The plant at Reading,[46] Mass., was installed by the Cumberland Manufacturing Company, and combines aeration, treatment with lime and sulphate of alumina and rapid filtration. The aeration is effected by pumping air through the water, after the water has received the lime. It afterwards receives sulphate of alumina and passes to a settling-tank holding 40,000 gallons, in which the water remains for about an hour. There are six filters of the Warren type, each with an effective filtering area of 54 square feet. The cost of coagulant is considerable. The chief disadvantage of the process is that it hardens the water, which is naturally soft. From the completion of the plant in July, 1896, to the end of the year the hardness of the water was increased, according to analyses of the State Board of Health, from 4.1 to 11.3 parts in 100,000, and for the year 1897 the increase was from 4.0 to 12.7. The iron, which is present in the raw water to the extent of about 0.26 part in 100,000, is removed sufficiently at all times. Prior to the erection of this plant Mr. Desmond FitzGerald advised aeration followed by sedimentation in two reservoirs holding half a million gallons each, and by rapid filtration. Mr. Bancroft states that in his opinion, if the reservoir recommended by Mr. FitzGerald had been built, the filters could be run with very little or no coagulation, and consequently without increase in hardness, which is the most obvious disadvantage to the procedure. The nominal capacity of the plant is one million gallons, and the average consumption about 200,000 gallons daily. The plant at Keyport, N. J., is similar, but smaller. CHAPTER XIII. TREATMENT OF WATERS. Having now reviewed the most important methods in use for the treatment of waters, we may take a general view of their application to various classes of waters. Different raw waters vary so much, and the requirements of filtration are so different, that it is not possible to outline any general procedure or combination of procedures, but each problem must be taken up by itself. Nevertheless, some general suggestions may be of service. In the first place, we may consider the case of waters containing very large quantities of oxidizable organic matter. Such waters are obtained from some reservoirs containing very active vegetable and animal growths, or from rivers receiving large amounts of sewage. Waters of both of these classes are, if possible, to be avoided for public water-supplies. When circumstances require their use, they can best be treated by intermittent filtration, this process being best adapted to the destruction by oxygen of excessive quantities of organic matter.