CHAPTER V

SEWAGE FILTERS

It has been shown that the selection of the type of plant best suited to solve the sewage-disposal problem at any given place depends on several factors and can be safely made only after a consideration and study of such local conditions as the character of the soil, the area available, the presence and nearness to the surface of ground water, and the local topographical conditions. If sub-surface irrigation is not feasible, when, for instance, the soil is nearly impervious to water or when, in the case of a wet soil, adequate underdrainage is not possible, some form of artificial filter must be constructed to complete the reduction of the sewage where the effluent from the settling tank may not properly be discharged directly into a stream.

If such a filter is to be constructed, the kind most suitable depends, in turn, upon several factors, such as the degree of purification to be attained, the suitability of the available areas or locations for the different types of filters, the operating head or fall available, and the relative cost of the sand, gravel, broken stone, or furnace slag which may be used as material for the filter bed.

With respect to the degree of purification of sewage that is desired it may be said that, of the three general methods of sewage purification, namely, intermittent sand filtration, treatment in contact beds, and filtration through sprinkling or trickling filters, the first method produces the most highly purified effluent. Such an effluent, if from a properly constructed and operated sand filter, may generally be considered sufficiently purified to allow its discharge into a stream, even if the stream is subsequently used as a source of potable water supply. In some instances, however, subsequent sterilization or disinfection of the effluent may be required, particularly if the waterworks intake is relatively near the point of discharge from the sewage filter, or if the flow of the stream is small in comparison with the sewage flow. However, if a stream used as a source of potable water supply receives the effluent from a properly operated sand filter, the further safeguarding of the quality of the water should generally be accomplished by filtration or sterilization of the water supply, or both.

In many cases the local conditions are such that contact beds or sprinkling filters may be constructed more easily or more economically than sand filters and, at the same time, the lesser efficiency of the contact bed or the sprinkling filter, owing to the fact that the stream is not used for water supply, may not preclude the adoption of these latter types of plants.

However, where natural deposits of sand of not too finely divided particles occur or where such sand may be readily procured, intermittent sand filters are most satisfactory for the final treatment of sewage.

INTERMITTENT SAND FILTERS

The agencies employed in purifying sewage by intermittent sand filtration involve its oxidation, or nitrification by bacterial action, while the mechanical straining effected by its passage through the sand plays a very small part in its reduction.

Where natural deposits of sand of suitable quality occur, sand filters are constructed by levelling off definite areas of sand and making embankments eighteen inches high to enclose these areas, the embankments being generally formed of the surface loam and subsoil which must usually be removed in order to expose the sand layer. There should be from three to five beds prepared in order to provide for alternating the discharge of the effluent from the settling tank over different portions of the filtration area and thus to provide resting periods for each bed while in operation. Also, the preparation of several equal areas permits discontinuing the use of any single area for several days or a week at a time in order to allow it to dry out and permanently retain its filtering capacity. In Fig. 37 is shown a view of a set of sand-filter beds arranged in terraces on sloping ground, the embankments being formed by the material excavated to uncover the natural sand layer.

The proper number of beds and the area of each bed corresponding to the number of persons to be served by the sewer are given in Table IV. This table also gives the required dimensions of siphon chambers (assuming that this chamber forms a separate compartment of the settling tank) for the capacities necessary in order that the effluent may be distributed in proper quantities over each bed or over each pair of beds, as in the case of plants serving two hundred or more persons. The widths of siphon chambers given correspond in general with the widths of settling tanks given in Table I. As in Table III, the dimensions of siphon chambers given are based on a drawing down of the effluent in the tank when the siphons discharge, amounting to from four to eight inches. The last column in the table gives the space which should be left between the roof of the tank and the top of the dividing wall between the settling tank and the siphon chamber to provide for this draught upon the settling-tank contents. It will be seen that no draught upon the contents of the settling tank when the siphons discharge is arranged for in the case of tanks serving from four to twenty-five persons.

TABLE IV

FOR USE IN CONSTRUCTING INTERMITTENT SAND FILTERS ═════════╤═════════╤═════════╤════════════╤═════════╤═════════ Persons │ No. of │ Area of │ Mean Width │Diameter │Distance Served by│ Beds. │Each Bed │ and Length │of Siphon│from Roof Sewer. │ │ (Square │ of Siphon │(Inches).│ of │ │ Feet). │ Chamber │ │Settling │ │ │ (Feet). │ │ Tank to │ │ │ │ │ Top of │ │ │ │ │ Wall │ │ │ │ │ between │ │ │ │ │Settling │ │ │ │ │Tank and │ │ │ │ │ Siphon │ │ │ │ │ Chamber │ │ │ │ │(Inches). ─────────┼─────────┼─────────┼────────────┼─────────┼───────── 4│ 3│ 60│ 3 × 3 │ 3│ 12 8│ 3│ 120│ 3 × 5 │ 3│ 12 12│ 3│ 180│ 4 × 5 │ 3│ 12 15│ 3│ 224│ 4 × 6.5│ 3│ 12 25│ 3│ 350│ 4 × 6 │ 5│ 12 35│ 3│ 480│ 4.5 × 5 │ 5│ 16 50│ 3│ 660│ 5 × 6 │ 5│ 16 75│ 3│ 1000│ 6 × 7 │ 5│ 18 100│ 3│ 1320│ 7 × 8 │ 5│ 18 125│ 3│ 1660│ 5.5 × 8 │ 6│ 20 150│ 3│ 2000│ 8 × 8 │ 6│ 20 175│ 3│ 2330│ 8 × 9 │ 6│ 20 200│ 5│ 1600│ 8 × 12 │ 8│ 20 250│ 5│ 2000│ 10.5 × 12 │ 8│ 20 300│ 5│ 2400│ 12 × 13 │ 8│ 20 350│ 5│ 2800│ 13 × 14 │ 8│ 20 400│ 5│ 3200│ 13 × 17 │ 8│ 20 450│ 5│ 3600│ 13 × 19 │ 8│ 20 500│ 5│ 4000│ 13 × 21 │ 8│ 20 ─────────┴─────────┴─────────┴────────────┴─────────┴─────────

The siphons in each instance should be so placed that the lower edge of the bell of the siphon will be at a distance below the roof of the tank equal to twelve inches plus the drawing depth or discharging depth of a siphon of the diameter indicated. There should be three inches of space between the siphon bell and the floor of the chamber. The discharging depths of siphons as used in forming Tables 2, 3, 4, and 5 are as follows:

Diameter of Siphon. │ Discharging Depth. │ 3 inches│ 13 inches 5 inches│ 23 inches 6 inches│ 30 inches 8 inches│ 35 inches 10 inches│ 60 inches 12 inches│ 72 inches

If the siphons installed are larger or smaller than those shown in these tables, or if the particular make of siphon purchased has the same diameter but a different discharging depth, proper allowance must be made in proportioning the size of the dosing chamber.

In order to quickly convey the dose from the siphon chamber to the filter beds at the rate at which the siphon discharges, the sewer from the siphon chamber should be of proper size and should have a sufficient gradient. For instance, with a 3–inch siphon the sewer should be 6 inches in diameter, with a gradient or fall of at least 12 inches per 100 feet; with a 5–inch siphon, the sewer should be 8 inches in diameter, with a gradient of at least 6 inches in 100 feet; with a 6–inch siphon, the diameter of the sewer should be 8 inches, and should have a gradient of at least 12 inches per 100 feet, or 10 inches with a gradient of at least 3 inches per 100 feet; with an 8–inch siphon, 12 inches, with a gradient of at least 12 inches per 100 feet.

Sewage is sometimes applied directly to the beds without treatment in settling tanks, generally, in such cases, after having been screened to remove the larger suspended matters, but it is decidedly preferable in the case of the smaller plants under discussion to pass the sewage first through settling tanks, as in the method of sub-surface irrigation. Therefore, the areas of beds given in the table are for sewage which has been passed through settling tanks. It is even necessary, in the case of sand filters for institutions where considerable grease and soaps are contained in the sewage, to provide grease traps through which the sewage must pass before it reaches the settling tank. The effluent from the tank should be discharged intermittently by means of a dosing chamber and siphon and should be distributed quickly over the surface of the bed as uniformly as possible. This is generally accomplished in the case of the larger beds by laying on the surface of the bed, wooden troughs, with short branches, as shown in Figs. 38 and 39. A detail of a portion of these distributing troughs is given in Fig. 40. This view shows the hinged gates which are used to effect a proportionate division of the flow of the various branches of the main trough. The view also shows the slots in the sides of the troughs which allow the sewage to flow out onto the bed.

If the ground-water level is within three feet of the surface at any time, or if the sand is very fine and contains a slight proportion of clay, underdrains should be laid at depths of four feet to prevent the beds from becoming waterlogged.

Where sand deposits do not occur at a point suitable for the location of the disposal plant, but where sand may be procured at a reasonable cost, the beds may be formed artificially similar to the natural sand beds heretofore described, but should not be less than three feet deep. It is generally necessary in the case of artificially constructed sand filters to provide underdrains as described below.

Two views of such an artificial sand filter are shown by Figs. 41 and 42. In Fig. 41 the settling tank and siphon chamber may be seen, situated between two of the four beds composing the filter. In Fig. 42 is shown a nearer view of one of the beds with the distributing trough and its branches on the surface of the bed. This bed, of the four composing the filter, was not in operation at the time the photograph was taken.

In Fig. 38 is shown a sand filter layout with three beds. In this drawing are shown the sewer leading from the house, the settling tank, the siphon chamber, in which are placed two siphons, the effluent sewers, and the diverting manhole, from which three pipe lines convey the sewage to the filter beds. In Fig. 39 is shown also a view of the three filter beds, one of the beds being shown in section. Figs. 43 and 44 show a plan and view of the diverting manhole.

Where sand must be carted in to form the filters, the embankments to retain the sand should generally be formed by excavating for a depth of two feet the whole area upon which the beds are to be placed. The material thus excavated will usually be sufficient to form the embankments. The embankments should usually be at least two feet wide on top and should have side slopes of one and a half to one; that is, the bottom width of the embankment should be two feet plus three times the height. In clay soils the pits for the filter beds may be excavated with the sides vertical, or nearly so. The bottom of each bed, as it is prepared for the placing of the sand which is to compose the filter, should slope slightly from the sides toward the centre line of the bed.

Where the character of the underlying strata of soil or the presence of ground water requires that sand filters, whether natural or artificial, should be underdrained, this may be accomplished by laying a longitudinal main drain through the centre of the bed at a depth of at least three or four feet below the surface, with branches each way at intervals of about fifteen feet. The main underdrain should be six inches in diameter, of agricultural tile or of vitrified sewer pipe, laid with open joints, and should have a fall of at least six inches per hundred feet. The branches may be of three-inch agricultural tile.

In large installations for cities and villages it is usual to install either plural alternating siphons or apparatus known as sewage feeds, by means of which the contents of the dosing chamber are discharged upon the different beds in rotation, there generally being four or five beds constructed in each unit. This requires a separate siphon or sewage feed for each bed, and entails considerable expense. However, for smaller plants such as are now being considered, two ordinary siphons may be placed in the same dosing chamber as described in Chapter III, and so primed as to discharge alternately. Then, by means of a diverting manhole or chamber through which the dose must pass, the effluent may be diverted onto two beds in rotation, allowing a third bed to rest, or, if there are five beds, it may be diverted onto two pairs of beds in rotation, allowing a fifth bed to rest. For instance, in the case of five beds, a diverting manhole may be constructed as shown in Fig. 45, and arrangements may be made to couple bed No. 3 with No. 2 or No. 4, allowing bed No. 1 or No. 5 to rest by means of the stop-plank to cut off the flow to either of these beds, as shown in the illustration. Then, when bed No. 3 is to be rested, stop-planks _A_ and _B_ are both closed, and the stop-planks against all pipe outlets are raised. If it is desired to throw bed No. 1 out of use, the stop-plank is placed against the end of the pipe leading to this bed, stop-plank A is raised, and stop-plank _B_ is lowered. One siphon will then discharge onto beds Nos. 4 and 5, and with the next filling of the siphon chamber the second siphon will discharge onto beds Nos. 2 and 3. By a proper combination of the stop-plank positions, any two sets of two beds each may receive alternately the discharge from the siphon chamber while the remaining single bed may be left resting. The method for operating the beds in rotation described above may, of course, be easily applied when only three beds are constructed. A provision for allowing one bed to be thrown out of use for a week or so at a time is very necessary for the reasons stated above.

At intervals of several weeks it will be found necessary to break up the surface of each bed by raking or else to remove a thin coating of clogging material. This should be done after the bed has been rested and dried out, when the surface matting may be taken off without removing much sand. To provide for operating the beds in winter, in the late fall, before the ground has frozen, ridges and furrows should be formed on the surface of the beds, similar to those shown in Fig. 51. The furrows should be two or three feet apart and eight to twelve inches deep. Then when effluent is discharged onto the beds in freezing weather, as it fills the furrows, an ice roof will gradually form, spanning the furrows and protecting the sides and bottoms of the furrows from freezing, especially if a snowfall occurs before severe weather sets in. It will sometimes be found necessary, especially with small beds that are well underdrained, to provide board coverings for the furrows to take the place of the natural ice roofs.

The effluent from the tank should be discharged in such quantities as to flood the entire bed to a depth of from one to two inches, except that some of this effluent will immediately begin to seep into the bed.

Respecting the quality and relative fineness of sand suitable for sewage filters, it should be noted that certain empirical methods of measurement have been developed for use in comparing the size and uniformity of particles of various sands. These measures are (1) the “effective size,” and (2) the “uniformity coefficient.” The “effective size” is the size of sand particle expressed in millimetres compared to which ten per cent by weight of the particles in the sample is finer. The “uniformity coefficient” is the ratio of the size of grain which has sixty per cent of the sample finer than itself to the size which has ten per cent finer than itself.

Concerning the grades of sand through which sewage may be successfully and properly treated by intermittent filtration, it has been found that the “effective size” should not be less than .20 of a millimetre, nor greater than .50 of a millimetre, and the “uniformity coefficient” should generally be from 1.5 to 3.0, when sewage is applied at the usual rate. If, however, the sand is clean and sharp, but has an “effective size” somewhat smaller than the limit above stated, it may sometimes be found suitable.

In the case of any sewage-disposal project of considerable magnitude, where any doubt exists as to the suitability of the sand available for use in sand filters, analyses of representative samples of the sand should be arranged for, and competent engineering advice should be sought before any large outlay is incurred. In general, however, it may be said that any clean, sharp sand suitable for building use is suitable for sand-filter beds in any situation. Obviously, the coarseness of the sand plays no part in its suitability as a filtering medium if the sand occurs in a natural bed and underdrains are not necessary, since no question of the discharging of an unpurified effluent would ordinarily arise in such cases.

CONTACT BEDS

The treatment of sewage in contact beds consists in distributing the effluent from settling tanks over beds of broken stone, furnace slag, or other similar material contained in water-tight compartments and allowing the beds to fill so that the spaces between the filtering material will be filled with the sewage effluent. These beds are so arranged that the effluent is held in contact with the filtering material for a fixed interval of time and then, usually by means of special siphons called “timed siphons,” or other automatic devices, it is discharged from the beds onto sand filters for further treatment, or into streams, as the case may be.

The process involves, as in intermittent sand filtration, the nitrifying or oxidizing agencies of bacterial action, and differs from intermittent filtration and from treatment of sewage on sprinkling filters principally in the fact that the flow of effluent through the beds is arrested and the liquid sewage held in contact with the filtering material, as noted above.

Much smaller areas of filter beds are required than in the case of sand filters, and for this reason this form of filter will often be found preferable. The conditions which result in its selection are usually either the unsuitable character of the soil or the presence of ground water, making the installation of sub-surface irrigation systems impracticable; or the absence of sand deposits or the high cost in any locality of sand suitable for sand-filtration beds, making their construction difficult or expensive.

The walls and floor of a contact bed are generally constructed of concrete, and the filter should be rectangular in form, as it is easier to distribute the effluent uniformly over a bed of this shape. The details given in Chapter II for constructing the walls and floors of settling tanks will serve as a general guide in the construction of contact beds.

The real work of the contact filter is carried on during the period of “resting empty,” that is, after the effluent has been withdrawn from the bed. While the effluent fills the beds, much of the suspended solid matter, together with a large proportion of the bacteria contained in the sewage, adheres to a gelatinous film which has formed on the surfaces of the stones or other materials forming the beds. This interval of “resting full” should usually be about two hours. Then, when the liquid portion is withdrawn from the bed, air is drawn in between the stones, enabling the nitrifying or aërobic bacteria to do their work of breaking down both the suspended and the partially dissolved organic matters which have been contained in the sewage and which have adhered to the filter material. It is believed, that some oxidation of that portion of the organic matter which is in true solution is also accomplished when the effluent passes over the gelatinous covering of the stones by reason of the oxygen which has been absorbed by this covering.

The interval when the bed is “resting empty” should be considerably longer than the combined intervals when the bed is filling, “resting full,” and emptying. For this reason there should be a series of from three to five beds in order that it will not be necessary to turn the effluent from the settling tank continuously onto one bed, which would result in the clogging of this bed with suspended matters. The additional third (or fifth) bed also gives opportunity for allowing each bed in turn to be thrown out of use for intervals of a week or so at a time, which is also necessary to keep the beds up to their proper efficiency and obviate the necessity of cleaning or renewing the filter material oftener than once in seven or eight years.

In Fig. 46 is shown in plan and section a sewage-disposal plant for the residence of Mr. Charles L. A. Whitney, of Albany, N. Y., consisting of a settling tank, dosing chamber, and contact beds. This plant is designed to serve twenty-five persons, although the settling tanks have a capacity for double the amount of sewage on the usual basis of design.

The depth of filtering material in the beds should preferably be four or five feet, although, where operating head or fall is limited, this depth may be decreased to three feet. The floor of the bed should slope toward the outlet end at a rate of about one-eighth of an inch per foot.

Various materials are used to form the body of the filter, such as broken stone, coke, broken brick, and furnace slag, but the material used should not be such as will disintegrate readily, and for this reason broken limestone, from one-half inch to one and one-half inches in size, with perhaps two-inch stones for the bottom six inches of the bed surrounding the underdrains, is most suitable for small plants.

These underdrains should be constructed of horse-shoe tiling, and in the case of beds more than eight or ten feet wide should preferably be laid with short branches reaching from a main drain laid along the centre of the floor of the bed; or these drains may be laid in parallel lines, as shown in Fig. 46.

In order to alternate the discharge of effluent from the settling tank onto different beds in turn and to provide for more uniformly distributing the effluent over all portions of the bed, the settling-tank effluent should be collected as in the other methods of disposal described, in a siphon or dosing chamber, from which, by means of alternating siphons, it may be delivered to the proper bed.

In the case of a group of three beds or five beds, diverting chambers with stop-planks, similar to those described in connection with intermittent sand filters, may be provided to allow the throwing out of use of each of the beds in turn for a week or so at a time. In the smaller plants accommodating up to one hundred and fifty persons, it is hardly necessary to provide for more than three beds, thus allowing opportunity for each one to rest for one week in every three to six weeks, which will result in a temporary increase of fifty per cent in the rate of application of effluent to the remaining beds. In the case of the larger plants, especially if they are to be operated continuously, it is better to construct five beds so that two pairs of two beds each may be used alternately, leaving one bed, or twenty per cent of the total area, out of use. This will result in an increase of but twenty-five per cent in the rate of application of effluent to the four beds in use.

With the usual rates of operation for contact beds, one filling per day of the beds will result, and, if the dosing of the beds is carried on as above and as described in the portion of this chapter dealing with the dosing of intermittent sand filters, but two siphons in the dosing tank, constituting double alternating siphons, will be necessary. Such an arrangement will eliminate the necessity of installing plural alternating siphons consisting of three or more siphons, the cost of which is not warranted in connection with small plants, since the double alternating siphons will insure proper operation of the beds at much less cost. Of course, in the larger plants where two beds are dosed at each discharge of a siphon, a larger siphon chamber is necessary with the two siphons, but the extra cost of a larger siphon chamber would in most cases be more than offset by the increased cost of plural alternating siphons.

The main effluent carrier from the siphon chamber to each contact bed should discharge into a half-tile carrier, with branches, laid on the surface of the contact bed, as shown in Fig. 46.

Each contact bed should be provided at the outlet end with a “timed” siphon set in a separate chamber of two compartments, as shown in the drawing. The diameter of the timed siphons should generally be that of the next larger size than that indicated for the dosing-chamber siphons. As shown in the illustration, where only three beds are necessary, the third timed siphon may be dispensed with if arrangements are made to permit the use of one siphon for discharging either the middle or the outside bed on that side, and to permit the use of the other siphon for discharging either the middle bed or the bed on the other side. In such installations gates or valves must be placed on the outlets of the contact beds to prevent the filling of the bed that is out of use by back flow from the timed siphon chamber used to discharge the adjacent bed.

The cost of contact beds is considerably greater than the cost of intermittent sand filters, especially when sand of proper quality is available, but their construction is advised in many cases where sub-surface irrigation is not feasible, where the premises are subject to overflow or the ground-water level is high, and where it is not practicable to construct sand filters.

In the following table are given the proper number of units or beds for contact filters of different-sized installations, together with the required area of each filter, the depth of the filter medium in all beds being four feet. The table also shows the dimensions of the siphon chamber adjacent to the settling tank and the diameter of the siphons necessary to discharge the effluent in proper volumes onto each contact bed, or each pair of contact beds. Where it is necessary to decrease the depth of the contact beds to three and one-half or three feet, owing to lack of operating head or fall, a proportionate increase should be made in the area of each bed.

TABLE V

FOR USE IN CONSTRUCTING CONTACT BEDS ══════════╤══════════╤══════════╤═══════════╤══════════╤═══════════════ Persons │ No. of │ Area of │Mean Width │ Diameter │ Distance from Served by │ Beds. │ Each Bed │and Length │of Siphons│ Roof of Sewer. │ │ (Square │ of Siphon │(Inches). │ Settling Tank │ │ Feet). │ Chamber │ │to Top of Wall │ │ │ (Feet). │ │ between │ │ │ │ │ Settling Tank │ │ │ │ │ and Siphon │ │ │ │ │ Chamber │ │ │ │ │ (Inches). ──────────┼──────────┼──────────┼───────────┼──────────┼─────────────── 4│ 3│ 20│3 × 4.5 │ 5│ 12 8│ 3│ 40│4 × 6.5 │ 5│ 12 12│ 3│ 60│ 6 × 7 │ 5│ 12 15│ 3│ 70│ 6 × 8 │ 5│ 12 25│ 3│ 100│ 6 × 8 │ 6│ 16 35│ 3│ 130│ 7 × 9 │ 6│ 16 50│ 3│ 180│ 8.5 × 10 │ 6│ 16 75│ 3│ 280│ 10 × 11 │ 8│ 18 100│ 3│ 370│ 12 × 12 │ 8│ 18 125│ 3│ 460│ 12 × 14 │ 8│ 20 150│ 3│ 550│12 × 16.5│ 8│ 20 175│ 5│ 390│ 13 × 14 │ 10│ 20 200│ 5│ 440│ 14 × 15 │ 10│ 20 250│ 5│ 550│ 15 × 18 │ 10│ 20 300│ 5│ 660│ 17 × 19 │ 10│ 20 350│ 5│ 770│ 18 × 21 │ 10│ 20 400│ 5│ 880│ 18 × 24 │ 10│ 20 450│ 5│ 990│ 20 × 21 │ 12│ 20 500│ 5│ 1110│ 20 × 23 │ 12│ 20 ──────────┴──────────┴──────────┴───────────┴──────────┴───────────────

In the above table, as in the previous tables, in indicating the height to which the dividing wall between the settling tank and siphon chamber should be carried, allowance is made for a draught upon the contents of the settling tank at each discharge of a siphon of from four to eight inches. In the discussion relating to siphon chambers in connection with the description of intermittent sand filters will be found the necessary details as to the discharging depths of siphons of different diameters and the necessary depths of the siphon chambers in which such siphons are to be placed. The construction of contact beds will naturally be approached with hesitancy by property owners and others not familiar with such work, and it is strongly recommended that where it is possible the services of a sanitary engineer be engaged to design and supervise the construction of a plant involving any considerable outlay, unless it is felt that the descriptions and directions given above have afforded a clear understanding of the design and construction of this type of sewage-disposal works.

SPRINKLING FILTERS

One of the more recently developed methods of sewage disposal,—the sprinkling- or trickling-filter system,—has for its principal feature the thorough aëration of the settling-tank effluent before its passage through the filter. This filter, like the contact filter, is of the rapid, coarse-grained type, but in its operation resembles the process of intermittent sand filtration in that the sewage effluent passes through the filter continuously without being held in contact with the filtering material as in the contact bed. The aëration of the sewage effluent, which very greatly aids the final process of nitrification or oxidation in the filter, is accomplished by spraying the sewage effluent over the surface of the beds through a series of riser pipes with nozzles, or allowing it to fall in fine streams on dash plates which cause it to sprinkle over the beds and thus to absorb oxygen from the air.

Sprinkling filters produce an effluent with a considerably less degree of purification than sand filters, but may in general be said to produce a more stable effluent, that is, one less liable to subsequent putrefaction than the effluent from contact beds. Furthermore, with the usual depths of contact beds and sprinkling filters, an area approximately four times greater is required to treat the same amount of sewage on contact beds than is necessary if sprinkling filters are constructed. However, since the effluent from sprinkling filters is much more turbid, necessitating, in most cases, subsequent sedimentation before discharge; since considerably greater operating head or fall is necessary; and since their operation requires much more supervision, the construction of sprinkling filters is not generally as advisable as contact beds for small installations, especially in cold climates.

The construction of sprinkling filters as compared to contact filters differs principally in the size and depth of filtering material, in the means provided for distributing the effluent over the beds, and in the arrangements for draining the filter.

The depth of filtering material in a sprinkling filter is usually from five to ten feet, preferably not less than eight feet. The material for the filter is the same as that used for contact filters but the fragments should be from one to three inches in diameter. Instead of a system of tile underdrains on the floor of the filter, a false floor of perforated tile, rectangular in section, or of half tile, circular in section, with drainage holes cut out along the sides, should be laid over the entire floor of the filter. As the settling-tank effluent is sprayed on the filter it passes downward between the spaces of the filtering material, and reaching the floor of the filter is collected in a main drainage channel, through which it passes to the outfall sewer, and thence to the final settling tank or into the stream.

As with contact beds and sand filters, intermittency of application of the effluent to the filter is essential for proper action of the filter, and this is accomplished as in the other types of filters by means of automatic siphons placed in dosing tanks. These tanks, however, are of special design in the case of sprinkling filters. Each siphon discharges into a main carrier of iron pipe which extends sometimes over the surface of the filter, but generally along the floor of the filter. The main carrier has branch pipes of smaller diameter extending at right angles nearly to the sides of the filter. On these branches vertical riser pipes spaced about ten feet apart are connected, and these riser pipes extend a few inches above the surface of the filter. Nozzles are fitted to the ends of the riser pipes by means of which the sewage effluent, under pressure, is sprinkled, at short intervals, in the form of a fine, umbrella-shaped spray, over the surface of the filter. This results in a thorough aëration of the effluent before it reaches the filtering material, and makes this form of filter very effective.

In Fig. 47 is shown a view of a sprinkling filter at Danville, Pa., operating when the temperature was 14° below zero. It is generally believed, however, that in cold climates it is advisable to house small sprinkling filters.

Owing to the rather complicated hydraulic features and the somewhat difficult engineering principles involved in the construction and operation of sprinkling or trickling filters, it is not deemed advisable to attempt to describe them in sufficient detail to furnish directions for their construction. The design of a sprinkling-filter system for small as well as for large installations should always be entrusted to an engineer conversant with this line of sanitary engineering. It is believed, however, that the description of such filters given above will aid those who are about to install sewage-disposal plants in the selection of the type of plant best suited to their particular needs and conforming to the conditions peculiar to their situation.

Summarizing the foregoing descriptions and directions with reference to sewage filters, it may be stated that where natural facilities for disposing of sewage by simpler methods do not exist, the construction of a sewage filter of some one of the above described types offers a solution of every problem thus encountered. It is well to repeat that sub-surface irrigation, where feasible, should be adopted, and this will be the method indicated in a large majority of cases where small disposal plants are to be constructed.

The principal point to be remembered in connection with sewage filters is that their construction is but a good beginning, and that their proper operation is very necessary to the success of the undertaking. They constitute, with the developed bacteria in the filter, a rather sensitive mechanism capable of efficient work if properly handled, but each filter unit must be carefully operated and must be regularly given extended periods of rest for the restoration of the void or open-space capacity of the filter, and to provide for the necessary aëration of the filtering material.

The peculiar action which takes place in a filter in the reduction of sewage, and which is not even yet fully known, is best evidenced by the fact that sewage filters, especially of the coarse-grained type, do not attain their highest efficiency until after several weeks or months of operation.

With a knowledge of these points, it will be seen that there should be little divergence from accepted standards in the construction and operation of sewage filters if they are to prove satisfactory when installed.