If a sewage disposal plant is operated in connection with a combined sewerage system grit chambers are usually necessary for the removal of sand, gravel and dirt before the sewage passes on for further treatment. Where a city has a separate system of sewerage grit chambers are held by some authorities to be unnecessary unless the first wash of the street after a storm is intercepted and the waste is treated. Gregory has expressed the belief that the safest plan under ordinary conditions seems to be to provide a grit chamber. It is generally agreed that the chambers should be so constructed that the sewage will flow through slowly enough for the grit to settle out, but fast enough to carry the organic matter in suspension. To insure proper operation the chamber must be cleaned out frequently. At the Cleveland Sewage Testing Station it has been found that velocities ranging from 30 to 60 feet per minute produce a grit of proper character. The California State Board of Health has advocated chambers with a capacity such that a net period of storage of at least three minutes be allowed and a velocity of not less than five feet per minute.
There are few cities which treat their sewage by the process of straining and roughing. This consists of removing the suspended matter by means of rapid straining through beds of coke or sand arranged like the rapid sand or mechanical water filter. Coke beds, especially in cold climates, have not been a success. The chief objection to the rapid sand filter is the wash water which contains much organic or mineral impurities of the sewage and which requires special treatment which experience has shown to be difficult and expensive. Difficulty has also been found in disposing of the sludge deposited upon the filter surface. Of this process the bulletin of the California State Board of Health says: “The process is an expensive one at best, both as respects construction and operation. The effluent from such works can be made fully equal to, if not better than the effluent of plain sedimentation basins from a sanitary point of view.” The experience of the Cleveland Testing Station with these filters was not favorable. The filters when operated at rates from 30 to 60 gallons per acre per 24 hours removed from 25 to 40 per cent. of suspended matter and their action was simply mechanical, there being no increase in the dissolved oxygen content. The report from the station says that the difficulties encountered in their operation were sufficient to eliminate the process as a method in itself or in combination with other processes.