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The septic tank is usually a rectangular shaped chamber with several 

 baffle boards, extending across to break the flow of the sewage. Such 

 tanks should be covered as the organisms that break down the solids 

 (known as anaerobic bacteria) thrive best in the absence of air and light. 

 Septic tanks usually take some time to become operative, a scum mat 

 forming at the top and sludge at the bottom. At intervals such tanks must 

 be cleaned of the sludge. It was formerly supposed that just as much 

 solid matter was turned to liquid and gas to offset the amount of solids 



Fig. 3. 

 Imhoff Tank and Sand Filters Installed at Indianapolis Country Club 



coming in. however, it has usually been found necessary to clean out the 

 resultant sludge at intervals. (See figure 1.) 



The Imhoff type of tank consists of two chambers, one for settling and 

 one for the deposit and digestion of sludge. Such a tank, while somewhat 

 more expensive than the plain tank is smaller and will give more uniform 

 results, besides offering better means for after-treatment of the liquid and 

 assuring a better solution of the sludge problem. (See figure 4.) 



This type of tank has an upper settling chamber with a slotted opening 

 at the bottom. The sewage in flowing through the upper chamber deposits 



