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 solution. Hence it is now more frequently called 

 a hydrolytic tank. It is, however, important in that it makes the succeed- 

 ing processes of nitrification 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 metbod. 



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 automatic 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 bigh 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. 



