Trickling, Percolating or Sprinkling Filters
Trickling or percolating filters consist of beds of coarse grained material such as pebbles or crushed stone, one-eighth to four inches in size, from four feet to ten feet deep and well underdrained. The character and strength of the sewage should determine the size of the material, the depth of the bed and the rate of operation. Some engineers give the capacity as about 20,000 persons per acre of stone surface; others say the rate of flow should be from one to two and one-half million gallons per acre. In some designs an auxiliary air supply is inducted into the filter material by tubes connected with the underground system. The Atlanta plant is equipped with ventilator hoods having weather vanes so that the mouth of each hood always points toward the wind. “This form of ventilation is of no particular value and may be detrimental in cold weather,” says Glenn D. Holmes, Chief Engineer of the Syracuse, N. Y., Sewer Board. By means of spray jets and moving sprinklers operated with some device for varying the pressure, such as a butterfly valve, or by means of an intermittent dosing tank operated by a siphon, the sewage is sprinkled or deposited on the surface of the bed in thin films and drops; thus the sewage is freed of objectionable gases and takes up oxygen as it passes through the air and through the filter. Sprinkling filters do not produce the best results when crude sewage is applied. They are most efficient when the suspended matter has been removed by some preparatory treatment. In some cities the screening process is first used, in others the sewage receives a preliminary treatment in tanks. Well designed and efficiently operated filters of this kind produce an effluent that is stable but not clear. Some plants are equipped with secondary settling tanks through which the effluent flows before final discharge and is freed of the humus-like particles it contains after leaving the filter. Reports agree that the effluent is not nearly so good in appearance and has a much higher percentage of bacteria than that produced by good intermittent sand filters. As compared with the double contact process the general opinion is that sprinkling filters are superior in respect to the removal of organic matter and cost less to operate. The chief advantages of a sprinkling filter are the high rate of filtration and the low cost of operation. The disadvantages are a possible nuisance, especially during hot weather, from odor when anything but fresh tank sewage is sprayed; and the development of insect life. Fowler says, “However economical their construction and maintenance it cannot be said that such a process meets all sanitary and æsthetic requirements.” The experience of Worcester, Massachusetts, at its experimental station was that more than twenty times as much sewage per unit of area was treated by the sprinkler filter as could be treated by intermittent sand filtration, and more than ten times as much per cubic yard of filter. Four times as much sewage was treated by these experimental filters as could be treated satisfactorily by experimental contact beds. In order to obtain equal nitrification with contact beds at least three contacts would be required.