Sewage disposal . 328 TREATMENT IN TRICKLING OR PERCOLATING BEDS and 36 inches in diameter. It is well to provide that under-drains should converge toward the center of the filter for econ-omy of head and where possible they should pass out throughthe walls at their upper ends to allow for flushing. At Fitchburg, Mass., Metcalf and Eddy laid out the filterfloors with shallow drains across which were placed narrowcement beams (17 inches x 4 inches x If inches) spaced 1£ inchesapart, with cobbles above to support the filtering material. Special facilities for aeration are provided in many Englis
Sewage disposal . 328 TREATMENT IN TRICKLING OR PERCOLATING BEDS and 36 inches in diameter. It is well to provide that under-drains should converge toward the center of the filter for econ-omy of head and where possible they should pass out throughthe walls at their upper ends to allow for flushing. At Fitchburg, Mass., Metcalf and Eddy laid out the filterfloors with shallow drains across which were placed narrowcement beams (17 inches x 4 inches x If inches) spaced 1£ inchesapart, with cobbles above to support the filtering material. Special facilities for aeration are provided in many English. Fig 92. Whittaker-Bryant Filter at Accrington (courtesy ofMr. Whittaker). filters by open construction of the walls. The walls of the Ducatfilter were built of open drainpipe inclined upward and connectedwith aerating drains at intervals in the body of the bed. TheWhittaker-Bryant filters at Accrington and elsewhere are octag-onal in shape, with walls of open brick and central open brickwork aerating wells (Fig. 92). The trickling filters at Atlanta have been provided with 8-inch cast-iron pipe risers (72 per acre) connected below withthe underdrains and terminating above the filter material inrevolving cowls. The necessity for such provision is somewhat FILLING MATERIAL 329 doubtful if free underdrainage is provided. In the treatment offresh sewage, as at Reading, it has been found that the sewageas it enters the filter after discharge from sprinkler nozzles is60-80 per cent saturated with oxygen. Rudolph Hering (1909)however believes that the open-side filters in England and thespec
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