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.
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.