The amount of purification to be obtained by dilution depends upon the size of the stream into which the sewage is discharged and also upon the amount of oxygen contained in the stream. The latter condition is controlled very largely by the rate of flow of the stream and its previous condition of pollution. Mr. E. P. Stearns in his report to the Massachusetts State Board of Health, 1890, on Pollution and Self-Purification of Streams, gave a table showing the calculated amount of free ammonia, dissolved solids and chlorine which sewage adds to running streams.

Much interest is being taken in the effect of the discharge of the sewage of Chicago, and the waters of Lake Michigan, into the Illinois River, and the outcomes of the analyses of samples taken along the river is awaited with interest. Published reports have not been given out, but information from the most reliable sources seem to show that a considerable purification takes place in the passage down the river.

No reliable data could be obtained giving the percent of purification by irrigation. At Berlin the sewage is purified by this method, and the effluent comes well within the requirements of the German law.

With chemical treatment about 90 percent of the matter in suspension and a small percent of that in solution is removed, and the purification is about 53 percent of the total organic matter.

The best examples of intermittent downward filtration show an efficiency of 95 percent on the total organic matter. However, if this process is used in connection with other methods the organic matter may be reduced 99 percent and the chemical analyses of the effluent may fill the drinking water requirements.

Results of analyses of sewage and effluent from septic tanks show an efficiency of from 85 to 90 percent in the organic matter in suspension. Very little change takes place in the matter in solution.

The result of experiments with a single contact bed at Sutton, England, from November 1896 to March 1898 shows a purification of 64 percent; if, however, two beds are used in series, a further purification of 50 percent is obtained or a total purification of 82 percent of the crude sewage.

The capacities of irrigation, and intermittent downward filtration plants, and contact beds are usually stated in terms of the number of gallons per acre per day.

Table III shows the capacity of representative plants. Best authorities consider 100,000 gallons per acre per day to be the maximum rate permissible under the best conditions for the treatment of crude sewage by the intermittent downward filtration system. With unfavorable conditions the quantity of sewage should be limited to as low as 20,000 gallons. Wherever too large a dose is applied to the bed the sewage is not properly purified and the beds soon become clogged up and unfit for further use.

When the beds are used for secondary purification, 750,000 gallons may be treated per acre per day.

The results of extensive experiments made at Manchester, England in 1898 and 1899 show that by means of contact beds crude sewage may be treated at the rate of 500,000 gallons per acre per day. When the beds are used for secondary purification, 750,000 gallons may be successfully treated, and storm water sewage treated at the rate of 2,500,000 gallons per acre per day.

The capacity of the chemical precipitation plant at Madison, Wisconsin is 68 percent of the daily flow, which in 1899 was estimated at 300,000.

The company that built the plant agreed that the plant should have a daily capacity of 1,200,000 for each and every day in the year. This shows the estimated capacity of the tank to be 28 percent.

At Worcester, one of the best examples of this process is in operation. The size of the tanks is equal to 28 percent of the total flow of 17.1 million gallons per day of which 10.1 million gallons is the out flow from a comparatively clear pond. If the actual amount of sewage is considered the tanks have a capacity of 65 percent of the total flow.

Rafter and Baker in their discussion of the size of tanks recommends that the total capacity should be nearly 50 percent of the average daily flow.

The two typical types of septic tanks are those at Exeter, England and at Champaign Illinois. The first has a cubic capacity of 93 percent of the total daily flow, while the second has a capacity of 7.5 percent of the daily flow.

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.

At Berlin, Germany where broad irrigation is used, the sewage has to be pumped to the farms and involves a considerable cost. Aside from this the receipts derived from the sale of farm products are enough to pay the running expenses of the farm.

The first cost of the intermittent downward filtration plant is estimated by the writers at $90,000 per million gallons per day of sewage treated, where the beds are artificial. Where natural beds can be used the first cost may be reduced to $7,500 per million gallons per day. The cost of maintenance is about $2.00 per 1,000,000 gallons.

The average cost for operating a chemical disposal plant is about 58 cents per inhabitant per year, or $16.00 per 1,000,000 gallons treated. At Manchester England where broad irrigation and chemical precipitation are used in combination, the average cost was $63.00 per 1,000,000 gallons treated.

A septic tank for treating 1,000,000 gallons of sewage per day will cost less than $10,000 and the cost of maintenance will be about $1.00 per 1,000,000 gallons treated.