PROTECTION OF RIVER AND HARBOR WATERS 11 



suspended solids, the oxidation of unstable organic compounds and the 

 destruction of pathogenic bacteria. 



SCREENING OF SEWAGE 



The first problem in almost every ease is the elimination of the coarser 

 floating particles by some form of straining or screening. Sometimes this 

 is accomplished roughly by the use of coarse bar screens with bars half an 

 inch or an inch apart. In England and Germany finer screens of wire cloth 

 with meshes as close as a tenth or even a twenty-fifth of an inch have been 

 used. Such screens are frequently arranged to revolve like an endless belt, 

 so that a fresh area is constantly brought into action and the accumulated 

 screenings are carried upward and automatically brushed off into a trough. 



Where it is necessary to remove a larger proportion of suspended solids 

 than can be held back by screening, sedimentation is the next process called 

 into play. Screening alone is sufficient for all practical purposes in some 

 cases, so in others screening and sedimentation will produce an effluent 

 pure enough to be discharged into adjoining waters. As a preliminary to 

 the processes used for final purification, sedimentation almost always plays 

 a part. 



SEDIMENTATION 



The purifying action of a sedimentation tank depends on the physical 

 factors of velocity and time. If the dimensions of the tank are such that 

 the flow is reduced only to a rate of thirty feet per minute the heavy mineral 

 matter — gravel, sand and the like — will be removed but the finer organic 

 particles will not be affected. Such a small tank as this is known as a 

 detritus tank or grit chamber, and forms a part of practically all sewage 

 works, generally in intimate connection with the screening process. 



True sedimentation of organic solids requires a velocity as low as six 

 feet per minute, or less maintained for a period of several hours; and the 

 tanks used for such sedimentation are usually rectangular basins of con- 

 crete or masonry with a capacity of four to twelve hours flow of sewage. 

 In place of shallow basins of this pattern English engineers, notably at the 

 city of Birmingham, have obtained very satisfactory results by the use of 

 deep tanks with conical or pyramidal bottoms. The sewage enters near the 

 bottom and as it rises and spreads out in the conical section, progressively 

 diminishes in velocity and leaves its suspended solid matter behind, so that 

 the effluent flows off clear at the top. These deep tanks have the added 

 advantage that the heavy sludge can be drawn oft' by a valve at the bottom 



