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.
[Illustration: FIG. 25.]
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.