The action in an intermittent sand filter is more complete than in other

forms of filters because a greater surface is exposed to the passage of

sewage by the fine sand particles, and the sewage is in contact with the

filtering material a longer time due to the lower rate of filtration and

the slow velocity of flow through the filter. It is essential that the

sewage be applied to the bed intermittently in order that air shall be

entrained in the filter. The period between doses should not be so long

that the filter becomes dry.

In the operation of an intermittent sand filter one dose per day is

considered an ordinary rate of application, although some plants operate

with as many as four doses per day per filter, and others on one dose at

long and irregular intervals. It is not always necessary to rest the

filter for any length of time unless signs of overloading and clogging

are shown. The intermittent dosing action may be obtained by the action

of an automatic siphon as is described in Chapter XXI. The sewage is

distributed on the beds through a number of openings in the sides of

distributing troughs resting on the surface of the filter. The sewage is

withdrawn from the bottom of the filter through a system of underdrains,

into which it enters after its passage through the bed. There are no

control devices on the outlet, as the rate of filtration is controlled

by the action of the dosing apparatus and the rate at which sewage is

delivered to it. The action of the dosing apparatus should respond

quickly to variations in sewage flow. As the doses are applied to a sand

filter, a mat of organic matter or bacterial zoöglea is formed on the

surface of the bed. The mat is held together by hair, paper, and the

tenacity of the materials. It may attain a thickness of ¼ to ½ an inch

before it is necessary to remove it. So long as the filter is draining

with sufficient rapidity this mat need not be removed, but if the bed

shows signs of clogging, the only cleaning that may be necessary will be

the rolling up of this dried mat. It is believed that the greater

portion of the action in the filter occurs in the upper 5 to 8 inches of

the bed, but occasionally the beds become so clogged that it is

necessary to remove ¾ of an inch to 2 inches of sand in addition to the

surface mat, or to loosen up the surface by shallow plowing or

harrowing. The necessity for such treatment may indicate that the filter

is being overloaded as a result of which the rate of filtration should

be decreased or the preliminary treatment should be improved. The

plowing of clogging material into the bed should be avoided as under

these conditions the final condition of the bed will be worse than its

condition when trouble was first observed.

In winter the surface of the bed should be plowed up into ridges and

valleys. The freezing sewage forms a roof of ice which rests on the

ridges and the subsequent applications of sewage find their way into the

filter through the valleys under the ice. In a properly operated bed the

filtering material will last indefinitely without change. If a filter is

operated at too high a rate, however, although the quality of the

effluent may be satisfactory, it will be necessary at some time to

remove the sand and restore the filter.

The rate of filtration depends on the character of the influent, the

desired quality of the effluent, and the depth and character of the

filtering material. Filters can be found operating at rates of 50,000

gallons per acre per day and others at eight times this rate. For sewage

which has had some preliminary treatment, the rate should not exceed

100,000 gallons per acre per day, whereas the rate for raw sewage should

be less than this. For rough estimates made without tests of the sewage

in question, the rate should not be taken at more than 1,000 persons per

acre. If the preliminary treatment of the sewage has been thorough and

the material of the sand filter is coarser than ordinary the rate of

filtration can be high. For less careful preliminary treatment and fine

filtering material the rates must be reduced. The sewage must undergo

sufficient preliminary treatment to remove large particles of solid

matter which would otherwise clog the dosing apparatus and the filter.

This treatment should include grit removal, screening, and some form of

tank treatment. Some plants have operated successfully with a stale

sewage and no preliminary treatment, as at Brockton, Mass. Septic tank

effluent can be treated successfully on an intermittent sand filter, but

not so satisfactorily as the effluent from a tank delivering a fresh

sewage.

The material of the filter should consist of clean, sharp, quartz or

silica sand with an effective size[164] of 0.2 to 0.4 mm., preferably

about 0.25 to 0.35 mm., and a uniformity coefficient[165] of 2 to 4.

Within the limits mentioned no careful attention need be given to the

size of the material. Natural sand found in place has been underdrained

and used successfully for sewage treatment. The size of the sand is

fixed by the rate of filtration rather than the bacteriological action

of the filter. A coarse sand will permit the sewage to pass through the

bed too rapidly, and a fine sand will hold it too long or will become

clogged. The same size of material should be used throughout the bed,

except that a layer of gravel from 6 to 12 inches thick, graded from

very small sizes to stones just passing a 2–inch ring should be placed

at the bottom to facilitate the drainage of the bed.

The thickness of the sand layer should not be less than 30 inches to

insure complete treatment of the sewage. In shallower beds the sewage

might trickle through without adequate treatment. Beds are ordinarily

made from 30 to 36 inches deep, but when deeper layers of sand are found

in place there is no set limit to the depth which may be used. The shape

and overall dimensions of the bed should conform to the topography of

the site and the rate of filtration adopted. A plan and cross-section of

an intermittent sand filter showing the distribution and under drainage

systems are given in Fig. 166 and 175.

The distribution system consists of a system of troughs on the surface

of the filter, laid out in a branching form, as shown in the figure. The

openings in the troughs should be so located that the maximum distance

from any point on the bed to the nearest opening should not exceed 20 to

30 feet. If the filters are small enough, troughs need not be used, the

sewage being distributed from one corner, or from mid-points on the

sides. Where troughs are used they should be supported from the bottom

of the filter in order to prevent uneven settling due to the washing of

the sand. The openings in the troughs are made adjustable by swinging

gates as shown in Fig. 176, or by other means so that after the filter

is in operation the intensity of the dose on any portion of the filter

can be changed. The troughs may be placed with their bottoms level with

the surface of the sand and with sides of sufficient height to give the

required gradient to the water surface, or they may be built up above

the surface of the filter and given the required slope so that the

surface of the flowing water is parallel to the bottom of the trough. In

either case a splash plate should be placed at each opening, so that not

less than 2 feet of the surface of the sand is protected in all

directions from the opening. A stone or concrete slab 2 to 4 inches

thick makes a satisfactory splash plate. Either wood or concrete may be

used for the construction of the troughs. The former is less durable,

but also less expensive in first cost. The capacity of the troughs may

be computed by Kutter’s formula with the quantity to be carried equal to

the maximum rate of discharge of the feeding siphon, with a reduction in

size below each branch or outlet proportional to the amount which will

be discharged above this point.

[Illustration:

FIG. 175.—Plan and Section of an Intermittent Sand Filter Showing

Central Location of Control House.

]

The operation of automatic devices for dosing the bed is explained in

Chapter XXI. The dosing tank should have a capacity sufficient to cover

the bed to a depth of about 1 to 3 inches at one dose, and the siphon

should discharge at a rate of about one second-foot for each 5,000

square feet of filter area. A dose should disappear within 20 minutes to

half an hour after it is applied to the filter. With the rate stated and

four applications per day to a depth of 1 inch at each dose, the rate

per acre per day will be 109,000 gallons.

[Illustration:

FIG. 176.—Distributing Trough with Adjustable Openings.

]

The filtration of sewage through sand in a manner similar to the _rapid

sand filtration_ of water is being attempted at the Great Lakes Naval

Training Station. No results of this treatment have been published and

the practical success of the method has not been assured.

=259. Cost of Filtration.=—Only comparative figures can be given in

stating the costs of filtration, as most data available are based on

pre-war conditions, and are therefore unreliable for present conditions.

The variations from the figures given may be very large but in general

the relative costs have not changed. The figures given in Table 90 are

suggestive of the relative costs of the different forms of filtration.

TABLE 90

RELATIVE COSTS OF DIFFERENT METHODS OF SEWAGE TREATMENT

Costs in Dollars per Million Gallons per Day

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

Form of Treatment │First Cost[166]│Operation and │ Total

│ │ Maintenance │

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

Coarse screens │ │ │ 0.20

Fine screens │ │ │ 3.00

Plain sedimentation │ 7.00│ 3.00│ 10.00

Chemical precipitation │ │ │ 22.00[167]

Septic tank │ 7.00│ 1.00│ 8.00

Imhoff tank │ 10.00│ 1.00│ 11.00

Contact bed │ 8.00│ 2.00│ 10.00

Trickling filter │ 4.00│ 2.00│ 6.00

Intermittent sand filter │ 15.00│ 10.00│ 25.00

Activated sludge │ 6.50│ 8.50│ 15.00[168]

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

IRRIGATION

=260. The Process.=—Broad irrigation is the discharge of sewage upon the

surface of the ground, from which a part of the sewage evaporates and

through which the remainder percolates, ultimately to escape in surface

drainage channels. Sewage farming is broad irrigation practiced with the

object of raising crops. Broad irrigation can be accomplished

successfully without the growing of crops, but it is seldom attempted as

some return and sometimes even a profit can be obtained from the crops

raised. Broad irrigation and sewage farming differ from intermittent

sand filtration in the intensity of the application of the sewage, the

method of preparing the area on which the sewage is to be treated, and

the care in operation. In broad irrigation and intermittent sand

filtration the paramount consideration is successful disposal of the

sewage. In sewage farming the paramount consideration is the growing of

crops. The growing of crops may be combined with irrigation and

filtration, however, but the crop should be sacrificed to the successful

disposal of the sewage.

The change which occurs in the characteristics of the sewage due to its

filtration through the ground is the same as occurs in aërobic

filtration. The effect on the crops is mainly that of an irrigant, as

the manurial value of the sewage is small.

=261. Status.=—The disposal of sewage by broad irrigation was practiced

in England previous to the development of any of the more intensive

biologic methods of treatment. It was considered the only safe and

sanitary method for the disposal of sewage, and as a result, areas

irrigated by sewage were common throughout England. Crops were grown on

these areas as a minor consideration, and sewage farming gained some of

its popularity from the apparent success of these disposal areas. The

success of sewage farms is due more to generous irrigation in dry years

than to fertilization by sewage.

The sewage farms of Paris and Berlin are frequently cited as examples of

the successful and remunerative disposal of sewage by farming in

connection with broad irrigation. Kinnicutt, Winslow, and Pratt[169]

state:

The Berlin Sewage farms offer examples of broad irrigation under

better conditions ... of 21,008 acres receiving sewage, 16,657

acres were farmed by the city, 3,956 acres were leased to farmers,

and only 395 acres were unproductive. The contributing population

at this time was 2,064,000 and the average amount of sewage

treated was 77,000,000 gallons, giving a daily rate of treatment

of about 3,700 gallons per acre of prepared land. The soil is

sandy and of excellent quality. A quarter of the area operated by

the authorities is devoted to pasturage, and about a third to the

cultivation of cereals, of which winter rye and oats are the most

important. Potatoes and beets are grown in considerable amounts

and a wide variety of other crops in smaller proportions.... Even

fish ponds are made to yield a part of the revenue, and the drains

on some of the farms have been successfully stocked with breed

trout.

The cost of the Berlin farms to March 31, 1910, was $17,470,000,

somewhat more than half being the purchase price of the land. The

expenses for this year amounted to $1,300,385 for maintenance, and

$741,818 for interest charges. The receipts were $1,240,773 and

there was an estimated increase of $122,593 in value of live stock

and other property.

The conditions at Berlin are quoted at length to indicate the success

which can accompany broad irrigation, and as an example of what is being

done abroad, where the rainfall is light and the soil is suitable.

In the United States success in sewage farming has not been marked. This

may be due partially to the relative weakness of American sewages, to

the cost of labor, to lack of satisfactory irrigation areas, and to

inattention to details. An attempt was made to grow crops on the sand

filters at Brockton, Mass., but it was finally abandoned as the

interests of the crops and the successful treatment of the sewage could

not both be satisfied. At Pullman, Illinois,[170] in 1880, there was

commenced probably the most extensive attempt at sewage farming in

eastern United States. The farm was a failure from the start, because of

the clay soil, and it was subsequently abandoned. Sewage farming, mainly

as a subsidiary consideration to the filtration of sewage, is practiced

in a few cities in the eastern portion of the United States to-day.

Among the cities mentioned by Metcalf and Eddy[171] are Danbury, Conn.,

and Fostoria, Ohio. In the western portion of the United States where

water is scarce and the ground is porous, sewage has been used as an

irrigant with some success. Such use of sewage cannot be considered as a

method of treatment since the prime consideration is the growing of

crops. In this process all sewage not used as an irrigant is discharged

without treatment into water courses. According to Metcalf and Eddy

there were 35 cities in California in 1914 that were operating sewage

farms. Among these are Pasadena, Fresno, and Pomona. Other farms,

notably the pioneer farm at Cheyenne, Wyo., have been abandoned because

of the local nuisance created and the lack of financial success.

=262. Preparation and Operation.=—A porous sandy soil on a good slope

and with good underdrainage is most suitable for broad irrigation.

Impervious clay or gumbo soils are unsuitable and should not be used.

They become clogged at the surface, forming pools of putrefying sewage,

or in hot weather form cracks which may permit untreated sewage to

escape into the underdrains.

The sewage may be distributed to the irrigated area in any one of five

ways which are known as: flooding, surface irrigation, ridge and furrow

irrigation, filtration, and subsurface irrigation. In flooding, sewage

is applied to a level area surrounded by low dikes. The depth of the

dose may be from 1 inch to 2 feet. In surface irrigation the sewage is

allowed to overflow from a ditch over the surface of the ground into

which it sinks or over which it flows into another ditch placed lower

down. This ditch conducts it to a point of disposal or to another area

requiring irrigation. Ridge and furrow irrigation consists in plowing a

field into ridges and furrows and filling the furrows with sewage while

crops are grown on or between the ridges. In filtration the sewage is

distributed in any desired fashion on the surface and is collected by a

system of underdrains after it has filtered through the soil. In

subsurface irrigation the sewage is applied to the land through a system

of open-joint pipes laid immediately below the surface, similarly to a

system of underdrains. Combinations of and modifications to these

methods are sometimes made. Underdrains may be used in connection with

any of these forms of distribution.

The preparation of the ground consists in: the construction of ditches

or dikes to permit of any of the above described methods of application,

grading of the surface to prevent pooling, the laying of underdrains,

and the grubbing and clearing of the land. The main carriers may be

excavated in open earth or earth lined with an impervious material. The

distribution of the sewage from the main carriers to groups of laterals

may be controlled by hand-operated stop planks. If the soil has a

tendency to become waterlogged it may be relieved by installing

underdrains at depths of 3 to 6 feet, and 40 to 100 feet apart. The tile

underdrains may discharge into open ditches excavated for the purpose

which serve also to drain the land. Drains should be used where the

ground water is within 4 feet of the surface, and the open ditches

should be cut below the drains to keep the ground water out of them.

Four or 6–inch open-joint farm tile may be used for underdrains. The

porosity of the soil will be increased by cultivation. Where particular

care is taken in the cultivation of the soil so that sewage can be

applied at a high rate, broad irrigation merges into the more intensive

intermittent filtration through sand.

Before being turned on to the land, sewage should be screened and

heavy-settling particles should be removed. The rate of application may

be increased as the intensity of the preliminary treatment is increased.

The rate at which sewage may be applied is dependent also on the

character of the soil, and may vary between 4,000 and 30,000 gallons per

acre per day, although higher rates have been used with the effluent

from treatment plants and on favorable soil. The sewage should be

applied intermittently in doses, the time between doses varying between

one day and two or three weeks or more, dependent on the weather and the

condition of the soil. The methods of dosing vary as widely as the

rates. The dose may be applied continuously for one or two weeks with

correspondingly long rests, or it may be applied with frequent

intermittency alternated with short rests, interspersed with long rest

periods at longer intervals of time. When applying the sewage to the

land the rate of application of the dose is about 10,000 to 150,000

gallons per acre per day. The area under irrigation at any one time may

be as much as 10 to 15 acres. The rate of the application of the sewage

is also dependent on the weather and may vary widely between seasons. It

is obvious that a rain-soaked pasture cannot receive a large dose of

sewage without danger of undue flooding. One of the principal

difficulties with the treatment or disposal of sewage by broad

irrigation is that the greatest load of sewage must be cared for in wet

seasons when the ground is least able to absorb the additional moisture.

=263. Sanitary Aspects.=—A well-operated sewage farm should cause no

offense to the eye or nose, and is not a danger to the public health. In

Berlin, a portion of the sewage farms are laid out as city parks. The

liquid in the drainage ditches or underdrains may be clear, odorless,

and colorless, high in nitrates and non-putrescible. Where the farm has

been improperly managed or overdosed the condition may be serious from

both esthetic and health considerations. Sewage may be spread out to

pollute the atmosphere and to supply breeding places for flying insects

which will spread the filth for long distances surrounding the farm. The

character of the crop is also a sanitary consideration.

=264. The Crop.=—From a sanitary viewpoint no crops which come in

contact with the sewage should be cultivated on a sewage farm. Such

products as lettuce, strawberries, asparagus, potatoes, radishes, etc.,

should not be grown. Grains, fruits, and nuts are grown successfully and

as they do not come in contact with the sewage there is no sanitary

objection to their cultivation in this manner. Italian rye grass and

other forms of hay are grown with the best success as they will stand a

large amount of water without injury. The raising of stock is also

advisable for sewage farms where hay and grain are cultivated. The stock

should be fed with the fodder raised on the irrigated lands and should

not be allowed to graze on the crops during the time that they are being

irrigated. This is due as much to the danger of injury to the

distributing ditches and the formation of bogs by the trampling of the

cattle, as to the danger to the health of the cattle.

CHAPTER XVIII

ACTIVATED SLUDGE

=265. The Process.=—In the treatment of sewage by the activated sludge

process the sewage enters an _aëration tank_ after it has been screened

and grit has been removed. As it enters the aëration tank it is mixed

with about 30 per cent of its volume of activated sludge. The sewage

passes through the aëration tank in about two to four hours during which

time air is blown through it in finely divided bubbles. The effluent

from the aëration tank passes to a _sedimentation tank_ where it remains

for one-half an hour to an hour to allow the sedimentation of the

activated sludge. The supernatant liquid from the sedimentation tank is

passed to the point of final disposal. A portion of the sludge removed

from the tank is returned to the influent of the aëration tank. The

remainder may be sent to any or all of the following: the _sludge drying

process_, the reaëration tanks, or to some point for final disposal.

Sections of the activated sludge plant at Houston, Texas, are shown in

Fig. 177.

The biological changes in the process occur in the aëration tank. These

changes are dependent on the aërobic organisms which are intensively

cultivated in the activated sludge. When placed in intimate contact with

fresh sewage, brought about by the agitation caused by the rising air,

and in the presence of an abundance of oxygen, the organic matter is

partially oxidized. The putrefactive stage of the organic cycle is

avoided. Colloids and bacteria are partially removed probably by the

agitation effected in the presence of activated sludge but the exact

action which takes place is not well understood.

[Illustration:

FIG. 177.—Activated Sludge Plant at Houston, Texas.

Eng. News, Vol. 77, p. 236.

]

=266. Composition.=—Activated sludge is the material obtained by

agitating ordinary sewage with air until the sludge has assumed a

flocculent appearance, will settle quickly, and contain aërobic and

facultative bacteria in such numbers that similar characteristics can be

readily imparted to ordinary sewage sludge when agitated with air in the

presence of activated sludge. Copeland described activated sludge as

follows:[172]

The sludge embodied in sewage and consisting of suspended organic

solids, including those of a colloidal nature, when agitated with

air for a sufficient period assumes a flocculent appearance very

similar to small pieces of sponge. Aërobic and facultative

bacteria gather in these flocculi in immense numbers—from 12 to 14

million per c.c.—some having been strained from the sewage and

others developed by natural growth. Among the latter are species

that have the power to decompose organic matter, especially of an

albuminoid or nitrogenous nature, setting the nitrogen free; and

others absorbing the nitrogen convert it into nitrites and

nitrates. These biological processes require time, air, and

favorable environment such as suitable temperature, food supply

and sufficient agitation to distribute them throughout all parts

of the sewage.

Ardern states that the sludge differs entirely from the usual tank

sludge. It is inoffensive and flocculent in character. The percentage of

moisture is from 95 to 99 per cent. American experience has generally

been that the sludge does not readily separate from its moisture by

treatment on fine-grain filters, but the results in England and at

Milwaukee, Wisconsin, are in conflict with this general experience. Upon

standing 24 hours or more partially dried activated sludge may start to

decompose accompanied by the production of offensive odors.

Duckworth states:

The activated sludge at Salford contained three times as much

nitrogen, twice as much phosphoric acid and one-half as much fatty

matter as ordinary sludge.

TABLE 91

COMPOSITION OF SEWAGE, IMHOFF SLUDGE, AND ACTIVATED SLUDGE AND EFFLUENT AT

MILWAUKEE

(W. R. Copeland, Eng. News, Vol. 76, p. 665)

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

Period│Source of │ Parts per Million

of │ Sample │

Test │ │

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

│ │ Sus- │ Nitrogen as │Nitrogen

│ │pended│ │ Reported as

│ │Matter│ │ Ammonia on a

│ │ │ │ Basis of

│ │ │ │ Sludge Dried

│ │ │ │ to 10 Per

│ │ │ │ Cent

│ │ │ │ Moisture.

│ │ │ │ Three

│ │ │ │ samples of

│ │ │ │ Sludge

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

│ │ │ Free │ Albu- │Organic │ Ni- │ Ni- │

│ │ │Ammonia│minoid │Nitrogen│trites│trates│

│ │ │ │Ammonia│ │ │ │

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

Aug., │Sewage │ 253│ 14.6│ 7.88│ 29│ 0.15│ 0.13│ │ │

1915│ │ │ │ │ │ │ │ │ │

│Imhoff │ 105│ 16.2│ 6.10│ 27│ 0.19│ 0.13│2.87│3.82│

│ effluent│ │ │ │ │ │ │ │ │

│Activated │ 14│ 3.8│ 3.19│ 6│ 0.29│ 6.00│5.71│4.97│7.04

│ sludge │ │ │ │ │ │ │ │ │

│ effluent│ │ │ │ │ │ │ │ │

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

Sept.,│Sewage │ 300│ 13.5│ 8.81│ 29│ 0.25│ 0.14│ │ │

1915│ │ │ │ │ │ │ │ │ │

│Imhoff │ 116│ 15.4│ 7.10│ 27│ 0.12│ 0.09│3.88│ │

│ effluent│ │ │ │ │ │ │ │ │

│Activated │ 8│ 5.7│ 2.22│ 9│ 0.24│ 5.01│8.69│9.00│

│ sludge │ │ │ │ │ │ │ │ │

│ effluent│ │ │ │ │ │ │ │ │

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

These results have been roughly checked by American experimenters as

shown in Table 91.[173] In the recovery of nitrogen from sewage the

activated sludge process is the most promising for satisfactory results.

In all other processes of sewage treatment the sludge is digested to

some extent and nitrogen lost in the gases or in the soluble matter

which passes off with the effluent. In the activated sludge process a

negligible amount of gasification and liquefaction take place and only a

small amount of nitrogen passes off with the effluent as compared with

the loss from the Imhoff process as shown in Table 91. The percentage of

nitrogen in dried activated sludge is shown in Table 92.

TABLE 92

NITROGEN CONTENT OF DRY ACTIVATED SLUDGE AND SLUDGE FROM OTHER

PROCESSES

(G. W. Fuller, Eng. News, Vol. 76, p. 667)

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

Source │ Per Cent Nitrogen

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

Milwaukee (Copeland) │ 4.40

Manchester, England (Ardern) │ 4.60

Salford, England (Melling) │ 3.75

Urbana, Illinois (Bartow) │ 3.5 to 6.4

Armour and Co. (Noble) │ 4.6

Approximate range of all other processes│ 1.0 to 3.0

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

These figures are expressed in terms of nitrogen and not of ammonia.

Nitrogen is only 82 per cent of the ammonia content.

Nitrifying bacteria and other species which have the power of destroying

organic matter have been isolated from the sludge. An analysis of the

dried sludge at Urbana[174] showed the following results after the

weight had been reduced 95.5 per cent by drying: 6.3 per cent nitrogen,

4.00 per cent fat, 1.44 per cent phosphorus, and 75 per cent volatile

matter or loss on ignition. Analyses of other domestic sewages have not

shown such high contents of these desirable constituents.

The dewatering of activated sludge is a problem which offers serious

obstacles to the successful operation of the process. It is its greatest

disadvantage. Five to ten times the volume of sludge may be produced by

the activated sludge process as by an Imhoff tank, and the activated

sludge contains a greater percentage of water. According to Copeland:

The best information now available points to a combination of

settling and decantation as a preliminary dewatering process. By

this means the water will be cut down from about 99 per cent to 96

per cent. On passing the concentrated residue through a pressure

filter the moisture can be cut down to 75 per cent. The press cake

can be dewatered in a heat drier to 10 per cent moisture or

less.[175]

The quantity of sludge produced at Milwaukee[176] is about 15 cubic

yards per million gallons of sewage, the sludge having about 98 per cent

moisture. On the basis of 10 per cent moisture it produces ½ ton of dry

sludge per million gallons of sewage treated. At Cleveland,[177] 20

cubic yards per million gallons at 97.5 per cent moisture are produced.

Methods of drying sludge are discussed in Chapter XX.

Chemical analyses and biological tests indicate that the fertilizing

value of the sludge is appreciable. Professor C. B. Lipman states, as

the result of a series of tests in which a sludge and a soil were

incubated for one month, as follows:[178]

The amounts of nitrates produced in one month’s incubation from

the soil’s own nitrogen and from the nitrogen from the sludge

mixed with the soil in the ratio of one part of sludge to 100 of

soil is, in milligrams of nitrate, as follows: Anaheim soil

without sludge 6.0, with sludge 10.0; Davis soil without sludge

4.2, with sludge 14.0; Oakley soil without sludge 2.2, with sludge

4.0.

The effect of the sludge on plant growth is shown in Table 93.[179] The

results represent the growth obtained after fifteen weeks from the

planting of 30 wheat seeds in each pot.

=267. Advantages and Disadvantages.=—Some of the advantages of the

process are: a clear, sparkling, and non-putrescible effluent is

obtained; the degree of nitrification is controllable within certain

limits; the character of the effluent can be varied to accord with the

quantity and character of the diluting water available; more than 90 per

cent of the bacteria can be removed; the cost of installation is

relatively low; and the sludge has some commercial value.

TABLE 93

FERTILIZING VALUE OF ACTIVATED SLUDGE

(E. Bartow, Journal Am. Water Works Ass’n, Vol. 3, p. 327)

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

Cultivating Medium │Grams Contained in Experimental

│ Pot

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

│ 1 │ 2 │ 3 │ 4

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

White sand │ 19,820│ 19,820│ 19,820│ 19,820

Dolomite │ 60│ 60│ 60│ 60

Bone meal │ 6│ 6│ 6│ 6

Potassium sulphate │ 3│ 3│ 3│ 3

Activated sludge │ 0│ 0│ 20│ 0

Activated sludge extracted with Ligroin│ 0│ 0│ 0│ 20

Dried blood │ 0│ 8.61│ 0│ 0

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

Number of heads of wheat │ 14│ 15│ 22│ 23

Number of seeds │ 85│ 189│ 491│ 518

Weight of seeds, grams │ 2.38│ 5.29│ 13.748│ 14.504

Bushels per acre, calculated │ 6.20│ 13.6│ 35.9│ 38.7

Average length of stalk, inches │ 19.40│ 23.0│ 35.4│ 37.1

Weight of straw, grams │ 2.25│ 8.25│ 26.75│ 26.21

Tons per acre, calculated │ 0.18│ 0.68│ 2.23│ 2.18

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

Among the disadvantages of the process can be included, uncertainty due

to the lack of information concerning the results to be expected under

all conditions, high cost of operation under certain conditions, the

necessity for constant and skilled attendance, and the difficulty of

dewatering the sludge.

=268. Historical.=—The most notable work in the aëration of sewage

within recent years was that performed by Black and Phelps for the

Metropolitan Sewerage Commission of New York, in 1910,[180] and by Clark

and Gage at the Lawrence, Massachusetts, Sewage Experiment Station in

1912 and 1913.[181] The results of these investigations showed that the

treatment of sewage by forced aëration might give a satisfactory

effluent, but that the time and expense in connection thereto rendered

the method impractical.

It remained for Messrs. Ardern and Lockett of Manchester, England, to

introduce the process of the aëration of sewage in the presence of

activated sludge, as a result of their connection with Dr. Fowler, who

attributes his inspiration to his visit to the Lawrence Experiment

Station and observing the work of Clark and Gage. Ardern and Lockett

commenced their experiments in 1913. Their results were published in the

_Journal of the Society of Chemical Industry_, May 30, 1914, Vol. 33, p.

523. Shortly thereafter experiments were started at the University of

Illinois by Dr. Edw. Bartow and Mr. F. W. Mohlmann of the Illinois State

Water Survey. At about the same time an experimental plant was started

at Milwaukee, by T. C. Hatton, Chief Engineer of the Milwaukee Sewerage

Commission. The United States Public Health Service became actively

interested in December, 1914, and on February 20, 1915, announced its

intention to co-operate with the Baltimore Sewerage Commission in the

conduct of experiments. In May, 1915, patent number 1,139,024 was

granted to Leslie C. Frank, Sanitary Engineer of the U. S. Public Health

Service, covering certain features of the process. Mr. Frank generously

donated this patent to the public for the use of municipalities.

The first full sized plant for the treatment of sewage by this method

was erected in Milwaukee in December, 1915. This plant had a capacity of

1,600,000 gallons per day. It was used for experimental purposes and is

not now in use. The Champaign, Illinois, septic tank, among the first of

its kind in the country, was converted into an activated sludge tank on

April 13, 1916. The changes, developments, and the results obtained from

these and other plants have been reported in the technical press from

time to time.

=269. Aëration Tank.=—The sewage on leaving the screen and grit chamber

enters the aëration tank, which is usually operated on the

continuous-flow principle, although in the early days of experimentation

the fill and draw method was practiced. This tank should be rectangular

with a depth of about 15 feet and a width of channel not to exceed 6 to

8 feet. Such proportions allow better air and current distribution than

larger tanks. The bottom should be level to insure an even distribution

of air. The velocity of flow of sewage through the tank is usually in

the neighborhood of 5 feet per minute, dependent on the length of the

tank and the period of retention. The period of retention is in turn

dependent on the desired quality of the effluent. The process is

flexible and the quality of the effluent can be changed by changing the

period of retention or by changing the rate of application of the air,

or both. The period of retention in the aëration tank is usually about 4

hours.

The bottom of the aëration tank is usually made of concrete arranged in

ridges and valleys, or small shallow hoppers, at the bottom of which the

air-diffusing devices are located, as shown in Fig. 177. The inlet and

outlet devices are similar to those in a plain sedimentation tank.

=270. Sedimentation Tank.=—It is evident that as no sedimentation is

permitted in the aëration tank, the settleable particles will be

discharged in the effluent unless some provision is made for their

detention. The effluent from the aëration tank is therefore run through

a plain sedimentation tank, usually with a hopper bottom, which has been

arranged to permit frequent and easy cleaning. An air lift or a

centrifugal sludge pump is satisfactory for this purpose. Another type

of sedimentation tank which has been used has a smooth bottom with a

slight slope towards the center. A revolving scraper collects the sludge

continuously, scraping it towards the center of the tank. Although this

arrangement gives better results than the hopper-bottom tank, its

expense has usually prevented its installation.[182]

The period of sedimentation in different plants varies from 30 minutes

to one hour, although the longer periods usually give the better

results. Approximately 65 per cent of the sludge will settle in the

first 10 minutes, 80 per cent in the first 30 minutes, and about 5 per

cent more in the next half hour.

The effluent from the sedimentation tank is ready for final disposal or

if desired, for further treatment by some other method. The sludge, or a

portion of it, is pumped back into the influent of the aëration tank,

provided the sludge is in a satisfactory state of nitrification.

Otherwise it should be pumped to the reaëration tanks. The remainder of

the sludge which is not to be used in the process is ready for drying

and final disposal.

=271. Reaëration Tank.=—The purpose of the reaëration or sludge aëration

tank is to reactivate the sludge which has gone through the aëration

tank. During the process of the aëration of the sewage in the aëration

tank the activated sludge may lose some of its qualities because of the

deficiency of oxygen to maintain aërobic conditions. By blowing air

through the sludge in the reaëration tank these properties are returned

and the sludge made available to be pumped back into the aëration tank.

The reactivation of the sludge obviates the necessity for supplying

sufficient air to the entire mass of the sewage to maintain aërobic

conditions, and results in an economy in the use of air. The use of

mechanical agitators has also been attempted both in the reaëration and

the aëration tanks with the expectation of saving in the use of air, but

with indifferent success.

It is difficult to say, without experimentation, what the size of the

reaëration tank should be, as the necessary amount or reactivation is

uncertain. In the experimental plant at Milwaukee, there were eight

units of aëration tanks, one sedimentation tank, and two reaëration

tanks, all of the same capacity and general design. This represents a

ration of about one reaëration tank to four aëration tanks.

=272. Air Distribution.=—Air is applied to the sewage at the bottom of

the aëration tank at a pressure in the neighborhood of 5.5 to 6.0 pounds

per square inch, dependent on the depth of the sewage, the loss of head

through the distributing pipes, and the rate of application. In

different experimental plants the pressure has varied from 3 to 30

pounds per square inch. Such pressures are on the line which divides the

use of direct blowers for low pressures from turbo and reciprocating

pressure machines for pressures above 10 pounds per square inch.

Positive-pressure blowers or direct blowers operate on the principle of

a centrifugal pump and because of the lighter specific gravity of air

they rotate at a very high speed. The Nash Hytor Turbo Blower consists

of a rotor with a large number of long teeth slightly bent in the

direction of rotation. The rotor, which has a circular circumference,

revolves in an elliptical casing. At the commencement of operation the

rotor and casing are partially filled with water. The revolution of the

rotor throws the water to the outside of the elliptical casing thus

forming a partial vacuum between any two teeth as the water is thrown

from near the center of the short diameter of the casing to the

extremity of the long diameter of the casing. Air is allowed to enter

through the inlet port to relieve the vacuum. As the teeth pass from the

long diameter to the short diameter of the ellipse, the water again

approaches the center of the rotor compressing the air trapped between

the teeth and forcing it out under pressure into the exhaust pipe. Among

the advantages of this compressor are the washing of the air, cooling,

and ease in operation. Reciprocating air compressors operate similarly

to direct-acting steam pumps or crank-and-fly-wheel pumps but at much

higher speeds, and they require more floor space than either of the

other types. Fig. 178 shows the field of serviceability of various types

of air compression machinery.

[Illustration:

FIG. 178.—Economic Range of Air Compressors.

From Eng. News, Vol. 74, p. 906.

]

For pressures up to about 10 pounds per square inch the positive blower

seems most desirable. It has a low first cost and a relatively high

efficiency of about 75 to 80 per cent of the power input. No oil or dirt

is added to the air to clog the distributing plates, as in the

reciprocating machine. A disadvantage is the difficulty of varying the

pressure or quantity of the output of the machine. As the required

pressure and volume of air increases the turbo blower becomes more and

more desirable within the limits of pressure which are ordinarily used

in this process. For small installations the best form of power is

probably the electric drive, but when the capacity becomes such as to

make turbo blowers advisable they should be driven by directly connected

steam turbines.

The quantity of air required varies between 0.5 to 6.0 cubic feet per

gallon of sewage, with from 3 to 6 hours of aëration. The quantity of

air depends on the degree of treatment required, the strength of the

sewage, the depth of the tank, and the period of aëration. The deeper

the tank the less the amount of air needed because of the greater travel

of the bubble in passing through the sewage, but the higher the pressure

at which the air must be delivered. Shallow tanks usually require a

longer period of retention. The depth of the tank then has very little

to do with economy in the use of air. Hatton states:[183]

The purification of sewage obtained varies decidedly with the

volume of air applied. Small volumes applied for 5 or 6 hours do

as well as larger volumes applied for 3 or 4 hours, but the time

of aëration required to obtain a like effluent does not vary

directly with the volume of air applied per unit of time. For

instance air applied at a rate of 2 cubic feet per minute purifies

the sewage in less time than one cubic foot of air per minute, but

will not accomplish an equal degree of purification in half the

time.

It has been found that although a low temperature has a deleterious

effect on the process, by the use of an additional quantity of air good

results can be maintained. The effect of changing the quantity of air

and the period of aëration are shown in Table 94 taken from Hatton.

The velocity of the air in the pipes should be about 1,000 feet per

minute. There should be relatively few sharp turns in the line, and the

distributing mains should be arranged without dead ends. It is desirable

to use as little piping as possible and at the same time to make the

travel of the sewage long in order to maintain a non-settling velocity

and intimate contact with the air. The piping should be accessible and

well provided with valves. It should be non-corrodible, particularly on

the inside, as flakes of rust will quickly clog the air diffusers. It

should drain to one point in order that it can be emptied when flooded,

as occasionally happens.

TABLE 94

EFFECT OF VARIOUS RATES AND PERIODS OF APPLICATION OF AIR ON THE RESULTS

OBTAINED FROM THE TREATMENT OF SEWAGE BY THE ACTIVATED SLUDGE PROCESS

(Milwaukee Results)

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

Time of │Cubic │Cubic │Appearance│Per Cent

Aëration,│ Feet │ Feet │of Settled│Removal

Hours │ Free │ Air │ Liquid │Bacteria

│ Air │ per │ │

│ Per │Gallon│ │

│Minute│ of │ │

│ │Sewage│ │

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

│ │ │ │

│ │ │ │

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

│ │ │ │

│ │ │ │

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

0│ 0│ 0.0│ Turbid │ 0

1│ 160│ 0.67│ Clear │ 52

2│ 160│ 1.32│ Clear │ 81

3│ 160│ 1.98│ Clear │ 92

4│ 160│ 2.64│ Clear │ 94

5│ 160│ 3.31│ Clear │ 98

2.5│ 90│ 1.07│ │ 92

3│ 90│ 1.28│ │ 96

4│ 90│ 1.71│ │ 98

4│ 80│ 1.82│ │ 97.7

4│ 70│ 1.60│ │ 99.6

4│ 46│ 1.67│ │ 88.3

4│ 105│ 1.75│ │ 92.7

3│ 140│ 1.75│ │ 91.2

2.5│ 168│ 1.74│ │ 96.7

│ │ 1.80│ │ 98.1

│ │ 1.53│ │ 99

│ │ 1.12│ │ 91

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

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

Time of │ Parts per Million │Stability,

Aëration,│ │ Hours

Hours │ │

│ │

│ │

│ │

│ │

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

│ Nitrogen as │Dissolved│Suspended│

│ │ Oxygen │ Matter │

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

│ Free │Nitrites│Nitrates│Organic│ │ │

│Ammonia│ │ │ │ │ │

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

0│ 22│ 0.08│ 0.08│ │ 0.00│ │ 000

1│ 17│ 0.00│ 0.04│ │ 0.30│ │ 2

2│ 15│ 0.95│ 0.70│ │ 1.90│ │ 33

3│ 11│ 1.75│ 2.80│ │ 4.30│ │ 120

4│ 7│ 2.20│ 5.60│ │ 5.90│ │ 120

5│ 5│ 2.50│ 8.20│ │ 6.70│ │ 120

2.5│ 11│ 0.05│ 2.00│ │ │ │ 69

3│ 9.9│ 0.12│ 2.9│ │ │ │ 95

4│ 1.8│ 0.14│ 5.2│ │ │ │ 120

4│ 1.95│ 0.08│ 8.5│ │ │ │ 120

4│ 5.79│ 0.14│ 9.0│ │ │ │ 120

4│ 7.90│ 0.02│ 2.0│ │ │ │ 61

4│ 4.86│ 0.36│ 4.9│ │ │ │ 120

3│ 9.39│ 0.60│ 3.0│ │ │ │ 120

2.5│ 11.2│ 0.36│ 1.1│ │ │ │ 84

│ │ │ 8.5│ 4│ │ 11│ 120

│ 5.79│ │ 9.0│ 8│ │ 9│ 120

│ 10.1│ │ 2.3│ 14│ │ 42│ 73

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

TABLE 95

COMPARATIVE RESULTS FROM THE AËRATION OF SEWAGE IN THE PRESENCE OF

ACTIVATED SLUDGE WITH THE USE OF DIFFERENT DISTRIBUTING MEDIA

(T. C. Hatton, Eng. Record, Vol. 73, p. 255)

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

Diffusers │Months in │ Pounds │ Air, │Per Cent│Nitrates,│Stability

│ 1915 │ per │ Cubic │Bacteria│Parts per│Effluent

│ │ Square │Feet per│Removed │ Million │in Hours

│ │ Inch │ Gallon │ │ │

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

Filtros plate│June 1 to│ 4.3│ 2.06│ 91│ 3.4│ 78

│ Aug. 15 │ │ │ │ │

Air jet │June 1 to│ 3.5│ 1.94│ 91│ 2.2│ 52

│ Aug. 15 │ │ │ │ │

Filtros plate│Nov. 18 to│ 4.6│ 1.71│ 90│ 0.3│ 113

│ Dec. 7 │ │ │ │ │

Monel metal │Nov. 18 to│ 3.0│ 1.71│ 80│ 0.2│ 63

│ Dec. 7 │ │ │ │ │

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

It is desirable to diffuse the air in small bubbles as by this means the

greatest efficiency seems to be obtained from the amount of air added. A

diameter 1/16 to ⅛ of an inch is approximately the maximum limit for the

size of an effective bubble. Monel metal cloth, porous wood blocks, open

jets, paddles, and other forms of diffusers have been tried, but none

have given the satisfaction of the filtros plate. The relative value of

different types of diffusers is shown in Table 95 taken from

Hatton.[184] The Filtros plates are a proprietary article manufactured

by the General Filtration Company of Rochester, N. Y. They are made of a

quartz sand firmly cemented together and can be obtained with

practically any degree of porosity, size of pore opening or dimension of

plate, but they are made in a standard size 12 inches square by 1½

inches thick. The frictional loss through the plate is not very great

for the amount of air ordinarily used. The plates are classified in

accordance with the volume of air which will pass through them, when

dry, per minute when under a pressure of 2 inches of water. These

classes run from ½ to 12 cubic feet of air per minute. The type usually

specified passes about 2 cubic feet of air per minute. The loss of head

through these plates as tested at Milwaukee showed an initial loss of ¾

of a pound and an additional loss of about ¼ of a pound for every cubic

foot of air per minute per square foot of surface. It is necessary to

screen and wash the air before blowing it through the filtros plate as

ordinary air is so filled with dirt as to clog the pores of the diffuser

quite rapidly.

The area of filtros plates required in the bottom of the tank is usually

expressed in terms of the free surface of the tank or as a ratio

thereto. In the Urbana tests the best ratio was found to be less than 1

: 3 and more than 1 : 9. In Milwaukee[185] the ratio adopted is in the

neighborhood of 1 : 4 or 1 : 5. At Fort Worth the ratio will be about 1

: 7 and at Chicago it will be 1 : 8. The exact ratio should be

determined by experiment and will depend on the construction of the tank

and the character of the raw sewage and the desired effluent. It is

essential that the filtros plates be placed level and at the same

elevation as otherwise the distribution of air will be uneven.

=273. Obtaining Activated Sludge.=—After a plant is once started

activated sludge is generated during the process of treatment and with

careful management a stock of activated sludge can be kept on hand. When

a plant is new, or if shut down for such a length of time that the

sludge loses its activation, it is necessary to activate some new

sludge. This is done by blowing air continuously through sewage either

on the fill and draw method with periodic decantations of the

supernatant liquid, or by the continuous-flow process, but more

preferably by the latter. Where activated sludge is to be obtained from

fresh sewage alone the time required is in the neighborhood of 10 to 14

days, and purification begins at the start. An estimate of the quantity

which will be obtained can not be made with accuracy. After the initial

quantity of sludge has been obtained activated sludge can be maintained

during the process of aëration of the raw sewage, or by means of the

reaëration tanks previously described.

The volume of activated sludge present in the aëration tank should be

about 25 per cent of the volume of the tank. The volume of the sludge is

measured in a somewhat arbitrary manner as the amount by volume which

will settle in 30 minutes in an ordinary test tube. It is found that

this is almost 90 per cent of the solids settling in 4 to 6 hours.

=274. Cost.=—The available information on the cost of the activated

sludge process is meager and unreliable. The factors entering into the

cost are: the price of fuel, the size of the plant, the period of

sedimentation, the amount of air per gallon of sewage, the air pressure,

and the percentage of sludge to be aërated in the mixture. In

Milwaukee[186] the cost of construction is estimated at $44,000 per

million gallons, and $4.75 per million gallons for operation. At

Houston, Texas, the cost is estimated at $24,000 per million gallons,

exclusive of the sludge drying plant, which may cost $40,000 per million

gallons. At Milwaukee, the cost of pressing the sludge is $4.82 per dry

ton and of drying is $3.93 per dry ton. The sludge may be sold at the

normal rate of $2.50 per unit of nitrogen. Based on the normal value the

evident profit will be $3.75 per ton. The net cost of disposing of

Milwaukee sewage is estimated at $9.64 per million gallons of which

$4.89 is chargeable to overhead and $4.75 to repairs, operation and

renewal. In a comparison of the costs of activated sludge and Imhoff

tanks with sprinkling filters,[187] the information given by Eddy has

been summarized in Table 96. In comparing the relative areas required

for different methods of sewage treatment, activated sludge should be

allowed about 15 million gallons per acre per day on the basis of

aëration tanks 15 feet deep. This figure represents approximately the

gross area of the plants at Milwaukee and at Cleveland.

TABLE 96

COMPARATIVE COSTS OF ACTIVATED SLUDGE, AND OF IMHOFF TANKS FOLLOWED BY

SPRINKLING FILTERS

(H. P. Eddy, Eng. Record, Vol. 74, p. 557)

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

Process │ First Cost │Operation per│Total Annual Cost at 4 Per

│ per Million │ Million │ Cent with Sinking Fund at

│ Gallons, │ Gallons, │ 2.5 Per Cent per

│ Dollars │ Dollars │

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

│ │ │ Million │ Capita,

│ │ │ Gallons, │ Dollars

│ │ │ Dollars │

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

Activated │ 57,100│ 20.00│ 29.85│ 1.09

sludge │ │ │ │

Imhoff tank │ 78,500│ 8.50│ 21.84│ 0.80

and │ │ │ │

sprinkling │ │ │ │

filter │ │ │ │

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

REFERENCES AND BIBLIOGRAPHY ON ACTIVATED SLUDGE

The following abbreviations will be used: A.S. for Activated Sludge,

E.C. for Engineering and Contracting, E.N. for Engineering News, E.R.

for Engineering Record, E.N.R. for Engineering News-Record, p. for page,

and V. for volume.

No.

1. Cooperation Sought in Conducting A.S. Experiments at Baltimore, by

Franks and Hendrick. E.R. V. 71, 1915, pp. 521, 724, and 784. V.

72, 1915, pp. 23, and 640.

2. Sewage Treatment Experiments with Aëration and A.S., by Bartow and

Mohlman. E.N. V. 73, 1915, p. 647, and E.R. V. 71, 1915, p. 421.

3. A.S. Experiments at Milwaukee, Wisconsin, by Hatton. E.N. V. 74,

1915, p. 134.

4. A.S. in America, An Editorial Survey, by Baker. E.N. V. 74, 1915,

p. 164.

5. Choosing Air Compressors for A.S., by Nordell, E.N. V. 74, 1915, p.

904.

6. A Year of A.S. at Milwaukee, by Fuller. E.N. V. 74, 1915, p. 1146.

7. A.S. Experiments at Urbana. E.N. V. 74, 1915, p. 1097.

8. Experiments on the A.S. Process, by Bartow and Mohlman. E.C. V. 44,

1915, p. 433.

9. Milwaukee’s A.S. Plant, the Pioneer Large Scale Installation, by

Hatton. E.R. V. 72, 1915, p. 481 and E.C. V. 44, 1915, p. 322.

10. A.S. Experiments at Milwaukee, by Hatton. Journal American

Waterworks Association and Proceedings Illinois Society of

Engineers, 1916. Also E.R. V. 73, 1916, p. 255. E.C. V. 45, 1916,

p. 104, and E.N. V. 75, 1916, pp. 262 and 306.

11. A.S. Defined. E.N. V. 75, 1916, p. 503, and E.N.R. V. 80, 1918, p.

205.

12. Status of A.S. Sewage Treatment, by Hammond. E.N. V. 75, 1916, p.

798.

13. Trial A.S. Unit at Cleveland, by Pratt. E.N. V. 75, 1916, p. 671.

14. Air Diffuser Experience with A.S. E.N. V. 76, 1916, p. 106.

15. Nitrogen from Sewage Sludge, Plain and Activated, by Copeland,

Journal American Chemical Society, Sept. 28, 1916. E.N. V. 76,

1916, p. 665. E.R. V. 74, 1916, p. 444.

16. Tests Show A.S. Process Adapted to Treatment of Stock Yards Wastes.

E.R. V. 74, 1916, p. 137.

17. Aëration Suggestions for Disposal of Sludge, by Hammond. Journal

American Chemical Society, Sept. 25, 1916. E.R. V. 74, 1916, p.

448.

18. Cost Comparison of Sewage Treatment. Imhoff Tank and Sprinkling

Filters vs. A.S., by Eddy. E.R. V. 74, 1916, p. 557.

19. Large A.S. Plant at Milwaukee. E.N. V. 76, 1916, p. 686.

20. A.S. Novelties at Hermosa Beach, Cal. E.N. V. 76, 1916, p. 890.

21. A.S. Experiments at University of Illinois, by Bartow, Mohlman, and

Schnellbach. E.N. V. 76, 1916, p. 972.

22. A.S. Results at Cleveland Reviewed, by Pratt and Gascoigne. E.N. V.

76, 1916, pp. 1061 and 1124.

23. Sewage Treatment by Aëration and Activation, by Hammond.

Proceedings American Society Municipal Improvements, 1916.

24. A.S., by Bartow and Mohlman, Proceedings Illinois Society of

Engineers, 1916.

25. The Latest Method of Sewage Treatment, by Bartow. Journal American

Waterworks Association, V. 3, March, 1916, p. 327.

26. Winter Experiences with A.S., by Copeland. Journal American Society

of Chemical Engineers, April 21, 1916. E.C. V. 45, 1916, p. 386.

27. A.S. Process Firmly Established, by Hatton. E.R. V. 75, 1917, p.

16.

28. Operate Continuous Flow A.S. Plant, by Bartow, Mohlman, and

Schnellbach. E.R. V. 75, 1917, p. 380.

29. Chicago Stock Yards Sewage and A.S., by Lederer. Journal American

Society of Chemical Engineers, April 21, 1916. E.C. V. 45, 1916,

p. 388.

30. The Patent Situation Concerning A.S. E.C. V. 45, 1916, p. 208.

31. “Sewage Disposal” by Kinnicutt, Winslow, and Pratt, published by

John Wiley & Sons. 2d Edition, Chapter 12.

32. A.S. Tests Made by California Cities. E.N.R. V. 79, 1917, p. 1009.

33. Conclusions on the A.S. Process at Milwaukee. Journal American

Public Health Association, 1917. E.N.R. V. 79, 1917, p. 840.

34. Dewatering A.S. at Urbana, by Bartow. Journal American Institute of

Chemical Engineers, 1917. E.N.R. V. 79, 1917, p. 269.

35. Milwaukee Air Diffusion Studies in A.S. E.N.R. V. 78, 1917, p. 628.

36. A.S. Bibliography (up to May 1, 1917) by J. E. Porter.

37. Air Diffusion in A.S. E.N.R. V. 78, 1917, p. 255.

38. A.S. Plant at Houston, Texas. E.N. V. 77, 1917, p. 236, E.N.R. 83,

1919, p. 1003, and V. 84, 1920, p. 75.

39. A.S. Power Costs, by Requardt. E.N. V. 77, 1917, p. 18.

40. A.S. at San Marcos, Texas, by Elrod. E.N. V. 77, 1917, p. 249.

41. Filtros Plates Made the Best Showing in Air Diffuser Tests. E.N.R.

V. 79, 1917, p. 269.

42. Results of Experiments on A.S., by Ardern and Lockett. Journal

Society for Chemical Research, V. 33, May 30, 1914, p. 523.

43. Final Plans at Milwaukee. E.N.R. V. 84, 1920, p. 990.

44. A.S. Bibliography, published by General Filtration Co., Rochester,

N. Y., 1921.

45. A.S. at Manchester, Eng. by Ardern. Journal Society Chemical

Industry, 1921. E.C. V. 55, 1921, p. 310.

46. The Des Plaines River A.S. Plant, by Pearse. E.N.R. V. 88, 1920, p.

1134.

47. Sewage Treatment by the Dorr System, by Eagles. Proceedings, Boston

Society of Engineers, 1920. Public Works V. 50, 1920, p. 53.

CHAPTER XIX

ACID PRECIPITATION, LIME AND ELECTRICITY, AND DISINFECTION

=275. The Miles Acid Process.=—The Miles Acid Process for the treatment

of sewage was devised and patented by G. W. Miles. It was tried

experimentally at the Calf Pasture sewage pumping station, Boston,

Mass., 1911 to 1914. In 1916 it was tried experimentally at the

Massachusetts Institute of Technology, and it has been tested

subsequently at other places, notably at New Haven, Conn., in 1917 and

1918. It is one of the most recent developments in sewage treatment and

no extensive experience has been had with it. The process consists in

the acidification of sewage with sulphuric or sulphurous acid, as the

result of which the suspended matter and grease are precipitated and

bacteria are removed. The equipment required for the process consists of

devices for the production of sulphur dioxide (SO_{2}), and for feeding

niter cake or other forms of acid; subsiding basins; sludge-handling

apparatus; sludge driers; grease extractors; grease stills; and tankage

driers and grinders.

The first step is the acidification of the sewage. The period of contact

with the acid is about 4 hours. Sulphurous acid seems to give better

results than sulphuric because of the ease in which it can be

manufactured on the spot. It seems also to be more virulent in attacking

bacteria than an equal strength of sulphuric acid. In experimental

plants the acidulation has been accomplished in different ways such as:

by the addition of compressed sulphur dioxide from tanks; by the

addition of sulphur dioxide made from burning sulphur; or by the

roasting of iron pyrite (FeS_{2}). The acidulation precipitates most of

the grease as well as the suspended matter and results in a sludge which

gives some promise of commercial value. In referring to the process R.

S. Weston states:[188]

(1) It disinfects the sewage by reducing the numbers of bacteria

from millions to hundreds per c.c.

(2) If the drying of the sludge and the extraction of the grease

can be accomplished economically, it is possible that a large

part, if not all, of the cost of the acid treatment may be met by

the sale of the grease and fertilizer recovered from the sewage.

(3) The use of so strong a deodorizer and disinfectant as sulphur

dioxide would prevent the usual nuisances of treatment works.

(4) The addition of sulphur dioxide to the sewage also avoids any

fly nuisance, which is a handicap to the operation of Imhoff tanks

and trickling filters.

The amount of acid used varies with the quality of the sewage and the

desired character of the effluent. At Bradford, England,[189] 5,500

pounds of sulphuric acid are used per million gallons, producing about

2,340 pounds of grease or 0.43 pound of grease per pound of sulphuric

acid. At Boston only 0.215 pound of grease were produced per pound of

sulphuric acid. The difference is probably due to the great difference

in the amount of grease in the raw sewage. In the East Street sewer at

New Haven, Conn.,[190] only 700 pounds of acid are used per million

gallons of sewage as the alkalinity is only 50 p.p.m. This amount of

acid secures an acidity of 50 p.p.m. whereas in the Boulevard sewer

1,130 pounds of acid had to be added to produce the same result. The

results obtained by the experiments conducted by the Massachusetts

State Board of Health in 1917 are shown in Table 97. The character of

the sludge from the same tests is shown in Table 98. After

acidification[191] the sewage contains bisulphites and some free

sulphurous acid, with some lime and magnesium soaps which are attacked

by the acid liberating the free fatty acids. Part of the bisulphites

and sulphurous acid are oxidized to bisulphates and sulphuric acid. It

was found as a result of the New Haven[191] experiments that the

presence of sulphur dioxide in the effluent caused an abnormal oxygen

demand from the diluting water and that this difficulty could be

partly overcome by the aëration of the effluent after acidulation and

sedimentation, without prohibitory expense. The effluent and sludge

are both stable for appreciable periods of time and are suitable for

disposal by dilution. The character of the sludge as determined by the

New Haven tests[192] is shown in Table 99.

TABLE 97

AVERAGE ANALYSIS OF SEWAGE ENTERING BOSTON HARBOR, BEFORE AND AFTER

TREATMENT, JULY 17 TO SEPTEMBER 27, 1917

(Eng. News-Record, Vol. 80, p. 319)

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

Sample │ Parts per Million │ Bacteria,

│ │ Millions

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

│ Ammonia │ Kjeldahl │Chlorine│ Oxygen │ │

│ │ Nitrogen │ │Consumed│ │

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

│Free │Albuminoid │ │ │ │ │ │

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

│Total│Total│Diss.│Total│Diss.│ │ │ 20° │ 37°

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

_Paddock’s Island_

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

Raw │ 14.0│ 3.3│ 1.8│ 6.8│ 3.6│ 134│ 23.1│1.86 │ 4.15

sewage │ │ │ │ │ │ │ │ │

Settled │ 12.2│ 1.6│ 1.1│ 3.5│ 2.2│ │ 15.4│ │

Sewage │ │ │ │ │ │ │ │ │

Acidified│ 20.9│ 5.2│ 3.9│ 10.0│ 7.5│ │ │units│units 91

and │ │ │ │ │ │ │ │ 94 │

settled│ │ │ │ │ │ │ │ │

sewage │ │ │ │ │ │ │ │ │

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

_Deer Island_

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

Raw │ 23.3│ 8.2│ 4.8│ 16.8│ 8.9│ 3100│ 87.3│2.63 │ 1.50

sewage │ │ │ │ │ │ │ │ │

Settled │ 21.1│ 5.6│ 3.9│ 10.7│ 7.3│ │ 62.2│ │

sewage │ │ │ │ │ │ │ │ │

Acidified│ 20.9│ 5.2│ 3.9│ 10.0│ 7.5│ │ │units│units 85

and │ │ │ │ │ │ │ │ 147 │

settled│ │ │ │ │ │ │ │ │

sewage │ │ │ │ │ │ │ │ │

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

_Calf Pasture_

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

Raw │ 18.0│ 4.5│ 2.0│ 9.7│ 4.1│ 3254│ 41.2│1.89 │ 0.98

sewage │ │ │ │ │ │ │ │ │

Settled │ 19.1│ 2.3│ 1.4│ 4.9│ 3.3│ │ 25.8│ │

sewage │ │ │ │ │ │ │ │ │

Acidified│ 17.8│ 2.4│ 1.6│ 4.9│ 3.3│ │ │units│units 149

and │ │ │ │ │ │ │ │ 277 │

settled│ │ │ │ │ │ │ │ │

sewage │ │ │ │ │ │ │ │ │

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

The success of the Miles Acid Process in comparison with other processes

is dependent on the commercial value of the sludge produced. The New

Haven experiments indicate that 16 to 21 per cent of the grease in the

sludge is unsaponifiable and seriously impairs the value of the process.

TABLE 98

AVERAGE AMOUNT OF SLUDGE AND FATS OBTAINED FROM SEWAGE ENTERING BOSTON

HARBOR AFTER EIGHTEEN HOURS SEDIMENTATION WITH AND WITHOUT

ACIDIFICATION

(Eng. News-Record, Vol. 80, p. 319)

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

│Paddock’s Island│ Deer Island │ Calf Pasture

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

│ Sedimentation │ Sedimentation │ Sedimentation

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

│Plain│Acidulated│Plain│Acidulated│Plain│Acidulated

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

Pounds of SO_{2} │ │ 818│ │ 1513│ │ 1189

used per million │ │ │ │ │ │

gallons of sewage │ │ │ │ │ │

treated │ │ │ │ │ │

Dry sludge per │ 782│ 959│ 1709│ 1939│ 1208│ 1427

million gallons │ │ │ │ │ │

Per cent Nitrogen in│ 3.10│ 3.38│ 3.57│ 3.45│ 3.18│ 2.83

sludge │ │ │ │ │ │

Per cent fats in │27.30│ 27.30│24.60│ 19.40│24.30│ 26.30

sludge │ │ │ │ │ │

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

TABLE 99

CHARACTER OF MILES ACID SLUDGE AT NEW HAVEN

(Eng. News-Record, Vol. 81, p. 1034)

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

│ East Street Sewer │Boulevard

│ │ Sewer

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

Length of run in days │ 25│ 24│ 44│ 70│ 29

Total sewage treated, │ 260│ 239.4│ 407.8│ 602.2│ 145.5

thousand gallons │ │ │ │ │

Gallons wet sludge per │ 3750│ 4025│ 3200│ 2600│ 5375

million gallons sewage │ │ │ │ │

Specific gravity │ 1.067│ 1.048│ 1.054│ 1.061│

Per cent moisture │ 86.6│ 88│ 86.3│ 85.7│ 92.5

Pounds of dry sludge per │ 503│ 483│ 439│ 368│ 403

million gallons sewage │ │ │ │ │

Ether extract, per cent │ 23.7│ 24.0│ 29│ 32.6│ 30.9

dry sludge │ │ │ │ │

Ether extract, pounds per│ 119│ 116│ 127│ 120│ 124

million gallons │ │ │ │ │

Volatile matter, per cent│ 47.2│ 51.2│ 57.3│ 63.8│ 78.5

dry sludge │ │ │ │ │

Nitrogen, per cent dry │ 1.6│ 1.6│ 2.4│ 2.0│ 3.0

sludge │ │ │ │ │

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

The conclusions reached as a result of the New Haven experiments

are:[193]

Our experience with New Haven sewage lends no color to the hope

that a net financial profit can be obtained by the use of the

Miles Acid Process, except with sewage of exceptionally high

grease content and low alkalinity. They do, however, suggest that

for communities where clarification and disinfection are

desirable—where screening would be insufficient and nitrification

unnecessary—the process of acid treatment comes fairly into

competition with the other processes of tank treatment, and that

it is particularly suited to dealing with sewages that contain

industrial wastes, and to use in localities where local nuisances

must be avoided at all costs and where sludge disposal could be

provided for only with difficulty.

The conclusions reached as a result of the Chicago experiments are:[194]

The results on hand indicate that treatment of this sewage with

acid results in a somewhat greater retention of fat. An apparent

reduction in the oxygen demand over that resulting from plain

sedimentation, while remarkable, is probably not real, being

simply due to a retardation of decomposition by the sterilization

of the bacteria present, the organic matter being left in

solution.... However, there appears the added cost of acid

treatment and the cost of recovery of the grease, as well as the

uncertainty of the price to be received for the grease recovered.

The cost of the treatment is estimated by Dorr to be $18 per million

gallons, and the value of the sludge obtained from the Boston sewage as

$24 per million gallons, giving a net margin of profit of $6 per million

gallons. At New Haven, the total return is estimated at $7.09 per

million gallons. Based on the production of sulphur dioxide by burning

sulphur (assumed to cost $36 per long ton) and on drying from 85 per

cent to 10 per cent moisture with coal assumed to cost $7.50 per ton, it

appears that the acid treatment of sewage should be materially cheaper

than either the Imhoff treatment or fine screening under the local

conditions. A comparison of the cost of the treatment of the East Street

and the Boulevard sewage at New Haven and the Calf Pasture sewage in

Boston is given in Table 100. The cost of construction was estimated by

Dorr and Weston in 1919 as greater than $15,000 per million gallons of

sewage per day capacity.

TABLE 100

ESTIMATED COST OF SEWAGE TREATMENT AT NEW HAVEN AND BOSTON BY THREE

DIFFERENT PROCESSES

Cost in Dollars per Million Gallons Treated

(Engineering and Contracting, Vol. 51, p. 510)

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

│ Miles Acid Process │ Imhoff Tank and │Fine Screens

│ │ Chlorination │ and

│ │ │Chlorination

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

│ East │Boul-│ Calf │ East │Boul-│ Calf │ East │Boul-

│Street│evard│Pasture│Street│evard│Pasture│Street│evard

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

Tanks and │ 2.47│ 2.47│ 2.47│ 5.28│ 4.44│ │ 4.60│ 4.60

Buildings Int.│ │ │ │ │ │ │ │

and Dep. │ │ │ │ │ │ │ │

Acid treatment │ 6.93│10.74│ 18.65│ │ │ │ │

Drying sludge │ 2.09│ 2.04│ 10.34│ │ │ │ │

Degreasing │ 1.78│ 1.91│ 9.12│ │ │ │ │

sludge │ │ │ │ │ │ │ │

Superintendence │ 1.06│ 2.65│ 1.06│ 0.46│ 1.15│ │ 0.47│ 1.15

Labor on tanks │ 1.00│ 1.00│ 1.00│ 1.20│ 1.50│ │ 1.42│ 2.05

and screens │ │ │ │ │ │ │ │

Disposal of │ │ │ │ 1.00│ 1.00│ │ 0.50│ 0.50

sludge or │ │ │ │ │ │ │ │

screenings │ │ │ │ │ │ │ │

Chlorination │ │ │ │ 4.05│ 4.05│ │ 4.05│ 4.05

Gross cost │ 15.50│20.98│ 42.75│ 11.99│12.14│ │ 11.03│12.35

Revenue │ 6.57│10.66│ 47.59│ │ │ │ │

Net cost │ 8.93│10.32│ 4.84│ 11.99│12.14│ │ 11.03│12.35

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

ELECTROLYTIC TREATMENT

=276. The Process.=—This process has been generally unsuccessful in the

treatment of sewage and has grown into disrepute. In the words of the

editor of the _Engineering News-Record_:[195]

Thirty years of experiments and demonstrations with only a few

small working plants built and most of them abandoned—such in

epitome is the record of the electrolytic process of sewage

treatment.

It is probably true that the process has never received a thorough and

exhaustive test on a large scale, but the small-scale tests have not

been promising of good results. Among the most extensive tests have been

those at Elmhurst, Long Island,[196] Decatur, Ill.,[197] and Easton,

Pa.[198]

Whatever degree of popularity the method has possessed has been due

possibly to the mystery and romance of “electricity” and to the

personality of its promoters. The process should, nevertheless, be

understood by the engineer in order that it may be explained

satisfactorily to the layman interested in its adoption.

In this process, sometimes called the direct-oxidation process, all grit

is removed and the sewage is passed through fine screens before entering

the electrolytic tank. In the electrolytic tank the sewage passes in

thin sheets between electrodes and an electric current is discharged

through it. A recent development has been the addition of lime to the

sewage at some point in its passage through the electrolytic tank. From

the electrolytic tank the sewage flows to a sedimentation tank, where

sludge is accumulated, and from which the liquid effluent is finally

disposed of.

It is claimed that the action of the electricity electrolyzes the

sewage, releasing chlorine, which acts as a powerful disinfectant. The

constituents of the sewage are oxidized so that the dissolved oxygen,

nitrates, and relative stability are increased and the sludge is

rendered non-putrescible. It is said that the addition of lime increases

the efficiency of sedimentation and enhances the effect of the electric

current. The results obtained by tests at Easton, Pa., are shown in

Table 101. It will be observed from this table that the combination of

lime and electricity does not have a more beneficial effect than either

one of them alone. The amount of sludge produced by the combination is

about the same as by chemical precipitation alone, but the character of

the sludge produced with electricity is less putrescible. The cost of

the treatment as estimated at Elmhurst is shown in Table 102.

As a result of the tests at Decatur, comparing lime alone with lime and

electricity together, Dr. Ed. Bartow stated:

The purification by treatment with lime alone was greater than

that obtained in several of the individual samples treated with

lime and electricity.

TABLE 101

COMPARATIVE RESULTS OBTAINED FROM THE TREATMENT OF SEWAGE BY LIME

ALONE, ELECTRICITY ALONE, AND LIME AND ELECTRICITY COMBINED

(Creighton and Franklin, Journal of the Franklin Institute, August,

1919)

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

│ Lime and │ Lime Alone │ Electricity

│ Electricity │ │ Alone

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

│Change,│Change,│Change,│Change,│Change,│Change,

│ Parts │ Per │ Parts │ Per │ Parts │ Per

│ per │ Cent │ per │ Cent │ per │ Cent

│Million│ │Million│ │Million│

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

Chlorine │ +1.2│ +1.9│ +12.3│ +18.2│ +1.6│ +2.2

Nitrites │ +0.014│ +58.3│ -.005│ –10.0│ –0.01│ –20.0

Nitrates │ +0.13│ +23.6│ +.005│ +0.8│ –0.15│ –20.0

Ammonia │ –3.3│ –18.3│ +0.2│ +1.3│ +0.9│ +6.6

Albuminoid ammonia │ –3.6│ –12.1│ –0.4│ –1.7│ –0.5│ –2.3

Oxygen demand │ –13.0│ –20.5│ –7.7│ –8.9│ –6.5│ –10.0

Dissolved oxygen │ +1.78│ +40.9│ –0.93│ –19.1│ +1.61│ +40.1

Total bacteria at 37° │ –343│ –92.7│ –373│ –82.4│ –165│ –37.8

(Thousands) │ │ │ │ │ │

Total bacteria at 20° │ –688│ –92.7│ –1074│ –90.1│ –635│ –70.0

(Thousands) │ │ │ │ │ │

B. Coli (Thousands) │ –77.9│ –99.85│ –96.3│ –92.3│ –45│ –81.8

Oxygen absorbed in 5 │ –3.40│ –81.6│ –1.03│ –21.│ +1.24│ +31

days │ │ │ │ │ │

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

DISINFECTION

=277. Disinfection of Sewage.=—Sewage is disinfected in order to protect

public water supplies, shell fish, and bathing beaches; to prevent the

spread of disease; to keep down odors, and to delay putrefaction.

Disinfection is the treatment of sewage by which the number of bacteria

is greatly reduced. Sterilization is the destruction of all bacterial

life, including spores. Ordinarily even the most destructive agents do

not accomplish complete sterilization. Chlorine and its compounds are

practically the only substances used for the disinfection of sewage. The

lime used in chemical precipitation, the acid used in the Miles Acid

Process, the aëration in the activated sludge process, all serve to

disinfect sewage, but are not used primarily for that purpose. Copper

sulphate has been used as an algaecide but never on a large scale as a

bactericide.[199] Heat has been suggested, but its high cost has

prevented its practical application to the disinfection of sewage.

TABLE 102

COST OF ELECTROLYTIC TREATMENT, ELMHURST, LONG ISLAND, AND EASTON,

PENNSYLVANIA

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

│ │ Three

Item │One Million Gallon │ Million

│ │ Gallon

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

│ unit at │ unit at │ unit at

│ Easton, │Elmhurst,│Elmhurst,

│ Dollars │ Dollars │ Dollars

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

Hydrated lime: │ │ │

Elmhurst, 1300 pounds at $7.90 ton. │ 12.56│ 5.14│ 15.42

Easton, 3720 pounds at $6.75 ton. │ │ │

Electric power electrolysis: │ │ │

Elmhurst, 85 kw-h. at 4 cents │ 4.19│ 3.40│ 9.60

Easton, 6.25 kw-h. at 8.05 cents │ │ │

Electric power, light and agitation: │ │ │

Elmhurst, 60 kw-h. at 4 cents │ 0.50│ 2.40│ 7.20

Easton, 6.25 kw-h at 8.05 cents │ │ │

Heating │ 1.25│ │

Labor and supervision │ 15.00│ 12.50│ 15.00

Maintenance, repairs and supplies │ 1.50│ 1.00│ 3.00

Sludge pressing and removal │ │ 5.11│ 15.33

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

Total │ 35.00│ 29.55│ 65.55

Cost per million gallons │ 35.00│ 29.55│ 21.85

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

The action which takes place on the addition to sewage of chlorine or

its compounds is not well understood. The idea that the bacteria are

burned up with “nascent” or freshly born oxygen, has been exploded.[200]

Likewise the idea that the toxic properties of chlorine have no effect

has not been borne out by experiments. It has been demonstrated,

particularly by tests on strong tannery wastes, that the action of

chlorine gas is more effective than the application of the same amount

of chlorine in the form of hypochlorite. All that we are certain of at

present is that the greater the amount of chlorine added under the same

conditions, the greater the bactericidal effect.

Chlorine is applied either in the form of a bleaching powder or a gas.

In ordinary commercial bleach (calcium hypochlorite) the available

chlorine is about 35 to 40 per cent by weight. In order to add one part

per million of available chlorine to sewage it is necessary to add about

25 pounds of bleaching powder or 8½ pounds of liquid chlorine per

million gallons of sewage. This can be computed as follows:

The molecular weight of calcium hypochlorite is 127.0. This reacts

to produce two atoms of available chlorine with a molecular weight

of 70.9. If the bleaching powder were pure the available chlorine

would therefore represent 70.9 ÷ 127, or 56 per cent of its

weight. Then to obtain one pound of chlorine it would be necessary

to have 1.79 pounds of pure bleaching powder. Since 1,000,000

gallons of water weigh approximately 8,300,000 pounds, in order to

apply one part per million of chlorine to 1,000,000 gallons of

sewage it is necessary to apply 1.79 × 8.3 or 14.9 pounds of pure

bleaching powder. Commercial bleaching powder is only about 60 per

cent calcium hypochlorite. It is therefore necessary to add 14.9 ÷

0.60 or about 25 pounds of commercial bleach.

Since liquid chlorine is very nearly pure, approximately 8½ pounds

of it applied to 1,000,000 gallons of sewage are equivalent to a

dose of one part per million.

Commercial bleaching powder is a dry white powder which absorbs moisture

slowly, and which loses its strength rapidly when exposed to the air. It

is packed in air-tight sheet iron containers, which should be opened

under water, or emptied into water immediately on being opened. The

strength of the solution should be from ½ to 1 per cent. The rate of the

application of the solution to the sewage may be controlled by automatic

feed devices, or by hand-controlled devices.

Commercial liquid chlorine is sold in heavy cast steel containers, which

hold 100 to 140 pounds of liquid chlorine under a pressure of 54 pounds

per square inch at zero degrees C. or 121 pounds per square inch at 20

degrees.

The amount of chlorine used is dependent on the character of the sewage

to be treated, the stage of decomposition of the organic matter, the

desired degree of disinfection, the period of contact, and the

temperature. The amount of chlorine is expressed in parts per million of

available chlorine, regardless of the form in which the chlorine is

applied. In general about 15 to 20 parts per million of available

chlorine with 30 minutes’ contact at a temperature of about 15° C. will

effect an apparent removal of 99 per cent of the bacteria from the raw

sewage. The effect is only apparent because many of the bacteria encased

in the solid matter of the sewage escape the effect of the chlorine, or

detection in the bacterial analysis. Stronger and older sewages, higher

temperatures, and shorter periods of contact will demand more chlorine

to produce the same results. A septic effluent will require more

chlorine than a raw sewage because of the greater oxygen demand by the

septic sewage. The results of experiments on disinfection made at

different testing stations have shown such wide variations in the amount

of chlorine necessary, as to demonstrate the necessity for independent

studies of any particular sewage which is to be chlorinated. For

instance, at Milwaukee approximately 13 p.p.m. of available chlorine

applied to an Imhoff tank effluent effected a 99 per cent removal of

bacteria, whereas the same result was obtained at Lawrence, Mass., on

crude sewage with only 6.6 p.p.m. and at Marion, Ohio, only 9 per cent

removal of bacteria was obtained by the addition of 4,815 p.p.m. to

crude sewage. The Ohio and Massachusetts reports show irrational

variations among themselves. For instance, 6.2 p.p.m. applied to a

septic effluent effected 88 per cent removal whereas in another case 7.6

p.p.m. effected only 36 per cent removal. At Lawrence in one case it

took 8.6 p.p.m. to remove 99 per cent from a sand filter effluent, but

only 6.3 p.p.m. to effect the same result in the effluent from a septic

tank. The most consistent results are those found at Milwaukee which

show a steadily increasing percentage removal with increasing amounts of

chlorine.

Some time after sewage has received its dose of chlorine the number of

bacteria may be greater than in the raw sewage. Such bacteria are called

aftergrowths. Certain forms of bacteria, particularly the pathogenic or

body temperature types, are most susceptible to disinfecting agents.

These are killed off and leave the sewage in a condition more favorable

to the growth of more resistant forms of bacteria. As the latter are

non-pathogenic and are generally aërobic their presence is usually more

beneficial than detrimental, as they hasten the action of

self-purification.

REFERENCES

The following abbreviations will be used: E.C. for Engineering and

Contracting, E.N. for Engineering News, E.R. for Engineering Record,

E.N.R. for Engineering News-Record, M.J. for Municipal Journal, p. for

page, and V. for volume.

No.

1. Grease and Fertilizer Base for Boston Sewage, by Weston, E.N. V.

75, 1916, p. 913 and Journal American Public Health Association,

April, 1916.

2. Getting Grease and Fertilizer from City Sewage, by Allen. E.N. V.

75, 1916, p. 1005.

3. New Haven Tests Five Processes of Sewage Treatment. E.N.R. V. 79,

1917, p. 829.

4. Recovery of Grease and Fertilizer from Sewage Comes to the Front.

E.N.R. V. 80, 1916, p. 319.

5. Miles Acid Process may Require Aëration of Effluent, by Mohlman.

E.N.R. V. 81, 1918, p. 235.

6. Promising Results with Miles Acid Process in New Haven Tests.

E.N.R. V. 81, 1918, p. 1034.

7. Baltimore Experiments on Grease from Sewage. E.N. V. 75, 1916, p.

1155.

8. Report on Industrial Wastes from the Stock Yards and Packingtown in

Chicago to the Trustees of the Sanitary District of Chicago,

1914, pp. 187–195.

9. The Separation of Grease from Sewage, by Daniels and Rosenfeld.

Cornell Civil Engineer. V. 24, p. 13.

10. The Separation of Grease from Sewage Sludge with Special Reference

to Plants and Methods Employed at Bradford and Oldham, England,

by Allen. E.C. V. 40, 1913, p. 611.

11. Acid Treatment of Sewage, by Dorr and Weston. Journal Boston

Society of Civil Engineers, April, 1919. E.C. V. 51, 1919, p.

510. M.J. V. 46, 1919, p. 365.

12. The Miles Acid Process for Sewage Disposal. Metallurgical and

Chemical Engineering, V. 18, p. 591.

13. Miles Acid Treatment of Sewage, by Winslow and Mohlman. Journal

American Society Municipal Improvements, Oct., 1918. M.J. V. 45,

1918, pp. 280, 297, and 321.

14. New Electrolytic Sewage Treatment. M.J. V. 37, 1914, p. 556.

15. Electrolytic Sewage Treatment. M.J. V. 47, 1919, p. 131.

16. Electrolytic Treatment of Sewage at Durant, Oklahoma, by Benham.

E.N. V. 76, 1916, p. 547. Municipal Engineering, V. 49, 1916, p.

141.

17. Electrolytic Treatment of Sewage at Elmhurst, Long Island, by

Travis. Report to the President of the Borough of Queens, Aug.

31, 1914. E.R. V. 70, 1914, pp. 292, 315, and 429. M.J. V. 39, p.

551. Municipal Engineering, V. 47, p. 281.

18. Tests of the Electrolysis of Sewage at Toronto, by Nevitt. E.N. V.

71, 1914, p. 1076.

19. Electrolytic Treatment of Sewage Little Better than Lime Alone, by

Bartow. E.R. V. 74, 1916, p. 596.

20. Electrolytic Sewage Treatment Not Yet an Established Process.

E.N.R. V. 83, 1919, p. 541.

21. Tests of Electrolytic Sewage Treatment Process at Easton, Pa.

Journal of the Franklin Institute, Aug., 1919. E.N.R. V. 83,

1919, p. 569.

22. The Disinfection of Sewage. U. S. Geological Survey, Water Supply

Paper, No. 229.

23. Sewage Disinfection in Actual Practice, by Orchard. E.R. V. 70,

1914, p. 164.

24. Water and Sewage Purification in Ohio. Report of the Ohio State

Board of Health, 1908, pp. 738–762.

25. Water Purification, by Ellms. Published in 1917 by McGraw-Hill Book

Co.

26. Electrolytic Sewage Treatment, A Half Century of Invention and

Promotion. E.N.R. V. 86, 1921, p. 25.

CHAPTER XX

SLUDGE

=278. Methods of Disposal.=—Sludge is the deposited suspended matter

which accumulates as the result of the sedimentation of sewage. The

methods for the disposal of sludge as discussed herein will include the

disposal of scum. Scum is a floating mass of sewage solids buoyed up in

part by entrained gas or grease, forming a greasy mat which remains on

the surface of the sewage.[201] The sludges formed by different methods

of sewage treatment are described in the chapter devoted to the

particular method. The disposal of sludge is a problem common to all

methods of sewage treatment involving the use of sedimentation tanks.

Sludge is disposed of by: dilution, burial, lagooning, burning, filling

land, and as a fertilizer or fertilizer base. Certain methods of

disposal, such as burning or as a fertilizer, demand that the sludge be

dried preparatory to disposal. Sludge is dried on drying beds, in a

centrifuge, in a press, in a hot-air dryer, or by acid precipitation.

=279. Lagooning.=—This is a method of sludge disposal in which fresh

sludge is run on to previously prepared beds to a depth of 12 to 18

inches or more, and allowed to stand without further attention. The

preparation of the lagoons requires leveling the ground, building of

embankments, and, if the ground is not porous, the placing of

underdrains laid in sand or gravel. At Reading, Pa.,[202] approximately

one acre was required for 1,700 cubic yards of wet sludge. The results

of lagooning at Philadelphia are given in Table 103.[202]

TABLE 103

RESULTS OF DRYING SLUDGE IN LAGOONS AT PHILADELPHIA

(“Sewage Sludge” by Allen)

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

Treatment │ Days │ Depth, │Per Cent,│Rainfall,│ Cubic

│ │ Inches │Moisture │ Inches │Yards per

│ │ │ │ │ Acre

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

Screened │ 0│ 12.20│ 82.8│ 0│ 1600

Screened │ 26│ 7.67│ 57.0│ 0│ 1000

Screened │ 49│ 3.50│ 51.6│ 0.43│ 470

Screened │ 0│ 13.50│ 90.1│ 0│ 1800

Screened │ 62│ 7.00│ 61.0│ 3.14│ 950

Crude │ 0│ 12.00│ 88.7│ 0│ 1600

Crude │ 59│ 4.70│ 62.8│ 2.59│ 640

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

During the period of standing in the lagoon the moisture drains out and

evaporates and the organic matter putrefies, giving off gases and foul

odors. In the course of three to six months, biological action ceases

and the sludge has become humified and reduced to about 75 per cent

moisture. In the utilization of this method of disposal the lagoons must

be removed from settled districts and should occupy land of little value

for other purposes. The odors created at the lagoons may be intense and

offensive. The land so used is rendered unfit for other purposes for

many years.

The digestion of sludge in special tanks is a form of lagooning in which

an attempt is made to maintain septic action as a result of which a

portion of the sludge is gasified or liquefied, leaving less to be cared

for by some of the other methods of treatment or disposal. The results

obtained by digestion tanks have not been entirely satisfactory. A

partial drying and consolidation of the sludge may be effected, however,

by the process of decantation, in which the supernatant liquid is run

off, followed by further sedimentation, rendering the final product more

compact.

=280. Dilution.=—In the disposal of sludge by dilution, as in the

disposal of sewage by dilution, there must be sufficient oxygen

available in the diluting water to prevent putrefaction, and a swift

current to prevent sedimentation. Such conditions exist in localities

along the sea coast, and in communities situated near rivers, when the

rivers are in flood. In some seacoast towns, for example at London and

Glasgow, the sludge is taken out to sea in boats, and dumped. Since it

is not necessary to discharge sludge continuously, it can be stored to

advantage in the digestion chamber of a tank, until the conditions in

the body of diluting water are suitable to receive it.

The amount of diluting water to receive sewage sludge has not been

sufficiently well determined to draw reliable general conclusions. A

dilution of 1,500 to 2,000 volumes may be considered sufficiently safe

to avoid a nuisance provided there is a sufficient velocity to prevent

sedimentation. Johnson’s Report on Sewage Purification at Columbus, Ohio

(1905), states that a dilution of 1 to 800 is sufficient to avoid a

nuisance. The character of the sludge has a marked effect on the proper

ratio of dilution, the sludge from septic and sedimentation tanks

requiring a greater dilution than that from Imhoff tanks.

=281. Burial.=—Sludge can be disposed of by burial in trenches about 24

inches deep with at least 12 inches of earth cover, without causing a

nuisance. The ground used for this purpose should be well drained. This

method of disposal is generally used as a makeshift and has not been

practiced extensively because of the large amount of land required.

Insufficient information is available to generalize on the amount of

land required or the time before the land can be used for further sludge

burial, or for other purposes. Indications are that the sludge may

remain moist and malodorous for years and that the land may be rendered

permanently unfit for further sludge burial. Under some conditions the

land may be used again for the same or other purposes. For example,

Kinnicutt, Winslow and Pratt[203] state that 500 tons of wet sludge can

be applied per acre and:

The same land, it is claimed, can be used again after a period of

a year and a half to two years, if in two months or so after

covering the sludge with earth, the ground is broken up, planted,

and, when the crop is removed, again plowed and allowed to remain

fallow for about a year.

=282. Drying.=—Before sludge can be disposed of to fill land, by

burning, or for use as a fertilizer filler it must be dried to a

suitable degree of moisture. The removal of moisture from the sludge

decreases its volume and changes its characteristics so that sludge

containing 75 per cent moisture has lost all the characteristics of a

liquid. It can be moved with a shovel or fork, and can be transported in

non-watertight containers. A reduction in moisture from 95 to 90 per

cent will cut the volume in half.

The change in volume on the removal of moisture can be represented as:

_V__{1} = _V_(100 − _P_)⁄(100 − _P__{1}),

in which _P_ = the original percentage of moisture;

_P__{1} = the final percentage of moisture;

_V_ = the original volume;

_V__{1} = the final volume.

The drying of sludge on coarse sand filter beds is more particularly

suited to sludge from Imhoff tanks. This sludge does not decompose

during drying, and is sufficiently light and porous in texture to permit

of thorough draining. The sludge from plain sedimentation or chemical

precipitation tanks is high in moisture, putrescible, and when placed on

a filter bed it settles into a heavy, compact, impervious mass which

dries slowly. In order to avoid this condition the sludge is run on to

the beds as quickly as possible, to a depth of not more than 6 to 10

inches. Lime is sometimes added to the sludge at this time as it aids

drying by assisting in the maintenance of the porosity of the sludge,

and it is advantageous in keeping down odors and insects.

Sludge filter beds are made up of 12 to 24 inches of coarse sand,

well-screened cinders, or other gritty material, underlaid by 6 inches

of coarse gravel and 6 or 8–inch open-joint tile underdrains, laid 4 to

10 feet apart on centers, dependent on the porosity of the subsoil. The

side walls of the filters are made of planks or of low earth

embankments. The sludge filters at Hamilton, Ontario, are shown in Fig.

179.

[Illustration:

FIG. 179.—Sludge drying Beds at Hamilton, Ontario.

Eng. News, Vol. 73, p. 426.

]

The size of the bed is dependent mainly upon the characteristics of the

sludge. For Imhoff tank sludge which comes from the tank with about 85

per cent moisture, the practice is to allow about 350[204] square feet

of filter surface per 1,000 population contributing sludge. For other

types of sludge the area varies from 900 to 9,000 square foot per 1,000

population contributing sludge, and only experiments with the sludge in

hand can determine the proper allowance. Imhoff recommends 1,080 square

feet per 1,000 population for septic tank sludge, and 6,480 square feet

for sludge from plain sedimentation tanks.[205] Kinnicutt, Winslow, and

Pratt in their book on Sewage Disposal state:

With an average depth of 10 inches per dose of sludge of 87 per

cent water content, one square foot of covered (glass) bed should

dry to a spadable condition one cubic yard of sludge per year.

The sludge is run on the bed in small quantities at periods from two

weeks to a month apart. In favorable weather Imhoff sludge will dry in

two weeks or less to approximately 50 to 60 per cent moisture. It is

then suitable for use as a filling material on waste land, for burning,

or for further drying by heat. Glass roofs, similar to those used on

green-houses, have been used to speed the drying process by preventing

the moistening of partly dried sludge during rainy weather. In some

instances sludge has dried to 10 per cent moisture on such beds. Imhoff

sludge can be removed from the drying beds with a manure or hay fork. It

has an odor similar to well-fertilized garden soil. It is stable, dark

brownish-gray in color, is of light coarse material, and is granular in

texture.

Sludge presses are suitable for removing moisture from the bulky wet

sludge obtained from plain sedimentation, chemical precipitation, and

the activated sludge process. The details of a typical sludge press are

shown in Fig. 180. The press shown is made up of a number of corrugated

metal plates about 30 inches in diameter with a hole in the center about

8 inches in diameter. The corrugations run vertically except for a

distance about 3 inches wide around the outer rim, which is smooth. To

this smooth portion is fastened, on each side of the plate, an annular

ring about an inch thick and 2 to 3 inches wide, of the same outside

diameter as the plate. A circular piece of burlap, canvas, or other

heavy cloth is fastened to this ring, covering the plate completely. A

hole is cut in the center of the cloth slightly smaller in diameter than

the center hole in the plate, and the edges of the cloth on opposite

sides of the plate are sewed together. The plates are then pressed

tightly together by means of the screw motion at the left end of the

machine, thus making a water-tight joint at the outer rim. Sludge is

then forced under pressure into the space between the plates, passing

through the machine by means of the central hole. The pressure on the

sludge may be from 50 to 100 pounds per square inch. This pressure

forces the water out of the sludge through the porous cloth from which

it escapes to the bottom of the press along the corrugations of the

separating plate. After a period of 10 to 30 minutes the pressure is

released, the cells are opened, and the moist sludge cake is removed.

The liquid pressed from the sludge is highly putrescible and should be

returned to the influent of the treatment plant. The pressing of wet

greasy sludges is facilitated by the addition of from 8 to 10 pounds of

lime per cubic yard of sludge. The cake thus formed is more cohesive and

easy to handle. The output of the press depends so much on the character

of the sludge that a definite guarantee of capacity is seldom given by

the manufacturer.

[Illustration:

FIG. 180.—Filter Press.

]

The simplest form of centrifugal sludge dryer is a machine which

consists of a perforated metal bowl lined with porous cloth in which the

sludge is placed. Surrounding this bowl is a second water-tight metal

bowl so arranged as to intercept the water thrown from the sides of the

inner bowl as it revolves. The peripheral velocity of the inner bowl is

about 6,000 feet per minute, which makes the effective weight of each

particle about 250 times its normal weight when at rest. Very few data

are available on the operation of such machines, and their use has not

been extensive because of the difficulty of starting and stopping the

machine at each filling, and the difficulty of removing the partially

dried sludge from the inner basket. The Besco-ter-Meer centrifuge,

manufactured by the Barth Engineering and Sanitation Co., can be

operated continuously and the difficulties of removing the dried sludge

from the machine have been overcome. According to the manufacturers the

centrifuge has been operated very successfully in Germany on plain

septic tank sludge. A removal of 70 per cent of suspended solids in the

raw sludge and a production of 3,600 pounds of sludge per hour,

containing 60 to 70 per cent of moisture, can be obtained at less than

900 r.p.m. with a consumption of 15 horse-power. Extensive tests of the

machine were made at Milwaukee from October, 1920, to September, 1921,

on activated sludge, but results of these tests are not as yet

available. Indications are that the centrifuge has acted as a

classifier. The coarser particles of sludge have been removed but the

finer particles have been continuously returned with the liquid to the

sedimentation tank, ultimately filling this tank with fine particles of

sludge. An illustration of the unit tested at Milwaukee is shown on this

page.

[Illustration:

Besco-ter-Meer Sludge Drying Centrifuge at Milwaukee, Wisconsin

Courtesy, Barth Engineering and Sanitation Co.

]

Experiments on the drying of sludge by acid flotation have not

progressed sufficiently to allow the installation of a working unit. The

method, which has been applied principally to activated sludge, consists

in adding a small amount of sulphuric acid to the sludge as it leaves

the storage tank. The sludge is coagulated by this action, the

coagulated material rising to the surface as a scum containing about 86

per cent moisture. The consistency is such that it can be removed with a

shovel. The liquid can be withdrawn continuously from below the scum.

[Illustration:

FIG. 181.—Direct-Indirect Sludge Dryer.

Courtesy, the Buckeye Dryer Co.

]

The moisture content of sludge to be used in the manufacture of

fertilizer must be reduced to 10 per cent or less. None of the methods

of drying described so far can be relied upon for such a product and it

becomes necessary to use direct or indirect heat dryers. There are

various types of dryers on the market. The details of a Buckeye dryer

are shown in Fig. 181. In the operation of this machine moist sludge is

fed in at the left end at the point marked “feed.” The hot gases pass

from the fire box up and around the cylinder which revolves at about

eight r.p.m. The gases are drawn into the inner cylinder through the

openings marked A which revolve with the two cylinders. The gases escape

from the inner cylinder through the openings to the right and flow

towards the left in the outer cylinder. They come in contact with the

sludge at this point. The gases then pass off through the fan at the

left. The sludge is lifted by the small longitudinal baffles fastened to

the outer cylinder, as the drying cylinders revolve. The right end of

the cylinder is placed lower than the left so that the drying sludge is

lifted and dropped through the cylinder at the same time that it moves

slowly toward the right hand end of the cylinder. These dryers require

about one pound of fuel for 10 pounds of water evaporated. The odors

from the dryer can be suppressed by passing the gases through a dust

chamber and washer.

A summary of the results from methods of sludge drying at Milwaukee[206]

follows:

Excess sludge produced, 12,100 gallons, having 97.5 per cent

moisture, per million gallons of sewage treated.

Sludge cake produced (by presses), 10,083 pounds having 80.3 per

cent moisture, per million gallons of sewage treated.

Dried sludge (from heat driers) produced, 2,521 pounds having 10

per cent moisture, per million gallons of sewage treated.

Press will produce 3 pounds of cake per square foot of filter

cloth in four and a half hours, or five operations per twenty-four

hours.

Dryers will reduce 6,700 pounds of sludge cake at 80 per cent

moisture to 10 per cent moisture, and will evaporate 8 pounds of

water per pound of combustible.

Thickening devices known as Dorr thickeners, patented and manufactured

by the Dorr Co. and originally intended for metallurgical purposes, have

been adapted to the thickening of sewage sludge. These thickeners are

circular sedimentation tanks, from 8 to 12 feet deep, more or less, and

are made in any diameter up to 200 feet or more. An arm, pivoted in the

center and extending to the circumference, is provided at the bottom

with a number of baffles or squeegees set at an angle with the arm. The

arm revolves at from one to fifteen revolutions per hour, and the

squeegees, in contact with the bottom of the tank, scrape the deposited

sludge towards a central sump, from which it is removed by a pump or by

gravity, without interrupting the operation of the thickener. The sludge

so thickened may be reduced to 95 or 96 per cent moisture. These devices

are ordinarily used only in the activated sludge process in which they

have been a pronounced success.

CHAPTER XXI

AUTOMATIC DOSING DEVICES

=283. Types.=—Automatic dosing devices are used to apply sewage to

contact beds, trickling filters, and intermittent sand filters. These

devices can be separated into two classes; those with moving parts and

those without moving parts. The latter are better known as air-locked

dosing devices. Simple devices without moving parts are less liable to

disorders and are nearer “fool-proof” than any device depending on

moving parts for its operation.

No one type of moving part device has been used extensively in different

sewage treatment plants. Designing engineers have exercised their

ingenuity at different plants, resulting in the production of different

types.[207] Among the best known forms is the apparatus designed by J.

W. Alvord for the intermittent sand filters at Lake Forest,

Illinois.[208] In its operation....

A float in the dosing chamber lifts an iron ball in one of a

series of wooden columns, and at a certain height the ball rolls

through a trough from one column to the next, in its passage

striking a catch, which opens an air valve attached to one of ten

bell-siphons in the dosing chamber. Each of the siphons discharges

on one of the ten sand beds, which are thus dosed in rotation.

Since air-locked dosing devices are in more general use their operation

will be explained in greater detail.

=284. Operation.=—The simplest form of these devices is the automatic

siphon used for flush-tanks, the operation of which is described in Art.

61.

In the operation of sand filters, sprinkling filters, or other forms of

treatment where there are two or more units to be dosed it is desirable

that the dosing of the beds be done alternately. A simple arrangement

for two siphons operating alternately is shown in Fig. 182. They operate

as follows: with the dosing tank empty at the start water will stand at

_bb′_ in siphon No. 2 and at _aa′_ in siphon No. 1. As the water enters

through the inlet on the left the tank fills. When the water rises

sufficiently, air is trapped in the bells, and as the water continues to

rise in the tank, surfaces _a_ and _b_ are depressed an equal amount.

When _b_ has been depressed to _d_, _a_ has been depressed to _c_. Air

is released from siphon No. 2 through the short leg, and siphon No. 2

goes into operation. Surface _c_ rises in siphon No. 1 as the tank

empties and when the action of Siphon No. 2 is broken by the admission

of air when the bottom of the bell is uncovered the water in siphon No.

1 has assumed the position of _bb′_ and that in No. 2 is at _aa′_. The

conditions of the two siphons are now reversed from that at the

beginning of the operation and as the tank refills siphon No. 1 will go

into operation. It is to be noted that these siphons are made to

alternate by weakening the seal of the next one to discharge and by

strengthening the seal of the one which has just discharged.

[Illustration:

FIG. 182.—Diagram Showing the Operation of Two Alternating Siphons.

]

[Illustration:

FIG. 183.—Diagram Showing the Operation of Three Alternating Siphons.

]

=285. Three Alternating Siphons.=—This principle can be extended to the

operation of three alternating siphons as shown in Fig. No. 183. These

operate as follows: with the dosing tank empty at the start and water at

_aa′_ in siphons 1 and 2, and at _bb′_ in siphon No. 3, the dosing tank

will be allowed to fill. As the water rises in the tank air is trapped

in all the bells and surfaces _a_ and _b_ are depressed. When surface

_b_ has been depressed to _d_, _a_ has been depressed to _c_. Air is

released from siphon No. 3 and this siphon goes into action. Surface _c_

rises in siphons 1 and 2 to the position _b_, as the dosing tank is

emptied. At the same time a small amount of water is passed from siphon

No. 3 to the short leg of siphon No. 1, through the small pipes shown,

thus filling this leg so that when siphon No. 3 ceases to operate the

water in siphons 1 and 3 stands at _aa′_ and that in No. 2 stands at

_bb′_. Siphon No. 2, having the weaker seal, will be the next to

operate. During its operation it will fill siphon No. 3, leaving No. 1

weak. When No. 1 operates it will refill No. 2, leaving No. 3 weak, thus

completing a cycle for the three siphons. This principle has not been

applied to the operation of more than three alternating siphons and is

seldom used on recent installations.

[Illustration:

FIG. 184.—Miller Plural Alternating Siphons.

Courtesy, Pacific Flush Tank Co.

]

=286. Four or More Alternating Siphons.=—An arrangement for the

alternation of four or more siphons is illustrated in Fig. 184. At the

commencement of the cycle it will be assumed that all starting wells are

filled with water except well No. 1, and that all main and all blow-off

traps are filled with water. The following description of the operation

of the siphons is taken from the catalog of the Pacific Flush Tank

Company:

The liquid in the tank gradually rises and finally overflows into

the starting well No. 1 and the starting bell being filled with

air, pressure is developed which is transmitted, as shown by the

arrows, to the blow-off trap connected with siphon No. 2. When the

discharge line is reached, sufficient head is obtained on the

starting bell to force the seal in blow-off trap No. 2, thus

releasing the air confined in siphon No. 2 and bringing it into

full operation.

During the time that siphon No. 2 is operating, siphonic action is

developed in the draining siphon connected with starting well No.

2 and as soon as the level in the tank is below the top of the

well it is drained down to a point below the bottom of starting

well No. 2. It can now be seen that after the first discharge

starting well No. 2 is empty, whereas the other three are full....

Therefore when the tank is filled the second time, pressure is

developed in starting bell No. 2, which forces the seal of

blow-off trap No. 3, thus starting siphon No. 3....

This alternation can be continued for any number of siphons. Other

arrangements have been devised for the automatic control of alternating

siphons, but these principles of the air-locked devices are fundamental.

=287. Timed Siphons.=—In the operation of a number of contact beds not

only must the dosing of the tanks be alternated, but some method is

needed by which the beds shall be automatically emptied after the proper

period of standing full. To fulfill this need the principle of the timed

siphon must be employed in conjunction with the alternating siphons.

Fig. 185 illustrates the operation of the Miller timed siphon. Its

operation is as follows: water is admitted to the contact bed and

transmitted to the main siphon chamber through the “opening into bed.”

Water flows from the main siphon chamber into the timing chamber at a

rate determined by the timing valve. The contact bed is held full during

this period. As the timing chamber fills with water air is caught in the

starting bell and the pressure is increased until the seal in the main

blow-off trap is blown and the main siphon is put into operation. As the

water level in the main siphon chamber descends, water flows from the

timing chamber into the main siphon through the draining siphon and the

timing chamber is emptied, ready to commence another cycle.

=288. Multiple Alternating and Timed Siphons.=[209]—The alternating and

timing of a number of beds is more complicated. The arrangement

necessary for this is shown in Fig. 186. It will be assumed at the start

that all beds are empty and that all feeds are air locked as shown in

Section _AB_ except that to bed No. 4 into which sewage is running. As

bed No. 4 fills, sewage is transmitted through the opening in the wall

into the timed siphon chamber No. 4. When the level of the water in the

bed and therefore in this chamber has reached the top of the withdraw

siphon leading to the compression dome chamber No. 4, this latter

chamber is quickly filled. The air pressure in starting bell No. 4_a_ is

transmitted to blow-off trap No. 1_a_. The seal of this trap is blown,

releasing the air lock in feed No. 1 and the flow into bed No. 1 is

commenced. At the same time the air pressure in compression dome No. 4

is transmitted to feed No. 4, air locking this feed and stopping the

flow into bed No. 4. The alternation of the feed into the different beds

is continued in this manner.

[Illustration:

FIG. 185.—Miller Timed Siphon.

Courtesy, Pacific Flush Tank Co.

]

Bed No. 4 is now standing full and No. 1 is filling. When compression

dome chamber No. 4 was filled, water started flowing through timing

siphon valve No. 4 into timing chamber No. 4 at a rate determined by the

amount of the opening of the timing valve. As this chamber fills

compression is transmitted to blow-off trap 4_b_ and when sufficiently

great this trap is blown and timed siphon No. 4 is put into operation.

Bed No. 4 is emptied by it, and compression dome chamber No. 4 is

emptied through the withdraw siphon at the same time. This completes a

cycle for the filling and emptying of one bed and the method of passing

the dose on to another bed has been explained. The principle can be

extended to the operation of any number of beds.

[Illustration:

FIG. 186.—Plural Timed and Alternating Siphons for Contact Bed

Control.

Courtesy, Pacific Flush Tank Co.

]

INDEX

A. B. C. process of sewage treatment, 4

Abandonment of contract, 225

Access to work, 228, 229

Accident, contractor’s responsibility, 221, 224

Acetylene, explosive, 347

Acid precipitation. _See_ Miles Acid Process.

of sludge, 503

Acids as disinfectants, 489, 490

Activated sludge. Chapter XVIII, 465–479

advantages and disadvantages, 469, 470

aëration tank, 471, 472

air diffusion, 475, 477

air distribution, 473–478

air quantity, 475, 476

area of filtros plates, 478

colloid removal, 358

composition, 465–469

cost, 478, 479

definition, 466

dewatering, 468, 469, 497–505

fertilizing value, 469, 470

historical, 470, 471

how obtained, 478

nitrogen content, 468

patent, 471

process, 465

quantity, 469

reaëration tank, 473

results, 467, 468, 476

sedimentation tank, 472

Advertisement, 214

Aëration, effect on oxygen dissolved, 373–375

of sewage, 371, 376, 465–479

Aërobes, 363

Aërobic decomposition, 366, 367

Aftergrowths, 492

Aggregates, specifications, 172–174

Air, see also ventilation, activated sludge, compressed air, etc.

ejectors, 150

lock dosing apparatus. Chap. XXI, 506–512

machinery for activated sludge, 473, 474

Algæ, 363

Alkalinity, 358

Alleys, sewers in, 80

Alum, 407, 408

Alvord tank, 427, 429

Ammonia, 366, 367, 374, 375, 410

explosives, 297

Analyses, bacteriological, 364

chemical, 354, 355

mechanical of sand, 182

physical, 352–354

sewage, 352–364

Anaërobes, 363, 365–367

Anaërobic, action, 410

bacteria, 363

conditions, 367

decomposition, 365–367

Ann Arbor, Michigan, Population, 14

Annual expense, method of financing, 157, 158

Ansonia air ejector, 150, 151

Antibiosis, definition, 363

Appurtenances to sewers. Chap. VI, 99–115

Arch, analyses, 204–208

elastic method, 206–208

vouissoir analysis, 204–206

brick construction, 312, 313

centers for brick sewers, 313

concrete construction, 318–321

Ardern and Lockett, development of activated sludge, 467, 468, 471

Area of cities, 31

Asphyxiation in sewer gas, 336

Assessments, special, 15, 16

Augers, earth, 21

Automatic, regulators, 117–121

siphons, flush-tanks, 110

double alternating, 507

multiple alternating, 508–512

timed, 510

timed and multiple alternating, 510–512

triple alternating, 508

Bacillus, definition and morphology, 362, 363

Backfilling, 328–331

Backfill, puddling, 330

weight of, 199, 201

Backwater curve, 73

Bacteria, definition and morphology, 362, 363

good and bad, 363, 364

nature of, 362, 363

nitrifying, 431, 432

sanitary significance of, 364

in sewage, 362, 363

total count, 364

Bacterial analyses, results in sewage, 364

Baffles, scum, 404, 413, 414, 421

in sedimentation tanks, 404

in septic tanks, 413, 414

in Imhoff tanks, 421

Balls, for cleaning sewers, 338

Band screen, 384

Barring, definition, 263

Bars for screens, 390

Basins, sedimentation, baffling, 404

bottoms, 404

cleaning arrangements, 404

depth, 401

economical dimensions, 401–403

inlets and outlets, 404

scum boards, 404

types, 395

Basket handle sewer section, 67, 69

Bathing beaches, pollution, 381

Bazin’s formula, 54

Bearings, for centrifugal pumps, 131, 137, 138

thrust, 138

Bellmouth, 121, 122

Bends in pipe, loss of head in, 116

Berlin, sewage farm, 460, 461

sewers, date of, 3

Bids, proposal, 217–219

Bidder’s duties, 215–217

Bio-chemical oxygen demand, 359–361

Biolysis of sewage, 366, 367

Black and Phelps dilution formulas, 377–379

Blasting and explosives, 294–304

caps, 297, 299, 300

detonators, 294, 297–300

firing, 302–304

fuses and detonators, 297–300

fuses, delayed action, 291, 300

fuses, electric, 299, 300

splicing, 303

gelatine, 296

loading holes, 303

powder, 295

precautions, 300–302

priming and loading, 303

rock, 269

size of charge, 304, 305

tunneling, 290, 291

Bleach, characteristics of for disinfection, 491

Block sewer, construction, 311–314

hollow tile as underdrains, 126

Blocks, vitrified clay, 189, 190

Boilers, steam, 147–150

Boilers, efficiencies, 149

horse-power, 149

Bond, contractor’s, 213, 214, 232

issues, 14

Bonds, definition and types, 14–16

Boring underground, 20

Bottom, activated sludge aëration tank, 472

Imhoff tanks, 423

sedimentation tanks, 404

trickling filter, 451, 452

Box sheeting, 272

Branch sewer, defined, 7

Breast boards, 288

Brick, arch construction, 312, 313

and block sewer construction, 311–315

invert construction, 311, 312

sewer construction, 311–315

arch centers, 313

invert, 311–312

organization, 314, 315

progress, 314

row lock bond, 312

specifications, 188, 189

sewers, life of, 351

Bricks for sewers, 316

British Royal Commission on Sewage Disposal, 4

Broad irrigation. _See_ under Irrigation.

Bucket excavators, 246, 255, 256

Building material, weight of, 201

Burkli-Ziegler formula, 47, 425

Butyrine, 366

Cableway excavators, 246, 250–252

Cage screen, 384, 385

Caisson excavation, 285, 286

Calcium carbide, explosive, 347

Calumet pumping station, 128, 142

Cameron septic patent, 411

Capacity of sewers, diagrams, 57–60

Capital, private invested in sewers, 17

Capitalization, method of financing, 157–160

Caps, blasting. _See_ blasting.

Carbohydrate, 366, 367

Carbon, analysis for, 356

dioxide, 366, 367

Carson Trench machine, 250, 251

Cast-iron pipe, 122, 164, 190, 191

joints, 164

quality, 101, 102, 190

Castings, iron, 101, 102

Catch-basins, 99, 107–108, 217

cleaning, 343, 344

inspection, 337

Catenary sewer section, 69

Cellars, depth of, 88

Cellulose, 367

Cement. _See also_ Concrete,

pipe, specifications, manufacture and sizes, 171–179

vs. concrete, 164

Centrifugal pumps. _See_ pumps, centrifugal.

Centrifuge for sludge drying, 501, 502

Cesspool, 411, 416, 417

Champaign, Illinois, septic tank, 415, 416

Changes in plan, 222, 223

Channeling, definition, 263

Character of surface, 44

Chemical analyses, 354–362

Chemical precipitation, 371, 405–409

chemicals used, 405–407

preparation of chemicals, 407, 408

results, 408, 409

at Worcester, 408

Chezy formula, 52, 53

Chicago. _See also_ Sanitary District of Chicago.

drainage canal, 374, 375

dilution requirement for sewage, 380

early sewers, 3

method of sewage disposal, 374

population and density, 29, 30

trench excavation in, 248

Chlorine. _See also_ Disinfection.

disinfectant, 489–493

in sewage, 358, 374, 375

Chlorine liquid, application, 491, 492

Cholera, transmittable disease, 364

Chromatin, 365

Chutes for concrete, 187

Circular sewer section, hydraulic elements, 65, 66, 69

types, 70, 71

City, growth of area, 31

growth of population, 24–28

legal powers, 219

Clay, life of pipe, 349–351

manufacture of pipe, 165–167

specifications for pipe, 168–170

unglazed for pipe, 165

vitrified blocks, 167, 189, 190

vitrified pipe, 165–171

Cleaning, grit chambers, 398, 400

sedimentation basins, 404

sewers, cost, 341

in N. Y. City, 332

methods, 337–343

tools, 338–340

up after completion of work, 228

Coccus, 362

Coefficient of uniformity of sand, 456

Coffin sewer regulator, 117, 118

Colloid, nature of, 358

treatment for, 358

Color of sewage, 352, 353

Combined sewer system, 78, 79

Commercial districts, characteristics of and sewage from, 32, 34, 35

Compensators for pumps, 142

Compressed air. _See also_ ventilation, tunneling, drilling, etc.

activated sludge, 473–475

for drilling, 264–268

in tunnels, 292–294

transporting concrete, 320, 321

Concentration, time of flood flow, 41–43, 96, 97

Concrete, aggregates, 172–174

mixing and placing, 184–188

pipe, details, 175–179

manufacture, 171–179

reinforcement, 177, 178, 209, 210

pipe, steam process, 176

sizes, 175

pressure against forms, 232, 323

Concrete, proportioning, 179–183

qualities, 179, 180

reinforcement, placing, 178, 326, 327

reinforcing steel, quality, 191

sewer construction, 314–328

arch, 318–321

form length, 319

labor costs, 327, 328

in open cut, 314–320

in tunnel, 320, 321

invert, 315–320

organization for, 328

working joints, 319

sewer costs, 327–329

strength, 181

waterproofing, 184

Conduits, special sections, 67, 70, 71

Connections to sewers, ordinances, 344, 345

record of 92, 238

Construction of sewers, Chap. XI, 233–331

Construction, elements of, 233

organizations, 315, 328

Contact bed, 432–437, 506

advantages and disadvantages, 432–434

automatic control, 437, 506

cleaning, 435

clogging, 435

construction, 434–436

control, 437, 506

cycle, 436, 437

depth, 434

description, 432, 433

design, 434–436

dimensions, 434, 435

loss of capacity, 435

material, 435, 436

multiple, 433, 435

operating conditions, 432–437

rate, 435

results, 433, 434

ripening, 432

Continuous bucket excavators, 246–250

Contour interval on maps, 79, 80