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work of the contact filter is carried on during the period of “resting empty,” that is, after the effluent has been withdrawn from the bed.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 Then, when the liquid portion is withdrawn from the bed, air is drawn in between the stones, enabling the nitrifying or aërobic bacteria to do their work of breaking down both the suspended and the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.
PLAN OF SEWAGE DISPOSAL WORKS
FOR
MR. CHARLES L. A. WHITNEY
ALBANY, N.Y.
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 Served by Sewer. | No. of Beds. | Area of Each Bed (Square Feet). | Mean Width and Length of Siphon Chamber (Feet). | Diameter of Siphons (Inches). | Distance from Roof of Settling Tank to Top of Wall 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.