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 (CaCO₃), 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 (FeSO₄) 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 gallons			4,781	5,317	8,893
Amount of sewage chemically treated, million gallons			3,650	3,574	7,300
Gallons of wet sludge per million gallons of sewage treated			4,450	4,185
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 gallons of sewage pumped			999 [^[148]]	762[^[147]]	534
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, 1904–1905			Aug.	Sept.	Oct.	Nov.	Dec.	Jan.	Feb.	March	April	May	June	Avg.
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 removal		18	28	23	15	16	31	16	17	6	–3	18	19
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 removal		–12	32	35	12	7	18	19	14	25	30	19	19
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 removal		–8	6	0	–6	3	–3	–7	–8	–21	–23	–5	–3

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 removal	6	6	10	5	6	6	5	8	16	12	12	8
	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 removal	11	13	20	15	12	18	14	18	31	16	11	16
	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 removal	4	5	8	2	5	4	3	5	11	1	12	6
Cubic yards wet sludge per million gallons:					0.10	1.24	1.09	1.17	0.65	0.63	0.57		1.34
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, cubic feet per day:										29	14	41	50

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 Gravity	Per Cent Moisture	Per Cent in Terms of Dry Matter				Cubic Yard per Million Gallons, Wet	Pounds per Million Gallons, Dry	Kind of Sludge	Reference
Place	Specific Gravity	Per Cent Moisture	Volatile	Fixed	Nitrogen	Fat	Cubic Yard per Million Gallons, Wet	Pounds per Million Gallons, Dry	Kind of Sludge	Reference
Mansfield, O.	1.11	80.8							Septic	1908 Report, State Board of Health
Chicago, Ill.	1.03	90	40	60	1.9	7.0	1.0	200	Septic
							1.5	300
Columbus, O.	1.09	83.3	4.4	16.7	0.25	0.94			Septic	G. A. Johnson 1905 Report
Atlanta, Ga.	1.02	87.1	39.1	60.9	1.25	6.11			Imhoff	Eng. Rec., V. 72, 1915, p. 4
Baltimore, Md.	1.02	91.9	66.2		2.45	4.02			Digestion Tank	Eng. News-Rec., V. 87, 1921, p. 98
Baltimore, Md.	1.02	92.4	62.7		2.75				Imhoff	do.
Baltimore, Md.		79.2	73.8		2.64	9.00			Raw Sludge	do.
Baltimore, Md.		92.4	58.0	3.19					Settling Basin	do.

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.

Fig. 159.—Septic Tank at Champaign, Illinois.

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 Recommended by State Board of Health	Number, Persons	Capacity, Gallons per Person	Period of Retention	Remarks
Wisconsin		30	24 hours
Ohio	4 to 10	50		Not less than 560 gallons
Kentucky			24 to 48 hours	Not more than 5 feet deep
Texas			24 hours
Illinois		45	24 hours
U.S. Dept. Agriculture.		40	24 hours	25 per cent additional
				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.

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.

Fig. 162.—Sketch of Radial Flow Imhoff Tank at Baltimore, Maryland.
Eng. Record, Vol. 70, p. 5.

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.

Fig. 164.—Plan and Section of Hydrolytic Tank at Luton, England.
Eng. News, Vol. 76, 1916, p. 194.

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.

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, Feet	Size of Material in Inches	Rate, Million Gallons per Acre per Day	Oxygen Consumed	Nitrogen as				Suspended Matter			Dissolved Oxygen
Filter	Depth, Feet	Size of Material in Inches	Rate, Million Gallons per Acre per Day	Oxygen Consumed	Organic	Free Ammonia	Nitrites	Nitrates	Total	Volatile	Fixed	Dissolved Oxygen
					Parts per Million

A	5	0.25–1.00	0.953	23	3.5	8.7	0.20	1.6	832	94	737	0.3
B	5	0.25–2.00	1.514	21	4.0	8.4	0.15	1.4	831	85	746	0.1
C	5	0.25–1.50	1.222	24	3.5	10.8	0.11	0.6	826	92	734	0.8
D	5	0.50–1.50	1.405	22	3.3	9.5	0.13	0.9	810	91	717	0.9

				Per Cent Removal of Constituents of Applied Sewage

A	5	0.25–1.00	0.953	48	49	10			73	70	76
B	5	0.25–2.00	1.514	52	40	11			80	77	83
C	5	0.25–1.50	1.222	47	31	12			70	70	70
D	5	0.50–1.50	1.405	46	37	19			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.

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 Fahrenheit	Parts per Million								Per Cent Saturation Dissolved Oxygen	Relative Stability
		Nitrogen as				Oxygen Consumed	Suspended Matter
		Organic	Free Ammonia	Nitrites	Nitrates	Oxygen Consumed	Total	Volatile	Fixed

Crude Sewage

1913
Maximum	77	15.6	21.8	0.1	3.0	100.0	371	154	163	47
Minimum	61	10.4	16.5	0.1	1.4	78.3	222	98	112	11
Average	70	12.8	18.8	0.1	2.2	90.6	285	126	138	28
1914 (7 months)
Maximum	74	16.0	33.4		2.3		431			48
Minimum	60	9.5	18.1		1.6		279			12
Average	66	13.4	27.1		2.0		351			30

Imhoff Effluent

1913
Maximum	78	13.2	21.9	0.2	3.1	68.0	90	50	41
Minimum	58	6.5	16.8	0.1	1.1	53.1	35	42	21
Average	68	9.0	20.0	0.2	2.1	60.1	68	46	33
1914 (7 months)
Maximum	77	10.3	30.3		2.0		73			48
Minimum	59	4.1	18.0		1.5		49			34
Average	65	7.7	25.9		1.8		65			43

Sprinkling Filter Effluent

1913
Maximum	79	5.6	14.2	0.8	11.3	32.1	60	31	28	76	99
Minimum	55	2.6	6.2	0.5	5.8	23.6	33	26	28	52	88
Average	66	3.8	9.9	0.7	8.2	28.2	49	28	28	64	89
1914 (7 months)
Maximum	77	8.5	20.7		11.2		106			79	99
Minimum	55	4.4	8.8		3.6		40			55	89
Average	63	5.7	15.2		7.2		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			Oxygen Consumed			Nitrites			Nitrates			Dissolved Oxygen			Per Cent Putrescible	Suspended Matter
																				Total			Volatile			Fixed
	Influent, Parts per Million	Effluent, Parts per Million	Per Cent Removed	Influent, Parts per Million	Effluent, Parts per Million	Per Cent Removed	Influent, Parts per Million	Effluent, Parts per Million	Per Cent Removed	Influent, Parts per Million	Effluent, Parts per Million	Per Cent Removed	Influent, Parts per Million	Effluent, Parts per Million	Per Cent Removed	Influent, Parts per Million	Effluent, Parts per Million	Per Cent Removed		Influent, Parts per Million	Effluent, Parts per Million	Per Cent Removed	Influent, Parts per Million	Effluent, Parts per Million	Per Cent Removed	Influent, Parts per Million	Effluent, Parts per Million	Per Cent Removed
1910
October	5.1	2.8	45	12.0	4.6	62	30	15	50		.90			7.8		0.0	8.5	∞	0	75	40	47	54	25	54	21	15	29
November	5.9	2.5	58	12.0	5.9	51	35	15	57		.76			5.9		0.0	8.1	∞	5	61	16	74	52	15	71	9	1	89
December	4.6	3.0	35	12.0	6.9	42	39	20	49	.07	.45	6.4	.15	2.6	17	2.0	8.4	4.2	35	85	40	53	60	26	57	25	14	44

1911
January	6.3	4.8	24	11.0	7.0	36	42	20	52	.08	.15	1.9	.27	2.2	8.2	3.0	7.8	2.9	38	112	43	63	68	29	57	44	13	70
February	9.0	4.8	47	10.0	7.2	28	46	20	56	.09	.15	1.7	.50	2.6	5.2	2.6	8.0	3.1	29	100	49	51	64	32	50	37	17	53
March	8.3	3.5	58	9.9	6.4	35	47	21	56	.09	.15	1.7	.34	3.2	9.4	2.2	6.6	3.0	28	106	37	65	63	22	65	43	15	65
April	6.4	4.0	37	8.3	3.6	69	38	21	45	.16	.21	1.3	.53	4.5	8.5	2.1	7.1	3.4	9	113	68	40	59	35	41	54	33	39
May	7.6	5.4	29	9.2	2.4	74	33	31	6	.08	.38	4.8	.15	7.5	4.3	0.1	7.7	77	6	88	150	1.7	54	70	1.3	34	80	2.4
June	5.9	3.2	46	11.0	0.6	95	28	16	43	.00	.30	∞	.16	8.3	5.2	0.0	7.6	∞	1	92	77	18	56	36	36	36	41	1.1
July	6.2	4.2	32	11.0	1.3	88	34	26	24	.00	.36	∞	.09	7.7	8.0	0.0	6.5	∞	4	155	130	16	74	61	18	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.

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.

Fig. 169.—Sprinkling Filter Nozzles.
Bulletin No. 3, Engineering Experiment Station, Purdue University.

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√(2gh);

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.

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 15.3m × 13.2n equals 43,560 or mn equals 215.

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;
	q₁	= the rate of discharge of the siphon in cubic feet per second;
	q₂	= the rate of inflow to the tank in cubic feet per second;
	q	= the rate of emptying the tank in cubic feet per second = (q₁ − q₂);
	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;
	h₁	= the initial head of water, in feet, on the siphon;
	h₂	= the final head of water in feet, on the siphon;
	t	= the time, in seconds, required to empty the tank,

then	dQ = -Adh = q₁dt − q₂dt,

and	dt = dQ q = − Adh q₁ − q₂,

but	q₁ = 0.4 A √(2gh),[^[163]]

therefore

but	A = 4h² + 4bh + b²,

therefore

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.

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.

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 Sample	Parts per Million						Rate of Filtration Gallons per Acre, per Day
	Nitrogen as				Oxygen Consumed	Oxygen Dissolved
	Free Ammonia	Albuminoid Ammonia	Nitrites	Nitrates	Oxygen Consumed	Oxygen Dissolved
Filter influent from grit chamber	11.0	8.6			59.
Filter effluent	1.12	0.88	0.08	11.5	6.9	6.3	0.081
Filter effluent	0.81	0.88	0.10	12.6	6.5	6.2	0.118
Filter influent from plain settling tank	9.7	5.4			33.
Filter effluent	0.62	0.77	0.11	14.9	6.0	8.2	0.139
Filter effluent	0.99	1.10	0.10	12.6	7.8	6.5	0.274
Filter effluent	2.61	1.39	0.09	9.0	9.7	3.9	0.357
Filter influent from septic tank	10.7	5.6			38.
Filter effluent	1.63	1.16	0.09	11.2	8.0	5.8	0.357
Filter influent from coke strainer	13.4	4.7			40.
Filter effluent	2.24	1.35	1.03	14.6	10.1	6.9	0.372
Filter influent from contact bed	8.6	3.6	0.19	1.6	24.	0.3
Filter effluent	2.62	1.35	0.31	8.1	8.3	5.8	0.516
Filter effluent	2.44	2.41	0.16	9.4	12.5	5.0	0.525
Filter effluent	3.40	1.15	0.20	10.9	9.7	5.2	0.525
Filter influent from sprinkling filter after sedimentation	9.0	4.8	0.42	1.3	27.	3.4
Filter effluent	2.95	1.25	0.19	7.0	8.8	3.8	0.675
Filter effluent	4.77	2.63	0.51	4.6	11.8	2.5	0.749
Filter effluent	3.47	1.61	0.31	7.2	11.9	3.7	1.129

The action in an intermittent sand filter is more complete than in other forms of filters because a greater surface is exposed to the passage of sewage by the fine sand particles, and the sewage is in contact with the filtering material a longer time due to the lower rate of filtration and the slow velocity of flow through the filter. It is essential that the sewage be applied to the bed intermittently in order that air shall be entrained in the filter. The period between doses should not be so long that the filter becomes dry.

In the operation of an intermittent sand filter one dose per day is considered an ordinary rate of application, although some plants operate with as many as four doses per day per filter, and others on one dose at long and irregular intervals. It is not always necessary to rest the filter for any length of time unless signs of overloading and clogging are shown. The intermittent dosing action may be obtained by the action of an automatic siphon as is described in Chapter XXI. The sewage is distributed on the beds through a number of openings in the sides of distributing troughs resting on the surface of the filter. The sewage is withdrawn from the bottom of the filter through a system of underdrains, into which it enters after its passage through the bed. There are no control devices on the outlet, as the rate of filtration is controlled by the action of the dosing apparatus and the rate at which sewage is delivered to it. The action of the dosing apparatus should respond quickly to variations in sewage flow. As the doses are applied to a sand filter, a mat of organic matter or bacterial zoöglea is formed on the surface of the bed. The mat is held together by hair, paper, and the tenacity of the materials. It may attain a thickness of ¼ to ½ an inch before it is necessary to remove it. So long as the filter is draining with sufficient rapidity this mat need not be removed, but if the bed shows signs of clogging, the only cleaning that may be necessary will be the rolling up of this dried mat. It is believed that the greater portion of the action in the filter occurs in the upper 5 to 8 inches of the bed, but occasionally the beds become so clogged that it is necessary to remove ¾ of an inch to 2 inches of sand in addition to the surface mat, or to loosen up the surface by shallow plowing or harrowing. The necessity for such treatment may indicate that the filter is being overloaded as a result of which the rate of filtration should be decreased or the preliminary treatment should be improved. The plowing of clogging material into the bed should be avoided as under these conditions the final condition of the bed will be worse than its condition when trouble was first observed.

In winter the surface of the bed should be plowed up into ridges and valleys. The freezing sewage forms a roof of ice which rests on the ridges and the subsequent applications of sewage find their way into the filter through the valleys under the ice. In a properly operated bed the filtering material will last indefinitely without change. If a filter is operated at too high a rate, however, although the quality of the effluent may be satisfactory, it will be necessary at some time to remove the sand and restore the filter.

The rate of filtration depends on the character of the influent, the desired quality of the effluent, and the depth and character of the filtering material. Filters can be found operating at rates of 50,000 gallons per acre per day and others at eight times this rate. For sewage which has had some preliminary treatment, the rate should not exceed 100,000 gallons per acre per day, whereas the rate for raw sewage should be less than this. For rough estimates made without tests of the sewage in question, the rate should not be taken at more than 1,000 persons per acre. If the preliminary treatment of the sewage has been thorough and the material of the sand filter is coarser than ordinary the rate of filtration can be high. For less careful preliminary treatment and fine filtering material the rates must be reduced. The sewage must undergo sufficient preliminary treatment to remove large particles of solid matter which would otherwise clog the dosing apparatus and the filter. This treatment should include grit removal, screening, and some form of tank treatment. Some plants have operated successfully with a stale sewage and no preliminary treatment, as at Brockton, Mass. Septic tank effluent can be treated successfully on an intermittent sand filter, but not so satisfactorily as the effluent from a tank delivering a fresh sewage.

The material of the filter should consist of clean, sharp, quartz or silica sand with an effective size [^[164]] of 0.2 to 0.4 mm., preferably about 0.25 to 0.35 mm., and a uniformity coefficient [^[165]] of 2 to 4. Within the limits mentioned no careful attention need be given to the size of the material. Natural sand found in place has been underdrained and used successfully for sewage treatment. The size of the sand is fixed by the rate of filtration rather than the bacteriological action of the filter. A coarse sand will permit the sewage to pass through the bed too rapidly, and a fine sand will hold it too long or will become clogged. The same size of material should be used throughout the bed, except that a layer of gravel from 6 to 12 inches thick, graded from very small sizes to stones just passing a 2–inch ring should be placed at the bottom to facilitate the drainage of the bed.

The thickness of the sand layer should not be less than 30 inches to insure complete treatment of the sewage. In shallower beds the sewage might trickle through without adequate treatment. Beds are ordinarily made from 30 to 36 inches deep, but when deeper layers of sand are found in place there is no set limit to the depth which may be used. The shape and overall dimensions of the bed should conform to the topography of the site and the rate of filtration adopted. A plan and cross-section of an intermittent sand filter showing the distribution and under drainage systems are given in Fig. 166 and 175.

The distribution system consists of a system of troughs on the surface of the filter, laid out in a branching form, as shown in the figure. The openings in the troughs should be so located that the maximum distance from any point on the bed to the nearest opening should not exceed 20 to 30 feet. If the filters are small enough, troughs need not be used, the sewage being distributed from one corner, or from mid-points on the sides. Where troughs are used they should be supported from the bottom of the filter in order to prevent uneven settling due to the washing of the sand. The openings in the troughs are made adjustable by swinging gates as shown in Fig. 176, or by other means so that after the filter is in operation the intensity of the dose on any portion of the filter can be changed. The troughs may be placed with their bottoms level with the surface of the sand and with sides of sufficient height to give the required gradient to the water surface, or they may be built up above the surface of the filter and given the required slope so that the surface of the flowing water is parallel to the bottom of the trough. In either case a splash plate should be placed at each opening, so that not less than 2 feet of the surface of the sand is protected in all directions from the opening. A stone or concrete slab 2 to 4 inches thick makes a satisfactory splash plate. Either wood or concrete may be used for the construction of the troughs. The former is less durable, but also less expensive in first cost. The capacity of the troughs may be computed by Kutter’s formula with the quantity to be carried equal to the maximum rate of discharge of the feeding siphon, with a reduction in size below each branch or outlet proportional to the amount which will be discharged above this point.

Fig. 175.—Plan and Section of an Intermittent Sand Filter Showing Central Location of Control House.

The operation of automatic devices for dosing the bed is explained in Chapter XXI. The dosing tank should have a capacity sufficient to cover the bed to a depth of about 1 to 3 inches at one dose, and the siphon should discharge at a rate of about one second-foot for each 5,000 square feet of filter area. A dose should disappear within 20 minutes to half an hour after it is applied to the filter. With the rate stated and four applications per day to a depth of 1 inch at each dose, the rate per acre per day will be 109,000 gallons.

Fig. 176.—Distributing Trough with Adjustable Openings.

The filtration of sewage through sand in a manner similar to the rapid sand filtration of water is being attempted at the Great Lakes Naval Training Station. No results of this treatment have been published and the practical success of the method has not been assured.

259. Cost of Filtration.—Only comparative figures can be given in stating the costs of filtration, as most data available are based on pre-war conditions, and are therefore unreliable for present conditions. The variations from the figures given may be very large but in general the relative costs have not changed. The figures given in Table 90 are suggestive of the relative costs of the different forms of filtration.

TABLE 90


Relative Costs of Different Methods of Sewage Treatment

Costs in Dollars per Million Gallons per Day
Form of Treatment	First Cost [^[166]]	Operation and Maintenance	Total
Coarse screens			0.20
Fine screens			3.00
Plain sedimentation	7.00	3.00	10.00
Chemical precipitation			22.00 [^[167]]
Septic tank	7.00	1.00	8.00
Imhoff tank	10.00	1.00	11.00
Contact bed	8.00	2.00	10.00
Trickling filter	4.00	2.00	6.00
Intermittent sand filter	15.00	10.00	25.00
Activated sludge	6.50	8.50	15.00 [^[168]]