An important fact in the septic tank effluent is that its compounds have been broken down and it can now be more readily purified.
The septic tank process is self-regulating, self-acting, requires no attention except the occasional cleaning out of the sludge.
As an illustration of this process the septic tank at Champaign Illinois may be described.
This tank although designed in 1895, has only been in operation since 1897. The dimensions of this tank are 37 by 16 by 5 feet and it has a cubic capacity of 22,000 gallons. It has taken care of a dry weather flow of 300,000 gallons per day, about 13 times its cubic contents. The tank has operated with a range of from 11 to 27 times its cubic capacity with a high efficiency of purification though much of this was ground water.
The reduction of the organic matter in suspension, as shown by the reduction of oxygen consumed, nitrogen as albuminoid ammonia, and total organic nitrogen in three analyses averaged 94 percent. However it is not considered that an average of more than 80 to 90 percent may generally be expected. The reduction of organic matter in solution is considerable, averaging 17 percent.
This sludge at the bottom is a black muddy looking silt-like deposit containing about 60 percent of water and 5 percent of organic matter. Its value as a fertilizer is small.
Although the amount of sludge is relatively small the accumulation requires removal occasionally, three or four times a year.
It is estimated that for every 1,000,000 gallons sewage there is about 3 cubic feet of dry matter. By chemical precipitation this would amount to from 20 to 25 cubic feet per 1,000,000 gallons.
Another good example of the septic tank method is the one in use at Exeter, England.
This plant has been in operation since 1896. It was designed for 98000 gallons capacity. The sewage is allowed to remain in the tank nearly 24 hours. The tank is used in conjunction with five beds in which further purification is used. The beds are filled with coke breeze and one is always resting for a week at a time. The tank is made nearly air tight by an arched roof and any gasses collecting may be burned at a vent.