THE SEPTIC TANK is designed to provide the first stage of bacterial action, mentioned just above, without the intervention of filtering ma terial. The sewage first enters a small grit chamber. where sand and like heavy matter is speedily deposited on account of its relatively great weight. The sewage then goes on to a nar row and rather long and shallow tank, having a trapped inlet and outlet, the better to exclude the air. The bulk of the suspended organic mat ter is deposited and retained in this tank. The anaerobic bacteria seize upon and break up the sludge, which is transformed into dissolved and gaseous matter. The former passes out with the tank effluent. As any sludge left behind remains in the tank week after week, there is no lack of opportunity for complete bacterial reduction. The sludge accumulates by slow degrees. The tank effluent, as has been stated, is about as well puri fied as that from chemical precipitation tanks, but it is in far better condition for further treat ment, while the sludge problem has been prac tically eliminated. Where further treatment is
required to prevent water pollution the tank effluent is generally passed through bacteria beds, sometimes being preceded by aeration in order to establish more favorable conditions for the aerobic bacteria.
The septic tank system was put in use at Exeter, England, in August, 1896, by Mr. Don ald Cameron, town surveyor. Since then many other septic tanks have been built. The Exeter tank, like others built under Mr. Cameron's pat ents, was tightly covered to exclude air and light. Covering, however, does not seem necessary.
It is asserted that the septic tank was de veloped independently at Urbana, 111., in 1894, by Professor A. N. Talbot. Certainly he built a tank there and then, which acted in much the same way as the tank. In 1895 he de signed a more pretentious one for Champaign, Ill., which was built in 1897. See Metcalf, "Anteced ents of the Tank," Proceedings of the icon Society of Ciril Engineers (New York, 1901).