Many eminent men have advocated special material as coal, coke, clinker slag, sand, and gravel, and even glass for filling material. The results do not differ materially and Prof. L. P. Kennicutt in an article in the Journal of the Association of Engineering Societies, February 1900 makes the following statement, “The material should be more or less porous so as to have a larger water absorbing area and have a jagged surface on which the gelatinized micro-organisms can be easily retained.” “The quantity of sewage that can be successfully treated by intermittent filtration has been shown not to be over 100,000 gallons per acre per day, a quantity so small as to be quite useless for towns and cities which would be obliged to construct beds with sand not “in situ”. This point was quickly perceived in England where sand “in situ” is not of common occurrence. These bacteria beds were not used long before the problem arose: Can the amount of land required by the intermittent filtration method be so reduced that the construction of artificial bacteria beds will be a practical possibility? “The results of the investigation started by this problem have given us what is known as the contact system of treatment and the septic tank treatment and have apparently shown not only that by combining these two methods the amount of area required for 100,000 gallons can be reduced from one acre to about one seventh of an acre but also that the bacterial treatment is possible with sewage containing manufacturing refuse, and have outlined how sewage containing storm water may be treated.” It has been found by experiment that after two or three weeks there is a marked reduction in the initial capacity of the tank due to breaking down of the filling material and also to the filling material becoming charged and coated over with a gelatinous slime consisting of living organisms and organic matter in the process of transformation. If the bed is not overworked the capacity of the tank will remain constant after the first two or three weeks, showing that the durability is unlimited. The beds are usually filled in half an hour, and the sewage allowed to remain on them about two hours when the effluent is run off and the beds allowed to rest several hours before again being filled. By contact beds in series, the number depending upon the kind of sewage, almost any degree of purification can be obtained. The Engineering Record of Jan. 27, 1900 gives a very interesting account of the sewage disposal works at Sutton, England. The works were constructed in 1891–93, and comprise an area of 28 acres, 18 acres only being capable of irrigation. They were originally designed for chemical precipitation and broad irrigation, but after giving these methods a two years’ trial the local board found itself unable to satisfy the requirements of the conservators of the River Thames, into which the effluent flowed.