536 IOWA DEPARTMENT OF AGRICULTURE. 



inlet and outlet are submerged about two feet below the surface of the 

 sewage in the tank. The filter box is filled with sand and gravel and has 

 an outlet at the bottom through which the water finally discharges. 



The operation of this system is simplicity simplified. The sewage enter- 

 ing the tank remains until it fills the tank and the weir box to a level with 

 the overflow from the curved tile outlets. In twenty-four hours or a little 

 over, after entering the tank, a scum will ha,ve formed on the surface, an 

 inch or more in thickness, consisting of a solid mass of filth bacteria, which 

 prey upon the poisonous matter and the solids contained in the sewage, 

 constantly fighting among themselves and destroying each other like the 

 Kilkenny cats, which devoured each other until nothing was left but the 

 tail, the tail in this case being represented by the two per cent of poisonous 

 matter left in the water as it escapes, and which is at once eliminated upon 

 exposure to the atmosphere. 



Light and air are fatal to these bacteria, hence the necessity of keeping 

 them in a dark, air-tight place that they may accomplish their work. For 

 this reason the tank must be air-tight. Again, to do their work effectively, 

 they must be left in perfect quiet, hence the inlet and outlet are submerged 

 below the surface in order that from inflow and outflow as little current as 

 possible may be caused, and this quiet is further assured by means of the 

 weir box. 



Upon emerging from the tiles the water is clear as crystal, and by chem- 

 ical arralysis contains but two per cent of bacteria that would be in the slight- 

 est degree injurious to the human system. This water is allowed to filter 

 through the sand and gravel, its exposure in this manner to the air destroy- 

 ing all remaining bacteria, so that it emerges from the final outlet absolutely 

 pure. 



Knowing its source, one would not care to drink it, though it is pure 

 enough for this purpose, and stock may drink it with perfect safety. 



A system of this kind will not freeze in winter, as the gases arising from 

 the sewage in the tank generate enough heat to counteract the cold and pre- 

 vent freezing. The water as it emerges will be found much warmer than 

 the air, in cold weather. 



In cases where the sewage discharge is scanty and intermittent there 

 might be danger of the water freezing in the filter box during a long cold 

 spell, and then it would be advisable to erect a small tight building, well 

 protected from frost, over the whole outfit, including both tank and filter, 

 but when the sewer is in constant use this would be unnecessary. 



The secret, if secret it may be called, of the whole system is the dark and 

 air-tight tank, the submerged inlet and submerged outlet, and that is all 

 there is to it. The bacteria will do their work if let alone. If stirred up 

 they refuse to perform as desired. When properly working the tank might 

 be opened, the bottom scraped and not a handful of solid matter be found. 



The tank should be large enough to hold all the sewage that is ever likely 

 to run into it within a period of twenty-four to thirty-six hours. For a pri- 

 vate residence this would rarely need to be larger than three feet wide, six 

 feet deep and eight to ten feet long. 



