SEWAGE TREATMENT — KEEFER 373 



Although it is possible to apply untreated sewage to intermittent sand 

 filters and produce a good effluent, the more general practice is to 

 give the sewage preliminary treatment by coarse screens and sedi- 

 mentation tanks. Intermittent sand filters are capable of removing 

 as much as 95 percent of the organic and suspended solids from 

 sewage with the production of an excellent effluent. The disad- 

 vantages of intermittent sand filters are that they require considerable 

 areas of land where large volumes of sewage must be treated, and 

 they produce disagreeable odors. One of the last large intermittent 

 sand Alteration plants in this country was in Worcester, Mass., where 

 74.3 acres of filter beds treated an average flow of over 4 million gal- 

 lons a day. They were superseded by trickling filters in 1925. 



TRICKLING FILTERS 



Trickling filters for many years have been and still are one of the 

 most important treatment facilities for oxidizing sewage. A trickling 

 filter consists of an artificial bed of durable material such as gravel, 

 crushed stone, or slag, on which sewage in the form of a spray is 

 intermittently or continuously applied. The sewage trickles down 

 over the surfaces of the stones and is collected in underdrains in the 

 filter bottom, from which it discharges for subsequent treatment. 

 The filtering medium usually varies in size from about 1 to 3 or 3^ 

 inches. Fine-grained material will produce a better effluent than a 

 coarse-grained medium. On the other hand filters with fine-grained 

 material are more liable to become clogged with sewage solids. The 

 depth of trickling filters varies from about 3 to 10 feet. 



Several ways of applying sewage to the surface of filters are avail- 

 able. One much-used method consists of distributing the sewage 

 through a network of pipes laid on or beneath the surface. Projecting 

 vertically upward from these pipes are equally spaced pipes, which 

 are usually 3 or 4 inches in diameter and which terminate some 2 or 3 

 feet above the filter. At the top of each vertical pipe there is a nozzle 

 through which the sewage discharges in a fine spray on the surface of 

 the stone. Various means are used to vary the pressure in these pipes 

 so that sewage is applied uniformly to the filter both close to and at a 

 distance from the nozzles. 



Sewage is also applied to trickling filters by means of rotary dis- 

 tributors (pi. 2, fig. 2) . This device consists of a vertical column, into 

 which the sewage enters at the bottom and is drawn upward. Attached 

 to this column are two or more horizontal pipes, which rotate about 

 the center of the column a few feet above the surface of the filter. 

 Sewage flows through these pipes and discharges through a series of 

 nozzles on the filter. The flow from the nozzles furnishes sufficient 

 impulse to rotate the distributor so that sewage is applied to the 

 entire surface of the filter. 



