The removal of iron from ground-water is ordinarily a very simple procedure. It is simply necessary to aerate the water, by which process the ferrous carbonate is decomposed, and oxidized with the formation of ferric hydrate, which forms a flocculent precipitate and is readily removed by filtration. The aeration required varies in different cases. The quantity of oxygen required to oxidize the iron is only a small fraction of the amount which water will dissolve, and allowing water to simply fall through the air for a few feet in fine streams will usually supply several times as much oxygen as is necessary for this purpose. Aerating devices of this kind have proved sufficient in a number of cases, as at Far Rockaway, L. I., and at Red Bank, N. J. In some cases, however, a further aeration is necessary, not for the purpose of getting more oxygen into the water, but to get the excess of carbonic acid out of it. Carbonic acid seems to retard in some way the oxidation of the iron, and it is occasionally present in ground-waters in considerable quantity, and quite seriously interferes with the process. It can be removed sufficiently by aeration, but the necessary amount of exposure to air is much greater than that required to simply introduce oxygen. Coke-towers have sometimes been used for this purpose. The towers are filled with coarse coke and have open sides, and water is sprinkled over the tops of them and allowed to drip through to the bottoms. In general the simple exposure of water to the air for a sufficient length of time, in any form of apparatus or simply in open channels, will accomplish the desired results. Mr. H. W. Clark[45] has called attention to the fact that in some cases coke seems to have a direct chemical action upon the water which is entirely independent of its aerating effect. In his experiments there seemed to be some property in the coke which caused the iron to oxidize and flocculate in many cases when it refused to do so with simple aeration and filtration. When the right conditions are reached the oxidation of the iron is very rapid, and it separates out in flakes of such size that they can be removed by filtration at almost any practicable rate. Mechanical filters have been used for this purpose, with rates of filtration of 100 million gallons per acre daily. In Germany, where plants for the removal of iron are quite common, modified forms of sand filters have usually been employed which have been operated at rates up to 25 million gallons per acre daily. In experiments made by the Massachusetts State Board of Health rates from 10 to 25 million gallons per acre daily have been employed. The sand used for filtration may appropriately be somewhat coarser than would be used for treating surface-waters, and the thickness of the sand layer may be reduced. Owing to the higher rates the underdrainage system must be more ample than is otherwise necessary.