452 BACTERIA PATHOGENIC TO MAN 



it is rare that such a pure effluent is required. Similar results may be 

 obtained by utilizing the septic tank method, running the sewage from 

 the septic tank to contact beds and thence to sand filter beds; where 

 because of the partial "self-purification of the sewage" in the septic tank 

 and contact beds 2,500,000 gallons of sewage can be filtered daily on 

 an acre of surface. In this process less land is required and both these 

 effluents can be safely turned into drinking-water streams. 



If, however, a merely non-putrescible effluent is required, one which, 

 though high bacterially, will not be offensive in any way, or subject to 

 further decomposition, it may be obtained by passing crude sewage 

 to septic tanks, thence to double contact beds, the resulting effluent 

 having merely an earthy, humus-like odor and being non-putrescible. 



Where acid wastes, tannery wastes, dyestuffs, etc., from various 

 factories enter into sewage, its disposal becomes a more complicated 

 problem and chemical precipitation by the use of lime or other chemi- 

 cals is generally employed for such sewage purification, which at best 

 is only partial and is sometimes supplemented by sand filtration. 



