SEWAGE TREATMENT — KEEFER 365 



chemical oxygen demand, is the amount of oxygen required for the 

 biochemical oxidation of the decomposible matter at a given tempera- 

 ture within a given time. 



SEWAGE TREATMENT PROCESSES 



The treatment of sewage can be accomplished in a variety of ways. 

 The type of treatment selected depends upon a number of factors such 

 as the relationship between the volume of sewage to be treated and the 

 minimum flow in the watercourse into which the sewage discharges, 

 the use to which the watercourse is put, the cost, and the land available 

 for a sewage-treatment plant and the nearness of built-up areas to the 

 plant site. A low flow of sewage from a small community, discharging 

 into a large watercourse, may require little if any treatment, whereas 

 a large flow discharging into a small watercourse will require a high 

 degree of treatment. The use to which the watercourse is put will 

 have a decided bearing on the type of treatment. Where there are 

 shellfish areas in the watercourse or where it is used for recreational 

 purposes such as swimming or bathing or is a source of water supply, 

 a high degree of treatment is usually necessary. Where land is costly 

 or where expensive foundations are required to support the sewage- 

 treatment structures, it may be desirable to select those types of treat- 

 ment units that occupy a small area of ground. The nearness of sew- 

 age plants to residential or business areas plays an important role in 

 the type of treatment selected. Where dwellings or business establish- 

 ments are close to a proposed sewage-treatment plant, that type of 

 treatment should be adopted which will be relatively free of odors and 

 other nuisances. 



SEWAGE SCREENS 



Practically every sewage plant is provided with a sewage screen. 

 The chief function of screens is to remove large suspended solids that 

 may clog pumps and small pipes. Screens can be classified into two 

 types : coarse and fine. Coarse screens are made of parallel steel or 

 wrought-iron bars with clear openings of about half an inch or more, 

 whereas fine screens have openings well under half an inch. 



Coarse screens can be either manually or mechanically cleaned. 

 They are generally placed (pi. 1, fig. 1) in a chamber or channel at 

 right angles to the flow of sewage in an inclined position to facilitate 

 ease of cleaning. Suspended solids in the sewage are caught on the 

 upstream surface of the screen and are raked off manually or 

 mechanically. 



Various types of fine screen are in use. One type consists of a drum 

 made of bronze plates, containing perforations varying from about 

 one-sixteenth to one-eighth inch in width. The drum (see pi. 6), 

 which is rotated by an electric motor, is partially submerged with its 



