158 
In many instances no doubt typhoid has been carried thirty miles by 
a river, and then has caused a serious epidemic. 
PROCESS OF PURIFICATION. 
While a certain amount of purification takes place in a septic tank, 
its office is rather that of changing the organic matter from the condition 
of suspension to one of soiution. Hence it is now more frequently called 
a hydrolytic tank. It is, however, important in that it makes the succeed- 
ing processes of nitrificaticn easier and permits of much more rapid treat- 
ment than would otherwise be possible. 
The second step is one of several types of filtration. First, we might 
place the slow sand filter, which was usually some 3 or 4 feet deep; over 
the surface sewage flowed either continuously or intermittently, the latter 
being the more efficient method. 
A second form was the contact filter, which was a tight tank filled 
with broken stone, coal or hard clinker. This tank was filled with sewage 
from the bottom, and after a time was emptied automatically. 
The third and most successful type of filter is formed of stone, about 
one-half inch in diameter. Over the surface sewage is sprayed or sprinkled 
periodically by automatie syphons. 
After filtration there is still left some organic matter, but, if the 
process is successful, it does not cause putrification. It is quite probable 
that some bacteria pass through the filter and thus gain access to the 
stream. Hence it has been proposed that where a high degree of purifica- 
tion is necessary the effluent from the filters should be sterilized. 
PURIFICATION PLANTS IN INDIANA. 
Two or three tanks were installed in Indiana some ten years ago, and 
a set of four small sand filters was at one time (about 1900) in operation 
at Indiana Harbor, but has since been abandoned. 
The oldest plant still in operation is at the Eastern Indiana Hospital 
for the Insane near Richmond. It consists of a concrete tank and inter- 
mittent sand filters. It treats the sewage of about 1,000 people and leaves 
the stream into which the effluent flows in a very satisfactory condition. 
It was built in 1901 at an expense of $9,000. The cost of operation has 
been negligible. 
