228. Purpose.—The first step in the treatment of sewage is usually that of coarse screening in order to remove the larger particles of floating or suspended matter. Screens and sedimentation basins are used to prevent the clogging of sewers, channels, and treatment plants; to avoid clogging of and injuries to machinery; to overcome the accumulation of putrefying sludge banks; to minimize the absorption of oxygen in diluting water; and to intercept unsightly floating matter.

By the plain sedimentation of sewage is meant the removal of suspended matter by quiescent subsidence unaffected by septic action or the addition of chemicals or other precipitants. In order to prevent septic action plain sedimentation tanks must be cleaned as frequently as once or twice a week in warm weather but not quite so often in cold weather.

Fine screening may take the place of sedimentation where insufficient space is available for sedimentation tanks, and it is desired to remove only a small portion of the suspended matter. Recent American practice has tended to restrict the field of fine screening to treatment requiring less than 10 per cent removal of suspended matter, thus eliminating screens from the field covered by plain sedimentation tanks. The practice is well expressed by Potter, who states:^[135]

Where a high degree of purification is sought, the use of fine screens is of doubtful value. A modern settling tank will give better results and at a less cost for a given degree of purification. A settled liquid is also superior to a screened liquid for subsequent biological treatment in filters.... Again the storing of large quantities of screenings must necessarily be more objectionable than the storing of the digested sludge of a modern settling tank.

229. Types of Screens.—The definitions of some types of screens as proposed by the American Public Health Association follow: A bar screen is composed of parallel bars or rods. A mesh screen is composed of a fabric, usually wire. A grating consists of 2 sets of parallel bars in the same plane in sets intersecting at right angles. A band screen consists of an endless perforated band or belt which passes over upper and lower rollers. A perforated plate screen is made of an endless band of perforated plates similar to a band screen. A wing screen has radial vanes uniformly spaced which rotate on a horizontal axis. A disc screen consists of a circular perforated disc with or without a central truncated cone of similar material mounted in the center. The Reinsch Wurl screen is the best known type of disc screen. A cage screen^[136] consists of a rectangular box made up of parallel bars with the upstream side of the box or cage omitted. Allen^[137] gives the following definitions: A drum screen is a cylinder or cone of perforated plates or wire mesh which rotates on a horizontal axis. A shovel vane screen is similar to a wing screen with semicircular wings and a different method of removing the screenings. Examples of a band screen, a wing screen, a shovel vane screen, a drum screen and a disc screen are shown in Fig. 150. A bar screen is shown in Fig. 151 and a cage screen is shown in Fig. 152.

Screens can be classed as fixed, movable, or moving. Fixed screens are permanently set in position and must be cleaned by rakes or teeth that are pulled between the bars. Movable screens are stationary when in operation, but are lifted from the sewage for the purpose of cleaning. Moving screens are in continuous motion when in operation and are cleaned while in motion. Fixed bar screens may be set either vertical, inclined, or horizontal.

Movable screens with a cage or box at the bottom are sometimes used. The box should be of solid material to prevent the forcing of screenings through it when the screen is being raised for cleaning. A mesh screen should be used only under special circumstances because of the difficulty in cleaning. Screens which must be raised from the sewage for cleaning should be arranged in pairs in order that one may be working when the other is being cleaned. Movable screens are undesirable for small plants because the labor involved in raising and lowering is greater than in cleaning with a rake and the screens are more likely to be neglected. In a large plant rakes operated by hand are too small for cleaning the screens. A fixed screen is sometimes used with moving teeth fastened to endless chains. The teeth pass between the parallel bars and comb out the screenings. If the screen chamber in a small plant is too deep for accessibility a movable cage or box screen may be desirable.

Moving screens are generally of fine mesh or perforated plates. They are kept moving in order to allow continuous cleaning. They are cleaned by brushes or by jets of air, water, or steam.

230. Sizes of Openings.—The area or size of the opening of a screen is dependent upon the character of the sewage to be treated and upon the object to be attained.

Large screens, with openings between 1½ inches and 6 inches are used to protect centrifugal pumps, tanks, automatic dosing devices, conduits, and gate valves from large objects such as pieces of timber, dead animals, etc., which are found in sewage. The quantity of material removed is variable, and is usually small.

Medium-size screens with openings from ¼ inch to 1½ inches are used to prepare sewage for passage through reciprocating pumps, complex dosing apparatus, contact beds, and sand filters. The amount of material removed varies from 0.5 to 10 cubic feet per million gallons of sewage treated, dependent on the character of the sewage and the size of the screen. Screenings before drying contain 75 to 90 per cent moisture and weigh 40 to 50 pounds per cubic foot. At times the amount removed may vary widely from the limits stated. Schaetzle and Davis^[138] state:

Screenings differ greatly both in amount and character.... The amount varies with the days of the week as well as during the course of the day. It reaches its maximum about noon or shortly before and commences to disappear about midnight, reaching a minimum about 4 or 5 a.m. The material is almost wholly organic and consists of scraps of vegetables or fruit, cloth, hair, wood, paper and lumps of fecal matter. The amount varies so widely that it is impossible to state just what to expect any definite size screen to remove. The amount of water contained is small compared with that in the sludge in sedimentation basins and amounts to from 70 per cent to 80 per cent. On account of its organic origin it is highly putrescible.

Medium-size screens are sometimes placed close together with the bars of the one opposite the openings in the other, thus approaching a fine screen.

Fine screens vary in size of opening from ¼ inch to 50 openings per linear inch or 2,500 per square inch. They are used for removing solids preparatory to disposal by dilution, to protect sprinkling filters, complex dosing apparatus, sand filters, sewage farms, and to prevent the formation of scum in subsequent tank treatment. In general, fine screens will remove from 0.1 to 1 cubic yard of wet material per million gallons of sewage treated. The wet screenings will contain about 75 per cent moisture and will weigh about 60 pounds per cubic foot. The dry weight of the screenings will therefore be about 10 to 400 pounds per million gallons of sewage treated. The effect of the removal of this amount of material is usually not detectable by methods of chemical analysis, the amount of suspended matter before and after screening being found unchanged.

In his conclusions on the discussion of the results to be expected from fine screens, Allen states:^[139]

With openings not more than 0.1 inch in size, fine screening should remove at least 30 per cent of the suspended solids and 20 per cent of the suspended organic solids from ordinary domestic sewage, or 0.1 cubic yard of screenings, containing 75 per cent water per thousand population daily.

The effect of the use of different size openings under the same conditions is shown in Fig. 153.^[140] Some data covering the amount of material removed by screening are given in Table 76. More extensive data are given in Volume III of “American Sewerage Practice” by Metcalf and Eddy.

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.

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.

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.

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.

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.

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.

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.

Fig. 157.—Diagram for the Computation of Economical Basin Dimensions.