Chapter XII: Removal of Iron From Ground-Waters

The filtration of ground-waters is a comparatively recent development. Ground-waters are filtered by their passage through soil generally much more perfectly than it is possible to filter other waters, and any further filtration of them is useless. Such waters, however, occasionally contain iron in solution as ferrous carbonate.

Waters containing iron have been used as mineral waters for a very long time. Such waters have an astringent taste, and have been esteemed for some purposes. As ordinary water-supplies, however, they are objectionable. The iron deposits in the pipes when the current is slow, and is flushed out when it is rapid, and makes the water turbid and disagreeable; and still worse, the iron often gets through the pipe-system in solution, and deposits in the wash-tub, coloring the linen a rusty brown and quite spoiling it.

An organism called crenothrix grows in pipes carrying waters containing iron, and after a while this organism dies, and decomposes, and gives rise to very disagreeable tastes and odors. It thus happens that ground-waters containing iron are unsatisfactory as public water-supplies, and are sources of serious complaint.

AMOUNT OF IRON REQUIRED TO RENDER WATER OBJECTIONABLE.

Three hundredths of a part in 100,000 of metallic iron very rarely precipitate or cause any trouble. Five hundredths occasionally precipitate, and this amount may be taken as about the allowable limit of iron in a satisfactory water. One tenth of a part is quite sure to precipitate and give rise to serious complaint. Two or three tenths make the water entirely unsuitable for laundry purposes, and are otherwise seriously objectionable, and will hardly be tolerated by a community. Under some conditions ground-waters carry as much as 1 part in 100,000 of iron, and such waters are hardly usable. In iron-removal plants an effluent containing less than 0.05 is regarded as satisfactory. One containing less than 0.02, as is the case with many plants, is all that can be desired. The percentage of removal is of no significance, but only the amount left in the effluent.

CAUSE OF IRON IN GROUND-WATERS.

Natural sands, gravels, and rocks almost always contain iron, often in considerable amount. The iron is usually combined with oxygen as ferric oxide, and in this condition it is insoluble in water. Water passing through iron containing materials will not ordinarily take up iron. When, however, the water contains a large amount of organic matter in solution, this organic matter takes part of the oxygen away from the iron, and reduces the ferric oxide to ferrous oxide. The ferrous oxide combines with carbonic acid, always present under these conditions, forming ferrous carbonate, which is soluble and which goes into solution.

Surface-waters nearly always carry free oxygen, and when such waters enter the ground they carry oxygen with them, and the organic matters in the water use up the free oxygen before they commence to take oxygen away from the iron of the ground. It is thus only in the presence of organic matters, and in the absence of free oxygen, that the solution of iron is possible. It sometimes happens that the organic matters which reduce the iron are contained in the soil itself, in which case iron may be taken up even by water originally very pure, as for instance, by rain-water.

Generally speaking, iron is everywhere present in sufficient quantity in the strata from which ground-waters are obtained, and wherever the conditions of the organic matters and oxygen necessary for solution occur, iron-containing waters are secured, and the iron is usually present in the earth in such quantity that the water can dissolve as much as it will take up for a long series of years, or for centuries, without exhausting the supply. There is thus little prospect of improvement of such waters from exhaustion of the supply of iron.

The circumstances which control the solution of iron are very complicated and difficult to determine. Wells near a river, and drawing their water largely from it by seepage, are apt to yield a water containing iron sooner or later, especially where the river-water carries a large amount of organic matter in solution. Waters drawn from extensive gravel deposits, in which the water is renewed principally by the rainfall upon the surface of the deposits themselves, often remain entirely free from iron indefinitely. The rain-water is almost free from organic matter, and the air is able to take care of decomposing organic matters in the surface soil, and below this there are no accumulations of organic matter sufficient to cause the solution of iron. Under other conditions there are subterranean sources of organic matter which result in the solution of iron under conditions which, on the surface, appear most favorable for securing good water. Wells are often used for many years without developing iron, when suddenly iron will appear. This appearance of iron is often connected with increasing consumption of water. In some cases it may result from drawing water from areas not previously drawn upon.

When iron once makes its appearance in a water, it seldom disappears completely afterward, although it often fluctuates widely at different seasons of the year and under different conditions of pumping. In some cases a decrease in the quantity of iron is noted after a number of years, but in other cases this does not happen.

In a few cases manganese has been found in ground-waters. Manganese in water behaves much like iron, but there are some points of difference, so that the possibility of the presence of this substance should be borne in mind.

Iron-containing waters are generally entirely free from oxygen, and when first drawn from the ground they are bright and clear and do not differ in appearance from other ground-waters. On exposure to the air they quickly become turbid from the oxidation of the iron, and its precipitation as ferric hydrate. At West Superior, Wisconsin, a water was found containing both iron and dissolved oxygen. It was turbid as pumped from the well. This condition of affairs seemed abnormal, but was repeatedly checked, and the theory was advanced by Mr. R. S. Weston, who made the observations, that it resulted from a mixture in the wells of two entirely different waters, namely, a water resulting from the rainfall on sand deposits back of the wells, containing dissolved oxygen and no iron, and water from the lake which had seeped through the sand, and which contained a considerable amount of iron in solution but no dissolved oxygen. The wells thus drew water from opposite directions, and the two waters were entirely different in character, and the mixture thus had a composition which would not have been possible in a water all of which came from a single source.

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.

IRON-REMOVAL PLANTS IN OPERATION.

Iron-removal plants are now in use at Amsterdam and The Hague in Holland, at Copenhagen in Denmark, at Kiel, Charlottenburg, Leipzig, Halle, and many other places in Germany; at Reading, Mass.; Far Rockaway, L. I.; Red Bank, Asbury Park, Atlantic Highlands, and Keyport, N. J.

Among the earliest plants for the removal of iron were the filters constructed at Amsterdam and The Hague in Holland. At Amsterdam the water is derived from open canals in the dunes draining a large area. The water has its origin in the rain-water falling upon the sand. The sand is very fine and contains organic matter in sufficient amount so that the ground-water is impregnated with iron. In flowing to a central point in the open canals the water becomes aerated and the iron oxidized. There are also algæ growths in the water which perhaps aid the process. Sand filters of ordinary construction are used, and remove both the iron and the algæ, and the rate of filtration is not higher than is usually used in the treatment of river-waters, although it could probably be largely increased without detriment to the supply.

The works at The Hague are very similar to those at Amsterdam, but covered collectors are used to supplement the open canals. Both of these plants were built before much was known about iron in ground-waters and the means for its removal, but they have performed their work with uniformly satisfactory results. In the more recent German works various aerating devices are employed, and filters similar in general construction to ordinary sand filters, but with larger connections suited to very high rates of filtration, are employed.

The plant at Asbury Park was the first of importance constructed in America. The water is raised from wells from 400 to 1100 feet deep by compressed air by a Pohle lift. It is delivered into a square masonry receiving-basin holding some hours’ supply. The aeration of the water by this means is very complete. It is afterwards pumped through Continental pressure filters direct into the service-pipes. The reservoir for the aerated water was not a part of the original plant, but was added afterwards to facilitate operation, and to give more complete aeration before filtration.

At Far Rockaway, L. I., the water is lifted from wells by a Worthington Pump, and is discharged over the bell of a vertical 16-inch pipe, from which it falls through the air to the water in a receiving chamber around it. The simple fall through the air aerates the water sufficiently. From the receiving-chamber the water is taken to either or both of two filters, each with an area of 20,000 square feet. These filters are open, with brick walls and concrete bottoms, three feet of sand and one foot of gravel, and the underdrains are of the usual type. The water flows through regulator-chambers to a well 25 feet in diameter and 12 feet deep, from which it is pumped to a stand-pipe in the town. The plant was built to treat easily three million gallons per day, and has occasionally treated a larger quantity. Either filter yields the whole supply while the other is being cleaned. The rate of filtration in this case was made lower than would have otherwise been necessary, as there was an alternate supply, namely, the water from two brooks, which could be used on occasions, and to purify which a lower rate of filtration was regarded necessary, than would have been required for the well-water. The removal of iron is complete.

The plant of the Rumson Improvement Company at Red Bank, N. J., is quite similar to that at Far Rockaway, but is much smaller. The outlet is a 6-inch pipe perforated with 1/4-inch holes which throws the water out in a pine-tree shape to the receiving-tank, thoroughly aerating it. Each of the two filters has 770 square feet of area. The filtering material is three feet of beach sand. From the regulator-chamber the water flows to a circular well 18 feet in diameter, covered by a brick dome and holding 17,000 gallons, from which it is pumped to the stand-pipe. Either of the filters will treat ten thousand gallons of water per hour, which is equal to the capacity of the pumps; and as the consumption is considerably less than this figure, they are only in use for a part of each day, the number of hours depending upon the consumption. These filters are shown by the accompanying plan. The cost of the work was as follows:

Filters and pure-water reservoir, with piping
and drains complete $3,799.47
New pump and connections 492.68
Engineering and superintendence 992.91
---------
Total cost of plant $5,285.06

The engineer who operates the pumps takes care of the filters, and no additional labor has been required. The entire cost of operation is thus represented by the additional coal required for the preliminary lift from the wells to the filters. The effluent is always free from iron.

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.