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.