Medium-size screens with openings from ¼ inch to 1½ inches are used to prepare sewage for passage through reciprocating pumps, complex dosing apparatus, contact beds, and sand filters. The amount of material removed varies from 0.5 to 10 cubic feet per million gallons of sewage treated, dependent on the character of the sewage and the size of the screen. Screenings before drying contain 75 to 90 per cent moisture and weigh 40 to 50 pounds per cubic foot. At times the amount removed may vary widely from the limits stated. Schaetzle and Davis[[138]] state:
Screenings differ greatly both in amount and character.... The amount varies with the days of the week as well as during the course of the day. It reaches its maximum about noon or shortly before and commences to disappear about midnight, reaching a minimum about 4 or 5 a.m. The material is almost wholly organic and consists of scraps of vegetables or fruit, cloth, hair, wood, paper and lumps of fecal matter. The amount varies so widely that it is impossible to state just what to expect any definite size screen to remove. The amount of water contained is small compared with that in the sludge in sedimentation basins and amounts to from 70 per cent to 80 per cent. On account of its organic origin it is highly putrescible.
Medium-size screens are sometimes placed close together with the bars of the one opposite the openings in the other, thus approaching a fine screen.
Fine screens vary in size of opening from ¼ inch to 50 openings per linear inch or 2,500 per square inch. They are used for removing solids preparatory to disposal by dilution, to protect sprinkling filters, complex dosing apparatus, sand filters, sewage farms, and to prevent the formation of scum in subsequent tank treatment. In general, fine screens will remove from 0.1 to 1 cubic yard of wet material per million gallons of sewage treated. The wet screenings will contain about 75 per cent moisture and will weigh about 60 pounds per cubic foot. The dry weight of the screenings will therefore be about 10 to 400 pounds per million gallons of sewage treated. The effect of the removal of this amount of material is usually not detectable by methods of chemical analysis, the amount of suspended matter before and after screening being found unchanged.
In his conclusions on the discussion of the results to be expected from fine screens, Allen states:[[139]]
With openings not more than 0.1 inch in size, fine screening should remove at least 30 per cent of the suspended solids and 20 per cent of the suspended organic solids from ordinary domestic sewage, or 0.1 cubic yard of screenings, containing 75 per cent water per thousand population daily.
The effect of the use of different size openings under the same conditions is shown in Fig. 153.[[140]] Some data covering the amount of material removed by screening are given in Table 76. More extensive data are given in Volume III of “American Sewerage Practice” by Metcalf and Eddy.
| TABLE 76 | ||||||||
|---|---|---|---|---|---|---|---|---|
| Data on Screens | ||||||||
| (Trans. Am. Society Civil Engineers, Vol. 78, Page 942) | ||||||||
| Type of Screen | Location | Clear Opening, in Inches | Screenings | Per Cent Moisture | Horse-Power Per Screen | Cost of Operation Per Million Gallons, Dollars | Remarks | |
| Per Million Gallons, y = Cubic Yard t = Tons | Per 1000 Population Daily, y = Cubic Yard t = Tons | |||||||
| Band | Hamburg | 0.6 | 0.34y | 0.018y | 87 | 2.5 | Note 1 | |
| Göttingen | 0.4 | 0.35y | 0.026y | 2.0 | ||||
| Sutton | 0.375[[141]] | 0.6y | ||||||
| Chicago | 2.4–3.1t | 79 | Stock Yard | |||||
| Wing | Frankfort | 0.40 | 0.7y | 0.040y | 5.0 | Note 2 | ||
| Elberfeld | 0.40 | 1.15y | 0.053y | 75 | Note 3 | |||
| Stralsund | 0.20 | 0.079y | 4.5 | |||||
| Wiesbaden | 0.60 | 1.1y | 0.033y | hand power | 1.64 | Note 4 | ||
| Shovel vane | Strassburg | 0.10 | 1.6y | 0.043y | 89.3 | 3.35 | Note 5 | |
| Gleiwitz | 0.12 | 0.192y | 0.90 | |||||
| Temesvar | 0.12 | 0.9–1.7y | 0.067–.133y | 60–70 | small | |||
| Drum | Bromberg | 0.08 | 4.75t | 40–60 | 2.45 | Experimental | ||
| Mainz | Note 6 | 0.52y | 75 | 5.2–6.8 | 0.89–3.42 | |||
| Trier | 0.10 | 0.39–0.42y | 0.13y | 50–60 | 2.41 | Experimental | ||
| Osnabruck | 0.08 | 3.2–4.0y | 0.08–.10y | 9.00 | Note 7 | |||
| Weand | Reading, Pa. | 36[[141]] | 1.0y | 89.5 | 2.0 | 1.00± | ||
| Brockton | 36[[141]] | 1.4t | ||||||
| Reinsch Wurl | Dresden | 0.08 | 0.97t | 0.09y | 84 | 2.5 | .325–1.76 | |
Notes:—1. After removal of ½ this volume of grit. 2. After removal of 16 per cent by the grit chamber. 3. Including 0.6 cubic yard grit per million gallons. 4. After passing 1.6 inch bar screen. 5. After removal of 0.132 cubic yard grit and coarse screenings per 1000 population. 6. 0.12, 0.04–0.08. 7. Before removal of 0.4 cubic yard grit per million gallons.
