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 
Underdrains 
Sub-sorl tile Onderdrain 
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 the 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. 
