TREATMENT OF IRON-CONTAINING WATERS.
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.
The rate of filtration employed is usually not a matter of vital importance, but by selecting a rate that is not too high it is possible to use a moderate loss of head. It is thus not necessary to clean the filters too often, and the expenses of operation are not as high as with an extreme rate. In some cases it is desired to accomplish other results than the removal of iron by filtration, and this may lead to the selection of a rate lower than would otherwise be used.
Under normal conditions of operation all of the iron separates on the top of the sand. No appreciable amount of it penetrates the sand at all. With open filters at Far Rockaway and at Red Bank there is an algæ growth in the water upon the filters which, with the iron, forms a mat upon the surface of the filter; and when the filter is put out of service and allowed to partially dry, this mat can be rolled up like a carpet and thrown off without removing any sand, and the filters have been in use for several years without renewing any sand and without any important decrease in the thickness of the sand layer.
Some waters contain iron in such a form that it cannot be successfully removed in this manner. Thus at Reading, Mass., it was reported by Dr. Thomas M. Drown that the iron was present in the form of ferrous sulphate instead of ferrous carbonate, and that it was not capable of being separated by simple aeration and filtration. A Warren mechanical filter was installed, and the water is treated by aeration and with the addition of lime and alum. The cost of the process is thereby much increased, and the hardness of the water is increased threefold.
Several other cases have been reported where it was believed that simple aeration and filtration were inadequate; but the advantages of the simple procedure are so great as to make it worth a very careful study to determine if more complete aeration, or the use of coke-towers and perhaps slower filtration, would not serve in these cases without resorting to the use of chemicals and their attendant disadvantages.