When the capacity of the tank is as small as at Champaign the bacteria do not have time to act upon the sewage as they would if the flow was not so rapid.
It is conceded now by the highest authorities on this subject that the tank should have a capacity equal to from one fourth to one half of the average daily flow.
Concerning the conditions for which the various processes are applicable, it may be said that the dilution process is used when advantage may be taken of natural resources. This method can be utilized by most cities situated along rivers or streams large enough to sufficiently dilute and carry away the sewage, and fulfill sanitary requirements.
As an example of the use of dilution the Chicago River may be cited. There the uncertain flow of the river was made to pass inward toward the Illinois River with a speed of 2½ miles per hour and a discharge of 20,000 gallons per minute per 100,000 inhabitants. The Blackstone River was used by the cities of Massachusetts until the pollution of the river became unbearable and the state was compelled to pass a law forcing the cities to purify their sewage.
Broad irrigation is a method which cannot generally be used on account of the large amount of land and labor required. This method is especially applicable to asylums, alms houses, and reformatories where cost of labor is small. It is used in the West where the value of all available water leads to the application of sewage to crops, and also on account of the low stage of western streams during the summer which renders sewage discharged into them an unbearable nuisance.
Intermittent downward filtration may be used where there is a considerable area of sandy soil, and also where a high degree of purification is necessary.
Chemical precipitation does not require a large area for its operation and used alone does not give a high degree of purification. Where land and material for beds is expensive, and partial purification is sufficient, this system may be used.
The septic tank requires a small area. The odors are not offensive and cannot be noticed 100 feet away. The effluent may be discharged into small streams and soon loses its identity.
Contact beds are used where a high degree of purity is required; if the effluent is emptied into rivers or sources of water supply. The beds do not require a large area and the process may be recommended if suitable material is at hand.
The cost of construction and maintenance of the different systems vary so largely according to local conditions that a fair estimate is not possible. As the most striking example of maximum and minimum cost the disposal system at Chicago and at Lowell, Massachusetts may be cited. The purification in both cases is by dilution, the cost being practically nothing at Lowell while the Chicago Drainage Canal cost $33,000,000. It may not be fair to charge this entire cost to the sewerage system; nevertheless up to date it has been used for no other purpose.