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.
Fig. 37.—View of Sand-filter Beds for Village in Massachusetts.
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 Served by Sewer. | No. of Beds. | Area of Each Bed (Square Feet). | Mean Width and Length of Siphon Chamber (Feet). | Diameter of Siphon (Inches). | Distance from Roof of Settling 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 |