Type of│ Location │ Clear │ Screenings
Screen │ │ Opening, │
│ │in Inches │
│ │ │
│ │ │
│ │ │
───────┼──────────┼──────────┼────────────┬─────────────
│ │ │Per Million │ Per 1000
│ │ │ Gallons, │ Population
│ │ │_y_ = Cubic │ Daily,
│ │ │ Yard │ _y_ = Cubic
│ │ │ _t_ = Tons │ Yard
│ │ │ │ _t_ = Tons
───────┼──────────┼──────────┼────────────┼─────────────
Band │Hamburg │ 0.6 │ 0.34_y_ │ 0.018_y_
│Göttingen │ 0.4 │ 0.35_y_ │ 0.026_y_
│Sutton │0.375[141]│ 0.6_y_ │
│Chicago │ │ 2.4–3.1_t_ │
Wing │Frankfort │ 0.40 │ 0.7_y_ │ 0.040_y_
│Elberfeld │ 0.40 │ 1.15_y_ │ 0.053_y_
│Stralsund │ 0.20 │ │ 0.079_y_
│Wiesbaden │ 0.60 │ 1.1_y_ │ 0.033_y_
Shovel │Strassburg│ 0.10 │ 1.6_y_ │ 0.043_y_
vane │ │ │ │
│Gleiwitz │ 0.12 │ │ 0.192_y_
│Temesvar │ 0.12 │ 0.9–1.7_y_ │0.067–.133_y_
Drum │Bromberg │ 0.08 │ 4.75_t_ │
│Mainz │ Note 6 │ 0.52_y_ │
│Trier │ 0.10 │0.39–0.42_y_│ 0.13_y_
│Osnabruck │ 0.08 │ 3.2–4.0_y_ │ 0.08–.10_y_
Weand │Reading, │ 36[141] │ 1.0_y_ │
│ Pa. │ │ │
│Brockton │ 36[141] │ 1.4_t_ │
Reinsch│Dresden │ 0.08 │ 0.97_t_ │ 0.09_y_
Wurl │ │ │ │
───────┴──────────┴──────────┴────────────┴─────────────
───────┬────────┬───────────┬─────────┬────────────
Type of│Per Cent│Horse-Power│ Cost of │ Remarks
Screen │Moisture│Per Screen │Operation│
│ │ │ Per │
│ │ │ Million │
│ │ │Gallons, │
│ │ │ Dollars │
───────┼────────┼───────────┼─────────┼────────────
│ │ │ │
│ │ │ │
│ │ │ │
│ │ │ │
│ │ │ │
│ │ │ │
───────┼────────┼───────────┼─────────┼────────────
Band │ 87 │ 2.5 │ │Note 1
│ │ 2.0 │ │
│ │ │ │
│ 79 │ │ │Stock Yard
Wing │ │ 5.0 │ │Note 2
│ 75 │ │ │Note 3
│ │ 4.5 │ │
│ │hand power │ 1.64 │Note 4
Shovel │ 89.3 │ 3.35 │ │Note 5
vane │ │ │ │
│ │ │ 0.90 │
│ 60–70 │ │ small │
Drum │ 40–60 │ │ 2.45 │Experimental
│ 75 │ 5.2–6.8 │0.89–3.42│
│ 50–60 │ │ 2.41 │Experimental
│ │ 9.00 │ │Note 7
Weand │ 89.5 │ 2.0 │ 1.00± │
│ │ │ │
│ │ │ │
Reinsch│ 84 │ 2.5 │.325–1.76│
Wurl │ │ │ │
───────┴────────┴───────────┴─────────┴────────────
Notes:—1. After removal of ½ this volume of grit.
2. After removal of 16 per cent by the grit chamber.
3. Including 0.6 cubic yard grit per million gallons.
4. After passing 1.6 inch bar screen.
5. After removal of 0.132 cubic yard grit and coarse
screenings per 1000 population.
6. 0.12, 0.04–0.08.
7. Before removal of 0.4 cubic yard grit per million gallons.
[Illustration:
FIG. 153.—Screenings Collected on Different Sized Openings.
1921 Report on Industrial Wastes Disposal, Union Stock Yards District,
Chicago, Illinois, to the Sanitary District of Chicago.
]
=231. Design of Fixed and Movable Screens.=—The determination of the
size of the opening is the first step in the design of a sewage screen.
This is followed by the computation of the net area of openings in the
screen. The final steps are the determination of the overall dimensions
of the screen; the size of the bar, wire, or support; and the dimensions
of the screen chamber. The net area of openings is fixed by the
permissible velocity of flow through the screen and the quantity of
sewage to be treated. In determining the velocity of flow the general
principle should be followed that the velocity should not be reduced
sufficiently to allow sedimentation in the screen chamber. The velocity
of grit bearing sewage in passing through coarse screens should not be
reduced below 2 or 3 feet per second. If the sewage contains no grit, or
the screen is placed below a grit chamber the velocity through a medium
or fine screen should be from ½ to 1½ feet per minute. The velocity
through the screen in a direction normal to the plane of the screen can
be reduced without reducing the horizontal velocity of the sewage by
placing the screen in a sloping position.
The final steps are the design of the screen bar and the determination
of the dimensions of the screen and of the screen chamber. The size of
the bar in a bar screen, or as a support to a wire mesh, is dependent on
the unsupported length of the bar. The stresses in the bars are the
results of impact and bending, caused by cleaning, and of the load due
to the backing up of the sewage when the screen is clogged. Allowance
should be made for a head of 2 or 3 feet of sewage against the screen. A
generous allowance should be made in addition for the indeterminate
stresses due to cleaning. The screen should be supported only at the top
and bottom, as intermediate supports in a bar screen are undesirable
unless they are so arranged as not to interfere with the teeth of the
cleaning devices.
Fixed screens should be placed at an angle between 30° and 60° with the
horizontal, with the direction of slope such that the screenings are
caught on the upper portion of the screen. A small slope is desirable in
order to obtain a low velocity through the screen. The slope is limited
since the smaller the slope the longer the bars of the screen and the
greater the difficulty of hand cleaning. Small slopes will tend to make
the screens self cleaning. As the screen clogs, the increasing head of
sewage will push the accumulated screenings up the screen. The use of
flat screens in a vertical position is not desirable because of the
difficulty of cleaning and the accumulation of material at inaccessible
points. If a flat screen is placed in a horizontal position with the
flow of sewage downward difficulties are encountered in cleaning and
solid matter is forced through the screen as clogging increases. An
upward flow through a horizontal screen is undesirable as the material
is caught in a position inaccessible for cleaning. Movable screens are
more easily handled when placed in a vertical position.
In the construction of small screens, round bars are sometimes used
where the unsupported length of the bar is less than 3 or 4 feet. They
are not recommended, however, as the efficient area and the amount of
material removed by the screen are diminished. Bars which produce
openings with the larger end upstream are undesirable as particles
become wedged in the screen, and are either forced through or become
difficult to remove.[142] Rectangular bars are easily obtained and give
satisfactory service except where they are of insufficient strength
laterally. For greater lateral thickness a pear-shaped bar is sometimes
used, with the thicker side upstream. Fine mesh screens or perforated
plates are supported on grids or parallel bars of stronger material
designed to take up the heavy stresses on the screen.
The dimensions of the bar may be selected arbitrarily. The length and
width of the screen are fixed to give desirable dimensions to the screen
chamber and to give the necessary net opening in the screen. The width
of the screen chamber and the screen should be the same. The screen
chamber should be sufficiently long to prevent swirling and eddying
around the screen. If the dimensions thus fixed permit an undesirable,
velocity in the screen chamber they should be changed. A sufficient
length of screen should be allowed to project above the sewage for the
accumulation of screenings. The bars may be carried up and bent over at
the top as shown in Fig. 151 to simplify the removal of screenings.
Coarse screens are usually placed above all other portions of a
treatment plant. They may be followed by grit chambers or finer screens.
Coarse screens are occasionally placed as a protection above medium or
fine screens. In sewage containing grit the smaller screens are
sometimes placed below the grit chamber. It is desirable to provide some
means of diverting the sewage from a screen chamber to allow of repairs
to the screen and the cleaning of the chamber. Screen chambers are
sometimes designed in duplicate to allow for the cleaning of one while
the other is operating.
PLAIN SEDIMENTATION
=232. Theory of Sedimentation.=—Sedimentation takes place in sewage
because some particles of suspended matter have a greater specific
gravity than that of water. All particles do not settle at the same
rate. Since the weights of particles vary as the cubes of their
diameters, whereas the surface areas (upon which the action of the water
takes place) vary only as the squares of the diameters, the amount of
the skin friction on small particles is proportionally greater than that
on large particles, because of the relatively greater surface area
compared to their weight. As a result the smaller particles settle more
slowly. The velocity of sedimentation of large particles has been found
to vary about as the diameter and of small particles as the square root
of the diameter. The change takes place at a size of about 0.01 mm.
Sedimentation is accomplished by so retarding the velocity of flow of a
liquid that the settling particles will be given the opportunity to
settle out. The slowing down of the velocity is accomplished by passing
the sewage through a chamber of greater cross-sectional area than the
conduit from which it came. The time that the sewage is in this chamber
is called the period of retention. Although the shape of a basin, the
arrangement of the baffles and other details have a marked effect on the
results of sedimentation, the controlling factors are the period of
retention and the velocity of flow. Another factor affecting the
efficiency of the process is the quality of the sewage. Usually the
greater the amount of sediment in the sewage the greater the per cent of
suspended matter removed. A method for the determination of the proper
period of sedimentation has been developed by Hazen in Transactions of
the American Society of Civil Engineers, Volume 53, 1904, page 45. The
results of his studies are summarized in Fig. 154 which shows the per
cent of sediment remaining in a treated water after a certain period of
retention. This period of retention is expressed in terms of the
hydraulic coefficient[143] of the smallest size particle to be removed.
Table 77 shows the hydraulic coefficients of various particles. In Fig.
154 _a_ represents the period of retention and _t_ the time that it
would take a particle to fall to the bottom of the basin. The different
lines of the diagram represent the results to be expected by various
arrangements of settling basins. The meaning of these lines is given in
Table 78.
TABLE 77
HYDRAULIC VALUES OF SETTLING PARTICLES IN MILLIMETERS PER SECOND
───────────────────────────────────┬───────────────────────────────────
Diameter in mm. │ Hydraulic Value
───────────────────────────────────┼───────────────────────────────────
1.00 │ 100
0.80 │ 83
0.60 │ 63
0.50 │ 53
0.40 │ 42
0.30 │ 32
0.20 │ 21
0.15 │ 15
0.10 │ 8
0.08 │ 6
0.06 │ 3.8
0.05 │ 2.9
0.04 │ 2.1
0.03 │ 1.3
0.02 │ 0.62
0.015 │ 0.35
0.010 │ 0.154
0.008 │ 0.098
0.006 │ 0.055
0.005 │ 0.0385
0.004 │ 0.0247
0.003 │ 0.0138
0.002 │ 0.0062
0.0015 │ 0.0035
0.001 │ 0.00154
0.0001 │ 0.0000154
───────────────────────────────────┴───────────────────────────────────
An example will be given to illustrate the method of using the diagram
and tables to determine the size of a sedimentation basin to perform
certain required work.
Let it be required to determine the period of retention in a
continuously operated sedimentation basin with good baffling,
corresponding to two properly baffled sedimentation basins in
series. The basins are to remove 60 per cent of the finest
particles which are to have a size of .01 mm. The quantity to be
treated daily is 3,000,000 gallons.
1st. Entering Table 77, we find that the hydraulic value of the
finest particles is .154 mm. per second.
2d. Since we wish to remove 60 per cent of the finest particles,
40 per cent will remain. Since Fig. 154 shows the per cent
remaining after the time _a_⁄_t_ we enter Fig. 154 at 40 per cent
on the ordinates and run horizontally until we encounter Line 4
corresponding to good baffling in Table 78. We then run down
vertically from this intersection and find that the ratio of
_a_⁄_t_ is 1.0.
Then _a_ equals _t_, which means that the period of retention
should equal the time that it takes a particle 0.01 mm. in
diameter to drop from the top to the bottom of the basin. Since
this depends on the depth of the basin it is necessary to
determine the depth before the other dimensions of the basin can
be fixed.
Although this method is seldom used in practice for the final design of
a sedimentation basin, it is a guide to judgment and can be used to
supplement the data obtained from tests.
[Illustration:
FIG. 154.—Hazen’s Diagram, Showing the Relation between the Time of
Settling and the Period of Retention in Various Types of
Sedimentation Basins.
Trans. Am. Society Civil Engineers, Vol. 53, 1904, p. 45.
]
TABLE 78
COMPARISON OF DIFFERENT ARRANGEMENTS OF SETTLING BASINS
(From Hazen)
────────────────────────────────────────┬────────────┬─────────────────
Description of Basins │Line in Fig.│ Values of
│ 154 │ _a_⁄_t_.
────────────────────────────────────────┼────────────┼─────────────────
│ │ Per Cent of
│ │ Matter Removed
────────────────────────────────────────┼────────────┼─────┬─────┬─────
│ │ 50 │ 74 │87.5
────────────────────────────────────────┼────────────┼─────┼─────┼─────
Theoretical maximum. Cannot be reached.│ A │ 0.50│ 0.75│0.875
Surface skimming. Rockner Roth system. │ B │ 0.54│ 0.98│ 1.37
Intermittent basins, reckoned on time of│ C │ │ │
service only. │ │ 0.63│ 1.26│ 1.89
Continuous basin. Theoretical limit. │ D │ 0.69│ 1.38│ 2.08
Close approximation to the above. │ 16 │ 0.71│ 1.45│ 2.23
Very well baffled basin. │ 8 │ 0.73│ 1.62│ 2.37
Good baffling. │ 4 │ 0.76│ 1.66│ 2.75
Two basins, tandem. │ 2 │ 0.82│ 2.00│ 3.70
One long basin, well controlled. │ 1.5 │ 0.90│ 2.34│ 4.50
Intermittent basin in service half time.│ E │ 1.26│ 2.50│ 3.80
One basin, continuous. │ 1 │ 1.0│ 3.00│ 7.00
────────────────────────────────────────┴────────────┴─────┴─────┴─────
The design of sedimentation basins should be based on experimental
observations made upon the quantity of sediment removed at certain rates
of flow and periods of retention in different types of basins. Hazen’s
mathematical analysis is serviceable in making preliminary estimates and
in checking the results. The shape of the tank, period of retention and
rate of flow producing the most desirable results should be duplicated
with the expectation of obtaining similar results or results but
slightly modified from those obtained in the tests. This is the most
satisfactory method of determining the proper period of retention.
=233. Types of Sedimentation Basins.=—A sedimentation basin is a tank
for the removal of suspended matter either by quiescent settlement or by
continuous flow at such a velocity and time of retention as to allow
deposition of suspended matter.[144] The difference between
sedimentation tanks and other forms of tank treatment is that no
chemical or biological action is depended on for the successful
operation of the tank. Sedimentation tanks may be divided into two
classes, grit chambers and plain sedimentation basins.
A grit chamber is a chamber or enlarged channel in which the velocity of
flow is so controlled that only heavy solids, such as grit and sand, are
deposited while the lighter organic solids are carried forward in
suspension. If the velocity of flow is more than about one foot per
second, the tank is a grit chamber and below this velocity it is a plain
sedimentation basin.
There are six general types of plain sedimentation basins:
1st. Rectangular flat-bottom tanks operated on the continuous-flow
principle.
2nd. Rectangular flat-bottom tanks operated on the fill and draw
principle.
3rd. Rectangular or circular hopper-bottom tanks operated on the
continuous-flow principle, with horizontal flow.
4th. Rectangular or circular hopper-bottom tanks operated on the
fill and draw principle, with horizontal flow.
5th. Rectangular or circular hopper-bottom tanks operated on the
continuous-flow principle with vertical flow.
6th. Circular hopper-bottom tanks operated on the continuous-flow
principle with radial flow.
TABLE 79
CRITICAL VELOCITIES FOR THE TRANSPORTATION OF DEBRIS
Sedimentation will not Occur at Higher Velocities
───────────┬───────────────────────────────────┬──────────────────────
Diameter of│Critical Velocity, Feet per Second.│ Size of Screen or
Particle in│ │ Number of Meshes per
Millimeters│ │ Inch
───────────┼───────────────────────────────────┼──────────────────────
│ Specific Gravity │
───────────┼────────┬────────┬────────┬────────┼──────────────────────
│ 1.5 │ 2.0 │ 3.0 │ 5.0 │
───────────┼────────┼────────┼────────┼────────┼──────────────────────
0.010 │0.13 │0.20 │0.22 │0.28 │
0.050 │0.23 │0.34 │0.39 │0.50 │More than 200
0.100 │0.30 │0.42 │0.50 │0.65 │More than 150
0.500 │0.55 │0.73 │0.91 │1.15 │More than 28
1.0 │0.71 │0.92 │1.18 │1.50 │More than 14
1.25 │0.77 │1.00 │1.30 │1.60 │
2.0 │0.92 │1.20 │1.50 │1.90 │More than 10
5.0 │1.30 │1.70 │2.20 │2.60 │More than 4
10 │1.70 │2.20 │2.8 │3.4 │
│ │ │ │ │
Diameter in Millimeters for a Velocity of 1 Foot per Second
│ │ │ │ │
│2.5 │1.25 │0.65 │0.32 │
───────────┴────────┴────────┴────────┴────────┴──────────────────────
=234. Limiting Velocities.=—Sand, clay, bits of metal and other
particles of mineral matter will commence to deposit in appreciable
quantities when the velocity of flow falls below 3 feet per second. The
amount deposited will increase as the velocity decreases. In Table 79
are given the approximate horizontal velocities at which certain size
particles of mineral matter will deposit. At a velocity of about one
foot per second organic matter will commence to deposit. It will be
noticed by interpolation in Table 79,[145] that particles with the same
specific gravity as sand (2.6), larger than one mm. in diameter will
deposit at a velocity of about one foot per second or less, and that
smaller and lighter particles will not deposit at velocity of one foot
per second or greater. It will also be noticed that a velocity of one
foot per minute is sufficiently slow to permit the deposit of the
smallest and lightest particles. For this reason velocities of 1 or 2 or
even 3 feet per second have been adopted as the velocities in grit
chambers and velocities less than 1 foot per minute in plain
sedimentation basins.
=235. Quantity and Character of Grit.=—The amount of material deposited
in grit chambers varies approximately between 0.10 and 0.50 cubic yard
per million gallons. It is to be noted that grit chambers are used only
for combined and storm sewage and for certain industrial wastes. They
are unnecessary for ordinary domestic sewage. The material deposited in
grit chambers operating with a velocity greater than one foot per second
is non-putrescible, inorganic, and inoffensive. It can be used for
filling, for making paths and roadways, or as a filtering material for
sludge drying beds. An analysis of a typical grit chamber sludge is
shown in Table 80.
TABLE 80
ANALYSIS OF GRIT CHAMBER SLUDGE
───────────┬───────────┬───────────┬───────────────────────────────────
Velocity │ Specific │ Per Cent │Calculated to Dry Weight, Per Cent
Feet per │ Gravity │ Moisture │
Second │ │ │
───────────┼───────────┼───────────┼──────────┬──────────┬─────────────
│ │ │ Nitrogen │ Fixed │Miscellaneous
│ │ │ │ Matter │
───────────┼───────────┼───────────┼──────────┼──────────┼─────────────
1.0 │ 1.5 │ 45 │ 20 │ 78 │ 2
───────────┴───────────┴───────────┴──────────┴──────────┴─────────────
=236. Dimensions of Grit Chambers.=—The quantity of sewage to be treated
and the amount and character of the settling solids which it contains
should be determined by measurement and analysis, and the amount of
settling solids to be removed should be determined by a study of the
desired conditions of disposal, in order that a grit chamber that will
accomplish the desired results may be designed. The period of retention
and the velocity of flow are the controlling features in the successful
operation of any grit chamber. These should be determined by experiment
or as the result of experience. Where neither are available, Hazen’s
method can be followed or a decision made based on a study of other grit
chambers. In general, the period of retention in grit chambers is from
30 to 90 seconds, and the velocity of flow is about one foot per second.
After having determined the quantity of sewage to be treated, the
quantity of grit to be stored between cleanings, the period of
retention, the arrangement of the chambers, and the velocity of flow to
be used, the overall dimensions of the chambers are computed. The
capacity of the chamber is fixed as the sum of the quantity of sewage to
be treated during the period of retention and the required storage
capacity for grit accumulated between cleanings. The length of the
chamber is fixed as the product of the velocity of flow and the period
of retention. The cross-sectional area of the portion of the chamber
devoted to sedimentation is fixed as the quotient of the quantity of
flow of sewage per unit time and the velocity of flow. Only the relation
between the width and depth of the portion devoted to sedimentation and
the portion devoted to the storage of grit remain to be determined.
These should be so designed as to give the greatest economy of
construction commensurate with the required results. They will be
affected by the local conditions such as topography, available space,
difficulties of excavation, etc. Common depths in use lie between 8 and
12 feet, although wide variations can be found. A study of the
proportions of existing grit chambers will be of assistance in the
design of other basins.
=237. Existing Grit Chambers.=—The details of some typical grit chambers
are shown in Figs. 155 and 156. The grit chamber at the foot of 58th
Street, in Cleveland, Ohio, is shown in Fig. 155. The special feature of
this structure is the shape of the sedimentation basin, the bottom of
which is formed by sloping steel plates forming a 6–inch longitudinal
slot above the grit storage chamber. Flows between 8,000,000 and
16,000,000 gallons per day are controlled by the outlet weir so that the
velocity of flow remains at one foot per second. This is accomplished by
increasing the depth of flow in the same ratio as the increase in the
rate of flow. The bottoms of the two chambers differ, one having a
special hopper for grit and the other a flat bottom. This is due to the
method of cleaning the chambers, it being necessary in the one with a
flat bottom to shut off the flow when removing the grit while in the one
with the hopper bottom it is hoped to remove the grit by the use of sand
ejectors without stopping the sewage flow. The details of the chamber at
Hamilton, Ontario, are shown in Fig. 156. In studying these drawings the
following features should be noted: 1st, the smooth curves in the
channel to prevent eddies, undue deposition of organic matter, and
difficulties in cleaning; 2nd, the hopper in the upper end of the grit
storage chamber and the slope of the bottom of at least 1:20; and 3rd,
the simplicity of the inlet and outlet devices which may be either stop
planks or cast-iron sluice gates.
[Illustration:
FIG. 155.—Grit Chamber at Cleveland, Ohio.
Eng. Record, Vol. 73, 1916, p. 409.
]
[Illustration:
FIG. 156.—Grit Chamber at Hamilton, Ontario.
Eng. News, Vol. 73, 1915, p. 425.
]
The drawings shown are merely representative of some satisfactory types.
The number and variety of grit chambers in existence is great. In
designing grit chambers consideration must be given to the method of
cleaning. They are ordinarily cleaned by such methods as have been
described for the cleaning of catch-basins in Chapter XII. Continuous
bucket scrapers similar to excavating machines are sometimes used for
the cleaning of large grit chambers. The period between cleanings is
variable. The design should be such as not to require more frequent
cleanings than twice a month under the worst conditions. The
fluctuations in quality and quantity of grit will vary the period
between cleanings.
=238. Number of Grit Chambers.=—The period of retention in grit chambers
is so short and the velocity of flow so near the maximum and minimum
limitations that the wide fluctuations in the rate of discharge in storm
and combined sewers necessitates the construction of a number of
chambers which should be operated in parallel in order to maintain the
velocity between the proper limits. Unless arrangements are made
permitting the cleaning of grit chambers during operation, more than one
grit chamber should be installed in order that when one is being cleaned
the others may be in operation. The number of grit chambers must be
determined by the desired conditions of operation and the cost of
construction. The larger the number of basins the more nearly the flow
in any one basin can be maintained constant, but the more expensive the
construction. The increase in velocity of flow with increasing quantity
is dependent on the outlet arrangements. In a shallow chamber with
vertical sides and a standard sharp-crested rectangular weir at the
outlet the velocity will vary approximately as the cube root of the rate
of flow. Similarly if the outlet is a V notch the velocity will vary as
the fifth root of the rate of flow. In all cases the deeper the basin
the more nearly the velocity varies directly as the rate of flow. The
outlet weir can be arranged as at Cleveland, so that the velocity
remains constant for all rates of flow within certain limits. It is
seldom that more than three grit chambers are necessary to care for the
fluctuations in flow.
=239. Quantity and Characteristics of Sludge from Plain
Sedimentation.=—The sludge removed from plain sedimentation basins is
slimy, offensive, not easily dried, and is highly putrescible and
odoriferous. It contains about 90 per cent moisture and has a specific
gravity from 1.01 to 1.05. The amount removed varies between 2 and 5
cubic yards per million gallons of sewage. The percentage of suspended
matter removed varies between 20 and 60. The total amount removed and
the percentage removal depend on the character of the sewage, the type
of basin, and the period of detention.
=240. Dimensions of Sedimentation Basins.=—The dimensions of a
sedimentation basin are determined by a method similar to the one given
for the determination of the dimensions of a grit chamber in Art. 236.
The capacity of the basin is first fixed upon to give the required
period of sedimentation and sludge storage capacity. The length of the
basin is the product of the velocity and the period of retention. The
length, width, and depth of the basin are normally fixed by
considerations of economy and the limitations of the local conditions,
such as available area, topography, foundations, etc., and examples of
good practice. A study of basins in use shows the relation between
length and width to vary normally between 2:1 and 4:1. Widths greater
than 30 to 50 feet are undesirable because of the danger of cross
currents and back eddies which will reduce the efficiency of the
sedimentation. Depths used in practice vary too widely to act as guides
for any particular design. Theoretically the shallower the basin the
better the result. Tanks abroad have been built as shallow as 3 feet and
some in this country as deep as 16 feet. The economical dimensions can
be determined by trial or by calculus. They will serve as a guide in the
adoption of the final dimensions.
The method to be pursued in determining the economical dimensions of any
engineering structure are:
I. Express the total cost of the structure in terms of as few
variables as possible.
II. Express all of the variables in terms of any one and rewrite
the expression for the total cost in terms of this one variable.
III. Equate the first derivative of the expression with regard to
this variable to zero and solve for the variable. The result will
be the economical value of the variable. The values of the other
variables can be computed from the relations already expressed.
[Illustration:
FIG. 157.—Diagram for the Computation of Economical Basin Dimensions.
]
For example, let it be desired to determine the dimensions of two
continuous-flow sedimentation basins as shown in Fig. 157, in which the
period of retention in each is to be 2 hours, the velocity of flow is
not to exceed one foot per second, and the sludge accumulated will be 3
cubic yards per million gallons of sewage treated. The quantity of
sewage to be treated is 18,000,000 gallons per day. The shortest time
between cleanings will be 2 weeks.
The capacity of each basin must be 2/24 of 18,000,000 gallons, or
200,000 cubic feet in order to allow a period of retention of 2 hours.
To this volume should be added sufficient capacity to allow for the 2
weeks of sludge storage between cleanings. When a basin is being cleaned
the load must be put on the remaining basins. Then if _Q_ represents the
rate of accumulation of sludge per day, _n_ represents the number of
days between cleanings, _m_ represents the number of basins, and _S_ the
sludge capacity of one basin, then
_S_ = (_Q_(_n_ − 1))⁄_m_ + _Q_⁄(_m_ − 1)
The sludge storage capacity for the example given will be approximately
11,000 cubic feet.
In expressing the total cost of the basins let
_h_ = the depth in feet.
_l_ = the length in feet.
_b_ = the width in feet.
The cost of land, floor, etc., per square foot = _p_ dollars.
The cost of wall per foot length = _qh_^2 dollars.
The cost of pipes, valves and appurtenances = _P_ dollars.
Then the total cost _C_ = (3_l_ + 4_b_)_qh_^2 + 2_plb_ + _P_.
It is now necessary to express the three variables _b_, _l_, and
_h_, in terms of one of them. From the relation _Q_ = 2_blh_ it is
possible to rewrite the expression for the total cost as:
_C_ = (3_Q_⁄(2_bh_) + 4_b_)_qh_^2 + (_pQ_)⁄_h_ + _P_.
_C_ = (3_l_ + 2_Q_⁄(_lh_))_qh_^2 + (_pQ_)⁄_h_ + _P_.
Holding _h_ constant and differentiating with regard to _b_ in the
first expression and with regard to _l_ in the second expression,
equating to zero and solving we get:
_b_ = √((3_Q_)⁄(8_h_)) and _l_ = √((2_Q_)⁄(3_h_)).
The economical relation between _b_ and _l_ is therefore
_b_ = 0.75_l_
regardless of the value of _h_.
Substituting these values of _l_ and _b_ in the original
expression for the total cost, it becomes
_C_ = (3√((2_Q_)⁄(3_h_)) + 4√((3_Q_)⁄(8_h_)))_qh_^2 + (_pQ_)⁄_h_ +
_P_.
Differentiating with regard to _h_, equating to zero, and solving
_h_ = 0.45((_pQ_^½)⁄_q_)^⅔.
In the example given if _q_ = 0.2 and _p_ = 1.0 then
_h_ = 11.6 feet, _b_ = 120 feet and _l_ = 160 feet.
Since these are reasonable dimensions and in accord with good practice
they should be used, unless other conditions are unsuitable or the
velocity of flow is too great. A width of channel of 120 feet as
compared to a length of 160 feet is conducive to a poor distribution of
velocity across the basin. A ratio of width to length of about 1:4 is
desirable. In this case, by the use of three baffles parallel to the
length of the basin, thus dividing it into channels 40 feet wide and
11.6 feet deep, the ratio of width to length is changed to 1:4 and the
velocity will be increased only to 0.06 foot per second or 3.6 feet per
minute, which is a reasonable velocity. It could be reduced by
increasing the spacing of the baffles or the depth of the chamber.
Complicated baffling is undesirable. Two or three overflow baffles may
be used to permit quiescent sedimentation in the space thus formed, and
hanging baffles may be placed before the inlet and outlet to break up
surface currents, or to prevent the movement of scum. The hanging
baffles should not extend more than 12 to 18 inches below the water
surface. The inlet and outlet are sometimes arranged to permit the
reversal of flow, and the connecting channels between basins to allow
the operation of any number of basins in series or in parallel, although
such arrangements are more important in water purification. Sewage
should enter and leave at the top of the basin.
[Illustration:
FIG. 158.—Section through a Dortmund Tank.
Depth 20 to 30 feet.
]
Cleaning is facilitated by the location of a central gutter in the
bottom of the basin with the slope of the bottom of the basin towards
the gutter from 1:25 to 1:80 or steeper. A pipe, 2 inches or larger in
diameter, containing water under pressure with connections for hose
placed at frequent intervals is a useful adjunct in flushing the sludge
from the sedimentation basins. For equal capacity, deep vertical flow
tanks are more expensive and difficult to construct than the shallower
rectangular type. Deep tanks are advantageous, however, in that sludge
can sometimes be removed by gravity or by pumping without stopping the
operation of the tank. They will also operate successfully with shorter
periods of detention and higher velocities. The upward velocity should
not be greater than the velocity of sedimentation of the smallest
particle to be removed. The efficiency of sedimentation in them will be
increased by the sedimentation of the larger particles which drag some
of the smaller particles down with them. The Dortmund tank shown in Fig.
158 is an example of this type.
Ordinarily it is not necessary to roof sedimentation basins as the odors
created are not strong, and difficulties with ice are seldom serious.
CHEMICAL PRECIPITATION
=241. The Process.=—Chemical precipitation consists in adding to the
sewage such chemicals as will, by reaction with each other and the
constituents of the sewage, produce a flocculent precipitate and thus
hasten sedimentation. The advantages of this process over plain
sedimentation are a more rapid and thorough removal of suspended matter.
Its disadvantages include the accumulation of a large amount of sludge,
the necessity for skilled attendance, and the expense of chemicals. The
process is not in extensive use as the conditions under which the
advantages outweigh the disadvantages are unusual. Sewage containing
large quantities of substances which will react with a small amount of
an added chemical to produce the required precipitate are the most
favorable for this method of treatment.
Chemical precipitation accomplishes the same result as plain
sedimentation, although the effluent from the chemically precipitated
sewage may be of better quality than that from a plain sedimentation
basin.
=242. Chemicals.=—Lime is practically the only chemical used for the
precipitation of the solid matter in sewage. Commercial lime used for
precipitation consists of calcium oxide (CaO), with large quantities of
impurities. It should be stored in a dry place and protected from undue
exposure to the air to prevent the formation of calcium carbonate
(CaCO_{3}), the formation of which is commonly known as air slacking.
The active work in the formation of the precipitate is performed by the
lime (CaO) or calcium hydroxide (Ca(OH)_{2}). The lime should therefore
be purchased on the basis of available CaO, which may be as low as 10 to
15 per cent in some commercial products. The amount of lime necessary
depends on the quality of the sewage, the period of retention in the
sedimentation basin, the method of application, the required results,
and other less easily measured factors. Full scale tests for the amount
of lime needed to produce certain results are the most satisfactory. In
practice the amount of lime necessary when lime alone is used as a
precipitant has been found to be about 15 grains per gallon. This may be
markedly different, dependent on the quality of the sewage. For acid
sewages, lime alone is not suitable as a precipitant since it is
necessary to add sufficient lime to neutralize the sewage before the
calcium carbonate will be precipitated.
The use of copperas (FeSO_{4}) together with lime, leads to economy in
the use of chemicals as the flocculent precipitate of ferrous hydroxide
(Fe(OH)_{2}) is more voluminous than the precipitate of calcium
carbonate. This is commonly known as the lime and iron process. The
presence of iron in certain trade wastes may reduce the cost of chemical
precipitation, as the necessary amount of copperas is reduced. Where 15
grains of lime alone will be needed per gallon of sewage, the total
amount of chemicals used will be reduced to 8 to 10 grains per gallon
with the use of lime and iron. This combination is less expensive than
the use of lime alone, and is even cheaper where the iron is already
present in the sewage. Such a condition is well illustrated by the
sewage at Worcester, Mass., where the oldest and best known chemical
precipitation plant in the United States is located. The amount of lime
used at this plant has varied between 6 and 10 grains per gallon of
sewage, the normal amount being about 7 grains. No iron is added because
of the amount already in solution.
The results of a series of experiments on the chemical precipitation of
sewage by Allen Hazen, are given in the 1890 Report of the Massachusetts
State Board of Health, on p. 737 of the volume on the Purification of
Water and Sewage. Hazen concludes as the result of his experiments:
concerning lime,
There is a certain definite amount of lime ... which gives as good
or better results than either more or less. This amount is that
which exactly suffices to form normal carbonates with all the
carbonic acid of the sewage. This amount can be determined in a
few minutes by simple titration.
Concerning lime and iron (copperas) he states:
Ordinary house sewage is not sufficiently alkaline to precipitate
copperas, and a small amount of lime must be added to obtain good
results. The quantity of lime required depends both upon the
composition of the sewage and the amount of copperas used, and can
be calculated from titration of the sewage. Very imperfect results
are obtained from too little lime, and, when too much is used, the
excess is wasted, the result being the same as with a smaller
quantity.
In precipitation by ferric sulphate and crude alum, the addition
of lime was found unnecessary, as ordinary sewage contains enough
alkali to decompose these salts. Within reasonable limits the more
of these precipitants used the better is the result, but with very
large quantities the improvement does not compare with the
increased cost.
Using equal values of different precipitants, applied under the
most favorable conditions for each, upon the same sewage, the best
results were obtained from ferric sulphate. Nearly as good results
were obtained from copperas and lime used together, while lime and
alum each gave somewhat inferior effluents.... When lime is used
there is always so much lime left in solution that it is doubtful
if its use would ever be found satisfactory except in case of an
acid sewage.
It is quite impossible to obtain effluents by chemical
precipitation which will compare in organic purity with those
obtained by intermittent filtration through sand.
It is possible to remove from one-half to two-thirds of the
organic matter by precipitation ... and it seems probable that ...
a result may be obtained which will effectually prevent a public
nuisance.
=243. Preparation and Addition of Chemicals.=—Lime is not readily
soluble in water. Therefore, it is not best to add the lime as a powder
to the sewage, but to form a milk of lime, that is, a supersaturated
solution containing from 2,000 to 4,000 grains per gallon, although dry
slaked lime has sometimes been applied directly. The solution is
prepared in tanks in a quantity sufficient for some part of the day’s
run, commonly sufficient to last through one shift of 8 or 10 hours. The
lime is prepared by placing the amount necessary to fill one storage
tank into a slaking tank containing some cold water. Sufficient water is
added to keep the solution just at the boiling point, or steam may be
added to make it boil. After slaking, it is run into the milk-of-lime
solution tank and sufficient water added to bring to the proper
strength. The milk of lime is added in measured quantities, being
controlled by a variable head on a fixed orifice or weir, so that it may
be varied with the amount of sewage flowing through the plant. The
amount of lime to be added is determined by titration with
phenolphthalein, experience indicating the color to be obtained when the
proper amount of lime has been added.
The use of either copperas or alum has been so rare, for the
precipitation of sewage, that a description of the methods of handling
these chemicals as a sewage precipitant is not warranted. An excellent
description of the methods of handling these chemicals in water
purification will be found in “Water Purification” by Ellms.
TABLE 81
RESULTS OF CHEMICAL PRECIPITATION AT WORCESTER, MASSACHUSETTS[146]
──────────────────────────────────────┬──────────┬──────────┬──────────
│ 1900 │ 1910 │ 1920
──────────────────────────────────────┼──────────┼──────────┼──────────
Amount of sewage treated, million │ 4,781 │ 5,317 │ 8,893
gallons │ │ │
Amount of sewage chemically treated, │ 3,650 │ 3,574 │ 7,300
million gallons │ │ │
Gallons of wet sludge per million │ 4,450 │ 4,185 │
gallons of sewage treated │ │ │
Per cent of solids in sludge │ 4.42 │ 8.20 │4.64[147]
Tons of solids │ 7,294 │ 4,182 │6,431[147]
Pounds of lime added per million │ 999[148] │ 762[147] │ 534
gallons of sewage pumped │ │ │
Per cent of organic matter removed: │ │ │
By albuminoid ammonia: │ │ │
Total │52.7[149] │ 58.4 │ 51.9
Suspended │90.0[149] │ 88.7 │ 83.6
By oxygen consumed: │ │ │
Total │62.8[149] │ 61.1 │ 62.5
Suspended │86.6[149] │ 89.7 │ 86.2
──────────────────────────────────────┴──────────┴──────────┴──────────
=244. Results.=—The results of Hazen’s experiments indicate that a
greater amount of suspended matter can be removed in the same time by
chemical precipitation than by plain sedimentation. The percentage of
removal of suspended matter may be as high as 80 to 90 per cent with a
period of retention of 6 to 8 hours and the addition of a proper amount
of chemical. That the method is not always a success is shown by the
results of some tests at Canton, Ohio.[150] The report states:
... lime treatment removes about 50 per cent of the suspended
matter, and in the main about 50 per cent of the organic
matter.... These data are instructive as indicating that the
addition of lime to the Canton sewage in quantities as previously
stated does not materially improve the character of the resulting
effluent over and above that which could be produced by plain
sedimentation alone.
The plant at Worcester, Mass., is the largest in the United States and
information from it is of value. A summary of the results at Worcester
for 1900, 1910, and 1920 are shown in Table 81.
CHAPTER XVI
SEPTICIZATION
=245. The Process.=—Septic action is a biological process caused by the
activity of obligatory or facultative anaërobes as the result of which
certain organic compounds are reduced from higher to lower conditions of
oxidation, some of the solid organic substances are rendered soluble,
and a quantity of gas is given off. Among these gases are: methane,
hydrogen sulphide, and ammonia. The biologic process in the septic tank
represents the downward portion of the cycle of life and death, in which
complex organic compounds are reduced to a more simple condition
available as food for low forms of plant life. The disposal of sewage by
septic action, when introduced, promised the solution of all problems in
sewage treatment. Septic action is now better understood, and it is
known that some of the early claims were unfounded.
The principal advantage of septic action in sewage treatment is the
relatively small amount of sludge which must be cared for compared to
that produced by a plain sedimentation tank. The sludge from a septic
tank may be 25 to 30 per cent and in some cases 40 per cent less in
weight, and 75 to 80 per cent less in volume than the sludge from a
plain sedimentation tank. The most important results of septic action
and the greatest septic activity occur in the deposited organic matter
or sludge. The biologic changes due to septic action which occur in the
liquid portion of the tank contents are of little or no importance. The
installation of a septic tank, although it may fail to prevent the
nuisance calling for abatement, has a remarkable psychological effect in
stilling complaints. Among other advantages are the comparative
inexpensiveness of the tanks and the small amount of attention and
skilled attendance required. The tanks need cleaning once in 6 months to
a year. If properly designed no other attention is necessary.
The septic tank has fallen into some disrepute because of the better
results obtainable by other methods, the occasional discharge of
effluents worse than the influent, the occasional discharge of sludge in
the effluent caused by too violent septic boiling, and on account of
patent litigation. This last difficulty has been overcome as the Cameron
patents expired in 1916. Occasionally the odors given off by the septic
process are highly objectionable and are carried for a long distance.
These odors can be controlled to a large extent by housing the tanks.
Over-septicization must be guarded against as an over-septicized
effluent is more difficult of further treatment or of disposal than a
comparatively fresh, untreated sewage. An over-septicized or stale
sewage is indicated by the presence of large quantities of ammonias,
either free or albuminoid, frequently accompanied by hydrogen sulphide
and other foul-smelling gases. The oxygen demand in an over-septicized
sewage is greater than that in a fresh or more carefully treated sewage.
=246. The Septic Tank.=—A septic tank is a horizontal, continuous-flow,
one-story sedimentation tank through which sewage is allowed to flow
slowly to permit suspended matter to settle to the bottom where it is
retained until anaërobic decomposition is established, resulting in the
changing of some of the suspended organic matter into liquid and gaseous
substances, and a consequent reduction in the quantity of sludge to be
disposed of.[151] It is to be noted that a continuous flow is essential
to a septic tank. Small tanks containing stagnant household sewage are
called cesspools, although sometimes erroneously spoken of as septic
tanks.
Septic and sedimentation tanks differ in their method of operation only
in the period of storage and the frequency of cleaning. The period of
flow in a septic tank is longer and it is cleaned less frequently. The
results obtained by the two processes differ widely. A septic tank can
be converted into a sedimentation tank, or vice versa, by changing the
method of operation, no constructional features requiring alteration.
The purpose of the tank is to store the sludge for such a period of time
that partial liquefaction of the sludge may take place, and thus
minimize the difficulty of sludge disposal. For this reason the sludge
storage capacity of a septic tank is sometimes greater than would be
necessary for a plain sedimentation tank.
TABLE 82
EFFICIENCIES AND PERFORMANCE OF SEPTIC TANK AT COLUMBUS, OHIO
(Report of Sewage Purification, by G. A. Johnson, Nov. 10, 1905)
──────────────┬────┬─────┬────┬────┬────┬────┬────┬─────┬─────┬───┬────┬────
Month, │Aug.│Sept.│Oct.│Nov.│Dec.│Jan.│Feb.│March│April│May│June│Avg.
1904–1905 │ │ │ │ │ │ │ │ │ │ │ │
──────────────┼────┼─────┼────┼────┼────┼────┼────┼─────┼─────┼───┼────┼────
Temperature, │ │ │ │ │ │ │ │ │ │ │ │
degrees F. │ │ │ │ │ │ │ │ │ │ │ │
Influent │ 69 │ 70 │ 65 │ 60 │ 54 │ 51 │ 48 │ 50 │ 57 │61 │ 67 │
Effluent │ 69 │ 68 │ 64 │ 59 │ 52 │ 48 │ 45 │ 49 │ 57 │62 │ 68 │
Oxygen │ │ │ │ │ │ │ │ │ │ │ │
consumed, │ │ │ │ │ │ │ │ │ │ │ │
parts per │ │ │ │ │ │ │ │ │ │ │ │
million: │ │ │ │ │ │ │ │ │ │ │ │
Influent │ 49 │ 50 │ 52 │ 47 │ 43 │ 51 │ 44 │ 47 │ 53 │33 │ 40 │ 47
Effluent │ 40 │ 36 │ 40 │ 39 │ 37 │ 35 │ 37 │ 39 │ 50 │34 │ 33 │ 38
Per cent │ 18 │ 28 │ 23 │ 15 │ 16 │ 31 │ 16 │ 17 │ 6 │–3 │ 18 │ 19
removal │ │ │ │ │ │ │ │ │ │ │ │
Organic │ │ │ │ │ │ │ │ │ │ │ │
nitrogen, │ │ │ │ │ │ │ │ │ │ │ │
parts per │ │ │ │ │ │ │ │ │ │ │ │
million: │ │ │ │ │ │ │ │ │ │ │ │
Influent │6.5 │ 8.2 │9.3 │8.4 │8.8 │8.5 │6.7 │ 6.4 │ 7.9 │6.1│6.7 │7.8
Effluent │7.3 │ 5.5 │6.0 │7.4 │8.2 │7.0 │5.4 │ 5.5 │ 5.2 │ │ │
Per cent │–12 │ 32 │ 35 │ 12 │ 7 │ 18 │ 19 │ 14 │ 25 │30 │ 19 │ 19
removal │ │ │ │ │ │ │ │ │ │ │ │
Free ammonia, │ │ │ │ │ │ │ │ │ │ │ │
parts per │ │ │ │ │ │ │ │ │ │ │ │
million: │ │ │ │ │ │ │ │ │ │ │ │
Influent │9.7 │12.2 │12.4│16.3│14.7│10.8│8.3 │ 9.9 │12.3 │6.9│8.3 │11.7
Effluent │10.5│11.5 │12.4│17.2│14.3│11.1│8.9 │10.7 │14.9 │9.0│8.7 │12.1
Per cent │ –8 │ 6 │ 0 │ –6 │ 3 │ –3 │ –7 │ –8 │ –21 │–23│ –5 │ –3
removal │ │ │ │ │ │ │ │ │ │ │ │
│ │ │ │ │ │ │ │ │ │ │ │
Residue on │ │ │ │ │ │ │ │ │ │ │ │
Evaporation,│ │ │ │ │ │ │ │ │ │ │ │
parts per │ │ │ │ │ │ │ │ │ │ │ │
million: │ │ │ │ │ │ │ │ │ │ │ │
Total: │ │ │ │ │ │ │ │ │ │ │ │
Influent │990 │ 952 │993 │961 │989 │949 │890 │ 850 │1067 │912│945 │946
Effluent │935 │ 891 │893 │916 │925 │886 │843 │ 782 │ 895 │800│835 │873
Per cent │ 6 │ 6 │ 10 │ 5 │ 6 │ 6 │ 5 │ 8 │ 16 │12 │ 12 │ 8
removal │ │ │ │ │ │ │ │ │ │ │ │
Volatile: │ │ │ │ │ │ │ │ │ │ │ │
Influent │231 │ 184 │162 │175 │156 │167 │156 │ 168 │ 212 │122│162 │166
Effluent │206 │ 160 │129 │148 │137 │137 │134 │ 137 │ 147 │103│144 │139
Per cent │ 11 │ 13 │ 20 │ 15 │ 12 │ 18 │ 14 │ 18 │ 31 │16 │ 11 │ 16
removal │ │ │ │ │ │ │ │ │ │ │ │
Mineral: │ │ │ │ │ │ │ │ │ │ │ │
Influent │759 │ 768 │831 │786 │833 │782 │734 │ 682 │ 855 │700│783 │780
Effluent │729 │ 731 │764 │768 │788 │749 │709 │ 645 │ 748 │697│691 │734
Per cent │ 4 │ 5 │ 8 │ 2 │ 5 │ 4 │ 3 │ 5 │ 11 │ 1 │ 12 │ 6
removal │ │ │ │ │ │ │ │ │ │ │ │
Cubic yards │ │ │0.10│1.24│1.09│1.17│0.65│0.63 │0.57 │ │1.34│
wet sludge │ │ │ │ │ │ │ │ │ │ │ │
per million │ │ │ │ │ │ │ │ │ │ │ │
gallons: │ │ │ │ │ │ │ │ │ │ │ │
Per cent │ │ │ │ │ │ │ │ │ │ │ │
removal of │ │ │ │ │ │ │ │ │ │ │ │
suspended │ │ │ │ │ │ │ │ │ │ │ │
matter: │ │ │ │ │ │ │ │ │ │ │ │
Total │ 59 │ 54 │ 56 │ 51 │ 42 │ 48 │ 32 │ 47 │ 56 │67 │ 53 │ 50
Volatile │ 60 │ 41 │ 48 │ 52 │ 44 │ 55 │ 47 │ 47 │ 62 │80 │ 15 │ 48
Fixed │ 75 │ 65 │ 60 │ 51 │ 40 │ 38 │ 19 │ 48 │ 53 │64 │ 67 │ 51
Gas evolved, │ │ │ │ │ │ │ │ 29 │ 14 │41 │ 50 │
cubic feet │ │ │ │ │ │ │ │ │ │ │ │
per day: │ │ │ │ │ │ │ │ │ │ │ │
──────────────┴────┴─────┴────┴────┴────┴────┴────┴─────┴─────┴───┴────┴────
=247. Results of Septic Action.=—The results obtained from the septic
tanks at the Columbus Sewage Experiment Station are given in Table 82.
The effluent is higher than the influent in free ammonia, but the
reduction of other constituents, particularly suspended matter, is
marked.
Septic action is sensitive to temperature changes, and to certain
constituents of the incoming sewage. Cold weather or an acid influent
will inhibit septicization. In winter the liquefaction of sludge may
practically cease, whereas in summer liquefaction may exceed deposition.
The amount of gas generated is a measure of the relative amount of
septic action. The rapid generation of gas in warm weather disturbs the
settled sludge and may cause a deterioration of the quality of the
effluent because of the presence of decomposed sludge. The results in
Table 82 show the effect of cold weather on the process. In warm weather
the violent ebullition of gas sometimes causes the discharge of sludge
in the effluent, resulting in a liquid more difficult of disposal than
the incoming sewage. Since septic action is dependent on the presence of
certain forms of bacteria, where these are absent there will be no
septic action. Sewage generally contains the forms of bacteria necessary
for this action but it has occasionally been found necessary to seed new
tanks in order to start septic action.
The sludge from septic tanks is usually black, with a slight odor,
though in some cases this odor may be highly offensive. The sludge will
flow sluggishly. It can be pumped by centrifugal pumps and it will flow
through pipes and channels. It has a moisture content of about 90 per
cent and a specific gravity of about 1.03. It is dried with difficulty
on open-air drying beds, and it is worthless as a fertilizer. The
composition of some septic sludges are shown in Table 83.
=248. Design of Septic Tanks.=—The sedimentation chambers of a septic
tank are designed on the same principles as the sedimentation basins
described in Art. 240. The velocity of flow should not exceed one foot
per minute. The channels should be straight and free from obstructions
causing back eddies. The ratio of length to width of channel should be
between 2 : 1 to 4 : 1 with a width not exceeding 50 feet, and desirably
narrower. The depths used vary between 5 and 10 feet, exclusive of the
sludge storage capacity. Hanging baffles should be placed, one before
the inlet and the other in front of the outlet, so as to distribute the
incoming sewage over the tank, and to prevent scum from passing into the
outlet. The baffles should hang about 12 inches below the surface of the
sewage. Intermediate baffles are sometimes desirable to prevent the
movement of sludge or scum towards the outlet. The placing of baffles
must be considered carefully as injudicious baffling may lessen the
effectiveness of a tank by so concentrating the currents as to prevent
sedimentation or the accumulation of sludge. Baffles should be built of
concrete or brick, as wood or metal in contact with septic sewage
deteriorates rapidly. In designing the sludge storage chambers it may be
assumed that one-half of the organic matter and none of the mineral
matter will be liquefied or gasified. The net storage volume allowed is
about 2 to 3 cubic yards per million gallons of sewage treated.
Variations between 0.1 and 10.0 cubic yards have been recorded, however.
If grit is carried in the sewage to be treated, it should be removed by
the installation of a grit chamber before the sewage enters the septic
tank.
TABLE 83
ANALYSIS OF TANK SLUDGES
──────────┬────────┬────────┬────────────────────────────
Place │Specific│Per Cent│ Per Cent in Terms of Dry
│Gravity │Moisture│ Matter
│ │ │
│ │ │
│ │ │
──────────┼────────┼────────┼────────┬─────┬────────┬────
│ │ │Volatile│Fixed│Nitrogen│Fat
──────────┼────────┼────────┼────────┼─────┼────────┼────
Mansfield,│ 1.11│ 80.8│ │ │ │
O. │ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
Chicago, │ 1.03│ 90│ 40│ 60│ 1.9│ 7.0
Ill. │ │ │ │ │ │
│ │ │ │ │ │
Columbus, │ 1.09│ 83.3│ 4.4│ 16.7│ 0.25│0.94
O. │ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
Atlanta, │ 1.02│ 87.1│ 39.1│ 60.9│ 1.25│6.11
Ga. │ │ │ │ │ │
│ │ │ │ │ │
Baltimore,│ 1.02│ 91.9│ 66.2│ │ 2.45│4.02
Md. │ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
Baltimore,│ 1.02│ 92.4│ 62.7│ │ 2.75│
Md. │ │ │ │ │ │
Baltimore,│ │ 79.2│ 73.8│ │ 2.64│9.00
Md. │ │ │ │ │ │
Baltimore,│ │ 92.4│ 58.0│ 3.19│ │
Md. │ │ │ │ │ │
──────────┴────────┴────────┴────────┴─────┴────────┴────
──────────┬────────┬────────┬─────────┬────────────
Place │ Cubic │ Pounds │ Kind of │ Reference
│Yard per│ per │ Sludge │
│Million │Million │ │
│Gallons,│Gallons,│ │
│ Wet │ Dry │ │
──────────┼────────┼────────┼─────────┼────────────
│ │ │ │
──────────┼────────┼────────┼─────────┼────────────
Mansfield,│ │ │Septic │1908 Report,
O. │ │ │ │ State
│ │ │ │ Board of
│ │ │ │ Health
Chicago, │ 1.0│ 200│Septic │
Ill. │ │ │ │
│ 1.5│ 300│ │
Columbus, │ │ │Septic │G. A.
O. │ │ │ │ Johnson
│ │ │ │ 1905
│ │ │ │ Report
Atlanta, │ │ │Imhoff │Eng. Rec.,
Ga. │ │ │ │ V. 72,
│ │ │ │ 1915, p. 4
Baltimore,│ │ │Digestion│Eng.
Md. │ │ │ Tank │ News-Rec.,
│ │ │ │ V. 87,
│ │ │ │ 1921, p.
│ │ │ │ 98
Baltimore,│ │ │Imhoff │ do.
Md. │ │ │ │
Baltimore,│ │ │Raw │ do.
Md. │ │ │ Sludge │
Baltimore,│ │ │Settling │ do.
Md. │ │ │ Basin │
──────────┴────────┴────────┴─────────┴────────────
Two or more tanks should be constructed to allow for the shut down of
one for cleaning and to increase the elasticity of the plant. The number
of tanks to be used is dependent on the total quantity of sewage and the
fluctuations in rate of flow. An average period of retention of about 9
to 10 hours with a minimum period of 6 hours during maximum flow is a
fair average to be assumed for design. The period of retention should
not exceed about 24 hours, as the sewage may become over-septicized. The
sludge storage period should be from 6 to 12 months.
A cover is not necessary to the successful operation of a septic tank.
Covers are sometimes used with success, however, in reducing the
dissemination of odors from the tank. They are also useful in retaining
the heat of the sewage in cold weather and thus aid in promoting
bacterial activity. Types of covers vary from a building erected over
the tank to a flat slab set close to the surface of the sewage. In the
design of a cover, good ventilation should be provided to permit the
escape of the gases, and easy access should be provided for cleaning.
Tightly covered tanks or tanks with too little ventilation have resulted
in serious explosions, as at Saratoga Springs in 1906 and at
Florenceville, N. C., in 1915.[152]
The sludge may be removed through drains in the bottom of the tank as
described for sedimentation basins, or where such drains are not
feasible the sludge and sewage are pumped out. For this purpose a pump
may be installed permanently at the tank, or for small tanks portable
pumps are sometimes used. Septic tanks should be cleaned as infrequently
as possible without permitting the overflow of sludge into the effluent.
The less frequent the cleaning the less the amount of sludge removed
since digestion is continuous throughout the sludge. It is necessary to
clean when the tank becomes so filled with sludge, that the period of
retention is materially reduced, or sludge is being carried over into
the effluent.
The details of the septic tank at Champaign, Illinois, are shown in Fig.
159. This tank was designed by Prof. A. N. Talbot, and was put in
service on Nov. 1, 1897. It was among the first of such tanks to be
installed in the United States. The tank shown in Fig. 159 is an example
of present day practice in single-story septic tank design.
[Illustration:
FIG. 159.—Septic Tank at Champaign, Illinois.
]
[Illustration:
FIG. 160.—Design for a Residential Septic Tank for a Family of Ten.
Illinois State Board of Health.
]
Small septic tanks for rural homes of 5 to 15 persons, or on a slightly
larger scale for country schools and small institutions, are little more
than glorified cesspools. Nevertheless much attention has been given to
the construction of such tanks by the National Government and by state
boards of health.[153] The recommendations of some of these boards have
been compiled in Table 84. A typical method for the construction of such
tanks, as recommended by the Illinois State Board of Health, is shown in
Fig. 160. A subsurface filter, into which the effluent is discharged, is
an important adjunct where no adequate stream is available to receive
the discharge from the tank.
TABLE 84
CAPACITIES OF SEPTIC TANKS FOR SMALL INSTALLATIONS
──────────────┬─────────────┬─────────┬─────────┬──────────────────────
Rule │ Number, │Capacity,│Period of│ Remarks
Recommended by│ Persons │ Gallons │Retention│
State Board of│ │ per │ │
Health │ │ Person │ │
──────────────┼─────────────┼─────────┼─────────┼──────────────────────
Wisconsin │ │ 30 │24 hours │
Ohio │ 4 to 10 │ 50 │ │Not less than 560
│ │ │ │ gallons
Kentucky │ │ │24 to 48 │Not more than 5 feet
│ │ │ hours │ deep
Texas │ │ │24 hours │
Illinois │ │ 45 │24 hours │
U.S. Dept. │ │ 40 │24 hours │25 per cent additional
Agriculture.│ │ │ │
│ │ │ │capacity for sludge
North Carolina│Large Schools│ 15 │ │Not less than 500
│ │ │ │ gallons
North Carolina│ 20 pupils │ 25 │ │
North Carolina│Medium School│ 20 │ │
North Carolina│ Homes │25 to 30 │ │
──────────────┴─────────────┴─────────┴─────────┴──────────────────────
=249. Imhoff Tanks.=—In the discussion of septic tanks it has been
brought out that one of the objections to their use is the unloading of
sludge into the effluent which occasionally causes a greater amount of
suspended matter in the effluent than in the influent. The Imhoff tank
is a form of septic tank so arranged that this difficulty is overcome.
It combines the advantages of the septic and sedimentation tanks and
overcomes some of their disadvantages. An Imhoff tank is a device for
the treatment of sewage, consisting of a tank divided into 3
compartments. The upper compartment is called the sedimentation chamber.
In it the sedimentation of suspended solids causes them to drop through
a slot in the bottom of the chamber to the lower compartment called the
_digestion_ chamber. In this chamber the solid matter is humified by an
action similar to that in a plain septic tank. The generated gases
escape from the digestion chamber to the surface through the third
compartment called the _transition_ or _scum_ chamber. Sections of
Imhoff tanks are shown in Fig. 161. It is essential to the construction
of an Imhoff tank that the slot in the bottom of the sedimentation
chamber does not permit the return of gases through the sedimentation
chamber, and that there be no flow in the digestion chamber.
[Illustration:
FIG. 161.—Typical Sections through Imhoff Tanks.
Eng. News, Vol. 75, p. 15.
]
The Imhoff tank was invented by Dr. Karl Imhoff, director of the Emscher
Sewerage District in Germany. Its design is patented in the United
States, the control of the patent being in the hands of the Pacific
Flush Tank Co. of Chicago, which collects the royalties which are
payable when construction work begins. The fee for a tank serving 100
persons is $10, for 1,000 persons is $80 and for 100,000 persons is
$2550. The rate of the royalty reduces in proportion as the number of
persons served increases.[154] As designed by Imhoff and used in Germany
the tanks were of the radial flow type and quite deep. The depth, as
explained by Imhoff, is one of the chief requirements for the successful
operation of the tank. As adapted to American practice the tanks are
generally of the longitudinal flow type and are not made so deep. An
isometric view of a radial flow Imhoff tank is shown in Fig. 162. The
sewage enters at the center of the tank near the surface and flows
radially outward under the scum ring and over a weir placed near the
circumference of the tank. One type of longitudinal flow tank is shown
in isometric view in Fig. 163.
[Illustration:
FIG. 162.—Sketch of Radial Flow Imhoff Tank at Baltimore, Maryland.
Eng. Record, Vol. 70, p. 5.
]
[Illustration:
FIG. 163.—Isometric View of Longitudinal Flow Imhoff Tank at Cleburne,
Texas.
Eng. News, Vol. 76, p. 1029.
]
=250. Design of Imhoff Tanks.=—The velocity of flow, period of
retention, and the quantity of sewage to be treated determine the
dimensions of the _sedimentation chamber_ as in other forms of tanks.
The velocity of flow should not exceed one foot per minute, with a
period of retention of 2 to 3 hours. A greater velocity than one foot
per minute results in less efficient sedimentation. A longer period of
retention than the approximate limit set may result in a septic or stale
effluent, and a shorter period may result in loss of efficiency of
sedimentation. The bottom of the sedimentation chamber should slope not
less than 1½ vertical to 1 horizontal, in order that deposited material
will descend into the sludge digestion chamber. Provision should be made
for cleaning these sloping surfaces by placing a walk on the top of the
tank from which a squeegee can be handled to push down accumulated
deposits. It is desirable to make the material of the sides and bottom
of the sedimentation chamber as smooth as possible to assist in
preventing the retention of sludge in the sedimentation chamber. Wood,
glass, and concrete have been used. The latter is the more common and
has been found to be satisfactory. The length of the sedimentation
chamber is fixed by the velocity of flow and the period of retention.
Tanks are seldom built over 100 feet in length, however, because of the
resulting unevenness in the accumulation of sludge. Where longer flows
are desired two or more tanks may be operated in series. The width of
the chamber is fixed by considerations of economy and convenience. It
should not be made so great as to permit cross currents. In general a
narrow chamber is desirable. Satisfactory chambers have been constructed
at depths between 5 and 15 feet. The depth of the sedimentation chamber
and the depth of the digestion chamber each equal about one-half of the
total depth of the tank. This should be made as deep as possible up to a
limit of 30 to 35 feet, with due consideration of the difficulties of
excavation. C. F. Mebus states:[155]
In 9 of the largest representative United States installations,
the depth from the flow line to the slot varies from 10 feet 10
inches to 13 feet 6 inches.
Imhoff states, concerning the depth of tanks:
Deep tanks are to be preferred to shallow tanks because in them
the decomposition of the sludge is improved. This is so because in
the deeper tanks the temperature is maintained more uniformly and
because the stirring action of the rising gas bubbles is more
intense.
The stirring action of the gas bubbles is desirable as it brings the
fresh sludge more quickly under the influence of the active bacterial
agents. The greater pressure on the sludge in deep tanks also reduces
its moisture content.
Two or more sedimentation chambers are sometimes used over one sludge
digestion chamber in order to avoid the depths called for by the sloping
sides of a single sedimentation chamber. An objection to multiple-flow
chambers is the possibility of interchange of liquid from one chamber to
another through the common digestion chamber.
The inlet and outlet devices should be so constructed that the direction
of flow in the tank can be reversed in order that the accumulated sludge
may be more evenly distributed in the hoppers of the digestion chamber.
The sewage should leave the sedimentation chamber over a broad crested
weir in order to minimize fluctuations in the level of sewage in the
tank. The gases in the digesting sludge are sensitive to slight changes
in pressure. A lowering of the level of sewage will release compressed
gas and will too violently disturb the sludge in the digestion chamber.
Hanging baffles, submerged 12 to 16 inches and projecting 12 inches
above the surface of the sewage, should be placed in front of the inlet
and outlet, and in long tanks intermediate baffles should be placed to
prevent the movement of scum or its escape into the effluent. An Imhoff
tank which is operating properly should not have any scum on the surface
of the sewage in the sedimentation chamber.
The _slot_ or opening at the bottom of the sedimentation chamber should
not be less than 6 inches wide between the lips. Wider slots are
preferable, but too wide a slot will involve too much loss of volume in
the digestion chamber. One lip of the slot should project at least 3
inches horizontally under the other so as to prevent the return of gases
through the sedimentation chamber. A triangular beam may be used as
shown in Fig. 161 A. This method of construction is advantageous in
increasing the available capacity for sludge storage.
The _digestion chamber_ should be designed to store sludge from 6 to 12
months, the longer storage periods being used for smaller installations.
In warm climates a shorter period may be used with success. The amount
of sludge that will be accumulated is as uncertain as in other forms of
sewage treatment. A widely quoted empirical formula, presented in
“Sewage Sludge” by Allen, states:
_C_ = 10.5 _PD_ for combined sewage;
_C_ = 5.25 _PD_ for separate sewage,
in which _C_ = the effective capacity of the digestion chamber in
cubic feet;
_P_ = the population served, expressed in thousands;
_D_ = the number of days of storage of sludge.
The effective capacity of the chamber is measured as the entire volume
of the chamber approximately 18 inches below the lower lip of the slot.
The capacity as computed from the above formula is assumed as
satisfactory for a deep tank. Frank and Fries[156] recommend the
increase of the capacity for shallow tanks to compensate for the
decreased hydrostatic pressure. In any event the formula can be no more
than a guide to design. No formula can be of equal value to data
accumulated from tests on the sewage to be treated. The Illinois State
Board of Health requires 3 cubic yards of sludge digestion space per
million gallons of sewage treated. Frank and Fries recommend an
allowance of 0.007 cubic foot of storage per inhabitant per day for
combined sewage and one-half that amount for separate sewage. If this is
based on 80 per cent moisture content, the volume for other percentages
of moisture can be easily computed. An average figure used in the
Emscher District is one cubic foot capacity for each inhabitant for the
combined system, and three-fourths of this for the separate system.
Metcalf and Eddy[157] recommend the following method for the
determination of the sludge storage capacity: (1) From analyses of the
sewage or study of the sources ascertain the amount of suspended matter.
(2) Assume, or determine by test, the amount which will settle in the
period of detention selected, say 60 per cent in 3 hours. (3) Estimate
the amount which will be digested in the sludge chamber at about 25 per
cent, leaving 75 per cent to be stored. (4) Estimate the percentage
moisture in the sludge conservatively, say 85 per cent. The total volume
of sludge can then be computed. This method is more rational than the
use of empirical formulas, but because of the estimates which must be
made its results will probably be of no greater accuracy than those
obtained empirically.
The digestion chamber is made in the form of an inverted cone or pyramid
with side slopes at most about 2 horizontal to 1 vertical and preferably
much steeper without necessitating too great a depth of tank. The
purpose of the steep slope is to concentrate the sludge at the bottom of
the hopper thus formed. Concrete is ordinarily used as the material of
construction as a smooth surface can be obtained by proper workmanship.
Where flat slopes have been used, a water pipe perforated at intervals
of 6 to 12 inches may be placed at the top of the slopes, and water
admitted for a short time to move the sludge when the tank is being
cleaned.
A cast-iron pipe, 6 to 8 inches in diameter, is supported in an
approximately vertical position with its open lower end supported about
12 inches above the lowest point in the digestion chamber. This is used
for the removal of sludge. A straight pipe from the bottom of the tank
to a free opening in the atmosphere is desirable in order to allow the
cleaning of the pipe or the loosening of sludge at the start, and to
prevent the accumulation of gas pockets. The sludge is led off through
an approximately horizontal branch so located that from 4 to 6 feet of
head are available for the discharge of the sludge. A valve is placed on
the horizontal section of the pipe. A sludge pipe is shown in Fig. 162
and 163. Under such conditions, when the sludge valve is opened the
sludge should flow freely. The hydraulic slope to insure proper sludge
flow should not be less than 12 to 16 per cent. Where it is not possible
to remove the sludge by gravity an air lift is the best method of
raising it.
The volume of the _transition_ or _scum_ chamber should equal about
one-half that of the digestion chamber. The surface area of the scum
chamber exposed to the atmosphere should be 25 to 30 per cent of the
horizontal projection of the top of the digestion chamber. Some tanks
have operated successfully with only 10 per cent, but troubles from
foaming can usually be anticipated unless ample area for the escape of
gases has been provided.
All portions of the surface of the tank should be made accessible in
order that scum and floating objects can be broken up or removed. The
gas vents should be made large enough so that access can be gained to
the sludge chamber through them when the tank is empty.
Precautions should be taken against the wrecking of the tank by high
ground water when the tank is emptied. With an empty tank and high
ground water there is a tendency for the tank to float. The flotation of
the tank may be prevented by building the tank of massive concrete with
a heavy concrete roof, by underdraining the foundation, or by the
installation of valves which will open inwards when the ground water is
higher than the sewage in the tank. Dependence should not be placed on
the attendant to keep the tank full during periods of high ground water.
Roofs are not essential to the successful operation of Imhoff tanks.
They are sometimes used, however, as for septic tanks, to assist in
controlling the dissemination of odors, to minimize the tendency of the
sewage to freeze, and to aid in bacterial activity. In the construction
of a roof, ventilation must be provided as well as ready access to the
tank for inspection, cleaning, and repairs.
=251. Imhoff Tank Results.=—The Imhoff tank has the advantage over the
septic tank that it will not deliver sludge in the effluent, except
under unusual conditions. The Imhoff tank serves to digest sludge better
than a septic tank and it will deliver a fresher effluent than a plain
sedimentation tank. Imhoff sludge is more easily dried and disposed of
than the sludge from either a septic or a sedimentation tank. This is
because it has been more thoroughly humified and contains only about 80
per cent of moisture. As it comes from the tank it is almost black,
flows freely and is filled with small bubbles of gas which expand on the
release of pressure from the bottom of the tank, thus giving the sludge
a porous, sponge-like consistency which aids in drying. When dry it has
an inoffensive odor like garden soil, and it can be used for filling
waste land, without further putrefaction. It has not been used
successfully as a fertilizer.
Offensive odors are occasionally given off by Imhoff tanks, even when
properly operated. They also have a tendency to “boil” or foam. The
boiling may be quite violent, forcing scum over the top of the
transition chamber and sludge through the slot in the sedimentation
chamber, thus injuring the quality of the effluent. The scum on the
surface of the transition chamber may become so thick or so solidly
frozen as to prevent the escape of gas with the result that sludge may
be driven into the sedimentation chamber.
Some chemical analyses of Imhoff tank influents and effluents are given
in Table 86 and the analyses of some sludges from Imhoff tanks are given
in Table 83. It is to be noted that the nitrites and nitrates are still
present in the effluent, whereas they are seldom present in the effluent
from septic tanks. The per cent of moisture in the Imhoff sludge is less
than that in the septic tank sludge, and its specific gravity is higher.
It is heavier and more compact because of the longer time and the
greater pressure it has been subjected to in the digestion chamber of
the Imhoff tank.
=252. Status of Imhoff Tanks.=—The introduction of the Imhoff tank into
the United States, like the introduction of the Burkli-Ziegler Run-Off
Formula, and Kutter’s Formula, is to be credited to Dr. Rudolph Hering.
He advised Dr. Imhoff to come to the United States to introduce his tank
and gave him material aid through recommendations and introductions to
engineers. Shortly after its introduction, in 1907, the tank became very
popular and installations were made in many cities. This popularity was
caused by a growing dissatisfaction with the septic tank, the litigation
then progressing over septic patents, the production of inoffensive
sludge, and the promising results which had been obtained in Germany. As
a result of the extended experience obtained in the use of Imhoff tanks
American engineers have learned that, like all other sewage treatment
devices introduced up to the present time, the Imhoff tank requires
experienced attention for its successful operation. These tanks are now
being installed in the place of septic tanks, and they are frequently
used in conjunction with sprinkling filters.
=253. Operation of Imhoff Tanks.=—The important feature in the
successful operation of an Imhoff tank is the proper control of the
sludge and transition chambers. During the ripening process, which may
occupy 2 weeks to 3 months after the start of the tank, offensive odors
may be given off, the tank may foam violently, and scum may boil over
into the sedimentation chamber. This is usually due to an acid condition
in the digestion chamber which may possibly be overcome by the addition
of lime. A very fresh influent will have a similar effect. Too violent
boiling is not likely to occur where the area for the escape of gas has
been made large and the gas is not confined. Any accumulation of scum
should be broken up and pushed down into the digestion chamber, or
removed from the tank. The stream from a fire hose is useful in breaking
up scum. The side walls of the sedimentation chamber should be squeegeed
as frequently as is necessary to keep them free from sludge, which may
be as often as once or twice a week. Material floating on the surface of
the sedimentation chamber should be removed from the tank or sunk into
the digestion chamber through the gas vents in the transition chamber.
No sludge should be removed, except for the taking of samples, until the
tank is well ripened. The ripening of the sludge can be determined by
examining a sample and observing its color and odor. An odorless, black,
granular, well humified sludge is indicative of a ripened tank. After
the tank has ripened, sludge should be removed in small quantities at 2
to 3–week intervals, except in cold or rainy weather. The sludge should
be drawn off slowly to insure the removal of the oldest sludge at the
bottom of the digestion chamber. After the drawing off of the sludge has
ceased the pipe should be flushed with fresh water to prevent its
clogging with dried sludge in the interim until the next removal. Under
no circumstances should all the sludge in the tank be removed at any
time. The removal of some sludge during foaming after ripening may
reduce or stop the foaming. The ripening of a tank can be hastened by
adding some sludge from a tank already ripened.
Sludge should not be allowed to accumulate within 18 inches of the slot
at the bottom of the digestion chamber. The elevation of the surface of
the sludge can be located by lowering into the tank, a stoppered,
wide-mouthed bottle on the end of a stick. The stopper is pulled out by
a string when the bottle is at some known elevation. The bottle is then
carefully raised and observed for the presence of sludge. The process is
repeated with the bottle at different elevations until the surface of
the sludge has been discovered. Another method is to place the suction
pipe of a small hand pump at known points, successively increasing in
depth, and to pump in each position until one position is found at which
sludge appears in the pump. When the sludge in one portion of the
digestion chamber has risen higher than in another portion, the
direction of flow in the sedimentation chamber should be reversed if
possible. In the ordinary routine of operation it is never necessary to
shut down an Imhoff tank. Sludge is removed while the tank is operating.
The shut down of a tank will be caused by accidents and breaks to the
structure or control devices.
=254. Other Tanks.=—The Travis Hydrolytic Tank represents a step in the
development from the septic tank to the Imhoff tank. The Doten tank and
the Alvord tank are recent developments, and are somewhat similar in
operation to the Imhoff tank.
The Travis Hydrolytic Tank when first designed differed from the later
design of the Imhoff tank in the slot between the sedimentation chamber
and the digestion chamber which was not trapped against the escape of
gas from the latter to the former, and in operation a small quantity of
fresh sewage was allowed to flow through the digestion chamber. The tank
is called a hydrolytic tank because some solids are liquefied in it. The
tank is mainly of historic interest as designs similar to it are rarely
made to-day. Better results are obtained from the use of the Imhoff
tank. Recent developments have altered the original design of the Travis
tank so that it is hardly recognizable. The Travis tank at Luton, Eng.,
is shown in Fig. 164. The detailed description given in the _Engineering
News_ in connection with this illustration shows that the governing
object of the design is to separate as quickly as possible the sludge
deposited by the sewage without septic action being set up. To aid in
the collection and settlement of flocculent matter vertical wooden grids
or colloiders are used. The suspended matter strikes these and forms a
slimy deposit on them that in a short time slips off in pieces large
enough to settle readily.
[Illustration:
FIG. 164.—Plan and Section of Hydrolytic Tank at Luton, England.
Eng. News, Vol. 76, 1916, p. 194.
]
[Illustration:
FIG. 165.—Doten Tank for Army Cantonment Sewage Disposal.
Eng. News-Record, Vol. 79, 1917, p. 931.
]
The Doten tank[158] is a single-storied, hopper-bottomed septic tank,
views of which are shown in Fig. 165. It was devised by L. S. Doten for
army cantonments during the War. Its chief purpose was to avoid the
foaming and frothing so common to Imhoff tanks when overdosed with fresh
sewage. The first Alvord tank was constructed in Madison, Wis., in
1913.[159] As now constructed the tank consists of three deep,
single-story compartments with hopper bottoms. These compartments are
arranged side by side in any one unit. Sewage enters at the surface of
one of the compartments and is retained here during one-half of the
total period of retention. It leaves the first compartment over a weir
and passes in a channel over the top of the intermediate compartment to
the third or effluent compartment, where it is held for the remainder of
the period of detention. Accumulated scum and sludge are drawn off into
the intermediate compartment at the will of the operator, this
compartment being used for sludge digestion only. Such tanks as the
Doten and the Alvord have been used for plants receiving very fresh
sewages such as is discharged from military cantonments, in order to
assist in the prevention of the foaming to be expected from an Imhoff
tank receiving such a fresh influent. The tanks are suitable for small
installations, or where excavation to the depth required for an Imhoff
tank is not practicable.
CHAPTER XVII
FILTRATION AND IRRIGATION
=255. Theory.=—The cycle through which the elements forming organic
matter pass from life to death and back to life again has been described
in Chapter XIII. It has been shown in Chapter XVI that septic action
occupies that portion of the cycle in which the combinations of these
elements are broken down or reduced to simpler forms and the lower
stages of the cycle are reached. The action in the filtration of sewage
builds up the compounds again in a more stable form and almost complete
oxidation is attained, dependent on the thoroughness of the filtration.
In the filtration of sewage only the coarsest particles of suspended
matter are removed by mechanical straining. The success of the
filtration is dependent on biologic action. The desirable form of life
in a filter is the so-called nitrifying bacteria which live in the
interstices of the filter bed and feed upon the organic matter in the
sewage. Anything which injures the growth of these bacteria injures the
action of the filter. In a properly constructed and operated filter, all
matter which enters in the influent, leaves with the effluent, but in a
different molecular form. A slight amount may be lost by evaporation and
gasification but this is more than made up by the nitrogen and oxygen
absorbed from the atmosphere. The nitrifying action in sewage filtration
is shown by the analysis of sewage passing through a trickling filter,
as given in Tables 86 and 87. It is shown by the reduction of the
content of organic nitrogen, free ammonia, oxygen consumed, and the
increase in nitrites, nitrates, and dissolved oxygen. The reduction of
suspended matter is interrupted periodically when the filter “unloads.”
The suspended matter in the effluent is then greater than in the
influent.
The nitrifying organisms have been isolated and divided into two
groups—_nitrosomonas_, the nitrite formers, and _nitrobacter_, the
nitrate formers. Experiments indicate that the growth of the nitrobacter
organisms is dependent on the presence of the nitrosomonas organisms,
which are in turn dependent on the presence of the putrefactive
compounds resulting from the action of putrefying bacteria. The
existence of these organisms is an example of symbiotic action in
bacterial growth. The organisms have been found to grow best on rough
porous material on which their zoögleal jelly can be easily deposited
and affixed. Sewage filters were constructed to provide these ideal
conditions before the action of a filter was thoroughly understood.
The action in irrigation is similar to that in filtration. Although more
strictly a method of final disposal rather than preliminary treatment,
the similarity of the actions which take place, and the grading of sand
filtration into broad irrigation with no distinct line of difference has
resulted in the inclusion of the discussion of irrigation in the same
chapter with filtration.
=256. The Contact Bed.=—A contact bed is a water-tight basin filled with
coarse material, such as broken stone, with which sewage and air are
alternately placed in contact in such a manner that oxidation of the
sewage is effected. A contact bed has some of the features of a
sedimentation tank and an oxidizing filter. As such it marks a
transitory step from anaërobic to aërobic treatment of sewage. A plan
and a section of a contact bed are shown in Fig. 166.
Because of its dependence on biologic action a contact bed must be
ripened before a good effluent can be obtained. The ripening or maturing
occurs progressively during the first few weeks of operation, the
earlier stages being more rapidly developed. The time required to reach
such a stage of maturity that a good effluent will be developed will
vary between one and six or eight weeks, dependent on the weather and
the character of the influent. During the period of maturing the load on
the bed should be made light.
The use of contact beds has been extensive where a more stable effluent
than could be obtained from tank treatment has been desired, yet the
best quality of effluent was not required. The sewage to undergo
treatment in a contact bed should be given a preliminary treatment to
remove coarse suspended matter. The efficiency of the contact treatment
can be increased by passing the sewage through two or three contact beds
in series. In double contact treatment the primary beds are filled with
coarser material and operate at a more rapid rate than the secondary
beds. Double contact gives better results than single contact, but
triple contact treatment, though showing excellent results, is hardly
worth the extra cost. An advantage which contact treatment has over all
other methods of sewage filtration is that the bed can be so operated
that the sewage is never exposed to view. As a result the odors from
well-operated contact beds are slight or are entirely absent and there
should be no trouble from flying insects. Such a method of treatment is
favorable to plants located in populous districts and to the fancies of
a landscape architect. Another advantage of the contact bed is the small
amount of head required for its operation, which may be as low as 4 to 5
feet. This low head consumption by a sewage filter is equaled only by
the intermittent sand filter.
[Illustration:
FIG. 166.—Plan and Section of Treatment Plant at Marion, Ohio, Showing
Septic Tank, Contact Bed, and Sand Filter.
1908 Report Ohio State Board of Health.
]
The quality of the effluent from some contact beds is shown in Table 85.
It is to be noted that nitrification has been carried to a fair degree
of completion, and that the reduction of oxygen consumed has been
marked. In comparison with the effluent from filters, contact effluent
contains a smaller amount of nitrogen as nitrites and nitrates, and
suspended solids. Contact effluent is usually clear and odorless, but it
is not stable without dilution. The absence of nitrites and nitrates is
sometimes advantageous as the effluent will not support vegetable
growths dependent on this form of nitrogen. The absence of suspended
solids obviates the use of secondary sedimentation basins which are
needed with trickling filters. The head of 5 to 8 feet required for
contact treatment is low in comparison to the 10 to 15 feet required for
trickling filters, but is slightly higher than the head required for
intermittent sand filtration. The cost of contact treatment is higher
than the cost of trickling filters but is lower than the cost of
intermittent sand filtration, as shown in Table 90.
TABLE 85
QUALITY OF EFFLUENTS FROM CONTACT BEDS
Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson, 1905.
──────┬──────┬─────────┬───────┬────────┬─────────────────────────────────
Filter│Depth,│ Size of │ Rate, │ Oxygen │ Nitrogen as
│ Feet │Material │Million│Consumed│
│ │in Inches│Gallons│ │
│ │ │ per │ │
│ │ │ Acre │ │
│ │ │per Day│ │
──────┼──────┼─────────┼───────┼────────┼───────┬───────┬────────┬────────
│ │ │ │ │Organic│ Free │Nitrites│Nitrates
│ │ │ │ │ │Ammonia│ │
──────┼──────┼─────────┼───────┼────────┼───────┴───────┴────────┴────────
│ │ │ │ │ Parts per Million
│ │ │ │ │ │ │ │
A │ 5│0.25–1.00│ 0.953│ 23│ 3.5│ 8.7│ 0.20│ 1.6
B │ 5│0.25–2.00│ 1.514│ 21│ 4.0│ 8.4│ 0.15│ 1.4
C │ 5│0.25–1.50│ 1.222│ 24│ 3.5│ 10.8│ 0.11│ 0.6
D │ 5│0.50–1.50│ 1.405│ 22│ 3.3│ 9.5│ 0.13│ 0.9
│ │ │ │ │ │ │ │
│ │ │Per Cent Removal of Constituents of Applied Sewage
│ │ │ │ │ │ │ │
A │ 5│0.25–1.00│ 0.953│ 48│ 49│ 10│ │
B │ 5│0.25–2.00│ 1.514│ 52│ 40│ 11│ │
C │ 5│0.25–1.50│ 1.222│ 47│ 31│ 12│ │
D │ 5│0.50–1.50│ 1.405│ 46│ 37│ 19│ │
──────┴──────┴─────────┴───────┴────────┴───────┴───────┴────────┴────────
──────┬──────┬────────────────────┬─────────
Filter│Depth,│ Suspended Matter │Dissolved
│ Feet │ │ Oxygen
│ │ │
│ │ │
│ │ │
│ │ │
──────┼──────┼─────┬────────┬─────┼─────────
│ │Total│Volatile│Fixed│
│ │ │ │ │
──────┼──────┴─────┴────────┴─────┴─────────
│ Parts per Million
│ │ │ │ │
A │ 5│ 832│ 94│ 737│ 0.3
B │ 5│ 831│ 85│ 746│ 0.1
C │ 5│ 826│ 92│ 734│ 0.8
D │ 5│ 810│ 91│ 717│ 0.9
│ │ │ │ │
│ Per Cent Removal of Constituents of Applied Sewage
│ │ │ │ │
A │ 5│ 73│ 70│ 76│
B │ 5│ 80│ 77│ 83│
C │ 5│ 70│ 70│ 70│
D │ 5│ 67│ 61│ 72│
──────┴──────┴─────┴────────┴─────┴─────────
The depth of the contact bed is generally made from 4 to 6 feet. The
deeper beds are less expensive per unit of volume, to construct, as the
cost of the underdrains and the distribution system is reduced in
relation to the capacity of the filter. The increased depth reduces the
aëration, and the periods of filling and emptying are so increased as to
limit the depths to the figures stated. The other dimensions of the bed
are controlled by economy and local conditions, as the success of the
contact treatment is not affected by the shape of the bed. Contact units
are seldom constructed larger than one-half an acre in area, as larger
beds require too much time for filling and emptying. A large number of
small units is also undesirable because of the increased difficulty of
control. In general it is well to build as large units as are compatible
with efficient operation, elasticity of plant, and which can be filled
within the time allowed at the average rate of sewage flow, or from
dosing tanks in which the storage period is not so long as to produce
septic conditions.
The interstices in a contact bed will gradually fill up, due to the
deposition of solid matter on the contact material, the disintegration
of the material, and the presence of organic growths. The period of rest
allowed every five or six weeks tends to restore partially some of this
lost capacity through the drying of the organic growths. It is
occasionally necessary to remove the material from the bed and wash it
in order to restore the original capacity. It may be necessary to do
this three or four times a year, in an overloaded plant, or as
infrequently as once in five or six years in a more lightly loaded bed.
The period is also dependent on the character of the contact material
and the quality of the influent. This loss of capacity may reduce the
voids from an original amount of 40 to 50 per cent of voids to 10 to 15
per cent. If the bed is not overloaded the loss of capacity will not
increase beyond these figures.
The rate of filtration depends on the strength of the sewage, the
character of the contact material, and the required effluent. It should
be determined for any particular plant as the result of a series of
tests. For the purposes of estimation and comparison the approximate
rate of filtration should be taken at about 94 gallons per cubic yard of
filtering material per day on the basis of three complete fillings and
emptyings of the tank. This is equivalent to 150,000 gallons per acre
foot of depth per day, or for a bed 5 feet deep to a rate of 750,000
gallons per acre per day. The net rate for double or triple filtration
is less than these figures, but on each filter the rates are higher.
The material of the contact bed should be hard, rough, and angular. It
should be as fine as possible without causing clogging of the bed.
Materials in successful use are: crushed trap rock or other hard stone,
broken bricks, slag, coal, etc. Soft crumbling materials such as coke
are not suitable as the weight of the superimposed material and the
movement of the sewage crushes and breaks it into fine particles which
accumulate in the lower portion of the filter and clog it. Roughness,
porosity, and small size are desirable, as the greater the surface area
the more rapid the deposition of material. After a short time, however,
the advantages of roughness and porosity are lost, as the sediment soon
covers all unevenness alike. The minimum size of the material is limited
by the tendency towards clogging. The sizes in successful use vary
between ¼ and ¾ of an inch, ½ inch being a common size. The same size of
material is used throughout the depth of the bed except that the upper 6
inches may be composed of small white pebbles or other clean material,
which does not come in contact with the sewage and which will give an
attractive appearance to the plant. In double or triple contact beds 3
or 4–inch material is sometimes used for the primary beds, and ¼-inch
material in the final bed.
Sewage may be applied at any point on or below the surface. The sewage
is withdrawn from the bottom of the bed. It is undesirable to have too
few inlet or outlet openings as the velocity of flow about the openings
will be so great as to disturb the deposit on the contact material. The
distribution system and the underdrains for the bed at Marion, Ohio, are
shown in Fig. 166.
The cycle of operation of a contact bed is divided into four periods. A
representative cycle might be: time of filling, one hour; standing full,
2 hours; emptying, one hour; standing empty, 4 hours. The length of
these periods is the result of long experience based on many tests and
are an average of the conclusions reached. Wide variations from them may
be found in different plants, and tests may show successful results with
different periods. The combination of these four periods is known as the
contact cycle.
The period of filling should be made as short as possible without
disturbing the material of the bed nor washing off the accumulated
deposits. The sewage should not rise more rapidly than one vertical foot
per minute. During the contact or standing full period sedimentation and
adsorption of the colloids are occurring on the area of surface exposed
to the sewage. This period should be of such length that septic action
does not become pronounced, and long enough to permit of thorough
sedimentation. The period of emptying should be made as short as
possible without disturbing the bed, on the same basis that the period
of filling is determined. During the period of standing empty, air is in
contact with the sediment deposited in thin layers on the contact
material, and the oxidizing activities of the filter are taking place.
The filter is given a rest period of one or two days every five or six
weeks, in order that it may increase its capacity and its biologic
activity.
The control of a contact bed may be either by hand or automatic, the
latter being the more common. Hand control requires the constant
attention of an operator and results in irregularity of operation,
whereas automatic control will require inspection not more than once a
day and insures regularity of operation. A number of automatic devices
have been invented which give more or less satisfaction. The air-locked
automatic siphons, without moving parts, have proven satisfactory and
are practically “fool-proof.” The operation of these devices is
explained in Chapter XXI.
=257. The Trickling Filter.=—A trickling or sprinkling filter is a bed
of coarse, rough, hard material over which sewage is sprayed or
otherwise distributed and allowed to trickle slowly through the filter
in contact with the atmosphere. A general view of a trickling filter in
operation at Baltimore is shown in Fig. 167. The action of the trickling
filter is due to oxidation by organisms attached to the material of the
filter. The solid organic matter of the sewage deposited on the surface
of the material, is worked over and oxidized by the aërobic bacteria,
and is discharged in the effluent in a more highly nitrified condition.
At times the discharge of suspended matter becomes so great that the
filter is said to be unloading. The action differs from that in a
contact bed in that there is no period of septic or anaërobic action and
the filter never stands full of sewage.
The effluent from a trickling filter is dark, odorless, and is
ordinarily non-putrescible. Analyses of typical effluents are given in
Tables 86 and 87. The unloading of the filter may occur at any time, but
is most likely to occur in the spring or in a warm period following a
period of low temperatures. It causes higher suspended matter in the
effluent than in the influent and may render the effluent putrescible.
The action is marked by the discharge of solid matter which has sloughed
off of the filter material and which increases the turbidity of the
effluent. Where the diluting water is insufficient to care for the
solids so carried in the effluent, they can be removed by a 2–hour
period of sedimentation. The effluent may become septic during this
time, however. The nitrogen in the effluent is almost entirely in the
form of nitrates, and the percentage of saturation with dissolved oxygen
is high. The effluent is more highly nitrified than that from a contact
bed, and its relative stability is also higher, thus demanding a smaller
volume of diluting water.
[Illustration:
FIG. 167.—Sprinkling Filter in Operation in Winter at Baltimore.
]
The principal advantage of a trickling filter over other methods of
treatment is its high rate which is from two to four times faster than a
contact bed, and about seventy times faster than an intermittent sand
filter. The greatest disadvantage is the head of 12 to 15 feet or more
necessary for its operation. Sedimentation of the effluent is usually
necessary to remove the settleable solids. During the period of
secondary sedimentation the quality of the filter effluent may
deteriorate in relative stability. In winter the formation of ice on the
filter results in an effluent of inferior quality, but as the diluting
water can care for such an effluent at this time the condition is not
detrimental to the use of the trickling filter. In summer the filters
sometimes give off offensive odors that can be noticed at a distance of
half a mile, and flying insects may breed in the filter in sufficient
quantities to become a nuisance if preventive steps are not taken. The
dissemination of odors is especially marked when treating a stale or
septic sewage. The treatment of a fresh sewage seldom results in the
creation of offensive odors.
TABLE 86
ANALYSIS OF CRUDE SEWAGE, IMHOFF TANK, AND SPRINKLING FILTER EFFLUENTS
AT ATLANTA, GEORGIA
(Engineering Record, Vol. 72, p. 4)
─────────┬───────────┬──────────────────────────────────────────
│Temperature│ Parts per Million
│Fahrenheit │
│ │
│ │
─────────┼───────────┼─────────────────────────────────┬────────
│ │ Nitrogen as │ Oxygen
│ │ │Consumed
─────────┼───────────┼───────┬───────┬────────┬────────┼────────
│ │Organic│ Free │Nitrites│Nitrates│
│ │ │Ammonia│ │ │
─────────┴───────────┴───────┴───────┴────────┴────────┴────────
_Crude Sewage_
─────────┬───────────┬───────┬───────┬────────┬────────┬────────
1913 │ │ │ │ │ │
Maximum │ 77│ 15.6│ 21.8│ 0.1│ 3.0│ 100.0
Minimum │ 61│ 10.4│ 16.5│ 0.1│ 1.4│ 78.3
Average │ 70│ 12.8│ 18.8│ 0.1│ 2.2│ 90.6
1914 (7 │ │ │ │ │ │
months)│ │ │ │ │ │
Maximum │ 74│ 16.0│ 33.4│ │ 2.3│
Minimum │ 60│ 9.5│ 18.1│ │ 1.6│
Average │ 66│ 13.4│ 27.1│ │ 2.0│
─────────┴───────────┴───────┴───────┴────────┴────────┴────────
_Imhoff Effluent_
─────────┬───────────┬───────┬───────┬────────┬────────┬────────
1913 │ │ │ │ │ │
Maximum │ 78│ 13.2│ 21.9│ 0.2│ 3.1│ 68.0
Minimum │ 58│ 6.5│ 16.8│ 0.1│ 1.1│ 53.1
Average │ 68│ 9.0│ 20.0│ 0.2│ 2.1│ 60.1
1914 (7 │ │ │ │ │ │
months)│ │ │ │ │ │
Maximum │ 77│ 10.3│ 30.3│ │ 2.0│
Minimum │ 59│ 4.1│ 18.0│ │ 1.5│
Average │ 65│ 7.7│ 25.9│ │ 1.8│
─────────┴───────────┴───────┴───────┴────────┴────────┴────────
_Sprinkling Filter Effluent_
─────────┬───────────┬───────┬───────┬────────┬────────┬────────
1913 │ │ │ │ │ │
Maximum │ 79│ 5.6│ 14.2│ 0.8│ 11.3│ 32.1
Minimum │ 55│ 2.6│ 6.2│ 0.5│ 5.8│ 23.6
Average │ 66│ 3.8│ 9.9│ 0.7│ 8.2│ 28.2
1914 (7 │ │ │ │ │ │
months)│ │ │ │ │ │
Maximum │ 77│ 8.5│ 20.7│ │ 11.2│
Minimum │ 55│ 4.4│ 8.8│ │ 3.6│
Average │ 63│ 5.7│ 15.2│ │ 7.2│
─────────┴───────────┴───────┴───────┴────────┴────────┴────────
─────────┬────────────────────┬──────────┬─────────
│ Parts per Million │ Per Cent │Relative
│ │Saturation│Stability
│ │Dissolved │
│ │ Oxygen │
─────────┼────────────────────┼──────────┼─────────
│ Suspended Matter │ │
│ │ │
─────────┼─────┬────────┬─────┼──────────┼─────────
│Total│Volatile│Fixed│ │
│ │ │ │ │
─────────┴─────┴────────┴─────┴──────────┴─────────
_Crude Sewage_
─────────┬─────┬────────┬─────┬──────────┬─────────
1913 │ │ │ │ │
Maximum │ 371│ 154│ 163│ 47│
Minimum │ 222│ 98│ 112│ 11│
Average │ 285│ 126│ 138│ 28│
1914 (7 │ │ │ │ │
months)│ │ │ │ │
Maximum │ 431│ │ │ 48│
Minimum │ 279│ │ │ 12│
Average │ 351│ │ │ 30│
─────────┴─────┴────────┴─────┴──────────┴─────────
_Imhoff Effluent_
─────────┬─────┬────────┬─────┬──────────┬─────────
1913 │ │ │ │ │
Maximum │ 90│ 50│ 41│ │
Minimum │ 35│ 42│ 21│ │
Average │ 68│ 46│ 33│ │
1914 (7 │ │ │ │ │
months)│ │ │ │ │
Maximum │ 73│ │ │ 48│
Minimum │ 49│ │ │ 34│
Average │ 65│ │ │ 43│
─────────┴─────┴────────┴─────┴──────────┴─────────
_Sprinkling Filter Effluent_
─────────┬─────┬────────┬─────┬──────────┬─────────
1913 │ │ │ │ │
Maximum │ 60│ 31│ 28│ 76│ 99
Minimum │ 33│ 26│ 28│ 52│ 88
Average │ 49│ 28│ 28│ 64│ 89
1914 (7 │ │ │ │ │
months)│ │ │ │ │
Maximum │ 106│ │ │ 79│ 99
Minimum │ 40│ │ │ 55│ 89
Average │ 62│ │ │ 65│ 95
─────────┴─────┴────────┴─────┴──────────┴─────────
TABLE 87
EFFICIENCY OF SPRINKLING FILTER CHICAGO, ILLINOIS
Depth of Filter 9 feet. Size of stone 2 in. to 3 in.
────────┬───────────────────────────┬───────────────────────────
Month │ Organic Nitrogen │ Free Ammonia
│ │
────────┼───────────────────────────┼───────────────────────────
│ │
────────┼─────────┬─────────┬───────┼─────────┬─────────┬───────
│Influent,│Effluent,│ Per │Influent,│Effluent,│ Per
│Parts per│Parts per│ Cent │Parts per│Parts per│ Cent
│ Million │ Million │Removed│ Million │ Million │Removed
────────┼─────────┼─────────┼───────┼─────────┼─────────┼───────
1910 │ │ │ │ │ │
October │ 5.1│ 2.8│ 45│ 12.0│ 4.6│ 62
November│ 5.9│ 2.5│ 58│ 12.0│ 5.9│ 51
December│ 4.6│ 3.0│ 35│ 12.0│ 6.9│ 42
│ │ │ │ │ │
1911 │ │ │ │ │ │
January │ 6.3│ 4.8│ 24│ 11.0│ 7.0│ 36
February│ 9.0│ 4.8│ 47│ 10.0│ 7.2│ 28
March │ 8.3│ 3.5│ 58│ 9.9│ 6.4│ 35
April │ 6.4│ 4.0│ 37│ 8.3│ 3.6│ 69
May │ 7.6│ 5.4│ 29│ 9.2│ 2.4│ 74
June │ 5.9│ 3.2│ 46│ 11.0│ 0.6│ 95
July │ 6.2│ 4.2│ 32│ 11.0│ 1.3│ 88
────────┴─────────┴─────────┴───────┴─────────┴─────────┴───────
────────┬───────────────────────────┬───────────────────────────
Month │ Oxygen Consumed │ Nitrites
│ │
────────┼───────────────────────────┼───────────────────────────
│ │
────────┼─────────┬─────────┬───────┼─────────┬─────────┬───────
│Influent,│Effluent,│ Per │Influent,│Effluent,│ Per
│Parts per│Parts per│ Cent │Parts per│Parts per│ Cent
│ Million │ Million │Removed│ Million │ Million │Removed
────────┼─────────┼─────────┼───────┼─────────┼─────────┼───────
1910 │ │ │ │ │ │
October │ 30│ 15│ 50│ │ .90│
November│ 35│ 15│ 57│ │ .76│
December│ 39│ 20│ 49│ .07│ .45│ 6.4
│ │ │ │ │ │
1911 │ │ │ │ │ │
January │ 42│ 20│ 52│ .08│ .15│ 1.9
February│ 46│ 20│ 56│ .09│ .15│ 1.7
March │ 47│ 21│ 56│ .09│ .15│ 1.7
April │ 38│ 21│ 45│ .16│ .21│ 1.3
May │ 33│ 31│ 6│ .08│ .38│ 4.8
June │ 28│ 16│ 43│ .00│ .30│ ∞
July │ 34│ 26│ 24│ .00│ .36│ ∞
────────┴─────────┴─────────┴───────┴─────────┴─────────┴───────
────────┬───────────────────────────┬───────────────────────────
Month │ Nitrates │ Dissolved Oxygen
│ │
────────┼───────────────────────────┼───────────────────────────
│ │
────────┼─────────┬─────────┬───────┼─────────┬─────────┬───────
│Influent,│Effluent,│ Per │Influent,│Effluent,│ Per
│Parts per│Parts per│ Cent │Parts per│Parts per│ Cent
│ Million │ Million │Removed│ Million │ Million │Removed
────────┼─────────┼─────────┼───────┼─────────┼─────────┼───────
1910 │ │ │ │ │ │
October │ │ 7.8│ │ 0.0│ 8.5│ ∞
November│ │ 5.9│ │ 0.0│ 8.1│ ∞
December│ .15│ 2.6│ 17│ 2.0│ 8.4│ 4.2
│ │ │ │ │ │
1911 │ │ │ │ │ │
January │ .27│ 2.2│ 8.2│ 3.0│ 7.8│ 2.9
February│ .50│ 2.6│ 5.2│ 2.6│ 8.0│ 3.1
March │ .34│ 3.2│ 9.4│ 2.2│ 6.6│ 3.0
April │ .53│ 4.5│ 8.5│ 2.1│ 7.1│ 3.4
May │ .15│ 7.5│ 4.3│ 0.1│ 7.7│ 77
June │ .16│ 8.3│ 5.2│ 0.0│ 7.6│ ∞
July │ .09│ 7.7│ 8.0│ 0.0│ 6.5│ ∞
────────┴─────────┴─────────┴───────┴─────────┴─────────┴───────
────────┬───────────┬───────────────────────────────────────────────────────
Month │ Per Cent │ Suspended Matter
│Putrescible│
────────┼───────────┼───────────────────────────┬───────────────────────────
│ │ Total │ Volatile
────────┼───────────┼─────────┬─────────┬───────┼─────────┬─────────┬───────
│ │Influent,│Effluent,│ Per │Influent,│Effluent,│ Per
│ │Parts per│Parts per│ Cent │Parts per│Parts per│ Cent
│ │ Million │ Million │Removed│ Million │ Million │Removed
────────┼───────────┼─────────┼─────────┼───────┼─────────┼─────────┼───────
1910 │ │ │ │ │ │ │
October │ 0│ 75│ 40│ 47│ 54│ 25│ 54
November│ 5│ 61│ 16│ 74│ 52│ 15│ 71
December│ 35│ 85│ 40│ 53│ 60│ 26│ 57
│ │ │ │ │ │ │
1911 │ │ │ │ │ │ │
January │ 38│ 112│ 43│ 63│ 68│ 29│ 57
February│ 29│ 100│ 49│ 51│ 64│ 32│ 50
March │ 28│ 106│ 37│ 65│ 63│ 22│ 65
April │ 9│ 113│ 68│ 40│ 59│ 35│ 41
May │ 6│ 88│ 150│ _1.7_│ 54│ 70│ _1.3_
June │ 1│ 92│ 77│ 18│ 56│ 36│ 36
July │ 4│ 155│ 130│ 16│ 74│ 61│ 18
────────┴───────────┴─────────┴─────────┴───────┴─────────┴─────────┴───────
────────┬───────────────────────────
Month │ Suspended Matter
│
────────┼───────────────────────────
│ Fixed
────────┼─────────┬─────────┬───────
│Influent,│Effluent,│ Per
│Parts per│Parts per│ Cent
│ Million │ Million │Removed
────────┼─────────┼─────────┼───────
1910 │ │ │
October │ 21│ 15│ 29
November│ 9│ 1│ 89
December│ 25│ 14│ 44
│ │ │
1911 │ │ │
January │ 44│ 13│ 70
February│ 37│ 17│ 53
March │ 43│ 15│ 65
April │ 54│ 33│ 39
May │ 34│ 80│ _2.4_
June │ 36│ 41│ _1.1_
July │ 81│ 69│ 15
────────┴─────────┴─────────┴───────
NOTE.—Italic figures represent increases.
Raw sewage cannot be treated successfully on a trickling filter. Coarse
solid particles should be screened and settled out, in order that the
distributing devices or the filter may not become clogged. The effluent
from an Imhoff tank has proven to be a satisfactory influent for a
trickling filter. A septic tank effluent may be so stale as to be
detrimental to the biologic action in the filter.
In the operation of a trickling filter the sewage is sprayed or
otherwise distributed as evenly as possible in a fine spray or stream,
over the top of the filtering material. The sewage then trickles slowly
through the filter to the underdrains through which it passes to the
final outlet. The distribution of the sewage on the bed is intermittent
in order to allow air to enter the filter with the sewage. The cycle of
operation should be completed in 5 to 15 minutes, with approximately
equal periods of rest and distribution. Cycles of too great length will
expose the filter to drying or freezing and will give poorer
distribution throughout the filter. Cycles which are too short will
operate successfully only with but slight variation in the rate of
sewage flow. In some plants it has been found advantageous to allow the
filters to rest for one day in 3 to 6 weeks or longer, dependent on the
quality of the effluent.
The rate of filtration may be as high as 2,000,000 gallons per acre per
day, which is equivalent to 200 gallons per cubic yard of material per
day in a bed 6 feet deep. This is more than double the rate permissible
in a contact bed. The exact rate to be used for any particular plant
should be determined by tests. It is dependent on the quality of the
sewage to be treated, on the depth of the bed, the size of the filling
material, the weather, and other minor factors.
The filtering material is similar to that used in a contact bed. It
should consist of hard, rough, angular material, about 1 to 2 inches in
size. Larger sizes will permit more rapid rates of filtration, but will
not produce so good an effluent. Smaller sizes will clog too rapidly.
The depth of the filter is limited by the possibility of ventilation and
the strength of the filtering material to withstand crushing. The deeper
the bed the less the expense of the distribution and collecting system
for the same volume of material, and the more rapid the permissible rate
of filtration. The depths in use vary between 6 and 10 feet, with 6 to 8
feet as a satisfactory mean. From a biologic standpoint the action of
the filter seems to be proportional to the volume of the filtering
material and therefore proportional to the depth of the bed, being
limited to a minimum depth of about 5 feet, below which sewage may pass
through the filter without treatment. The shape and other dimensions of
the filter depend on the local conditions and the economy of
construction. The filters need not be broken up into units by
water-tight dividing walls. One filter can be constructed sufficient for
all needs and various portions of it can be isolated as units by the
manipulation of valves in the distribution system. Ventilation is
provided by the air entrained with the sewage as it falls upon the
surface. If the sides of the filter are built of open stone crib work
the ventilation will be greatly improved, but it will not be possible to
flood the filters to keep down flies, and in cold climates these
openings must be covered in winter to prevent freezing. Filters have
been constructed without side walls, the filtering material being
allowed to assume its natural angle of repose. This has usually been
found to be more expensive than the construction of side retaining
walls, due to the unused filling material and the extra underdrains
required.
The distribution of sewage is ordinarily effected by a system of pipes
and spray nozzles as shown in Fig. 168 and 169. Other methods of
distribution have been used. At Springfield, Mo.,[160] a moving trough
from which the sewage flows continuously is drawn back and forth across
the bed by means of a cable. In England circular beds have been
constructed and the sewage distributed on them through revolving
perforated pipes. At the Great Lakes Naval Training Station[161] the
distributing pipes in the plant, now abandoned, were supported above the
surface of the filter. The sewage fell from holes in the lower side of
these pipes on to brass splash plates 14 inches above the filter. It was
deflected horizontally from these plates over the filter surface. Pipes
and spray nozzles have been adopted almost universally in the United
States. Splash plates, traveling distributors, and other forms of
distribution have been used only in exceptional cases. In a distributing
system consisting of pipes and nozzles, a network of pipes is laid out
somewhat as shown in Fig. 168, in such a manner that the head loss to
all points is approximately equal. The number of valves required should
be reduced to a minimum. The pipes may be laid out with the main feeders
leading from a central point and branches at right angles to them,
somewhat on the order of a spider’s web, or they may be laid out on a
rectangular or gridiron system. The radial system is advantageous
because of the central location of the control house, but it does not
always lend itself favorably to the local conditions, and the piping and
nozzle location are not so simple. The gridiron system lends itself
favorably to the equalization of head losses. The pipes used should be
larger than would be demanded by considerations of economy alone, both
for the purpose of reduction of head loss and ease in cleaning. No pipe
less than 6 inches in diameter should be used, and the average velocity
of flow should not exceed one foot per second. Cast-iron, concrete, or
vitrified clay pipe may be used, but cast iron is the material commonly
used. The system should be arranged for easy flushing and cleaning and
the pipes so sloped that the entire system can be drained in case of a
shut down in cold weather.
[Illustration:
FIG. 168.—Section through Sprinkling Filter at Fitchburg, Mass.,
Showing Distribution System.
Eng. Record, Vol. 67, p. 634.
]
The pipes are placed far enough below the surface of the filling
material so that the top of the spraying nozzle is 6 to 12 inches above
the surface of the filter. If the pipes are placed near the surface they
are accessible for repairs, but are exposed to temperature changes. If
the pipes are large their presence near the surface of the filter may
seriously affect the distribution of the sewage through the filter. If
the distributing pipes are placed near the bottom of the filter they are
inaccessible for repairs and the nozzles must be connected to them by
means of long riser pipes. The distributing pipes should be supported by
columns extending to the foundation of the filter bed, there being a
column at every pipe joint with such intermediate supports as may be
required. In some plants the pipes have been supported by the filtering
material. Although slightly less expensive in first cost the practice of
so supporting the pipes is poor, as settling of the material may break
the pipe or cause leaks, and if the bed becomes clogged, removal of the
material is made more difficult. Valves should be placed in the
distributing system in such a manner that different sets of nozzles can
be cut out at will, thus resting those portions of the filter and
permitting repairs without shutting down the entire filter.
The spacing of the nozzles is fixed by the type and size of the nozzle,
the available head, and the rate of filtration. Various types of
sprinkler nozzles are shown in Fig. 169 and the discharge rates, head
losses, and distances to which sewage is thrown for the Taylor nozzles,
are shown in Fig. 170. Nozzles are available which will throw circular,
square, or semicircular sprays. In the use of circular sprays there is
necessarily some portion of the filter which is underdosed if the
nozzles are placed at the corners of squares with the sprays tangent,
and there is an overdosing of other portions if the sprays are allowed
to overlap so that no portion of the filter is left without a dose.
Rectangular sprays will apparently overcome these difficulties, but
studies have shown that circular sprays with some overlapping, and the
nozzles placed at the apexes of equilateral triangles as shown in Fig.
172 will give as satisfactory distribution as other forms.
[Illustration:
FIG. 169.—Sprinkling Filter Nozzles.
Bulletin No. 3, Engineering Experiment Station, Purdue University.
]
[Illustration:
FIG. 170.—Diagram Showing the Discharge and Spacing of Taylor Nozzles.
]
The nozzles should be selected to give the best distribution, to consume
all of the head available, and to give the proper cycle of operation.
The entire head available should be consumed in order that the fewest
number of nozzles may be used. An excellent study of the characteristics
of various types of nozzles has been published in Bulletin No. 3 of the
Engineering Experiment Station at Purdue University, 1920. As a result
of the tests on the nozzles shown in Fig. 169, it was determined for all
nozzles, except No. 8, that
_Q_ = _Ca_√(2_gh_);
in which _Q_ = the rate of discharge in cubic feet per second;
_C_ = a coefficient shown in Table 88;
_a_ = the net cross-sectional opening of the nozzle in square
feet;
_h_ = the pressure on the nozzle in feet of water.
TABLE 88
COEFFICIENTS OF DISCHARGE FOR SPRINKLER NOZZLES SHOWN IN FIG. 169
──────────────────────┬──────┬──────┬──────┬──────┬──────┬──────┬──────
Nozzle Number │ 1 │ 2 │ 3 │ 4 │ 5 │ 6 │ 7
──────────────────────┼──────┼──────┼──────┼──────┼──────┼──────┼──────
Coefficient │ .648 │ .756 │ .696 │ .666 │ .675 │ .598 │ .569
──────────────────────┴──────┴──────┴──────┴──────┴──────┴──────┴──────
It is evident that if the head on the nozzles is constant and the nozzle
throws a circular spray, the intensity of dosing at the circumference
will be greater than nearer the center. This difficulty is overcome by
so designing the dosing tank from which the sewage is fed that the head
on the nozzle and the quantity thrown will vary in such a manner that
the distribution over the bed is equalized. Intermittent action is
obtained by an automatic siphon which commences to discharge when the
tank is full and empties the tank in the period allowed for dosing.
Under such conditions the tank should discharge for a longer time at the
higher heads than at the lower heads as there is more territory to be
covered at the higher heads. The design of the tank to do this with
exactness is difficult, and the construction of the necessary curved
surfaces is expensive. Where a dosing tank is used for such conditions
it has been found satisfactory to construct the tank with plane sides
sloping at approximately 45 degrees from the vertical (or horizontal). A
tank with curved surfaces is shown in Fig. 171. The dosing siphon is
usually placed in the tank as shown in the figure. The head and quantity
of discharge through the nozzles can be varied also by maintaining a
constant depth in a dosing tank by means of a float feed valve, and
varying the head and quantity discharged to the nozzles by a butterfly
valve in the main feed line, or by the use of a Taylor undulating valve
designed for this purpose. The butterfly valve is opened and closed by a
cam so designed and driven at such a rate that the required distribution
is obtained. The Taylor undulating valve is opened and closed at a
constant rate, the shape of the valve giving the required variations in
head and discharge. Other methods of control have been attempted but
have not been used extensively.
[Illustration:
FIG. 171.—Section of 12–inch Siphon and Dosing Tank, for King’s Park,
Long Island.
]
An example of the design of the nozzle layout and dosing tank for a
sprinkling filter follows:
Let it be required to determine the nozzle layout for one acre of
sprinkling filters with 5 feet available head on the nozzles.
The selection of the type of nozzle and the size of opening is a
matter of judgment and experience. Nozzles with large openings are
less liable to clog and fewer nozzles are needed than where small
nozzles are used, but the distribution of sewage is not so even as
with the use of small nozzles. In this example Taylor circular
spray nozzles will be selected. Fig. 170 shows that a Taylor
circular spray nozzle will discharge 22.3 g.p.m. under a head of 5
feet, and that the economical nozzle spacing will be 15.3 feet.
The least number of nozzles at this spacing required for a bed of
one acre in area is found as follows: In Fig. 172, let _n_ equal
the number of nozzles in a horizontal row, counting half-spray
nozzles as ½, and let _m_ equal the number of rows counting rows
of half-spray nozzles as half rows.[162] Then the number of
nozzles, _N_, equals _mn_, and 15.3_m_ × 13.2_n_ equals 43,560 or
_mn_ equals 215.
[Illustration:
FIG. 172.—Typical Sprinkler Nozzle Layout.
]
The next step should be the design of the dosing tank and siphon. It is
possible to design a tank which will give equal distribution over equal
areas of filter surface. It has been found, however, that the expense of
this refinement is unwarranted as there are a number of outside factors
which tend to overcome the theoretical design. The effect of wind,
unequal spacing, and irregularities in the elevation of the nozzles have
a tendency to offset refinements in the design of a dosing tank. It is
therefore the general practice to slope the sides of the tank at an
angle of about 45 degrees as previously stated. The dosing tank is
generally designed to have a capacity which will give a complete cycle
of operation once in 15 minutes. In the ordinary design the factors
given are the rate of inflow and the given time of filling. In the
following example the time of filling will be taken as 10 minutes, the
time of emptying as 5 minutes, and the rate of flow as 1,000,000 gallons
per day. The capacity of the tank will therefore be (1,000,000)⁄24 x 6 =
7,000 gallons. The diameter of the siphon to be selected can be computed
as follows:
Let _Q_ = the capacity of the tank in cubic feet;
_q__{1} = the rate of discharge of the siphon in cubic feet per
second;
_q__{2} = the rate of inflow to the tank in cubic feet per second;
_q_ = the rate of emptying the tank in cubic feet per second =
(_q__{1} − _q__{2});
_A_ = the cross-sectional area of the free surface of the water
in the tank at any instant, in square feet;
_a_ = the cross-sectional area of the siphon in square feet;
_b_ = the small dimension of the base of the tank in feet;
_h_ = the head of water, in feet, on the discharge siphon;
_h__{1} = the initial head of water, in feet, on the siphon;
_h__{2} = the final head of water in feet, on the siphon;
_t_ = the time, in seconds, required to empty the tank,
then _dQ_ = -_Adh_ = _q__{1}_dt_ − _q__{2}_dt_,
and _dt_ = (_dQ_)⁄_q_ = − _Adh_⁄(_q__{1} − _q__{2}),
but _q__{1} = 0.4 _A_ √((2_gh_)),[163]
therefore _t_ = ∫_{_h__{2}}^{_h__{1}} -_Adh_⁄(0.4_a_√(2_gh_) −
_q__{2}),
but _A_ = 4_h_^2 + 4_bh_ + _b_^2,
therefore _t_ = ∫_{_h__{1}}^{_h__{2}} ((_b_^2 + 4_bh_ +
4_h_^2)_dh_)⁄0.4_a_√(2_gh_) − _q__{2}.
The integration of this expression is tedious. Its solution for siphons
between 6 inches and 12 inches operating under heads commencing from 3
feet to 6 feet, with a time of emptying of 5 minutes and time of filling
of 10 minutes is given in Fig. 173. In the example given the rate of
inflow is 1.55 sec. feet and the head is 5 feet. Then from Fig. 173 the
size of the siphon to be used is 12 inches. Where a siphon of the size
required to empty the tank in the time fixed is not available,
combinations of available sizes can sometimes be used.
[Illustration:
FIG. 173.—Diagram for the Determination of the Capacities of Dosing
Tanks for Trickling Filters.
Time of emptying, 5 minutes. Time of filling, 10 minutes. Shape of
tank is a right pyramid or a truncated right pyramid with all four
sides making an angle of 45 degrees with the vertical. All
horizontal cross-sections are squares.
]
For example, if the given head is 6 feet, and the rate of inflow
is 1.4 sec. feet, it is evident from Fig. 173 that a 6,300–gallon
dosing tank and two 8–inch siphons will give the required cycle.
The method used for the design of the setting of Taylor nozzles by the
Pacific Flush Tank Co., is less rational but more simple and probably as
satisfactory. In this method the steps are as follows:
(1) Divide the maximum daily rate of sewage flow by 1,000 to get
the maximum minute inflow.
(2) The number of nozzles required is determined by dividing the
preceding figure by 6. Generally a Taylor nozzle with an orifice
of ⅞ of an inch will discharge about 20 g.p.m. at the high head
and about 8 g.p.m. at the low head, and as the nozzles must have a
capacity which will take care of the inflow at the low head, the
divisor 6 is used as a factor of safety instead of using 8 as the
divisor.
(3) The type of nozzle to be used is selected from experience or
as a matter of judgment. Circular-spray nozzles are more generally
used.
(4) The spacings are determined from Fig. 170.
(5) The dosing tank of the shape described is then designed. The
capacity is such as to give a complete cycle once every 15
minutes. The method of this design is similar to that followed
previously.
(6) The dosing siphons are designed so that they will have a
capacity at the minimum head of from 40 to 50 per cent in excess
of the maximum minute inflow, and the draining depth of the siphon
will be limited to a maximum of 5 to 5½ feet. The siphons are all
made adjustable with a variation of 6 inches or more on either
side of the normal discharge line so that the spraying area and
cycle can be varied to secure the best results.
The underdrainage of a trickling filter should consist of some form of
false bottom such as the types shown in Fig. 174. Where possible the
underdrains should be open at both ends for the purpose of ventilation
and flushing. It is desirable that the drains be so arranged that a
light can be seen through them in order that clogging can be easily
located. The drains should be placed on a slope of approximately 2 in
100 towards a main collector. The length of the drains is limited by
their capacity to carry the average dose from the area drained by them.
The main collecting conduits must be designed in accordance with the
hydraulic principles given in Chapter IV. No valves, or other
controlling apparatus, are placed on the underdrains or outlets from the
filter.
Covers have been provided in winter for some trickling filters in cold
climates. The Taylor sprinkling nozzle has been found to work
successfully in extremely cold weather, and it is generally accepted
that the covering of filters is unnecessary, if the filter is not to be
shut down for any length of time in cold weather.
The operation of devices for automatically controlling the operation of
a trickling filter is explained in Chapter XXI.
[Illustration:
FIG. 174.—Types of False Bottoms for Trickling Filters.
Eng. News, Vol. 74, p. 5.
]
=258. Intermittent Sand Filter.=—An intermittent sand filter is a
specially prepared bed of sand, or other fine grained material, on the
surface of which sewage is applied intermittently, and from which the
sewage is removed by a system of underdrains. It differs from broad
irrigation in the character of the material, the care and preparation of
the bed, and the thoroughness of the underdrainage. A distinctive
feature of the intermittent sand filter is the quality of the effluent
delivered by it. In a properly designed and operated plant the effluent
is clear, colorless, odorless, and sparkling. It is completely
nitrified, is stable and contains a high percentage of dissolved oxygen.
It contains no settleable solids except at widely separated periods when
a small quantity may appear in the effluent. The percentage removal of
bacteria may be from 98 to 99 per cent. Some analyses of sand filter
effluents are given in Table 89. The dissolved solids, the remaining
bacteria, and the antecedents of the effluent are the only differences
between it and potable water. An effluent from an intermittent sand
filter is the most highly purified effluent delivered by any form of
sewage treatment. The effluent can be disposed of without dilution, on
account of its high stability. The treatment of sewage to so high a
degree is seldom required, so that the use of intermittent filters is
not common. Other drawbacks to their use are the relatively large area
of land necessary and the difficulty of obtaining good filter sand in
all localities.
TABLE 89
QUALITY OF EFFLUENTS FROM SAND FILTERS
(Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson, 1905)
────────────┬───────────────────────────────────────────────────────┬───────
Source of │ Parts per Million │Rate of
Sample │ │Filtra-
│ │ tion
│ │Gallons
│ │ per
│ │ Acre,
│ │per Day
────────────┼────────────────────────────────────┬────────┬─────────┼───────
│ Nitrogen as │ Oxygen │ Oxygen │
│ │Consumed│Dissolved│
────────────┼───────┬──────────┬────────┬────────┼────────┼─────────┼───────
│ Free │Albuminoid│Nitrites│Nitrates│ │ │
│Ammonia│ Ammonia │ │ │ │ │
────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼───────
Filter │ 11.0 │ 8.6 │ │ │ 59. │ │
influent │ │ │ │ │ │ │
from grit │ │ │ │ │ │ │
chamber │ │ │ │ │ │ │
Filter │ 1.12 │ 0.88 │ 0.08 │ 11.5 │ 6.9 │ 6.3 │ 0.081
effluent │ │ │ │ │ │ │
Filter │ 0.81 │ 0.88 │ 0.10 │ 12.6 │ 6.5 │ 6.2 │ 0.118
effluent │ │ │ │ │ │ │
────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼───────
Filter │ 9.7 │ 5.4 │ │ │ 33. │ │
influent │ │ │ │ │ │ │
from plain│ │ │ │ │ │ │
settling │ │ │ │ │ │ │
tank │ │ │ │ │ │ │
Filter │ 0.62 │ 0.77 │ 0.11 │ 14.9 │ 6.0 │ 8.2 │ 0.139
effluent │ │ │ │ │ │ │
Filter │ 0.99 │ 1.10 │ 0.10 │ 12.6 │ 7.8 │ 6.5 │ 0.274
effluent │ │ │ │ │ │ │
Filter │ 2.61 │ 1.39 │ 0.09 │ 9.0 │ 9.7 │ 3.9 │ 0.357
effluent │ │ │ │ │ │ │
────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼───────
Filter │ 10.7 │ 5.6 │ │ │ 38. │ │
influent │ │ │ │ │ │ │
from │ │ │ │ │ │ │
septic │ │ │ │ │ │ │
tank │ │ │ │ │ │ │
Filter │ 1.63 │ 1.16 │ 0.09 │ 11.2 │ 8.0 │ 5.8 │ 0.357
effluent │ │ │ │ │ │ │
────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼───────
Filter │ 13.4 │ 4.7 │ │ │ 40. │ │
influent │ │ │ │ │ │ │
from coke │ │ │ │ │ │ │
strainer │ │ │ │ │ │ │
Filter │ 2.24 │ 1.35 │ 1.03 │ 14.6 │ 10.1 │ 6.9 │ 0.372
effluent │ │ │ │ │ │ │
────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼───────
Filter │ 8.6 │ 3.6 │ 0.19 │ 1.6 │ 24. │ 0.3 │
influent │ │ │ │ │ │ │
from │ │ │ │ │ │ │
contact │ │ │ │ │ │ │
bed │ │ │ │ │ │ │
Filter │ 2.62 │ 1.35 │ 0.31 │ 8.1 │ 8.3 │ 5.8 │ 0.516
effluent │ │ │ │ │ │ │
Filter │ 2.44 │ 2.41 │ 0.16 │ 9.4 │ 12.5 │ 5.0 │ 0.525
effluent │ │ │ │ │ │ │
Filter │ 3.40 │ 1.15 │ 0.20 │ 10.9 │ 9.7 │ 5.2 │ 0.525
effluent │ │ │ │ │ │ │
────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼───────
Filter │ 9.0 │ 4.8 │ 0.42 │ 1.3 │ 27. │ 3.4 │
influent │ │ │ │ │ │ │
from │ │ │ │ │ │ │
sprinkling│ │ │ │ │ │ │
filter │ │ │ │ │ │ │
after │ │ │ │ │ │ │
sedimen- │ │ │ │ │ │ │
tation │ │ │ │ │ │ │
Filter │ 2.95 │ 1.25 │ 0.19 │ 7.0 │ 8.8 │ 3.8 │ 0.675
effluent │ │ │ │ │ │ │
Filter │ 4.77 │ 2.63 │ 0.51 │ 4.6 │ 11.8 │ 2.5 │ 0.749
effluent │ │ │ │ │ │ │
Filter │ 3.47 │ 1.61 │ 0.31 │ 7.2 │ 11.9 │ 3.7 │ 1.129
effluent │ │ │ │ │ │ │
────────────┴───────┴──────────┴────────┴────────┴────────┴─────────┴───────