594 │ 0.37 │5″ shell│5″ shell│4″ shell

Daily │ │ │ │ │ │

progress, │ │ │ │ │ │

feet │ 47 │24 to 36│ │ │ │

Cost per foot,│ │ │ │ │ │

dollars │ 1.20 │ 4.40 │ 2.97 │ 1.35 │ 1.08 │ 0.91

Per cent of │ │ │ │

total cost: │ │ │ │

Labor │39.0[103]│ 33.5 │ 33.0 │ =17.1=

Tools │ 1.5 │ 11.5 │ │

Sand and │ │ │ │

gravel │ 12.4 │ 15.5 │ 18.9 │ =19.3=

Lumber │ 0.9 │ │ │

Water │ 0.7 │ 11.5 │ │

Reinforcing │ 0.0 │ │ 14.5 │ =22.3=

Cement │ 23.0 │ 20.0 │ 27.5 │ =32.0[104]=

Frost │ │ │ │

prevention│ 2.0 │ │ │

Forms │ 12.5 │ 8.0 │ 6.1 │ =9.3=

Engineering │ 8.0 │ │ │

Length of day,│ │ │ │

hours │ 8 │ 10 │ │

Year of │ │ │

construction│ 1908 │ 1906 │ Pre-war conditions

──────────────┴─────────┴────────┴─────────────────────────────────────

Above a point 2 feet above the top of the sewer the method pursued and

the care observed in backfilling will depend on the character of the

backfilling material and the location of the sewer. If the sewer is in a

paved street the backfill is spread in layers 6 inches thick and tamped

with rammers weighing about 40 pounds with a surface of about 30 square

inches. One man tamping for each man shoveling is frequently specified.

If no pavement is to be laid but it is required that the finished

surface shall be smooth, slightly less care need be taken and only one

man tamping is specified for each two men shoveling. On paved streets a

reinforced concrete slab with a bearing of at least 12 inches on the

undisturbed sides of the trench may be designed to support the pavement

and its loads. This is of great help in preventing the unsightly

appearance and roughness due to an improperly backfilled trench. On

unpaved streets the backfill is crowned over the trench to a depth of

about 6 inches and then rolled smooth by a road roller. In open fields,

in side ditches, or in locations where obstruction to traffic or

unsightliness need not be considered, after the first 2 feet of backfill

have been placed with proper care, the remainder is scraped or thrown

into the trench by hand or machine, care being taken not to drop the

material so far as to disturb the sewer.

If the top of the sewer, manhole, or other structure comes close to or

above the surface of the ground, an earth embankment should be built at

least 3 feet thick over and around the structure. The embankment should

have side slopes of at least 1½ on 1 and should be tamped to a smooth

and even finish.

If sheeting is to be withdrawn from the trench it should be withdrawn

immediately ahead of the backfilling, and in trenches subject to caving

it may be pulled as the backfilling rises.

Puddling is a process of backfilling in which the trench is filled with

water before the filling material is thrown in. It avoids the necessity

for tamping and can be used satisfactorily with materials that will

drain well and will not shrink on drying. Sand and gravel are suitable

materials for puddling, heavy clay is unsatisfactory. Puddling should

not be resorted to before the first 2 feet of backfill has been

carefully placed. More compact work can be obtained by tamping than with

puddling.

Frozen earth, rubbish, old lumber, and similar materials should not be

used where a permanent finished surface is desired as these will

decompose or soften resulting in settlement. Rocks may be thrown in the

backfill if not dropped too far and the earth is carefully tamped around

and over them. In rock trenches fine materials such as loam, clay, sand,

etc., must be provided for the backfilling of the first portion of the

trench for 2 feet over the top of the pipe. More clay can generally be

packed in an excavation than was taken out of it, but sand and gravel

occupy more space than originally even when carefully tamped.

Tamping machines have not come into general use. One type of machine

sometimes used consists of a gasoline engine which raises and drops a

weighted rod. The rod can be swung back and forth across the trench

while the apparatus is being pushed along. It is claimed that two men

operating the machine can do the work of six to ten men tamping by hand.

The machine delivers 50 to 60 blows per minute, with a 2 foot drop of

the 80 to 90 pound tamping head.

Backfilling in tunnels is usually difficult because of the small space

available in which to work. Ordinarily the timbering is left in place

and concrete is thrown in from the end of the pipe between the outside

of the pipe and the tunnel walls and roof. If vitrified pipe is used in

the tunnel, the backfilling is done with selected clayey material which

is packed into place around the pipe by workmen with long tamping tools.

The backfilling should be done with care under the supervision of a

vigilant inspector in order that subsequent settlement of the surface

may be prevented.

CHAPTER XII

MAINTENANCE OF SEWERS

=198. Work Involved.=—The principal effort in maintaining sewers is to

keep them clean and unobstructed. A sewerage system, although buried,

cannot be forgotten as it will not care for itself, but becoming clogged

will force itself on the attention of the community. Besides the

cleaning and repairing of sewers and the making of inspections for

determining the necessity for this work, ordinances should be prepared

and enforced for the purpose of protecting the sewers from abuse.

Inspections to determine the amount of the depreciation of sewers with a

view towards possible renewal, or to determine the capacity of a sewer

in relation to the load imposed upon it are sometimes necessary. The

valuation of the sewerage system as an item in the inventory of city

property may be assigned to the engineer in charge of sewer maintenance.

The work involved in the inspection and cleaning of sewers in New York

City for the year ending May, 1914, included the removal of 22,687 cubic

yards of material from catch-basins, and 14,826 catch-basin cleanings.

This made an average of two and one-half cleanings per catch-basin per

year, or 1½ cubic yards removed at each cleaning. The 6,432 catch-basins

were inspected 71,890 times. There were 4,112 cubic yards of material

removed from 517 miles of sewers, or about 8 cubic yards per mile.

Inspection of 194 miles of brick sewers were made, 4.4 miles were

flushed, and 27 miles were cleaned. Inspections of 198 miles of pipe

sewers were made, 80 miles were examined more closely, 37 miles were

flushed, and 91 miles were cleaned. The field organization for this work

consisted of 17 foremen, 8 assistant foremen, 29 laborers, 71 cleaners,

13 mechanics, 7 inspectors of construction, 3 inspectors of sewer

connections, 13 horses and wagons, and 28 horses and carts.[105]

=199. Causes of Troubles.=—The complaints most frequently received about

sewers are caused by clogging, breakage of pipes, and bad odors. Sewers

become clogged by the deposition of sand and other detritus which

results in the formation of pools in which organic matter deposits,

aggravating the clogged condition of the sewers and causing the odors

complained of. Grease is a prolific cause of trouble. It is discharged

into the sewer in hot wastes, and becoming cooled, deposits in thick

layers which may effectively block the sewer if not removed. It can be

prevented from entering the sewers by the installation of grease traps

as described in Chapter VI. The periodic cleaning of these traps is as

important as their installation.

Tree roots are troublesome, particularly in small pipe sewers in

residential districts. Roots of the North Carolina poplar, silver leaf

poplar, willow, elm, and other trees will enter the sewer through minute

holes and may fill the sewer barrel completely if not cut away in time.

Fungus growths occasionally cause trouble in sewers by forming a network

of tendrils that catches floating objects and builds a barricade across

the sewer. Difficulties from fungus growths are not common, but constant

attention must be given to the removal of grit, grease, and roots. Tarry

deposits from gas-manufacturing plants are occasionally a cause of

trouble, as they cement the detritus already deposited into a tough and

gummy mass that clings tenaciously to the sewer.

Broken sewers are caused by excessive superimposed loads, undermining,

and progressive deterioration. The changing character of a district may

result in a change of street grade, an increase in the weight of

traffic, or in the construction of other structures causing loads upon

the sewer for which it was not designed. The presence of corrosive acids

or gases may cause the deterioration of the material of the sewer.

=200. Inspection.=—The maintenance of a sewerage system is usually

placed under the direction of a sewer department. In the organization of

the work of this department no regular routine of inspection of all

sewers need be followed ordinarily. Attention should be given regularly

to those sewers that are known to give trouble, whereas the less

troublesome sewers need not be inspected more frequently than once a

year, preferably during the winter when labor is easier to obtain.

The routine inspection of sewers too small to enter is made by an

examination at the manhole. If the water is running as freely at one

manhole as at the next manhole above, it is assumed that the sewer

between the manholes is clean and no further inspection need be given

unless there is some other reason to suspect clogging between manholes.

If the sewage is backed up in a manhole it indicates that there is an

obstruction in the sewer below. If the sewage in a manhole is flowing

sluggishly and is covered with scum it is an indication of clogging,

slow velocity and septic action in the sewer. Sludge banks on the

sloping bottom of the manhole or signs of sewage high upon the walls

indicate an occasional flooding of the sewer due to inadequate capacity

or clogging.

[Illustration:

FIG. 140.—Inspecting Sewers with Reflected Sunlight.

]

If any of the signs observed indicate that the sewer is clogged, the

manhole should be entered and the sewer more carefully inspected. Such

inspection may be made with the aid of mirrors as shown in Fig. 140 or

with a periscope device as shown in Fig. 141. Sunlight is more brilliant

than the electric lamp shown in Fig. 141, but the mirror in the manhole

directs the sunlight into the eyes of the observer, dazzling him and

preventing a good view of the sides of the sewer. The observers’ eyes

can be protected against the direct rays of the electric light, which

can be projected against the sides of the pipe by proper shades and

reflectors. It is possible with this device to locate house connection,

stoppages, breaks of the pipe, and to determine fairly accurately the

condition of the sewer without discomfort to the observers.

Sewers that are large enough to enter should be inspected by walking

through them where possible. The inspection should be conducted by

cleaning off the sewer surface in spots with a small broom, and

examining the brick wall for loose bricks, loose cement or cement lost

from the joints, open joints, broken bond, eroded invert, and such other

items as may cause trouble. An inspection in storm sewers is sometimes

of value in detecting the presence of forbidden house connections.

[Illustration:

FIG. 141.—Inspecting Sewers with Periscope and Electric Light. The G-K

System.

]

Certain precautions should be taken before entering sewers or manholes.

If a distinct odor of gasoline is evident the sewer should be ventilated

as well as possible by leaving a number of manhole covers open along the

line until the odor of gasoline has disappeared. The strength of

gasoline odor above which it is unsafe to enter a sewer is a matter of

experience possessed by few. A slight odor of gasoline is evident in

many sewers and indicates no special danger. A discussion of the amount

of gasoline necessary to create explosive conditions is given in Art.

206. In making observations of the odor it should also be noted whether

air is entering or leaving the manhole. The presence of gasoline cannot

be detected at a manhole into which air is entering.

As soon as it is considered that the odors from a sewer indicate the

absence of an explosive mixture, a lighted lantern or other open flame

should be lowered into the manhole to test the presence of oxygen.

Carbon monoxide or other asphyxiating gases may accumulate in the sewer,

and if present will extinguish the flame. If the flame burns brilliantly

the sewer is probably safe to enter, but if conditions are unknown or

uncertain, the man entering should wear a life belt attached to a rope

and tended by a man at the surface. Asphyxiating or explosive gases are

sometimes run into without warning due to their lack of odor, or the

presence of stronger odors in the sewer. Breathing masks and electric

lamps are precautions against these dangers, the masks being ready for

use only when actually needed. More deaths have occurred in sewers due

to asphyxiating gases than by explosions, as the average sewer explosion

is of insufficient violence to do great damage, although on occasion,

extremely violent explosions have occurred. During inspections of sewers

there should always be at least one man at the surface to call help in

case of accident and the inspecting party should consist of at least two

men.

It must not be felt that entering sewers is fraught with great danger,

as it is perfectly safe to enter the average sewer. The air is not

unpleasant and no discomfort is felt, but conditions are such that

unexpected situations may arise for which the man in the sewer should be

prepared. It is therefore wise to take certain precautions. These may

indicate to the uninitiated, a greater danger than actually exists.

The inspection of sewers should include the inspection of the

flush-tanks, control devices, grit chambers, and other appurtenances. A

common difficulty found with flush-tanks is that the tank is “drooling,”

that is to say the water is trickling out of the siphon as fast as it is

entering the tank, and the intermittency of the discharge has ceased.

If, when the tank is first inspected the water is about at the level of

the top of the bell it is probable that the siphon is drooling. A mark

should be made at the elevation of the water surface and the tank

inspected again in the course of an hour or more. If the water level is

unchanged the siphon is drooling. This may be caused by the clogging of

the snift hole or by a rag or other obstacle hanging over the siphon

which permits water to pass before the air has been exhausted, or a

misplacement of the cap over the siphon, or other difficulty which may

be recognized when the principle on which the siphon operates is

understood. Occasionally it is discovered that an over zealous water

department has shut off the service.

Control devices, such as leaping or overflow weirs, automatic valves,

etc., may become clogged and cease to operate satisfactorily. They

should be inspected frequently, dependent upon their importance and the

frequency with which they have been found to be inoperative. An

inspection will reveal the obstacle which should be removed. Floats

should be examined for loss of buoyancy or leaks rendering them useless.

Grit and screen chambers should be examined for sludge deposits.

Catch-basins on storm sewers are a frequent cause of trouble and need

more or less frequent cleaning. Cleanings are more important than

inspections for catch-basins for if they are operating properly they are

usually in need of cleaning after every storm of any magnitude, and a

regular schedule of cleaning should be maintained.

A record should be kept of all inspections made. It should include an

account of the inspection, its date, the conditions found, by whom made

and the remedies taken to effect repairs.

=201. Repairs.=—Common repairs to sewerage systems consist in replacing

street inlets or catch-basin covers broken by traffic; raising or

lowering catch-basin or manhole heads to compensate for the sinking of

the manhole or the wear of the pavement; replacing of broken pipes,

loosened bricks or mortar which has dropped out; and other miscellaneous

repairs as the necessity may arise. Connections from private drains are

a source of trouble because either the sewer or the drain has broken due

to careless work or the settlement of the foundation or the backfill.

=202. Cleaning Sewers.=—Sewers too small to enter are cleaned by

thrusting rods or by dragging through them some one of the various

instruments available. The common sewer rod shown in Fig. 142 is a

hickory stick, or light metal rod, 3 or 4 feet long, on the end of which

is a coupling which cannot come undone in the sewer. Sections of the rod

are joined in the manhole and pushed down the sewer until the

obstruction is reached and dislodged. Occasionally pieces of pipe

screwed together are used with success. The end section may be fitted

with a special cutting shoe for dislodging obstructions. In extreme

cases these rods may be pushed 400 to 500 feet, but are more effective

at shorter distances. Obstructions may be dislodged by shoving a fire

hose, which is discharging water under high pressure through a small

nozzle, down the sewer toward the obstruction. The water pressure

stiffens the hose, which, together with the support from the sides of

the conduit, make it possible to push the hose in for effective work 100

feet or more from the manhole. A strip of flexible steel about ½ inch

thick and 1½ to 2 inches wide is useful for “rodding” a short length of

crooked sewer.

[Illustration:

FIG. 142.—Sewer Rods

]

Sewers are seldom so clogged that no channel whatever remains. As a

sewer becomes more and more clogged, the passage becomes smaller,

thereby increasing the velocity of flow of the sewage around the

obstruction and maintaining a passageway by erosion. This phenomenon has

been taken advantage of in the cleaning of sewers by “pills.” These

consist of a series of light hollow balls varying in size. One of the

smaller balls is put into the sewer at a manhole. When the ball strikes

an obstruction it is caught and jammed against the roof of the sewer.

The sewage is backed up and seeks an outlet around the ball, thus

clearing a channel and washing the ball along with it. The ball is

caught at the next manhole below. A net should be placed for catching

the ball and a small dam to prevent the dislodged detritus from passing

down into the next length of pipe. The feeding of the balls into the

sewer is continued, using larger and larger sizes, until the sewer is

clean. This method is particularly useful for the removal of sludge

deposits, but it is not effective against roots and grease. The balls

should be sufficiently light to float. Hollow metal balls are better

than heavier wooden ones.

[Illustration:

FIG. 143.—Cable and Windlass Method of Cleaning Sewers.

The cable is held to the bottom of the sewer by bracing a 2 x 4

upright in the sewer, with a snatch block attached. A trailer is

attached to the scoop to prevent loss of material.

]

Plows and other scraping instruments are dragged through pipe sewers to

loosen banks of sludge and detritus and to cut roots or dislodge

obstructions. One form of plow consists of a scoop[106] similar to a

grocer’s sugar scoop, which is pushed or dragged up a sewer against the

direction of flow. As fast as the scoop is filled it is drawn back and

emptied. The method of dragging this through a sewer is indicated in

Fig. 143. At Atlantic City the crew operating the scoop comprises five

men, two are at work in each manhole and one on the surface to warn

traffic and wait on the men in the manholes. The outfit of tools is

contained in a hand-drawn tool box and includes sewer rods, metal scoops

for all sizes of sewers, picks, shovels, hatchets, chisels, lanterns,

grease and root cutters, etc., and two winches with from 400 to 600 feet

of ⅜-inch wire cable.

[Illustration:

FIG. 144.—Sewer Cleaning Device.

Eng. News, Vol. 42, 1899, p. 328.

]

[Illustration:

FIG. 145.—Tools for Cleaning Sewers.

]

[Illustration:

FIG. 146.—Turbine Sewer Machine Connected to Forcing Jack.

The forcing jack is used when windlass and cable cannot be used.

Courtesy, The Turbine Sewer Machine Co.

]

Another form of plow or drag consists of a set of hooks or teeth hinged

to a central bar as shown in Fig. 144. A root cutter and grease scraper

in the form of a spiral spring with sharpened edges, and other tools for

cleaning sewers are shown in Fig. 145. A turbine sewer cleaner shown in

Fig. 146 consists of a set of cutting blades which are revolved by a

hydraulic motor of about 3 horse-power under an operating pressure of

about 60 pounds per square inch. The turbine is attached to a standard

fire hose and is pushed through the sewer by utilizing the stiffness of

the hose, or by rods attached to a pushing jack as shown in the figure.

This machine was invented and patented by W. A. Stevenson in 1914. Its

performance is excellent. The blades revolve at about 600 R.P.M.,

cutting roots and grease. The revolving blades and the escaping water

also serve to loosen and stir up the deposits and the forward helical

motion imparted to the water is useful in pushing the material ahead of

the machine and in scrubbing the walls of the sewer. In Milwaukee four

men with the machine cleaned 319 feet of 12–inch sewer in 16 hours, and

in Kansas City 7,801 feet of sewers were cleaned in 14 days.

Sewers large enough to enter may be cleaned by hand. The materials to be

removed are shoveled into buckets which are carried or floated to

manholes, raised to the surface and dumped. In very large sewers

temporary tracks have been laid and small cars pushed to the manhole for

the removal of the material. Hydraulic sand ejectors may also be used

for the removal of deposits, similar to the steam ejector pump shown in

Fig. 97. The water enters the apparatus at high velocity, under a

pressure of about 60 pounds per square inch, leaps a gap in the machine

from a nozzle to a funnel-shaped guide leading to the discharge pipe.

The suction pipe of the machine leads to the chamber in which the leap

is made. In leaping this gap the water creates a vacuum that is

sufficient to remove the uncemented detritus large enough to pass

through the machine, and will lift small stones to a height of 10 to 12

feet. Occasionally barricades of logs, tree branches, rope, leaves, and

other obstructions which have piled up against some inward projecting

portion of the sewer, must be removed by hand either by cutting with an

axe or by pulling them out. Projections from the sides of sewers are

objectionable because of their tendency to catch obstacles and form

barricades.

Little authentic information on the cost of cleaning sewers is

available. A permanent sewer organization is maintained by many cities.

The division of their time between repairs, cleaning, and other duties

is seldom made a matter of record. From data published in Public

Works[107] it is probable that the cost varies from $3 to $15 per cubic

yard of material removed. From the information in Vol. II of “American

Sewerage Practice” by Metcalf and Eddy the combined cost of cleaning and

flushing will vary between $10 and $40 per mile; the expense of either

flushing or cleaning alone being about one-half of this.

=203. Flushing Sewers.=—Sewers can sometimes be cleaned or kept clean by

flushing. Flushing may be automatic and frequent, or hand flushing may

be resorted to at intervals to remove accumulated deposits. Automatic

flush-tanks, flushing manholes, a fire hose, a connection to a water

main, a temporary fixed dam, a moving dam, and other methods are used in

flushing sewers. The design, operation, and results obtained from the

use of automatic flush-tanks and flushing manholes are discussed in

Chapter VI.

The method in use for cleaning a sewer by thrusting a fire hose down it

can also be used for flushing sewers. It is an inexpensive and fairly

satisfactory method. There is, however, some danger of displacing the

sewer pipe because of the high velocity of the water. An easier and

safer but less effective method is to allow water to enter at the

manhole and flow down the sewer by gravity. Direct connections to the

water mains are sometimes opened for the same purpose.

Sewers are sometimes flushed by the construction of a temporary dam

across the sewer, causing the sewage to back up. When the sewer is half

to three-quarters full the dam is suddenly removed and the accumulated

sewage allowed to rush down the sewer, thus flushing it out. The dam may

be made of sand bags, boards fitted to the sewer, or a combination of

boards and bags. The expense of equipment for flushing by this method is

less than that by any other method, but the results obtained are not

always desirable. Below the dam the results compare favorably with those

obtained by other methods, but above the dam the stoppage of the flow of

the sewage may cause depositions of greater quantities of material than

have been flushed out below. A time should be chosen for the application

of this method when the sewage is comparatively weak and free from

suspended matter. The most convenient place for the construction of a

dam is at a manhole in order that the operator may be clear of the rush

of sewage when the dam is removed.

Movable dams or scrapers are useful in cleaning sewers of a moderate

size, but are of little value in small sewers. The scraper fits loosely

against the sides of the sewer and is pushed forward by the pressure of

the sewage accumulated behind it. The iron-shod sides of the dam serve

to scrape grease and growths attached to the sewer and to stir up sand

and sludge deposited on the bottom. The high velocity of the sewage

escaping around the sides of the dam aids in cleaning and scrubbing the

sewer.

A natural watercourse may be diverted into the sewer if topographical

conditions permit, or where sewers discharge into the sea below high

tide a gate may be closed during the flood and held closed until the

ebb. The rush of sewage on the opening of the gate serves to flush the

sewers and stir up the sludge deposited during high tide. Other methods

of flushing sewers may be used dependent on the local conditions and the

ingenuity of the engineer or foreman in charge.

In some sewers it is not necessary to remove the clogging material from

the sewer. It is sufficient to flush and push it along until it is

picked up and carried away by higher velocities caused by steeper grades

or larger amounts of sewage.

=204. Cleaning Catch-basins.=[108]—Catch-basins have no reason for

existence if they are not kept clean. Their purpose is to catch

undesirable settling solids and to prevent them from entering the

sewers, on the theory that it is cheaper to clean a catch-basin than it

is to clean a sewer. If the cleaning of storm sewers below some inlet to

which no catch-basin is attached becomes burdensome, the engineer in

charge of maintenance should install an adequate catch-basin and keep it

clean. Catch-basins are cleaned by hand, suction pumps, and grab

buckets. In cleaning by hand the accumulated water and sludge are

removed by a bucket or dipper and dumped into a wagon from which the

surplus settled water is allowed to run back into the sewer. The grit at

the bottom of the catch-basin is removed by shoveling it into buckets

which are then hoisted to the surface and emptied.

Suction pumps in use for cleaning catch-basins are of the hydraulic

eductor type. The eductor works on the principle of the steam pump shown

in Fig. 97, except that water is used instead of steam. The material

removed may be discharged into settling basins constructed in the

street, or may be discharged directly into wagons.[109] In Chicago a

special motor-driven apparatus is used. This consists of a 5–yard body

on a 5–ton truck, and a centrifugal pump driven by the truck motor. In

use, the truck, about half filled with water, drives up to the

catch-basin, the eductor pipe is lowered and water pumped from the truck

into the eductor and back into the truck again, together with the

contents of the catch-basin. The surplus water drains back into the

sewer. The Chicago Bureau of Sewers reports a truck so equipped to have

cleaned 1013 catch-basins, removing 1763 cubic yards of material, and

running 1380 miles, during the months of August, September and October,

1917. The cost, including all items of depreciation, wages, repairs,

etc., was $1,393.89. Orange-peel buckets, about 20 inches in diameter,

operated by hand or by the motor of a 3½ to 5–ton truck with a

water-tight body, are used for cleaning catch-basins in some cities.

Catch-basins in unpaved streets and on steep sandy slopes should be

cleaned after every storm of consequence. Basins which serve to catch

only the grit from pavement washings require cleaning about two or three

times per year, and from one to three cubic yards of material are

removed at each cleaning. The cost of cleaning ordinary catch-basins by

hand may vary from $15 to $25, but with the use of eductors or

orange-peel buckets the cost is somewhat lower. In Seattle the cost of

cleaning large detritus basins by hand is said[110] to vary from $45 to

$60. With the use of eductors this cost has been reduced to one-third or

one-fifth the cost of cleaning by hand.

=205. Protection of Sewers.=[111]—City ordinances should be wisely drawn

and strictly enforced for the protection of sewers against abuse and

destruction. The requirements of some city ordinances are given in the

following paragraphs.

Washington, D. C.,[112] sewer ordinances provide that:

No person shall make or maintain any connection with any public

sewer or appurtenance thereof whereby there may be conveyed into

the same any hot, suffocating, corrosive, inflammable or explosive

liquid, gas, vapor, substance or material of any kind ... provided

that the provisions of this act shall not apply to water from

ordinary hot water boilers or residences.

The following extracts from the ordinances of Indianapolis are typical

of those from many cities:

2950. No connection shall be made with any public sewer without

the written permission of the Committee on Sewers and the Sewerage

Engineer.

2953. No person shall be authorized to do the work of making

connections until he has furnished a satisfactory certificate that

he is qualified for the duties. He shall also file bond for not

less than $1,000 that he will indemnify the City from all loss or

damage that may result from his work and that he will do the work

in conformity to the rules and regulations established by the City

Council.

2955. It shall be unlawful for any person to allow premises

connected to the sewers or drains to remain without good fixtures

so attached as to allow a sufficiency of water to be applied to

keep the same unobstructed.

2956. No butcher’s offal or garbage, or dead animals, or

obstructions of any kind shall be thrown in any receiving basin or

sewer in penalty not greater than $100. Any person injuring,

breaking, or removing any portion of any receiving basin, manhole

cover, etc., shall be fined not more than $100.

2962. No person shall drain the contents of any cesspool or privy

vault into any sewer without the permission of the Common Council.

The Cleveland ordinances are similar and contain the following in

addition:

1251. Rule 4. All connections with the main or branch sewers shall

be made at the regular connections or junctions built into the

same, except by special permit.

Rule 16. No steam pipe, nor the exhaust, nor the blow off from any

steam engine shall be connected with any sewer.

Evanston, Illinois, protects its sewers against the additions of grease

and other undesirable substances as follows:

1444. It is unlawful for any person to use any sewer or

appurtenance to the sewerage system in any manner contrary to the

orders of the Commissioner of Public Works.

1446. Wastes from any kitchen sinks, floor drains, or other

fixtures likely to contain greasy matter from hotels, certain

apartment houses, boarding houses, restaurants, butcher shops,

packing houses, lard rendering establishments, bakeries,

laundries, cleaning establishments, garages, stables, yard and

floor drains, and drains from gravel roofs shall be made through

intervening receiving basins constructed as prescribed in par.

VIII of this code.

Receiving basins suitable for the work required in the code are

illustrated in Chapter VI.

=206. Explosions in Sewers.=—Disastrous explosions in sewers were first

recorded about 1886.[113] Up to about 1905 explosions were infrequent

and were considered as unavoidable accidents and so rare as to be

unworthy of study. For a decade or more after 1905 explosions occurred

with increasing violence and frequency causing destruction of property,

but by some freakish chance, but little loss of life. A violent and

destructive explosion occurred in Pittsburgh on Nov. 25, 1913,[114] and

another on March 12, 1916. The property damage amounted to $300,000 to

$500,000 on each occasion, but there was no loss of life. Two miles of

pavement were ripped up, gas, water, and other sewer pipes were broken,

buildings collapsed and the streets were flooded. The streets were

rendered unserviceable for long periods during the expensive repairs

that were necessary. In recent years the number of explosions in sewers

has been smaller, due probably to the gain in knowledge of the causes

and intelligent methods of prevention.

The three principal causes of explosions in sewers are: gasoline vapor,

illuminating gas, and calcium carbide. It is probable that gasoline

vapor is by far the most troublesome. Explosions caused by these gases

are not so violent as those caused by dynamite or other high explosives,

as the volume of gas and the temperature generated are much less. The

violence of sewer explosions may be increased somewhat by the sudden

pressures that are put upon them.

Gasoline finds its way into sewers from garages and cleaning

establishments. A mixture of 1½ per cent gasoline vapor and air may be

explosive. It needs only the stray spark of an electric current, a

lighted match, or a cigar thrown into the sewer to cause the explosion.

As the result of a series of experiments on 2,706 feet of 8–foot sewer,

Burrell and Boyd conclude.[115]

One gallon of gasoline if entirely vaporized produces about 32

cubic feet of vapor at ordinary temperature and pressure. If 1½

per cent be adopted as the low explosive limit of mixtures of

gasoline vapor and air, 55 gallons or a barrel of gasoline would

produce enough vapor to render explosive the mixture in 1,900 feet

of 9 foot sewer provided the gasoline and the air were perfectly

mixed. Many different factors, however, govern explosibility, such

as: size of the sewer, velocity of the sewage, temperature of the

sewer, volatility and rate of inflow of the gasoline. Only under

identical conditions of tests would duplicate results be obtained.

A large amount of gasoline poured in at one time is less dangerous

than the same amount allowed to run in slowly. With a velocity of

flow of about 6½ feet per second it was evident that 55 gallons of

gasoline poured all at once into a manhole rendered the air

explosive only a few minutes (less than 10) at any particular

point. With the same amount of gasoline run in at the rate of 5

gallons per minute, an explosive flame would have swept along the

sewer if ignited 15 minutes after the gasoline had been dumped.

With a slow velocity of flow and a submerged outlet the gasoline

vapor being heavier than air accumulated at one point and

extremely explosive conditions could result from a small amount of

gasoline. Comparatively rich explosive mixtures were found 5 hours

after the gasoline had been discharged. High-test gasoline is much

more dangerous than the naphtha used in cleaning establishments,

yet on account of the large quantity of waste naphtha the sewage

from cleaning establishments may be very dangerous.

Illuminating gas is not so dangerous as gasoline vapor as it is lighter

than air and it is more likely to escape from the sewer than to

accumulate in it. It requires about one part of illuminating gas to

seven parts of air to produce an explosive mixture.

Calcium carbide is dangerous because it is self igniting. The heat of

the generation of gas is sufficient to ignite the explosive mixture. The

gases are highly explosive and cause a relatively powerful explosion.

Fortunately large amounts of this material seldom reach a sewer, the gas

being generated in garage drains or traps and escaping in the

atmosphere.

A hydrocarbon oil used by railroads in preventing the freezing of

switches, if allowed to reach the sewers, may cause explosions

therein.[116] The oil crystallizes and in this form it is soluble in

water. It will thus pass traps and on volatilization will produce

explosive mixtures.

Methane, generated by the decomposition of organic matter, is a feebly

explosive gas occasionally found in sewers. Its presence may add to the

strength of other explosive mixtures.

Sewer explosions may be prevented by the building of proper forms of

intercepting basins to prevent the entrance of gasoline and calcium

carbide gases, and by ventilation to dilute the explosive mixtures which

may be made up in the sewer. There are no practical means to predict

when an explosion is about to occur, and after an explosion has occurred

it is difficult to determine the cause as all evidence is usually

destroyed.

=207. Valuation of Sewers.=—The necessity for the valuation of a

sewerage system may arise from the legal provisions in some states

limiting the amount of outstanding bonds which may be issued by a

municipality to a certain percentage of the present worth of municipal

property. The investment in the sewerage system is usually great and

forms a large portion of the City’s tangible property. It may be

desirable also to determine the depreciation of the sewers with a view

towards their renewal.

The most valuable work on the valuation of sewers has been done in New

York City[117] by the engineers of the Sewer Department. The committee

of engineers appointed to do the work recommended: (1) that the original

cost be made the basis of valuation, and that (2), in fixing this cost

the cost of pavement should be omitted or at most the cost of a cheap

(cobblestone) pavement should be included. Trenches previously excavated

in rock were considered as undepreciated assets.

The present worth of sewers depends on many factors aside from the

effects of age, such as the care exercised in the original construction,

the material used, the kind and quantity of sewage carried, the care

taken in maintenance, and finally the injury caused by the careless

building of adjoining substructures. During the progress of the

inspections the examination of brick sewers, due to their accessibility,

yielded better results than the examination of pipe sewers. The routine

of the examination of the brick sewers consisted in cleaning off the

bricks with a short broom, tapping the brick with a light hammer to

determine solidity, and testing the cement joints by scraping with a

chisel. In addition, measurements of height and width were taken every

30 feet. The bricks in the invert at and below the flow line were

examined for wear.

A study of the reports of these examinations disclosed that the

following defects were noticeable:

1. Cement partly out at water line.

2. Cement partly out above water line.

3. Depressed arch and sewer slightly spread.

4. Large open joints.

5. Loose brick.

6. Bond of brick broken.

7. Distorted sides, uneven bottom, joints out of line.

[Illustration:

FIG. 147.—Diagrams used in Estimating Depreciation of Brick Sewers Due

to Age, Manhattan Borough, New York City.

_a._ Proportionate deterioration from various causes.

_b._ Percentage of depreciation based on examination of sewers, use of

deterioration curve (Fig. a), and age of sewers examined.

Eng. News, Vol. 71, p. 84.

]

Inspection of pipe sewers from manholes, the pipe being illuminated by

floating candles, was found to be unsatisfactory. Reliance was placed on

the reports of men experienced in making connections and repairs to the

sewers. Early pipe sewers in New York were laid directly on the bottom

of the trench. Under these circumstances a small leak at a joint was

sufficient to wash the earth away and to drop the pipe, causing serious

conditions along the line. No wear or deterioration of pipe sewers were

noted, the only defects being cracking of the pipes at the center line

due to poor foundation and to defects in the pipe itself.

[Illustration:

FIG. 148.—Diagram Showing Rate of Depreciation of Pipe Sewers.

Eng. News, Vol. 71, p. 86.

]

The depreciation of brick sewers as studied in New York, is shown

graphically in Fig. 147. At zero the sewer is in good condition and at

100 it is in such a state of dilapidation as to require instant

rebuilding. Repairs are not considered economical in this condition. In

the preparation of this diagram each condition on the list above was

given a certain number of points, which when added together represented

the state of depreciation of the sewer. These sums were plotted as

ordinates and the corresponding ages of the sewer were plotted as

abscissas. The various points were taken cumulatively, and where the

bond of the brickwork was broken (given a value of 72) plus other

defects gave a total of 164 the sewer was considered as valueless and

not worth repair. The scale of 164 was later reduced to a percentage

basis as shown on the right of the figure. Fig. 148 shows a similar

diagram for the depreciation of pipe sewers.

It was concluded that the life of a brick sewer in New York is 64 years.

Some of the sewers examined were over 200 years old. The total original

cost of 483 miles of brick, pipe and wood sewers was figured as

$23,880,000 with a present worth of $18,665,000 and an average annual

depreciation of 2.2 per cent. In figuring these amounts no account was

taken of obsolescence. The deterioration of catch-basins proceeded at

about the same rate as for brick sewers.

CHAPTER XIII

COMPOSITION AND PROPERTIES OF SEWAGE

=208. Physical Characteristics.=—Sewage is the spent water supply of a

community containing the wastes from domestic, industrial, or commercial

use, and such surface and ground water as may enter the sewer.[118]

Sewages are classed as: domestic sewage, industrial waste, storm water,

surface water, street wash, and ground water. Domestic sewage is the

liquid discharged from residences or institutions and contains water

closet, laundry, and kitchen wastes. It is sometimes called sanitary

sewage. Industrial sewage is the liquid waste resulting from processes

employed in industrial establishments. Storm water is that part of the

rainfall which runs over the surface of the ground during a storm and

for such a short period following a storm as the flow exceeds the normal

and ordinary run-off. Surface water is that part of the rainfall which

runs over the surface of the ground some time after a storm. Street wash

is the liquid flowing on or from the street surface. Ground water is

water standing in or flowing through the ground below its surface.

Ordinary fresh sewage is gray in color, somewhat of the appearance of

soapy dish water. It contains particles of suspended matter which are

visible to the naked eye. If the sewage is fresh the character of some

of the suspended matter can be distinguished as: matches, bits of paper,

fecal matter, rags, etc. The amount of suspended matter in sewage is

small, so small as to have no practical effect on the specific gravity

of the liquid nor to necessitate the modification of hydraulic formulas

developed for application to the flow of water. The total suspended

matter in a normal strong domestic sewage is about 500 parts per

1,000,000. It is represented graphically in Fig. 149. The quantity of

organic or volatile suspended matter is about 200 parts per 1,000,000.

It is shown graphically in the smaller cube in Fig. 149.

[Illustration:

FIG. 149.—Graphical Representation of Relative Volumes of Liquids and

Solids in Sewage.

]

The odor of fresh sewage is faint and not necessarily unpleasant. It has

a slightly pungent odor, somewhat like a damp unventilated cellar.

Occasionally the odor of gasoline, or some other predominating waste

matter may hide all other odors. Stale sewage is black and gives off

nauseating odors of hydrogen sulphide and other gases. If the sewage is

so stale as to become septic, bubbles of gas will be seen breaking the

surface and a black or gray scum may be present. Before the South Branch

of the Chicago River was cleaned up and flushed this scum became so

thick in places, particularly in that portion of the Stock Yards where

the river became known as Bubbly Creek, that it is said that weeds and

small bushes sprouted in it, and chickens and small animals ran across

its surface.

A physical analysis of sewage should include an observation of its

appearance, and a determination of its temperature, turbidity, color,

and odor, both hot and cold. The temperature is useful in indicating

certain of the antecedents of the sewage, its effect on certain forms of

bacterial life, and its effect on the possible content of dissolved

gases. Temperatures higher than normal are indicative of the presence of

trades wastes discharged while hot into the sewers. A low temperature

may indicate the presence of ground water. If the temperature is much

over 40° C. bacterial action will be inhibited and the content of

dissolved gases will be reduced. Turbidity, color, and odor

determinations may be of value in the control of treatment devices, or

to indicate the presence of certain trades wastes, which give typical

reactions. Since all normal sewages are high in color and turbidity, the

relative amounts of these two constituents in two different sewages has

little significance regarding the relative strengths of the two sewages

or the proper method of treating them. A fresh domestic sewage should

have no highly offensive odor. The presence of certain trades wastes can

be detected sometimes in fresh sewages, and a stale sewage may sometimes

be recognized by its odor.

Sewage is a liability to the community producing it. Although some

substances of value can be obtained from sewage[119] the cost of the

processes usually exceed the value of the substances obtained. Where it

becomes necessary to treat sewage the value of these substances may be

helpful in defraying the cost of treatment.

=209. Chemical Composition.=—Sewage is composed of mineral and organic

compounds which are either in solution or are suspended in water. In

making a standard chemical analysis of sewage only those chemical

radicals and elements are determined which are indicative of certain

important constituents. Neither a complete qualitative nor quantitative

analysis is made. A sewage analysis will not show, therefore, the number

of grams of sodium chloride present or any other constituent. A complete

standard sanitary chemical analysis will report the constituents as

named in the first column of Table 71. The quantities of these materials

found in average strong, medium and weak sewages are also shown in this

table. These values are not intended as fixed boundaries between sewages

of different strengths. They are presented merely as a guide to the

interpretation of sewage analyses.

The principal objects of a chemical analysis of sewage are to determine

its strength and its state of decomposition. The influents and effluents

of a sewage treatment device are analyzed to aid in the control of the

device and to gain information concerning the effect of the treatment.

Chemical and other analyses, in connection with the desired conditions

after disposal, will indicate the extent of treatment which may be

required. The standard methods of water and sewage analysis adopted by

the American Public Health Association have been generally accepted by

sanitarians. These uniform methods make possible comparisons of the

results obtained by laboratories working according to these standards.

CHEMICAL ANALYSIS OF SEWAGES

(Parts per million)

From Report on Industrial Wastes from the Stock Yards and Packingtown,

Chicago by the Sanitary District of Chicago in 1921, page 231.

──────────┬──────────────────┬───────┬────────┬──────────

│ │ │ │

│ │ │ │

│ │ │ │

│ │ │ │Waterbury,

│ │Boston │Columbus│ Conn.,

│ Typical Analyses │1905–7 │ 1904–5 │ 1905–6

──────────┼──────┬──────┬────┼───────┼────────┼──────────

│Strong│Medium│Weak│ │ │

──────────┼──────┼──────┼────┼───────┼────────┼──────────

Nitrogen │ │ │ │ │ │

as │ │ │ │ │ │

Organic │ │ │ │ │ │

Nitrogen│ 35│ 20│ 10│ 9.1│ 9.0│ 14.8

Free │ │ │ │ │ │

Ammonia │ 50│ 30│ 15│ 13.9│ 11.0│ 7.8

Nitrites │ 0.10│ 0.05│ 0.0│ 0.0│ 0.09│ 0.14

Nitrates │ 0.40│ 0.20│ 0.1│ 0.20│ 0.20│ 1.52

Oxygen │ │ │ │ │ │

consumed│ 75│ 50│ 30│56[120]│ 51[121]│ 46[120]

Oxygen │ │ │ │ │ │

demand │ 300│ 200│ 100│ │ │

Chlorine │ 175│ 100│ 15│ 2300│ 65│ 48

Suspended │ │ │ │ │ │

matter │ 500│ 300│ 150│ 135│ 209│ 165

Volatile│ │ │ │ 91│ 79│ 115

Fixed │ │ │ │ 44│ 130│ 50

Alkalinity│ 200│ 100│ 50│ 125│ 350│ 41

Fats │ 40│ 20│ │ │ 25│ 26

──────────┴──────┴──────┴────┴───────┴────────┴──────────

──────────┬─────────────┬──────────┬───────────┬───────────

│ │ │ │ Chicago,

│ │ │ │ Center

│ │ │ Chicago, │ Avenue.

│Gloversville,│Worcester,│ 39th St. │Industrial.

│ N. Y. │ Mass. │Residential│Day Sewage

│ 1908–9 │ 1908 │ 1909–12 │ 1913

──────────┼─────────────┼──────────┼───────────┼───────────

│ │ │ │

──────────┼─────────────┼──────────┼───────────┼───────────

Nitrogen │ │ │ │

as │ │ │ │

Organic │ │ │ │

Nitrogen│ 23.0│ │ 7.8│ 79

Free │ │ │ │

Ammonia │ 12.0│ 22.2│ 9.1│ 22

Nitrites │ 0.38│ │ 0.10│ 0.49

Nitrates │ 0.88│ │ 0.33│ 3.04

Oxygen │ │ │ │

consumed│ 95[120]│ 117│ 43│ 268

Oxygen │ │ │ │

demand │ │ │ │

Chlorine │ 158│ 57│ 40│ 1100

Suspended │ │ │ │

matter │ 406│ 258│ 144│ 605

Volatile│ 229│ 166│ 90│ 46

Fixed │ 177│ 92│ 54│ 144

Alkalinity│ 233│ │ 212│ 291

Fats │ 48│ │ 23[122]│ 198[123]

──────────┴─────────────┴──────────┴───────────┴───────────

=210. Significance of Chemical Constituents.=—Organic nitrogen and free

ammonia taken together are an index of the organic matter in the sewage.

Organic nitrogen includes all of the nitrogen present with the exception

of that in the form of ammonia, nitrites, and nitrates. Free ammonia or

ammonia nitrogen is the result of bacterial decomposition of organic

matter. A fresh cold sewage should be relatively high in organic

nitrogen and low in free ammonia. A stale warm sewage should be

relatively high in free ammonia and low in organic nitrogen. The sum of

the two should be unchanged in the same sewage.

Nitrites (RNO_{2}) and nitrates (RNO_{3})[124] are found in fresh

sewages only in concentrations of less than one part per million. In

well-oxidized effluents from treatment plants the concentration will

probably be much higher. Nitrates contain one more atom of oxygen than

nitrites. They represent the most stable form of nitrogenous matter in

sewage. Nitrites are not stable and are reduced to ammonias or are

oxidized to nitrates. Their presence indicates a process of change. They

are not found in large quantities in raw sewage because their formation

requires oxygen which must be absorbed from some other source than the

sewage. In an ordinary sewer or sluggishly flowing open stream this

absorption cannot take place from the atmosphere with sufficient

rapidity to supply the necessary oxygen.

Oxygen consumed is an index of the amount of carbonaceous matter readily

oxidizable by potassium permanganate. It does not indicate the total

quantity of any particular constituent, but it is the most useful index

of carbonaceous matter. Carbonaceous matter is usually difficult of

treatment and a high oxygen consumed is indicative of a sewage difficult

to care for. The amount of oxygen consumed, as expressed in the

analysis, is dependent on the amount of oxidizable carbonaceous matter

present, the oxidizing agent used, and the time and temperature of

contact of the sewage and the oxidizing agent. It is essential therefore

that the test be conducted according to some standard method, since the

results are of value only as compared with results obtained under

similar conditions.

Total solids (residue on evaporation) are an index of the strength of

the sewage. They are made up of organic and inorganic substances. The

inorganic substances include sand, clay, and oxides of iron and

aluminum, which are usually insoluble, and chlorides, carbonates,

sulphates and phosphates, which are usually soluble. The insoluble

inorganic substances are undesirable in sewage because of their sediment

forming properties which result in the clogging of sewers, treatment

plants, pumps, and stream beds. The soluble inorganic substances are

generally harmless and cause no nuisance, except that the presence of

sulphur may permit the formation of hydrogen sulphide, which has a

highly offensive odor. The organic substances are: carbohydrates, fats,

and soaps, which are carbonaceous and are difficult of removal by

biological processes; and the nitrogenous substances such as urea,

proteins, amines, and amino acids. The inorganic and organic substances

may be either in solution or suspension or in a colloidal condition.

Volatile solids are used as an index of the organic matter present, as

it is assumed that the organic matter is more easily volatilized than

the inorganic matter. The amount of volatile inorganic matter present is

usually so small as to be negligible.

Fixed solids are reported as the difference between the total and

volatile solids. They are therefore representative of the amount of

inorganic matter present.

Suspended matter is the undissolved portion of the total solids. High

volatile suspended matter is an indication of offensive qualities in the

nature of putrefying organic matter, whereas fixed suspended matter is

indicative of inoffensive inorganic matter. It is difficult to obtain a

sample of sewage which will represent the amount of suspended matter in

the sewage, since a sample taken from near the surface will contain less

inorganic matter and grit than a sample taken near the bottom.

Settling solids are indicative of the sludge forming properties of the

sewage and of the probable degree of success of treatment by plain

sedimentation. Volatile settling solids indicate the property of the

formation of offensive putrefying sludge banks. There is no chemical

test which will indicate the scum-forming properties of sewage. Fixed

settling solids indicate the presence of inorganic matter, probably

gritty material such as sand, clay, iron oxide, etc.

Colloidal matter is material which is too finely divided to be removed

by filtration or sedimentation, yet is not held in solution. It can

sometimes be removed by violent agitation in the presence of a

flocculent precipitate, as in the treatment with activated sludge, or by

the flocculent precipitate alone, as in chemical precipitation, or by

the acidulation of the sewage so as to precipitate the colloids.

Colloidal matter is probably the result of the constant abrasion of

finely divided suspended matter while flowing through the sewer or other

channel. High colloidal matter may therefore indicate a stale sewage, or

the presence of a particular trades waste. Colloids are difficult of

removal. For this reason, where sewage is to be treated, turbulence in

the tributary channels should be avoided.

Alkalinity may indicate the possibility of success of the biologic

treatment of sewage, since bacterial life flourishes better in a

slightly alkaline than in a slightly acid sewage. Within the normal

limits of the amount of alkalinity in sewage the exact amount has little

significance in sewage analyses. Sewages are normally slightly alkaline.

An abnormal alkalinity or acidity may indicate the presence of certain

trades wastes necessitating special methods of treatment. A method of

sewage treatment may be successful without changing the amount of

alkalinity in the sewage since the amount of alkalinity is not

inherently an objection.

Chlorine, in the form of sodium chloride, is an inorganic substance

found in the urine of man and animals. The amount of chlorine above the

normal chlorine content of pure waters in the district is used as an

index of the strength of the sewage. The chlorine content may be

affected by certain trades wastes such as ice-cream factories,

meat-salting plants, etc., which will increase the amount of chlorine

materially. Since chlorine is an inorganic substance which is in

solution it is not affected by biological processes nor sedimentation.

Its diminution in a treatment plant or in a flowing stream is indicative

of dilution and the reduction of chlorine will be approximately

proportional to the amount of dilution.

Fats have a recoverable market value when present in sufficient quantity

to be skimmed off the surface of the sewage. Ordinarily fats are an

undesirable constituent of sewage as they precipitate on and clog the

interstices in filtering material, they form objectionable scum in tanks

and streams, and they are acted on very slowly by biological processes

of sewage treatment. Although fats are carbonaceous matter they are not

indicated by the oxygen consumed test because they are not easily

oxidized. They are therefore determined in another manner; by

evaporation of the liquid and extracting the fats from the residue by

dissolving them in ether.

Relative stability and bio-chemical oxygen demand are the most important

tests indicating the putrefying characteristics of sewage. Since

stability and putrescibility have opposite meanings the relative

stability test is sometimes called the putrescibility test. The relative

stability of a sewage is an expression for the amount of oxygen present

in terms of the amount required for complete stability.

A relative stability of 75 signifies that the sample examined

contains a supply of available oxygen equal to 75 per cent of the

amount of oxygen which it requires in order to become perfectly

stable. The available oxygen is approximately equivalent to the

dissolved oxygen plus the available oxygen of nitrate and

nitrite.[125]

TABLE 72

RELATIVE STABILITY NUMBERS

──────────────────────────────────┬──────────────────────────────────

Time Required for Decolorization │

at 20° C. Days │ Relative Stability Number

──────────────────────────────────┼──────────────────────────────────

0.5│ 11

1.0│ 21

1.5│ 30

2.0│ 37

2.5│ 44

3.0│ 50

4.0[126]│ 60

5.0│ 68

6.0│ 75

7.0│ 80

8.0│ 84

9.0│ 87

10.0│ 90

11.0│ 92

12.0│ 94

13.0│ 95

14.0│ 96

16.0│ 97

18.0│ 98

20.0│ 90

──────────────────────────────────┴──────────────────────────────────

The relative stability numbers, given in Table 72, are computed from the

expression, _S_ = 100(1 − 0.794_t_) in which _S_ is the stability number

and _t_ is the time in days that the sample has been incubated at 20° C.

The bio-chemical oxygen demand is more directly an index of the

consumption of available oxygen by the biological and chemical changes

which take place in the decomposition of sewage or polluted water. As

such it is a more valuable, though less easily performed test than the

test of relative stability.

The methods for the determination of the relative stability and the

bio-chemical oxygen demand are given to show more clearly what these

tests represent. The procedure in the relative stability test is to add

0.4 c.c. of a standard solution of methylene blue to 150 c.c. of the

sample. The mixture is then allowed to stand in a completely filled and

tightly stoppered bottle at 20° C. for 20 days or until the blue fades

out due to the exhaustion of the available oxygen. There are three

methods in use for the determination of the bio-chemical oxygen

demand;[127] the relative stability method, the excess nitrate method,

and the excess oxygen method. In the relative stability method the

sample to be treated should have a relative stability of at least 50. If

it is lower than this the sample should be diluted with water containing

oxygen until the relative stability has been raised to or above this

point. The oxygen demand in parts per million is then expressed as

_O′_ = ((1 − _P_)_O_)⁄_RP_,[128]

in which _O′_ is the oxygen demand, _O_ is the initial oxygen in parts

per million (p.p.m.) in the diluting water or sewage, _P_ is the

proportion of sewage in the mixture expressed as a ratio, and _R_ is the

relative stability of the mixture expressed as a decimal. For the

effluents from sewage treatment plants, polluted waters, and similar

liquids, the total available oxygen expressed as the sum of the

dissolved oxygen, nitrites, and nitrates, divided by the relative

stability expressed as a decimal will give the bio-chemical oxygen

demand. The excess nitrate method requires the determination of the

total oxygen available as dissolved oxygen, nitrites, and nitrates and

the addition of a sufficient amount of oxygen in the form of sodium

nitrate to prevent the exhaustion of oxygen during a 10–day period of

incubation. At the end of the period the total available oxygen is again

determined. The difference between the original and the final oxygen

content represents the bio-chemical oxygen demand. The excess oxygen

test requires the determination of the total available oxygen as before,

and the addition of a sufficient amount of oxygen, in the form of

dissolved oxygen in the diluting water, to prevent exhaustion of the

oxygen in a 10–day period of incubation. The difference between the

original and final oxygen content represents the bio-chemical oxygen

demand. Theriault concludes as a result of his tests, that the relative

stability and excess nitrate methods are open to objections but that the

excess oxygen method yields very accurate and consistent results with as

little or less labor than is required by other methods.

Dissolved oxygen represents what its name implies, the amount of oxygen

(_O__{2}) which is dissolved in the liquid. Normal sewage contains no

dissolved oxygen unless it is unusually fresh. It is well, if possible,

to treat a sewage before the original dissolved oxygen has been

exhausted. Normal pure surface water contains all of the oxygen which it

is capable of dissolving, as shown in Table 73. The presence of a

smaller amount of oxygen than is shown in this table indicates the

presence of organic matter in the process of oxidation, which may be in

such quantities as ultimately to reduce the oxygen content to zero.

Normal pure ground waters may be deficient in dissolved oxygen because

of the absence of available oxygen for solution. The presence of certain

oxygen-producing organisms in polluted or otherwise potable surface

waters may cause a supersaturation with oxygen however.

The dissolved-oxygen test for polluted water is probably the most

significant of all tests. If dissolved oxygen is found in a polluted

water it means that putrefactive odors will not occur, since

putrefaction cannot begin in the presence of oxygen. It is possible for

different strata in a body of water to have different quantities of

dissolved oxygen, and putrefaction may be proceeding in the lower strata

before the oxygen is exhausted from the upper strata. The oxygen content

of a river water will indicate the ability of the river to receive

sewage without resulting in a nuisance.

TABLE 73

SOLUBILITY OF OXYGEN IN WATER

Under an atmospheric pressure of 760 mm. of mercury, the atmosphere

containing 20.9 per cent of oxygen.

───────────────────────────────────┬───────────────────────────────────

Temperature, degrees C │ Oxygen in parts per million

───────────────────────────────────┼───────────────────────────────────

0│ 14.62

1│ 14.23

2│ 13.84

3│ 13.48

4│ 13.13

5│ 12.8

6│ 12.48

7│ 12.17

8│ 11.87

9│ 11.59

10│ 11.33

11│ 11.08

12│ 10.83

13│ 10.6

14│ 10.37

15│ 10.15

16│ 9.95

17│ 9.74

18│ 9.54

19│ 9.35

20│ 9.17

21│ 8.99

22│ 8.83

23│ 8.68

24│ 8.53

25│ 8.38

26│ 8.22

27│ 8.07

28│ 7.92

29│ 7.77

30│ 7.63

───────────────────────────────────┴───────────────────────────────────

=211. Sewage Bacteria.=—A slight knowledge of the nature of bacteria is

necessary in order that the biological changes which occur in the

treatment of sewage may be understood. Bacteria are living organisms

which are so small that it is difficult or impossible to study them

either with the eye alone or with the aid of powerful microscopes. They

are studied by means of cultures, stains, and certain characteristic

phenomena such as the production of a gas, the production of a red

colony on litmus lactose agar, etc. Bacteria occur in three forms:

spherical, called coccus; cylindrical, called bacillus; and spiral,

called spirillum. In size they vary from the largest at about 1⁄10,000

of an inch to sizes so small as to be invisible under the most powerful

microscope. An ordinary size is 1⁄25,000 of an inch. The cylindrical or

rod bacteria are about four times as long as they are wide. Some

bacteria possess the power of motion due to the presence of flagella or

hairs which can be moved and cause the cell to progress at a rate as

high as 18 cm. per hour, but usually the rate is very much less than

this. The composition of the bacterial cell has never been definitely

determined.

Bacteria are unicellular plants. They possess no digestive organs and

apparently obtain their food by absorption from the surrounding media.

Reproduction is by the division of the cell into two approximately equal

portions. This reproduction may occur as frequently as once every half

hour and if unchecked would quickly mount to unimaginable numbers. The

natural cause limiting the growth of bacteria is the generation by the

bacterium of certain substances such as the amino acids, which are

injurious to cell life. The exhaustion of the food supply is not

considered as an important cause of inhibition of multiplication. The

products of growth of one species of bacteria may be helpful or harmful

to other forms. Where the products are helpful the effect is known as

symbiosis, and where harmful the effect is known as antibiosis. In

sewage the presence of both aërobic and anaërobic bacteria is usually

mutually helpful and the condition is an example of symbiosis. The

aërobes, sometimes called obligatory aërobes, are bacteria which demand

available oxygen for their growth. The anaërobes, or obligatory

anaërobes, can grow only in the absence of oxygen. There are other forms

that are known as facultative anaërobes (or aërobes) whose growth is

independent of the presence or absence of oxygen.

Spores are formed by some bacteria when they are subjected to an

unfavorable environment such as high temperatures, the absence of food,

the absence of moisture, etc. Spores are cells in which growth and

animation are suspended but the life of the cell is carried on through

the unsuitable period, somewhat similar to the condition in a plant

seed.

=212. Organic Life in Sewage.=—Living organisms, both plants and

animals, exist in sewage. Bacteria are the smallest of these organisms.

Others, which can be studied easily under the microscope or can be seen

with difficulty by the naked eye but which do not require special

cultures for their study, are classed as microscopic organisms or

plankton. Organisms which are large enough to be studied without the aid

of a microscope or special cultures are classed as macroscopic. The part

taken in the biolysis of sewage by macroscopic organisms belonging to

the animal kingdom, such as birds, fish, insects, rodents, etc., which

feed upon substances in the sewage is so inconsequential as to be of no

importance. Both plants and animals are found among the macroscopic

organisms.

Organisms in sewage may be either harmful, harmless, or beneficial. From

the viewpoint of mankind the harmful organisms are the pathogenic

bacteria. Their condition of life in sewage is not normal and in general

their existence therein is of short duration. It may be of sufficient

length, however, to permit the transmission of disease. The diseases

which can be transmitted by sewage are only those that are contracted

through the alimentary canal, such as typhoid fever, dysentery, cholera,

etc. Diseases are not commonly contracted by contact of sewage with the

skin nor by breathing the air of sewers. It is safe to work in and

around sewage so long as the sewage is kept out of the mouth, and

asphyxiating or toxic gases are avoided.

The beneficial organisms in sewage are those on which dependence is

placed for the success of certain methods of treatment. These organisms

have not all been isolated or identified.

The total number of bacteria in a sample of sewage has little or no

significance. In a normal sewage the number may be between 2,000,000 and

20,000,000 per c.c. and because of the extreme rapidity of

multiplication of bacteria a sample showing a count of 1,000,000 per

c.c. on the first analysis may show 4 to 5 times as many 3 or 4 hours

later. A bacterial analysis of sewage is ordinarily of little or no

value, since pathogenic organisms are practically certain to be present,

there is no interest in the harmless organisms, and the helpful

nitrifying and aërobic bacteria will not grow on ordinary laboratory

media. Occasionally the presence of certain bacteria may indicate the

presence of certain trades wastes. In general, the total bacterial

count, as sometimes reported, represents only the number of bacteria

which have grown under the conditions provided. It bears no relation to

the total number of bacteria in the sample.

The presence of bacteria in sewage is of great importance however, as

practically all methods of treatment depend on bacterial action, and all

sewages which do not contain deleterious trades wastes, contain or will

support the necessary bacteria for their successful treatment, if

properly developed.

=213. Decomposition of Sewage.=—If a glass container be filled with

sewage and allowed to stand, open to the air, a black sediment will

appear after a short time, a greasy scum may rise to the surface, and

offensive odors will be given off. This condition will persist for

several weeks, after which the liquid will become clear and odorless.

The sewage has been decomposed and is now in a stable condition. The

decomposition of sewage is brought about by bacterial action the exact

nature of which is uncertain.

It[129] is well established that many of the chemical effects

wrought by bacteria, as by other living cells, are due, not to the

direct action of the protoplasm, but to the intervention of

soluble ferments or enzymes.

Enzymes are soluble ferments produced by the growth of the bacterial

cell.

In[130] many cases the enzymes diffuse out from the cell and exert

their effort on the ambient substances ... in others the enzyme

action occurs within the cell and the products pass out, (for

example) ... the alcohol-producing enzymes of the yeast cell act

upon sugar within the cell, the resulting alcohol and carbon

dioxide being ejected.

Other chemical effects may be brought about by the direct action of the

living cells, but this has never been well established.

Metabolism is the life process of living cells by which they absorb

their food and convert it into energy and other products. It is the

metabolism of bacterial growth that in itself or by the production of

enzymes hastens the putrefactive or oxidizing stages of the organic

cycles in sewage treatment. Bacteria can assimilate only liquid food

since they have no digestive tract through which solid food can enter.

The surrounding solids are dissolved by the action of the enzymes, the

resulting solution diffusing through the chromatin or outer skin, and

being digested throughout the interior cytoplasm.

Bacteria are sometimes classified as parasites and saprophytes. The

parasites live only on the growing cells of other plant or animal life.

The saprophytes obtain their food only from the life products of living

organisms and do not exist at the expense of the organisms themselves.

Facultative saprophytes (or parasites) may exist on either living or

dead tissue.

The decomposition of sewage may be divided into anaërobic and aërobic

stages. These conditions are usually, but not always, distinctly

separate. The growth of certain forms of bacteria is concurrent, while

the growth of other forms is dependent on the results of the life

processes of other bacteria in the early stages of decomposition.

When sewage is very fresh it contains some oxygen. This oxygen is

quickly exhausted so that the first important step in the decomposition

of sewage is carried on under anaërobic conditions. This is accompanied

by the creation of foul odors of organic substances, ammonia, hydrogen

sulphide, etc.; other odorless gases such as carbon dioxide, hydrogen,

and marsh gas, the latter being inflammable and explosive; and other

complicated compounds. An exception to the rule that putrefaction takes

place only in the absence of oxygen is the production of other

foul-smelling substances by the putrefactive activity of obligatory and

facultative aërobes. Hydrogen sulphide may be produced apparently in the

presence of oxygen the action which takes place not being thoroughly

understood.

The biolysis of sewage is the term applied to the changes through which

its organic constituents pass due to the metabolism of bacterial life.

Organic matter is composed almost exclusively of the four elements:

carbon, oxygen, hydrogen, and nitrogen (COHN) and sometimes in addition

sulphur and phosphorus. The organic constituents of sewage can be

divided into the proteins, carbohydrates, and fats. The proteins are

principally constituents of animal tissue, but they are also found in

the seeds of plants. The principal distinguishing characteristic of the

proteins is the possession of between 15 and 16 per cent of nitrogen. To

this group belong the albumens and casein. The carbohydrates are organic

compounds in which the ratio of hydrogen to oxygen is the same as in

water, and the number of carbon atoms is 6 or a multiple of 6. To this

group belong the sugars, starches and celluloses. The fats are salts

formed, together with water, by the combination of the fatty acids with

the tri-acid base glycerol. The more common fats are _stearin_,

_palmatin_, _olein_, and _butyrine_. The soaps are mineral salts of the

fatty acids formed by replacing the weak base glycerol with some of the

stronger alkalies.

The first state in the biolysis of sewage is marked by the rapid

disappearance of the available oxygen present in the water mixed with

organic matter to form sewage. In this state the urea, ammonia, and

other products of digestive or putrefactive decomposition are partially

oxidized and in this oxidation the available oxygen present is rapidly

consumed, the conditions in the sewage becoming anaërobic. The second

state is putrefaction in which the action is under anaërobic conditions.

The proteins are broken down to form urea, ammonia, the foul-smelling

mercaptans, hydrogen sulphide, etc., and fatty and aromatic acids. The

carbohydrates are broken down into their original fatty acid, water,

carbon dioxide, hydrogen, methane, and other substances. Cellulose is

also broken down but much more slowly. The fats and soaps are affected

somewhat similarly to the hydrocarbons and are broken down to form the

original acids of their make up together with carbon dioxide, hydrogen,

methane, etc. The bacterial action on fats and soaps is much slower than

on the proteins, and the active biological agents in the biolysis of the

hydrocarbons, fats, and soaps are not so closely confined to anaërobes

as in the biolysis of the proteins. The third state in the biolysis of

sewage is the oxidation or nitrification of the products of

decomposition resulting from the putrefactive state. The products of

decomposition are converted to nitrites and nitrates, which are in a

stable condition and are available for plant food. It must be understood

that the various states may be coexistent but that the conditions of the

different states predominate approximately in the order stated. In the

biolysis of sewage there is no destruction of matter. The same elements

exist in the same amount as at the start of the biolytic action.

=214. The Nitrogen Cycle.=—Nitrogen is an element that is found in all

organic compounds. Its presence is necessary to all plant and animal

life. The nitrogenous compounds are most readily attacked by bacterial

action in sewage treatment. The non-nitrogenous substances such as soaps

and fats, and the inorganic compounds are more slowly affected by

bacterial action alone. The element nitrogen passes through a course of

events from life to death and back to life again that is known as the

Nitrogen Cycle. It is typical of the cycles through which all of the

organic elements pass.

Upon the death of a plant or animal, decomposition sets in accompanied

by the formation of urea which is broken down into ammonia. This is

known as the _putrefactive stage_ of the Nitrogen Cycle. The next state

is _nitrification_ in which the compounds of ammonia are oxidized to

nitrites and nitrates, and are thus prepared for plant food. In the

state of _plant life_ the nitrites and nitrates are denitrified so as to

be available as a plant or animal food. The highest state of the

Nitrogen Cycle is _animal life_, in which nitrogen is a part of the

living animal substance or is charged from protein to urea, ammonia,

etc., by the functions of life in the animal. Upon the death of this

animal organism the cycle is repeated. The Nitrogen Cycle, like the

cycle of Life and Death, is purely an ideal condition as in nature there

are many short circuits and back currents which prevent the continuous

progression of the cycle. The conception of this cycle is an aid,

however, in understanding the processes of sewage treatment.

=215. Plankton and Macroscopic Organisms.=—In general the part played by

these organisms in the biolysis of sewage is not sufficiently well

understood to aid in the selection of methods of sewage treatment

involving their activities. The presence in bodies of water receiving

sewage, of certain plankton which are known to exist only when

putrefaction is not imminent, indicates that the body of water into

which the discharge of sewage is occurring is not being overtaxed. The

control of sewage treatment plant effluents so as to avoid the poisoning

of fish life or the contamination of shell fish is likewise important.

The study of plankton and macroscopic life in the treatment of sewage is

an open field for research.

=216. Variations in the Quality of Sewage.=—The quality of sewage varies

with the hour of the day and the season of the year. Some of the causes

of these variations are: changes in the amount of diluting water due to

the inflow of storm water or flushing of the streets or sewers;

variations in domestic activities such as the suspension of

contributions of organic wastes during the night, Monday’s wash, etc.;

characteristics of different industries which discharge different kinds

of wastes according to the stage of the manufacturing process, etc. In

general night sewage is markedly weaker than day sewage in both domestic

and industrial wastes, but in specific cases the varying strength

depends entirely upon the characteristics of the district. Some analyses

are given in Table 74, which emphasize these points.

TABLE 74

SEWAGE ANALYSES SHOWING HOURLY, DAILY, AND SEASONAL VARIATIONS IN

QUALITY

────────────┬────────────┬─────┬────────┬─────────┬──────────┬─────────

Place │ Time │ │ │Suspended│ Remarks │Reference

│ Nitrogen │Total│Chlorine│ Matter │ │

────────────┼────────────┼─────┼────────┼─────────┼──────────┼─────────

Marion, Ohio│Mid’t-noon, │ │ │ │Industrial│ 1

│ 5–21–06. │ 45│ 53│ 190│ │

│Noon-mid’t │ │ │ │Domestic │ 1

│ 5–21–06. │ 37│ 94│ 133│ │

Westerville,│Day │ │ │ │college │ 1

Ohio │ │ 10.2│ 76│ 118│ town │

│Night │ 2.6│ 74│ 41│ │ 1

Columbus, │1904–1905 │ │ │ │ │

Ohio │ │ │ │ │ │

│Mid’t to 2 │ │ │ │ │ 2

│ a.m. │ 4.6│ 50│ 131│ │

│2 a.m. to 4 │ │ │ │ │ 2

│ a.m. │ 3.0│ 52│ 95│ │

│4 a.m. to 6 │ │ │ │ │ 2

│ a.m. │ 2.3│ 51│ 83│ │

│6 a.m. to 8 │ │ │ │ │ 2

│ a.m. │ 2.7│ 48│ 83│ │

│8 a.m. to 10│ │ │ │ │ 2

│ a.m. │ 16.3│ 66│ 476│ │

│10 a.m. to │ │ │ │ │ 2

│ noon │ 11.4│ 100│ 324│ │

│Noon to 2 │ │ │ │ │ 2

│ p.m. │ 11.3│ 86│ 246│ │

│2 p.m. to 4 │ │ │ │ │ 2

│ p.m. │ 12.3│ 78│ 246│ │

│4 p.m. to 6 │ │ │ │ │ 2

│ p.m. │ 22.0│ 78│ 368│ │

│6 p.m. to 8 │ │ │ │ │ 2

│ p.m. │ 8.2│ 71│ 209│ │

│8 p.m. to 10│ │ │ │ │ 2

│ p.m. │ 7.8│ 80│ 120│ │

│10 p.m. to │ │ │ │ │ 2

│ mid’t │ 6.2│ 56│ 117│ │

│ │ │ │ │ │

Center Ave.,│Mid’t to 3 │ │ │ │ │ 3

Chicago. │ a.m. │ │ │ 123│ │

│4 a.m. to 7 │ │ │ │ │ 3

│ p.m. │ │ │ 316│ │

│8 a.m. to 11│ │ │ │ │ 3

│ p.m. │ │ │ 608│ │

│Noon to 3 │ │ │ │ │ 3

│ p.m. │ │ │ 785│ │

│4 p.m. to 7 │ │ │ │ │ 3

│ p.m. │ │ │ 717│ │

│8 p.m. to 11│ │ │ │ │ 3

│ p.m. │ │ │ 287│ │

│ │ │ │ │ │

Columbus, │Sunday │ │ │ │ │ 2

Ohio │ │ 6.7│ 55│ 858│ │

│Monday │ 9.1│ 66│ 1048│ │ 2

│Tuesday │ 9.4│ 69│ 1024│ │ 2

│Wednesday │ 9.6│ 68│ 1005│ │ 2

│Thursday │ 9.2│ 66│ 990│ │ 2

│Friday │ 9.2│ 67│ 1018│ │ 2

│Saturday │ 9.3│ 67│ 1016│ │ 2

│ │ │ │ │ │

Baltimore, │Aug. 1 to │ │ │ │ │ 4

1907–1908 │ Sept. 1 │ 16.0│ │ 246│ │

│Sept. 4 to │ │ │ │ │ 4

│ Oct. 3 │ 19.0│ │ 190│ │

│Oct. 6 to │ │ │ │ │ 4

│ Nov. 4 │ 20.0│ │ 188│ │

│Nov. 15 to │ │ │ │ │ 4

│ Nov. 29 │ 20.0│ │ 164│ │

│Dec. 3 to │ │ │ │ │ 4

│ Dec. 29 │ 20.0│ │ 123│ │

│Jan. 6 to │ │ │ │ │ 4

│ Jan. 21 │ 19.0│ │ 127│ │

│Feb. 2 to │ │ │ │ │ 4

│ Feb. 26 │ 20.0│ │ 149│ │

│Feb. 29 to │ │ │ │ │ 4

│ Mar. 24 │ 28.0│ │ 274│ │

│Mar. 27 to │ │ │ │ │ 4

│ April 29 │ 25.0│ │ 165│ │

│April 30 to │ │ │ │ │ 4

│ May 26 │ 19.0│ │ 104│ │

│June 8 to │ │ │ │ │ 4

│ July 11 │ 15.0│ │ 88│ │

│July 13 to │ │ │ │ │ 4

│ Aug. 8 │ 9.5│ │ 124│ │

────────────┴────────────┴─────┴────────┴─────────┴──────────┴─────────

References:

1. 1908 Report of the Ohio State Board of Health.

2. Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson,

1905.

3. Report on Industrial Wastes from the Stock Yards and Packingtown in

Chicago, by the Sanitary District of Chicago. 1921.

4. Report of the Baltimore Sewerage Commission, 1911.

=217. Sewage Disposal.=—Previous to the development of the

water-carriage method for removing human excreta and other liquid wastes

the solid matter was disposed of by burial and the liquid wastes were

allowed to seep into the ground or to run away over its surface.

Following the development of the water-carriage system, which

necessitated the development of sewers, the problem of ultimate disposal

was rendered more serious by the concentration of human excreta together

with a large volume of water. The unthinking citizen believes the

problem of sewage disposal is solved when the toilet is flushed or the

bath tub is drained. The problem may more truly be said to commence at

this point.

It would appear that the simplest method of disposal of sewage would be

to discharge it into the nearest water course. Unfortunately the nature

of sewage is such that it may be either highly offensive to the senses

or dangerous to health or both, when discharged in this manner. Only the

most fortunate communities are favored with a body of water of

sufficient size to receive sewage without creating a nuisance.

The problems of sewage disposal are to prevent nuisances causing offense

to sight and smell; to prevent the clogging of channels; to protect

pumping machinery; to protect public water supplies; to protect fish

life; to prevent the contamination of shell fish; to recover valuable

constituents of the sewage; to enrich and to irrigate the soil; to

safeguard bathing and boating; for other minor purposes; and in some

cases to comply with the law. Sewage may be treated to attain one or

more of these objects by methods of treatment varying as widely as the

objects to be attained.

=218. Methods of Sewage Treatment.=—In studying the subject of sewage

treatment it must be borne in mind that it is impossible to destroy any

of the elements present. They may be removed from the mixture only by

gasification, straining or sedimentation. Their chemical combinations

may be so changed, however, as to result in different substances than

those introduced to the treatment plant. It is with these chemical

changes that the student of sewage treatment is interested.

The methods of sewage treatment can be classified as mechanical,

chemical and biological. These classifications are not separated by

rigid lines but may overlap in certain treatment devices or methods.

Mechanical methods of treatment are exemplified by sedimentation, and

screening. Chemical precipitation and sterilization are examples of

chemical methods. The biological methods, the most important of all,

include dilution, septicization, filtration, sewage farming, activated

sludge, etc. If for any reason it is desired to treat sewage by more

than one of these methods the procedure should follow as nearly as

possible the order of the occurrence of the phenomena in the natural

biolysis of sewage. For example, in one treatment plant the sewage would

first pass through a grit chamber where the coarse sediment would be

removed, then through a screen where the floating matter and coarse

suspended matter would be removed, then to a sedimentation basin where

some finer suspended matter might settle out, then to a digestive tank

where the solid matter deposited would be worked upon by bacterial

action and partially liquefied. Simultaneous to the liquefaction of the

deposited solid matter the liquid effluent from the digestive tank might

proceed to an aërating device to expedite oxidation, then to an aërobic

filter, and finally to disposal by dilution.

CHAPTER XIV

DISPOSAL BY DILUTION

=219. Definition.=—Disposal of sewage by dilution is the discharge of

raw sewage or the effluent from a treatment plant into a body of water

of sufficient size to prevent offense to the senses of sight and smell,

and to avoid danger to the public health.

=220. Conditions Required for Success.=—Among the desired conditions for

successful disposal by dilution are: adequate currents to prevent

sedimentation and to carry the sewage away from all habitations before

putrefaction sets in, or sufficient diluting water high in dissolved

oxygen to prevent putrefaction; a fresh or non-septic sewage; absence of

floating or rapidly settling solids, grease or oil; and absence of back

eddies or quiet pools favorable to sedimentation in the stream into

which disposal is taking place. The conditions which should be prevented

are: offensive odors due to sludge banks, the rise of septic gases, and

unsightly floating or suspended matter. In some instances the pollution

of the receiving body of water is undesirable and the sewage must be

freed from pathogenic organisms and the danger of aftergrowths minimized

before disposal. Such conditions are typified at Baltimore, where the

sewage is discharged into Back Bay, an arm of Chesapeake Bay. One of the

important industries of the state of Maryland is the cultivation of

oysters. The pollution of the Bay was therefore so objectionable that

careful treatment of the Baltimore sewage has been a necessary

preliminary to final disposal by dilution. It is unwise to draw public

water supplies, without treatment, from a stream receiving a sewage

effluent, no matter how careful or thorough the treatment of the sewage.

The treatment of the sewage is a safeguard, and lightens the load on the

water purification plant, but under no considerations can it be depended

upon to protect the community consuming the diluted effluent.

The sewer outlet should be located well out in the current of the

stream, lake, or harbor. Deeply submerged outlets are usually better

than an outlet at the surface, as a better mixture of the sewage and

water is obtained. The discharge of sewage into a body of water of which

the surface level changes, alternately covering and exposing large areas

of the bottom is unwise, as the sludge which is deposited during

inundation will cause offensive odors when uncovered. Such conditions

must be carefully guarded against when selecting a point of disposal in

tidal estuaries because of the frequent fluctuations in level.

=221. Self-Purification of Running Streams.=—The self-purification of

running streams is due to dilution, sedimentation, and oxidation. The

action is physical, chemical, and biological. When putrescible organic

matter is discharged into water the offensive character of the organic

matter is minimized by dilution. If the dilution is sufficiently great,

it alone may be sufficient to prevent all nuisance. The oxidation of the

organic matter commences immediately on its discharge into the diluting

water due to the growth and activity of nitrifying and other oxidizing

organisms and to a slight degree to direct chemical reaction. So long as

there is sufficient oxygen present in the water septic conditions will

not exist and offensive odors will be absent. When the organic matter is

completely nitrified or oxidized there will be no further demand on the

oxygen content of the stream and the stream will be said to have

purified itself. At the same time that this oxidation is going on some

of the organic matter will be settling due to the action of

sedimentation. If oxidation is completed before the matter has settled

on the bottom the result will be an inoffensive silting up of the river.

If oxidation is not complete, however, the result will be offensive

putrefying sludge banks which may send their stinks up through the

superimposed layers of clean water to pollute the surrounding

atmosphere.

The most important condition for the successful self-purification of a

stream is an initial quantity of dissolved oxygen to oxidize all of the

organic matter contributed to it, or the addition of sufficient oxygen

subsequent to the contribution of sewage to complete the oxidation.

Oxygen may be added through the dilution received from tributaries,

through aëration over falls and rapids, or by quiescent absorption from

the atmosphere. The rapidity of self-purification is dependent on the

character of the organic matter, the presence of available oxygen, the

rate of reaëration, temperature, sedimentation, and the velocity of the

current. Sluggish streams are more likely to purify themselves in a

shorter distance and rapidly flowing turbulent streams are more likely

to purify themselves in a shorter time, other conditions being equal.

Although the absorption of oxygen by a stream whose surface is broken is

more rapid than through a smooth unbroken surface, the growth of algæ,

biological activity, the effect of sunlight, and sedimentation are more

potent factors and have a greater effect in sluggish streams than the

slightly more rapid absorption of oxygen in a turbulent stream. It is

frequently more advantageous to discharge sewage into a swiftly moving

stream, however, regardless of the conditions of self-purification, as

the undesirable conditions which may result occur far from the point of

disposal and may be offensive to no one.

The sewage from a population of about 3,000,000 persons residing in and

about Chicago is discharged into the Chicago Drainage Canal. It

ultimately reaches tide water through the Des Plaines, the Illinois, and

the Mississippi Rivers. The action occurring in these channels is one of

the best illustrations known of the self-purification of a stream. In

Table 75 are shown the results of analyses of samples taken at various

points below the mouth of the Chicago River where the diluting water

from Lake Michigan enters, to Grafton, Illinois, at the junction of the

Illinois and Mississippi Rivers about 40 miles above St. Louis. The

effect of the physical characteristics of the stream on its chemical

composition is well illustrated in this table. The rise in the chlorine

content between Lake Michigan and the entrance to the Drainage Canal is

a measure of the addition of sewage. Since the chlorine is an inorganic

substance which is not affected by biologic action, its loss in

concentration in the lower reaches of the rivers is due to dilution by

tributaries and sedimentation, e.g., between the end of the canal at

Lockport and the sampling point at Joliet, the entrance of the Des

Plaines River reduces the concentration of chlorine from 124.5 to 41.5

parts per million. The entrance of the Kankakee River at Dresden Heights

further reduces the chlorine to 24.5 p.p.m. The increase of albuminoid

and ammonia nitrogen accompanied by a decrease in nitrites and nitrates,

between the upper end of the canal at Bridgeport and its lower end at

Lockport indicates the reducing action proceeding therein. The oxidizing

action over the various dams and the effect of dilution with water

containing oxygen is shown between miles 34 and 38, at mile 79, and at

mile 294. The excellent effect of quiescent sedimentation and aëration

in Peoria Lakes is shown between miles 145, 161 and 165.

TABLE 75

ANALYSES OF CHICAGO, DES PLAINES AND ILLINOIS RIVERS

(Parts per million)

────────────┬────────┬──────────────────────────────────────────────

Sampling │Distance│January-June, 1900, from “Sewage Disposal,” by

Point │in Miles│ Kinnicutt, Winslow and Pratt

│ from │

│ Lake │

│Michigan│

────────────┼────────┼────────┬────────┬──────────┬────────┬────────

│ │Chlorine│ Ammonia│Albuminoid│Nitrates│Nitrates

│ │ │Nitrogen│ Nitrogen│ │

│ │ │ │ │ │

────────────┼────────┼────────┼────────┼──────────┼────────┼────────

Lake │ 0│ 3.0│ 0.03│ 60.13│ 0.002│ 0.008

Michigan │ │ │ │ │ │

│ │ │ │ │ │

Canal, │ 5│ 96.6│ 8.05│ 2.05│ .021│ .074

Bridgeport│ │ │ │ │ │

Canal, │ 34│ 124.5│ 10.90│ 2.07│ .013│ .066

Lockport │ │ │ │ │ │

Joliet │ 38│ 41.5│ 4.22│ 0.83│ .021│ .086

│ │ │ │ │ │

│ │ │ │ │ │

│ │ │ │ │ │

│ │ │ │ │ │

│ │ │ │ │ │

Dresden │ 52│ │ │ │ │

Heights │ │ │ │ │ │

Dresden │ 52│ │ │ │ │

Heights │ │ │ │ │ │

Morris │ 62│ 24.5│ 2.46│ .60│ .075│ .424

│ │ │ │ │ │

Marseilles │ 79│ │ │ │ │

Marseilles │ 79│ │ │ │ │

Ottawa │ 85│ 15.3│ 1.55│ .41│ .197│ .966

La Salle │ 100│ 17.5│ 1.05│ .43│ .109│ .979

Henry │ 129│ 13.3│ .92│ .38│ .102│ .800

Chillicothe │ 145│ │ │ │ │

│ │ │ │ │ │

│ │ │ │ │ │

Averyville │ 161│ 13.5│ .81│ .37│ .004│ 1.150

│ │ │ │ │ │

│ │ │ │ │ │

Wesley │ 165│ 12.0│ .57│ .41│ .083│ 1.03

│ │ │ │ │ │

Pekin │ 175│ 12.3│ .70│ .43│ .060│ .990

Havana │ 205│ 11.2│ .60│ .36│ .065│ .570

Beardstown │ 237│ 10.7│ .69│ .44│ .106│ .685

La Grange │ 249│ │ │ │ │

Kampsville │ 294│ 11.3│ .66│ .44│ .044│ .870

Kampsville │ 294│ │ │ │ │

Grafton │ 325│ 9.8│ .46│ .42│ .031│ 1.06

│ │ │ │ │ │

Grafton │ 325│ │ │ │ │

│ │ │ │ │ │

────────────┴────────┴────────┴────────┴──────────┴────────┴────────

────────────┬─────────────────────┬───────────

Sampling │ Dissolved Oxygen │ Remarks

Point │ │

│ │

│ │

│ │

────────────┼───────┬──────┬──────┼───────────

│ Jan. │ July │ Nov. │

│30–Feb.│ 8–15 │12–19,│

│2, 1912│ 1912 │ 1912 │

────────────┼───────┼──────┼──────┼───────────

Lake │ 14.1│ │ 10.8│Typical

Michigan │ │ │ │ chemical

│ │ │ │ analysis

Canal, │ │ │ 6.9│Kedzie

Bridgeport│ │ │ │ Avenue

Canal, │ 9.9│ │ 1.7│Above dam

Lockport │ │ │ │

Joliet │ │ 1.4│ 5.6│Aëration

│ │ │ │ over dam.

│ │ │ │ Dilution

│ │ │ │by Des

│ │ │ │ Plaines

│ │ │ │ River

Dresden │ │ 1.0│ 4.1│Des Plaines

Heights │ │ │ │ River

Dresden │ │ │ 10.4│Kankakee

Heights │ │ │ │ River

Morris │ 7.8│ │ 5.7│Illinois

│ │ │ │ River

Marseilles │ 5.7│ 0.6│ 6.8│Above dam

Marseilles │ 8.2│ 4.5│ 9.3│Below dam

Ottawa │ 10.0│ │ 8.1│

La Salle │ 5.4│ │ 7.8│

Henry │ │ │ 7.9│

Chillicothe │ 3.4│ 1.5│ 5.9│Above

│ │ │ │ Peoria

│ │ │ │ Lakes

Averyville │ 3.3│ 8.2│ 8.9│Below

│ │ │ │ Peoria

│ │ │ │ Lakes

Wesley │ │ │ 7.1│Below

│ │ │ │ Peoria

Pekin │ 4.9│ 3.2│ 8.9│

Havana │ 4.8│ │ 8.8│

Beardstown │ 6.5│ │ 9.1│

La Grange │ │ 4.1│ 9.4│Below dam

Kampsville │ │ 4.1│ 10.0│Above dam

Kampsville │ │ 4.6│ 10.0│Below dam

Grafton │ 6.6│ 4.7│ 10.4│Illinois

│ │ │ │ River

Grafton │ │ 7.3│ 12.0│Mississippi

│ │ │ │ River

────────────┴───────┴──────┴──────┴───────────

=222. Self-Purification of Lakes.=—Sewage may be disposed of into lakes

with as great success as into running streams if conditions exist which

are favorable to self-purification. Lakes and rivers purify themselves

from the same causes; oxidation, sedimentation, etc., but in the former

the currents are much less pronounced and may be entirely absent. In

shallow lakes (20 feet or less in depth) dependence must be placed on

horizontal currents and the stirring action of the wind to keep the

water in motion in order that the sewage and the diluting water may be

mixed. In deeper bodies of water, currents induced by the wind are

helpful but entire dependence need not be placed upon them. Vertical

currents, and the seasonal turnovers in the spring and fall completely

mix the waters of the lake above those layers of water whose temperature

never rises higher than 4° C.

In the early winter the cold air cools the surface waters of a lake. The

cooling increases the density of the surface water causing it to sink,

and allowing the warmer layers below to rise and become cooled. After

the temperature of the entire lake has reached 4° C. the vertical

currents induced by temperature cease, as continued cooling decreases

the density of the surface water maintaining the same layer at the

surface. In the spring as the temperature of the surface water rises to

4° C. and above it becomes heavier and drops through the colder water

below causing vertical currents. These phenomena are known as the fall

and spring turnovers. The former is more pronounced. These turnovers are

effective in assisting in the self-purification of lakes.

=223. Dilution in Salt Water.=—The oxygen content in salt water is about

20 per cent less than in fresh water at the same temperature. The

greater content of matter in solution in salt water reduces its capacity

to absorb many sewage solids. This, together with the chemical reaction

between the constituents of the salt water and those of the sewage serve

to precipitate some of the sewage solids and to form offensive sludge

banks. The evidence of the action which takes place in the absorption of

oxygen from the atmosphere by salt water and its effect on dissolved

sewage solids is conflicting, but in general fresh water is a better

diluting medium than salt water.

Black and Phelps have made valuable studies of the relative rates of

absorption of oxygen from the air by fresh and salt water. The results

of their experiments are published in a Report to the Board of Estimate

and Apportionment of N. Y. City, made March 23, 1911.[131] Concerning

these rates they conclude:

Therefore there is no reason to believe that the reaëration of

salt water follows any other laws than those we have determined

mathematically and experimentally for fresh water. In the absence

of fuller information on the effect of increased viscosity upon

the diffusion coefficient, it can only be stated that the rate of

reaëration of salt water is less than that of fresh water, in

proportion to the respective solubilities of oxygen in the two

waters, and still less, but to an unknown extent, by reason of the

greater viscosity and consequent small value of the diffusion

coefficient.

=224. Quantity of Diluting Water Needed.=—In a large majority of the

problems of disposal of sewage by dilution it is not necessary to add

sufficient diluting water to oxidize completely all organic matter

present. Ordinarily it is sufficient to prevent putrefactive conditions

until the flow of the stream, lake, or tidal current, has reached some

large body of diluting water or where putrefaction is no longer a

nuisance. It is never desirable to allow the oxygen content of a stream

to be exhausted as putrescible conditions will exist locally before

exhaustion is complete. The exact point to which oxygen can be reduced

in safety is in some dispute. Black and Phelps have assumed 70 per cent

of saturation as the allowable limit; Fuller has placed it at 30 per

cent; Kinnicutt, Winslow, and Pratt have placed it at 50 per cent. Since

the reaction between the oxygen and the organic matter is quantitative,

others have placed the limit in terms of parts per million of oxygen.

Wisner,[132] has recommended a minimum of 2.5 p.p.m. as the limit for

the sustenance of fish life, which is not far from Fuller’s limit for

hot-weather conditions.

Formulas of various types have been devised to express the rate of

absorption of oxygen with a given quantity of diluting water which is

mixed with a given quantity and quality of sewage. The quantity of

sewage is sometimes expressed in terms of the tributary population or in

other ways. Knowing the rate at which oxygen is exhausted and the

velocity of flow of the stream, the point at which the oxygen will be

reduced to the limit allowed is easily determined. The accuracy of none

of these formulas has been proven, and their use, without an

understanding of the effect of local conditions, may lead to error. They

may be used as a check on the bio-chemical oxygen demand determinations,

which should be conclusive.

The following formula, based on the work of Black and Phelps, is a guide

to the amount of sewage which can be added to a stream without causing a

nuisance. It is:

_C_ = (log(_O′_⁄_O_))⁄_kt_,

in which _C_ = per cent of sewage allowed in the water;

_O′_ = per cent of saturation or the p.p.m. of oxygen in the

mixture at the time of dilution;

_O_ = per cent of saturation or the p.p.m. of oxygen in the

stream after period of flow to point beyond which no

nuisance can be expected;

_t_ = time in hours required for the stream to flow to this

point;

_k_ = constant determined by test determinations of the

factors in the following expression:

_k_ = (log(_O′__{1}⁄_O__{1}))⁄_C__{1}_t__{1},

in which _O′__{1} = per cent of saturation or the p.p.m. of oxygen in

the diluting water before mixing with the sewage;

_O__{1} = per cent of saturation or the p.p.m. of oxygen in

an artificial mixture made in the laboratory,

after _t__{1} hours of incubation;

_C__{1} = per cent of sewage in the mixture;

_t__{1} = number of hours of incubation of the mixture of

sewage and diluting water under laboratory

conditions.

In the solution of these formulas it is desired to determine the

permissible amount of sewage to discharge into a given quantity of

diluting water. This value is expressed by C in the first equation. In

solving this equation:

_O′_ is determined by laboratory tests and should represent

the conditions to be expected during various seasons

of the year;

_O_ is determined by judgment. It may be 30 per cent or 50

per cent or more as previously explained;

_t_ is determined by float tests or other measurements of

the stream flow;

_k_ is determined by laboratory tests in which mixtures of

various strengths are incubated for various periods

of time. Different values of _k_ will be obtained for

different characteristics of the sewage; but for the

same sewage the value of _k_ should be unchanged for

different periods of incubation.

Rideal devised the formula:[133]

_XO_ = _C_(_M_ − _N_)_S_

in which _X_ = flow of the stream expressed in second-feet;

_O_ = grams of free oxygen in one cubic foot of water;

_S_ = rate of sewage discharge in second-feet;

_M_ = grams of oxygen required to consume the organic matter

in one cubic foot of diluted sewage as determined by

the permanganate test with 4 hours boiling;

_N_ = grams of oxygen available in the nitrites and nitrates

in one cubic foot of diluted sewage;

_C_ = ratio between the amount of oxygen in the stream and

that required to prevent putrefaction. Where _C_ is

equal to or greater than one, satisfactory conditions

have been attained.

In using this formula it is necessary to make analyses of trial mixtures

of sewage and water until the correct mixture has been found.

Hazen’s formula is:[134]

_D_ = _x_⁄_S_ = 4_m_⁄_O_,

in which _D_ = dilution ratio;

_x_ = volume of water;

_S_ = volume of sewage;

_m_ = result of the oxygen consumed test expressed in p.p.m.

after 5 minutes, boiling with potassium permanganate;

_O_ = amount of dissolved oxygen in the diluting water

expressed in p.p.m.

For comparison with Rideal’s formula the factor of 7 should be used

instead of 4 to allow for the increased time of boiling.

Since the amount of oxygen needed is dependent on the amount of organic

matter in the sewage rather than the total volume of the sewage, and

since the amount of organic matter is closely proportional to the

population, the amount of diluting water has sometimes been expressed in

terms of the population. Hering’s recommendation for the quantity of

diluting water necessary for Chicago sewage was 3.3 cubic feet of water

per second per thousand population. Experience has proven this to be too

small. Between a minimum limit of 2 second-feet and a maximum of 8

second-feet of diluting water per thousand population the success of

dilution is uncertain. Above this limit success is practically assured

and below this limit failure can be expected.

Even with these carefully devised formulas and empirical guides, the

factors of reaëration, dilution, sedimentation, temperature, etc., may

have so great an effect as to vitiate the conclusions. As shown in Table

75 dilution in winter is far more successful than in summer. The lower

temperatures so reduce the activity of the putrefying organisms that

consumption of oxygen is greatly retarded.

=225. Governmental Control.=—A comprehensive discussion of the legal

principles governing the pollution of inland waters is contained in “A

Review of the Laws Forbidding the Pollution of Inland Waters,” by E. B.

Goodell, published by the United States Geological Survey in 1905, as

Water Supply Paper No. 152.

The disposal of sewage by dilution is subject to statutory limitations

in many states. The enforcement of these laws is usually in the hands of

the state board of health, which is frequently given discretionary

powers to recommend and sometimes to enforce measures for the abatement

of an actual or potential nuisance. Such recommendations usually take

the form of a specification of certain forms of treatment preliminary to

disposal by dilution. No project for the disposal of sewage by dilution

should be consummated until the local, state, national, and in the case

of boundary waters, international laws have been complied with. The

attitude of the courts in different states has not been uniform. Little

guidance can be taken from the personal feeling of the persons

immediately interested. The opinion of the riparian owner 5 miles down

stream may differ materially from the popular will of the voters of a

city, and it is likely to receive a more favorable hearing from the

court. Statutes and legal precedents are the safest guides.

=226. Preliminary Treatment.=—If the sewage to be disposed of by

dilution contains unsightly floating matter, oil, or grease, no amount

of oxygen in the diluting water will prevent a nuisance to sight, or the

formation of putrefying sludge banks. Under such conditions it will be

necessary to introduce screens or sedimentation basins, or both, in

order to remove the floating and the settling solids. Biologic tanks,

filtration, or other methods of treatment may be necessary for the

removal of other undesirable constituents.

=227. Preliminary Investigations.=—Before adopting disposal of sewage by

dilution without preliminary treatment, or before considering the proper

form of treatment necessary to render disposal by dilution successful, a

study should be made of the character of the body of water into which

the sewage or effluent is to be discharged. This study should include:

measurements of the quantity of water available at all seasons of the

year; analyses of the diluting water to determine particularly the

available dissolved oxygen; observations of the velocity and direction

of currents, and the effect of winds thereon; a study of the effect on

public water supplies, bathing beaches, fish life, etc. Good judgment,

aided by the proper interpretation of such information should lead to

the most desirable location for the sewer outlet. If preliminary

treatment is found to be necessary tests should be made to determine the

necessary extent and thoroughness of the treatment.

CHAPTER XV

SCREENING AND SEDIMENTATION

=228. Purpose.=—The first step in the treatment of sewage is usually

that of coarse screening in order to remove the larger particles of

floating or suspended matter. Screens and sedimentation basins are used

to prevent the clogging of sewers, channels, and treatment plants; to

avoid clogging of and injuries to machinery; to overcome the

accumulation of putrefying sludge banks; to minimize the absorption of

oxygen in diluting water; and to intercept unsightly floating matter.

By the plain sedimentation of sewage is meant the removal of suspended

matter by quiescent subsidence unaffected by septic action or the

addition of chemicals or other precipitants. In order to prevent septic

action plain sedimentation tanks must be cleaned as frequently as once

or twice a week in warm weather but not quite so often in cold weather.

Fine screening may take the place of sedimentation where insufficient

space is available for sedimentation tanks, and it is desired to remove

only a small portion of the suspended matter. Recent American practice

has tended to restrict the field of fine screening to treatment

requiring less than 10 per cent removal of suspended matter, thus

eliminating screens from the field covered by plain sedimentation tanks.

The practice is well expressed by Potter, who states:[135]

Where a high degree of purification is sought, the use of fine

screens is of doubtful value. A modern settling tank will give

better results and at a less cost for a given degree of

purification. A settled liquid is also superior to a screened

liquid for subsequent biological treatment in filters.... Again

the storing of large quantities of screenings must necessarily be

more objectionable than the storing of the digested sludge of a

modern settling tank.

[Illustration:

FIG. 150.—Types of Moving Screens.

Trans. Am. Society Civil Engineers, Vol. 78, 1915, p. 893.

]

=229. Types of Screens.=—The definitions of some types of screens as

proposed by the American Public Health Association follow: A _bar

screen_ is composed of parallel bars or rods. A _mesh screen_ is

composed of a fabric, usually wire. A _grating_ consists of 2 sets of

parallel bars in the same plane in sets intersecting at right angles. A

_band screen_ consists of an endless perforated band or belt which

passes over upper and lower rollers. A _perforated plate screen_ is made

of an endless band of perforated plates similar to a band screen. A

_wing screen_ has radial vanes uniformly spaced which rotate on a

horizontal axis. A _disc screen_ consists of a circular perforated disc

with or without a central truncated cone of similar material mounted in

the center. The Reinsch Wurl screen is the best known type of disc

screen. A _cage screen_[136] consists of a rectangular box made up of

parallel bars with the upstream side of the box or cage omitted.

Allen[137] gives the following definitions: A _drum screen_ is a

cylinder or cone of perforated plates or wire mesh which rotates on a

horizontal axis. A _shovel vane screen_ is similar to a wing screen with

semicircular wings and a different method of removing the screenings.

Examples of a band screen, a wing screen, a shovel vane screen, a drum

screen and a disc screen are shown in Fig. 150. A bar screen is shown in

Fig. 151 and a cage screen is shown in Fig. 152.

[Illustration:

FIG. 151.—Sketch of a Bar Screen.

]

[Illustration:

FIG. 152.—Sketch of a Cage Screen.

]

Screens can be classed as fixed, movable, or moving. Fixed screens are

permanently set in position and must be cleaned by rakes or teeth that

are pulled between the bars. Movable screens are stationary when in

operation, but are lifted from the sewage for the purpose of cleaning.

Moving screens are in continuous motion when in operation and are

cleaned while in motion. Fixed bar screens may be set either vertical,

inclined, or horizontal.

Movable screens with a cage or box at the bottom are sometimes used. The

box should be of solid material to prevent the forcing of screenings

through it when the screen is being raised for cleaning. A mesh screen

should be used only under special circumstances because of the

difficulty in cleaning. Screens which must be raised from the sewage for

cleaning should be arranged in pairs in order that one may be working

when the other is being cleaned. Movable screens are undesirable for

small plants because the labor involved in raising and lowering is

greater than in cleaning with a rake and the screens are more likely to

be neglected. In a large plant rakes operated by hand are too small for

cleaning the screens. A fixed screen is sometimes used with moving teeth

fastened to endless chains. The teeth pass between the parallel bars and

comb out the screenings. If the screen chamber in a small plant is too

deep for accessibility a movable cage or box screen may be desirable.

Moving screens are generally of fine mesh or perforated plates. They are

kept moving in order to allow continuous cleaning. They are cleaned by

brushes or by jets of air, water, or steam.

=230. Sizes of Openings.=—The area or size of the opening of a screen is

dependent upon the character of the sewage to be treated and upon the

object to be attained.

Large screens, with openings between 1½ inches and 6 inches are used to

protect centrifugal pumps, tanks, automatic dosing devices, conduits,

and gate valves from large objects such as pieces of timber, dead

animals, etc., which are found in sewage. The quantity of material

removed is variable, and is usually small.

Medium-size screens with openings from ¼ inch to 1½ inches are used to

prepare sewage for passage through reciprocating pumps, complex dosing

apparatus, contact beds, and sand filters. The amount of material

removed varies from 0.5 to 10 cubic feet per million gallons of sewage

treated, dependent on the character of the sewage and the size of the

screen. Screenings before drying contain 75 to 90 per cent moisture and

weigh 40 to 50 pounds per cubic foot. At times the amount removed may

vary widely from the limits stated. Schaetzle and Davis[138] state:

Screenings differ greatly both in amount and character.... The

amount varies with the days of the week as well as during the

course of the day. It reaches its maximum about noon or shortly

before and commences to disappear about midnight, reaching a

minimum about 4 or 5 a.m. The material is almost wholly organic

and consists of scraps of vegetables or fruit, cloth, hair, wood,

paper and lumps of fecal matter. The amount varies so widely that

it is impossible to state just what to expect any definite size

screen to remove. The amount of water contained is small compared

with that in the sludge in sedimentation basins and amounts to

from 70 per cent to 80 per cent. On account of its organic origin

it is highly putrescible.

Medium-size screens are sometimes placed close together with the bars of

the one opposite the openings in the other, thus approaching a fine

screen.

Fine screens vary in size of opening from ¼ inch to 50 openings per

linear inch or 2,500 per square inch. They are used for removing solids

preparatory to disposal by dilution, to protect sprinkling filters,

complex dosing apparatus, sand filters, sewage farms, and to prevent the

formation of scum in subsequent tank treatment. In general, fine screens

will remove from 0.1 to 1 cubic yard of wet material per million gallons

of sewage treated. The wet screenings will contain about 75 per cent

moisture and will weigh about 60 pounds per cubic foot. The dry weight

of the screenings will therefore be about 10 to 400 pounds per million

gallons of sewage treated. The effect of the removal of this amount of

material is usually not detectable by methods of chemical analysis, the

amount of suspended matter before and after screening being found

unchanged.

In his conclusions on the discussion of the results to be expected from

fine screens, Allen states:[139]

With openings not more than 0.1 inch in size, fine screening

should remove at least 30 per cent of the suspended solids and 20

per cent of the suspended organic solids from ordinary domestic

sewage, or 0.1 cubic yard of screenings, containing 75 per cent

water per thousand population daily.

The effect of the use of different size openings under the same

conditions is shown in Fig. 153.[140] Some data covering the amount of

material removed by screening are given in Table 76. More extensive data

are given in Volume III of “American Sewerage Practice” by Metcalf and

Eddy.

TABLE 76

DATA ON SCREENS

(Trans. Am. Society Civil Engineers, Vol. 78, Page 942)