SEWAGE TREATMENT — KEEFER 



371 



of anaerobic bacterial action. From time to time after the sludge has 

 digested, it is removed and disposed of. The septic tank was in- 

 vented by Donald Cameron in 1895 and was adopted by many large 

 cities. The objections to septic tanks are that they produce un- 

 pleasant odors and the effluent often becomes contaminated with sludge 

 solids, which, after decomposing on the bottom of the tank, rise to 

 the surface of the sewage and flow out with the effluent. They have 

 not been installed at medium- and large-size sewage plants for many 

 years. Their use has been restricted to treating the sewage from in- 

 dividual dwellings, small real-estate developments, schools, hospitals, 

 etc. 



IMHOFF TANKS 



An Imhoff tank (fig. 4) is a two-story tank, designed to remove 

 the suspended solids from sewage in an upper or sedimentation com- 

 partment and to provide space in a lower compartment for the di- 

 gestion and stabilization of the solids. The sedimentation compart- 

 ment is usually of such a size that the sewage takes 2 or 3 hours to 

 pass through it. During this detention period the velocity of the 

 flow is such that the suspended solids settle on the sloping floor and 

 slide through a slot in the floor into the lower compartment. The 

 solids are retained in the digestion compartment for several weeks 

 or months until they have decomposed and have lost much of their 



2^T 



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S J 



GQ5 Vent ■■■■■. .. Settling 

 ■ i. Chamber 



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Section I- 1 



Figure 4. — Typical Imhoff tank. 



Section 2-2 



