367 



location of adjacent property and the size of the water course into which 

 the treated sewage is finally discharged ; all are important and enter into 

 the proper solution of this important question. ? 



Tank treatment, therefore, is essential as the first step in sewage 

 reduction, and is necessary in order to retain and break down the solids, 

 but it must not be supposed that it purifies the remaining sewage liquor. 

 The tank treatment is necessary in preparing the sewage liquor for fur- 

 ther purification. Such tanks can be made in the form of plain settling 

 tanks, a septic tank, or a combination of both. 



The public will universally call any tank (even a cesspool) a septic- 

 tank, and usually they believe that a septic tank absolutely purifies the 

 sewage. Such is not the case, a reduction from thirty to sixty per cent, of 



Onderdrains 



'>imm 



mmR 



ILlevalion 



'"^'iWkiiiyiiyfc. 



foroos gravel or sand 



^Sub-soil hi 



Ie Onderdrarn-^ 



Fig. 2. 



Septic Tank and Natural Sand Filter for Small Installations. 



suspended matter and around thirty per cent, in organic matter is usually 

 what takes place. The tank will not take care of very fine particles or col- 

 loidal matter. Such matter (colloidal) being in condition just between 

 suspension and solution. The best results are obtained when tbe solids are 

 taken out or retained as quickly as possible and the subsequent liquor 

 remaining immediately treated. It is important that liquor be not retained 

 too long or it will become in a toxic condition. Time is an important 

 element in the proper design of a tank, also the state of the sewage in 

 reaching the tank. 



Septic tanks are usually designed for a rate of flow, of from eight 

 to sixteen hours. The more modern type of tank with two compartments, 

 one for settling and one for sludge digestion, are usually designed with a 

 rate of flow of half (or even less) than the above. 



