The operation of devices for automatically controlling the operation of a trickling filter is explained in Chapter XXI.
Fig. 174.—Types of False Bottoms for Trickling Filters.
Eng. News, Vol. 74, p. 5.
258. Intermittent Sand Filter.—An intermittent sand filter is a specially prepared bed of sand, or other fine grained material, on the surface of which sewage is applied intermittently, and from which the sewage is removed by a system of underdrains. It differs from broad irrigation in the character of the material, the care and preparation of the bed, and the thoroughness of the underdrainage. A distinctive feature of the intermittent sand filter is the quality of the effluent delivered by it. In a properly designed and operated plant the effluent is clear, colorless, odorless, and sparkling. It is completely nitrified, is stable and contains a high percentage of dissolved oxygen. It contains no settleable solids except at widely separated periods when a small quantity may appear in the effluent. The percentage removal of bacteria may be from 98 to 99 per cent. Some analyses of sand filter effluents are given in Table 89. The dissolved solids, the remaining bacteria, and the antecedents of the effluent are the only differences between it and potable water. An effluent from an intermittent sand filter is the most highly purified effluent delivered by any form of sewage treatment. The effluent can be disposed of without dilution, on account of its high stability. The treatment of sewage to so high a degree is seldom required, so that the use of intermittent filters is not common. Other drawbacks to their use are the relatively large area of land necessary and the difficulty of obtaining good filter sand in all localities.
| TABLE 89 | |||||||
|---|---|---|---|---|---|---|---|
| Quality of Effluents from Sand Filters | |||||||
| (Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson, 1905) | |||||||
| Source of Sample | Parts per Million | Rate of Filtration Gallons per Acre, per Day | |||||
| Nitrogen as | Oxygen Consumed | Oxygen Dissolved | |||||
| Free Ammonia | Albuminoid Ammonia | Nitrites | Nitrates | ||||
| Filter influent from grit chamber | 11.0 | 8.6 | 59. | ||||
| Filter effluent | 1.12 | 0.88 | 0.08 | 11.5 | 6.9 | 6.3 | 0.081 |
| Filter effluent | 0.81 | 0.88 | 0.10 | 12.6 | 6.5 | 6.2 | 0.118 |
| Filter influent from plain settling tank | 9.7 | 5.4 | 33. | ||||
| Filter effluent | 0.62 | 0.77 | 0.11 | 14.9 | 6.0 | 8.2 | 0.139 |
| Filter effluent | 0.99 | 1.10 | 0.10 | 12.6 | 7.8 | 6.5 | 0.274 |
| Filter effluent | 2.61 | 1.39 | 0.09 | 9.0 | 9.7 | 3.9 | 0.357 |
| Filter influent from septic tank | 10.7 | 5.6 | 38. | ||||
| Filter effluent | 1.63 | 1.16 | 0.09 | 11.2 | 8.0 | 5.8 | 0.357 |
| Filter influent from coke strainer | 13.4 | 4.7 | 40. | ||||
| Filter effluent | 2.24 | 1.35 | 1.03 | 14.6 | 10.1 | 6.9 | 0.372 |
| Filter influent from contact bed | 8.6 | 3.6 | 0.19 | 1.6 | 24. | 0.3 | |
| Filter effluent | 2.62 | 1.35 | 0.31 | 8.1 | 8.3 | 5.8 | 0.516 |
| Filter effluent | 2.44 | 2.41 | 0.16 | 9.4 | 12.5 | 5.0 | 0.525 |
| Filter effluent | 3.40 | 1.15 | 0.20 | 10.9 | 9.7 | 5.2 | 0.525 |
| Filter influent from sprinkling filter after sedimentation | 9.0 | 4.8 | 0.42 | 1.3 | 27. | 3.4 | |
| Filter effluent | 2.95 | 1.25 | 0.19 | 7.0 | 8.8 | 3.8 | 0.675 |
| Filter effluent | 4.77 | 2.63 | 0.51 | 4.6 | 11.8 | 2.5 | 0.749 |
| Filter effluent | 3.47 | 1.61 | 0.31 | 7.2 | 11.9 | 3.7 | 1.129 |
The action in an intermittent sand filter is more complete than in other forms of filters because a greater surface is exposed to the passage of sewage by the fine sand particles, and the sewage is in contact with the filtering material a longer time due to the lower rate of filtration and the slow velocity of flow through the filter. It is essential that the sewage be applied to the bed intermittently in order that air shall be entrained in the filter. The period between doses should not be so long that the filter becomes dry.
In the operation of an intermittent sand filter one dose per day is considered an ordinary rate of application, although some plants operate with as many as four doses per day per filter, and others on one dose at long and irregular intervals. It is not always necessary to rest the filter for any length of time unless signs of overloading and clogging are shown. The intermittent dosing action may be obtained by the action of an automatic siphon as is described in Chapter XXI. The sewage is distributed on the beds through a number of openings in the sides of distributing troughs resting on the surface of the filter. The sewage is withdrawn from the bottom of the filter through a system of underdrains, into which it enters after its passage through the bed. There are no control devices on the outlet, as the rate of filtration is controlled by the action of the dosing apparatus and the rate at which sewage is delivered to it. The action of the dosing apparatus should respond quickly to variations in sewage flow. As the doses are applied to a sand filter, a mat of organic matter or bacterial zoöglea is formed on the surface of the bed. The mat is held together by hair, paper, and the tenacity of the materials. It may attain a thickness of ¼ to ½ an inch before it is necessary to remove it. So long as the filter is draining with sufficient rapidity this mat need not be removed, but if the bed shows signs of clogging, the only cleaning that may be necessary will be the rolling up of this dried mat. It is believed that the greater portion of the action in the filter occurs in the upper 5 to 8 inches of the bed, but occasionally the beds become so clogged that it is necessary to remove ¾ of an inch to 2 inches of sand in addition to the surface mat, or to loosen up the surface by shallow plowing or harrowing. The necessity for such treatment may indicate that the filter is being overloaded as a result of which the rate of filtration should be decreased or the preliminary treatment should be improved. The plowing of clogging material into the bed should be avoided as under these conditions the final condition of the bed will be worse than its condition when trouble was first observed.
In winter the surface of the bed should be plowed up into ridges and valleys. The freezing sewage forms a roof of ice which rests on the ridges and the subsequent applications of sewage find their way into the filter through the valleys under the ice. In a properly operated bed the filtering material will last indefinitely without change. If a filter is operated at too high a rate, however, although the quality of the effluent may be satisfactory, it will be necessary at some time to remove the sand and restore the filter.
The rate of filtration depends on the character of the influent, the desired quality of the effluent, and the depth and character of the filtering material. Filters can be found operating at rates of 50,000 gallons per acre per day and others at eight times this rate. For sewage which has had some preliminary treatment, the rate should not exceed 100,000 gallons per acre per day, whereas the rate for raw sewage should be less than this. For rough estimates made without tests of the sewage in question, the rate should not be taken at more than 1,000 persons per acre. If the preliminary treatment of the sewage has been thorough and the material of the sand filter is coarser than ordinary the rate of filtration can be high. For less careful preliminary treatment and fine filtering material the rates must be reduced. The sewage must undergo sufficient preliminary treatment to remove large particles of solid matter which would otherwise clog the dosing apparatus and the filter. This treatment should include grit removal, screening, and some form of tank treatment. Some plants have operated successfully with a stale sewage and no preliminary treatment, as at Brockton, Mass. Septic tank effluent can be treated successfully on an intermittent sand filter, but not so satisfactorily as the effluent from a tank delivering a fresh sewage.
The material of the filter should consist of clean, sharp, quartz or silica sand with an effective size[[164]] of 0.2 to 0.4 mm., preferably about 0.25 to 0.35 mm., and a uniformity coefficient[[165]] of 2 to 4. Within the limits mentioned no careful attention need be given to the size of the material. Natural sand found in place has been underdrained and used successfully for sewage treatment. The size of the sand is fixed by the rate of filtration rather than the bacteriological action of the filter. A coarse sand will permit the sewage to pass through the bed too rapidly, and a fine sand will hold it too long or will become clogged. The same size of material should be used throughout the bed, except that a layer of gravel from 6 to 12 inches thick, graded from very small sizes to stones just passing a 2–inch ring should be placed at the bottom to facilitate the drainage of the bed.