Straining or Roughing
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.