THE COAGULATION OF WATERS.
The coagulation of water consists in the addition to it of some
substance which forms an inorganic precipitate in the water, the
presence of which has a physical action upon the suspended matters, and
allows them to be more readily removed by subsidence or filtration.
The most common coagulant is sulphate of alumina. When this substance
is added to water it is decomposed into its component parts, sulphuric
acid and alumina, the former of which combines with the lime or other
base present in the water, or in case enough of this is lacking, it
remains partly as free acid and partly undecomposed in its original
condition; while the alumina forms a gelatinous precipitate which draws
together and surrounds the suspended matters present in the water,
including the bacteria, and allows them to be much more easily removed
by filtration than would otherwise be the case. In addition, the
alumina has a chemical attraction for dissolved organic matters, and
the chemical purification may be more complete at very high rates than
would be possible with sand filtration without coagulant at any rate,
however low.
Coagulants have been employed in connection with filtration from
very early times. As early as 1831 D’Arcet published in the “Annales
d’hygiène publique,”[32] an account of the purification of Nile water
in Egypt by adding alum to the water, and afterwards filtering it
through small household filters. More recently alum has been repeatedly
used in connection with sand filters, particularly
at Leeuwarden, Groningen, and Schiedam in Holland, where the river
waters used for public supplies are colored by peaty matter which
cannot be removed by simple filtration.
SUBSTANCES USED FOR COAGULATION.
Mr. Fuller[33] has given a very full account of the substances which
can be used for the clarification of waters. Without taking up all of
the unusual substances which have been suggested, the most important of
the coagulants will be briefly described below.
_Lime._—Lime has been extensively used in connection with the
purification of sewage, and also for softening water. Lime is first
slaked and converted into calcium hydrate, which is afterwards
dissolved in water, and applied to the water under treatment. The
amount of lime to be used is fixed by the amount of carbonic acid in
the water. So much lime is always used as will exactly convert the
whole of the carbonic acid of the water into normal carbonate of lime.
This substance is but slightly soluble in water and it precipitates.
The precipitate is crystalline rather than flocculent, and is not as
well adapted to aid in the removal of clayey matters as some other
substances, although its action in this respect is considerable. The
precipitate is quite heavy, and is largely removed by sedimentation,
although filtration must be used to complete the process. Water which
has been treated with lime is slightly caustic; that is to say, there
is a deficiency of carbonic acid in it, and it deposits lime in the
pipes, in pumps, etc.; and although the precipitated calcium carbonate
is much softer than steel, it rapidly destroys pumps used for lifting
it.
Principally for these reasons it is necessary to supply carbonic acid
to water which has been treated in this way, and this is done by
bringing it in contact with flue-gases, or by the direct addition of
carbonic acid.
The use of lime for softening waters is known as Clark’s process. It
was patented in England many years ago, and the
patent has now expired. Various ingenious devices have been constructed
for facilitating various parts of the operation. The process has hardly
been used in the United States, but there is a large field for it in
connection with the softening of very hard waters, and where such
waters also contain iron or clay, these substances will be incidentally
removed by the process.
Larger quantities of lime have an action upon the suspended matters
which is entirely different from that secured in Clark’s process, and
the action upon bacteria is particularly noteworthy. This action was
noted in experiments at Lawrence,[34] where it was found that sewage
was almost completely sterilized by the application of considerable
quantities of lime. An extremely interesting series of experiments upon
the application of large quantities of lime to water was made by Mr.
Fuller in 1899.[35] The bacterial results were extremely favorable,
although the necessity for removing the excess of lime afterward is a
somewhat serious matter, and in these experiments it was not entirely
accomplished.
_Aluminum Compounds._—Sulphate of alumina is most commonly employed.
It can be obtained in a state of considerable purity at a very
moderate price, and important improvements in the methods used for its
manufacture have been recently introduced. Potash and soda alums have
no advantage over sulphate of alumina, and, in fact, are less efficient
per pound, while their costs are greater. Chloride of alumina is
practically equivalent to the sulphate in purifying power, but is more
expensive.
_Sodium Aluminate_ has been examined by Mr. Fuller, who states that
experience has shown that its use is impracticable in the case of the
Ohio River water.
_Compounds of Iron._—Iron forms two classes of compounds, namely,
ferrous and ferric salts. When the ferrous salts are applied to water,
under certain conditions, ferrous hydrate is precipitated, but this
substance is not entirely insoluble in water containing carbonic acid.
Under some conditions the precipitated ferrous hydrate is oxidized
by oxygen present in the water to ferric hydrate, and so far as this
is the case, good results can be obtained. Ferrous sulphate is not
as readily oxidized when applied to water as is the ferric carbonate
present in many natural waters, and for this reason ferrous sulphate
has not been successfully used in water purification. In the treatment
of sewage, where the requirements are somewhat different, it has been
one of the most satisfactory coagulants.
Ferric sulphate acts in much the same way as sulphate of alumina,
and is entirely suitable for use where sulphate of alumina could be
employed, but it has not been used in practice, due probably to its
increased cost as compared with its effect, and to the practical
difficulties of applying it in the desired quantities due to its
physical condition.
_Metallic Iron: The Anderson Process._—The use of metallic iron for
water purification in connection with a moderately slow filtration
through filters of the usual form is known as Anderson’s process
(patented), and has been used at Antwerp and elsewhere on a large
scale, and has been experimentally examined at a number of other places.
The process consists in agitating the water in contact with metallic
iron, a portion of which is taken into solution as ferrous carbonate.
Upon subsequent aeration this is supposed to become oxidized and
precipitate out as ferric hydrate, with all the good and none of the
bad effects which follow the use of alum. The precipitate is partially
removed by sedimentation, while filtration completes the process.
The process is admirable theoretically, and in an experimental way
upon a very small scale often gives most satisfactory results, muddy
waters very difficult of filtration, and colored peaty waters yielding
promptly clear and colorless effluents.
In applying the process on a larger scale, however, with peaty waters
at least, it seems impossible to get enough iron to go into solution
in the time which can be allowed, and the small quantity which is
taken up either remains in solution or else slowly and incompletely
precipitates out, without the good effects which follow the sudden and
complete precipitation of a larger quantity, and in this case the color
is seldom reduced, and may even be increased above the color of the raw
water by the iron remaining in solution.
The ingenuity of those who have studied the process has not yet found
any adequate means of avoiding these important practical objections;
and even at Antwerp a great extension of the filtering area, as well
as the use of alum at times of unusual pollution, is good evidence
that simple filtration, in distinction from the effect of the iron, is
relied upon much more than formerly.
At Dordrecht also, where the process has been long in use, the rate of
filtration does not exceed the ordinary limits; nor is the result, so
far as I could ascertain, in any way superior to that obtained a few
miles away at Rotterdam, by ordinary filtration, with substantially the
same raw water.
The results obtained at Boulogne-sur-Seine, near Paris, have been
closely watched by the public chemist and bacteriologist of Paris,
and have been very favorable, and a number of new plants of very
considerable capacity have been built, to supply some of the suburbs of
Paris, but even in these cases only moderate rates of filtration are
employed which would yield excellent effluents without the iron.
_Compounds of Manganese._—Manganese forms compounds similar to those
of iron, that is to say manganous and manganic salts, but their use
in connection with water filtration has not been found possible. In
addition, manganese forms a series of compounds, known as manganates
and permanganates, quite different in their structure and action from
the others. These compounds contain an excess of oxygen which they
give up very readily to organic matters capable of absorbing oxygen,
and because of this power, they have been extensively used in the
treatment of sewage. Applied to the treatment of waters their action is
very slight, and the compounds are so expensive that they have not been
employed for this purpose. Theoretically the action is very attractive,
as the oxygen liberated by their decomposition oxidizes some of the
organic matter of the water, thereby purifying it in part, while the
manganese is precipitated as a flocculent precipitate having all of
the advantages pertaining to a precipitate of hydrate of alumina, and
without the disadvantage of adding acid to the water, as is the case
with the compounds of alumina and iron. These chemicals, when used in
comparatively concentrated condition, have powerful germicidal actions,
but in water purification the amounts which can be used are so small
that no action of this kind results. The amount which can be applied to
a water is limited to the amount which can be decomposed by the organic
matters present in the water, and is not large.
_The Use of Metallic Iron and Aluminum, with the Aid of
Electricity._—Elaborate experiments were made at Louisville with
metallic iron and aluminum oxidized and made available by the aid of
electric currents. The use of iron with electric currents was tried
in sewage purification some years ago, under the name of the Webster
process, but was never put to practical use. The theory is to oxidize
the iron or aluminum in contact with the water, with the formation
of flocculent hydrates, by the aid of an electric current, thereby
securing the advantages of the application of salts of these metals to
the water without the disadvantage of the addition of acid.
_Other Chemicals Employed._—A solution containing chlorine produced
by electrical action has been suggested. Chlorine is a powerful
disinfectant, and when used in large quantities kills bacteria. It is
not possible to use enough chlorine to kill the bacteria in the water
without rendering it unfit for human use. The nature of this treatment
has been concisely described by Dr. Drown,[36] who shows that the
electrically prepared fluids do not differ in their action in any way
from well-known chemicals, the use of which would be hardly considered.
The use of ozone and peroxide of hydrogen have also been suggested, but
I do not know that they have been successfully used on a large scale.
The same is true of many other chemicals, the consideration of which is
hardly necessary in this connection.
COAGULANTS WHICH HAVE BEEN USED.
In actual work sulphate of alumina is practically the only coagulant
which has been employed, excepting the alums, which are practically its
equivalent in action, differing only in strength. Nearly all important
experiments upon the coagulation of water have been made with sulphate
of alumina, and in the further discussion of this subject only this
coagulant will be considered.
AMOUNT OF COAGULANT REQUIRED TO REMOVE TURBIDITY.
In the coagulation of turbid waters a certain definite amount of
coagulant must be employed. If less than this amount is used either no
precipitate will be formed, or it will not be formed in sufficient bulk
to effect the desired results. It is necessary that the precipitate
should be sufficient, and that it should be formed practically all at
one time. The amount of coagulant necessary to accomplish this purpose
is dependent upon the turbidity of the raw water. With practically
clear waters sulphate of alumina of the ordinary commercial strength,
that is to say, with about 17 per cent soluble oxide of aluminum,
used in quantities as small as 0.3 or 0.4 of a grain per gallon, will
produce coagulation. As the turbidity increases larger amounts must be
employed.
A special study was made of this point in connection with the Pittsburg
experiments.[37] As an average of these results it was found that two
grains per gallon of sulphate of alumina were
required to properly coagulate waters having turbidities of 1.00, so
that they could be filtered by the Jewell filter, and 2.75 grains were
required for the Warren filter.
[Illustration: FIG. 20.—AMOUNT OF COAGULANT REQUIRED TO REMOVE
TURBIDITY.]
Aside from the amount required to produce a precipitate in the clearest
waters, the amount of coagulant required was proportional to the
turbidity. As an average for the two filters the required quantity was
approximately 0.30 of a grain, and in addition 0.02 of a grain for each
0.01 of turbidity. Thus a water having a turbidity of 0.20 requires
0.70 of a grain per gallon; a water having a turbidity of 0.50 requires
1.30 grains; of 1.00, 2.30 grains; of 2.00, 4.30 grains, etc. These are
average minimum results. Occasionally clear effluents were produced
with smaller quantities of coagulant, while at other times larger
quantities were necessary for satisfactory results.
The amount of coagulant required for clarification at Cincinnati has
been stated by Mr. Fuller in his report. A number of his results are
brought together in the following table, to which has also been added a
column showing approximately the corresponding results at Pittsburg.
ESTIMATED AVERAGE AMOUNTS OF REQUIRED CHEMICAL FOR DIFFERENT GRADES OF
WATER.
----------+---------------------------------------------
| Chemical Required, Grains per Gallon.
+----------+----------+----------+------------
Suspended | Raw | Subsided | Subsided |Minimum for
Matter, |Water for |Water for |Water for |Raw Water
Parts in | Sand | Sand |Mechanical| for
100,000. | Filters. | Filters. | Filters. |Mechanical
|Cincinnati|Cincinnati|Cincinnati| Filters.
| Report, | Report, | Report, |
|Page 290. |Page 290. |Page 341. | Pittsburg.
----------+----------+----------+----------+------------
1.0 | 0 | 0 | 0.75 | 0.40
2.5 | 0 | 0 | 1.25 | 0.50
5.0 | 0 | 0 | 1.50 | 0.70
7.5 | 0 | 1.30 | 1.95 | 0.90
10.0 | 1.50 | 1.60 | 2.20 | 1.00
12.5 | 1.60 | 1.80 | 2.45 | 1.15
15.0 | 1.70 | 2.00 | 2.65 | 1.30
17.5 | 1.80 | 2.10 | 2.85 | 1.40
20.0 | 1.95 | 2.20 | 3.00 | 1.60
30.0 | 2.25 | 2.45 | 3.80 | 2.00
40.0 | 2.50 | 2.75 | 4.40 | 2.50
50.0 | 2.80 | | |
60.0 | 3.05 | | |
75.0 | 3.40 | | |
100.0 | 4.00 | | |
120.0 | 4.75 | | |
----------+----------+----------+----------+------------
Mr. Fuller’s results seem to show that a greater amount of coagulant
is required for the preparation of water for mechanical filters than
is necessary in connection with sand filters. The results with sand
filters indicate that settled waters and raw waters containing equal
amounts of suspended matters are about equally difficult to treat. The
results at Pittsburg indicate that the raw waters required much smaller
quantities of coagulant for given amounts of suspended matters than was
the case with subsided waters at Cincinnati, the results agreeing more
closely with the amounts required to prepare raw water for sand filters
at Cincinnati.
AMOUNT OF COAGULANT REQUIRED TO REMOVE COLOR.
The information upon this point is, unfortunately, very inadequate. In
some experiments made by Mr. E. B. Weston at Providence in 1893 with a
mechanical filter,[38] with quantities of sulphate of alumina averaging
0.6 or 0.7 of a grain per gallon, the removal of color was usually
from 70 to 90 per cent. The standard used for the measurement of color
is not stated, and there is no statement of the basis of the scale,
consequently no means of determining the absolute color of the raw
water upon standards commonly used.
At Westerly, R. I., with a New York filter, the actual quantity of
potash alum employed from Oct. 10, 1896, to March 1, 1897, was 1.94
grains per gallon, the amount being regulated to as low a figure as it
was possible to use to secure satisfactory decolorization. There is no
record of the color of the raw water. A very rough estimate would place
it at 0.50 upon the platinum scale. The chemical employed in this case
was alum, and two thirds as large a quantity of sulphate of alumina
would probably have done corresponding work, had suitable apparatus for
applying it been at hand.
At Superior, Wisconsin, the water in the bay coming from the St.
Louis River, having a color of 2.40 platinum scale, was treated
experimentally with quantities of sulphate of alumina up to 4 grains
per gallon, by Mr. R. S. Weston in January, 1899, but even this
quantity of coagulant utterly failed to coagulate and decolorize it.
At Greenwich, Conn., during 1898 the average amount of sulphate of
alumina employed, as computed from quantities stated in the annual
report of the Connecticut State Board of Health for 1898, was about
0.44 of a grain per gallon, and this quantity sufficed to reduce the
color of the raw water from 0.40 to 0.30, platinum standard. This
reduction is very slight, and it is obvious that this quantity of
coagulant was not enough for decolorization.
Some experiments bearing on color removal were made at East Providence,
R. I., by Mr. E. B. Weston, and are described in the Proceedings of the
American Society of Civil Engineers for September, 1899. In this case
the color is reported to have been reduced from 0.58 to 0.10 platinum
standard by the use of one grain of sulphate of alumina, containing 22
per cent of effective alumina, equivalent to about 1.30 grains of the
ordinary article per gallon.
The various experiments seem to indicate that a removal from 80 to
90 per cent of the color can be effected by the use of a quantity of
sulphate of alumina equal to rather more than two grains per gallon
for waters having colors of 1.00, platinum standard, and proportionate
quantities for more and less deeply colored waters. With much less
sulphate of alumina decolorization is not effected, and even larger
quantities do not remove all of the color.
The data are much less complete than could be desired, and it is to be
hoped that experiments will be undertaken to throw more light upon this
important subject.
SUCCESSIVE APPLICATION OF COAGULANT.
Mr. Fuller, in his experiments at Louisville, has ascertained that when
sulphate of alumina is added to extremely muddy water the sediment
absorbs some of the chemical before it has time to decompose, and
carries it to the bottom, and so far as this is the case, no benefit
is derived from that part of the coagulant which is absorbed. In other
words, it is necessary to add more coagulant than would otherwise be
necessary because of this action. The data showed that different kinds
of suspended matters took up very different amounts of coagulant in
this way. With only moderately turbid waters the loss of chemical
from this source is unimportant. Hardly any trace of it was found at
Pittsburg with the Allegheny River water. At Louisville, however, it
was an important factor, as shown by Mr. Fuller’s results.
To avoid this loss of chemical Mr. Fuller has suggested the removal of
the greater part of the suspended matters by sedimentation, without
chemicals, or with the aid of a small quantity of chemical, followed by
the application of the final coagulant prior to filtration. With the
worst waters encountered at Louisville the saving in coagulant to be
effected in this way is very great.
Mr. Fuller states in “Water Purification at Louisville,” p. 417: “The
practical conclusions to be drawn from this experience are that with
preliminary coagulation, followed by subsidence for a period of about
three hours, the application of coagulants may be divided to advantage,
and a considerable portion of the suspended matter kept off the filter,
when the total amount of required coagulant ranges from 2 to 2.5
grains or more of ordinary sulphate of alumina per gallon. In the case
of a water requiring more than this amount of coagulating treatment,
a proper division of the application would increase the saving of
coagulants and would diminish the frequency of washing the filter.”
In his final summary and conclusions, page 441, Mr. Fuller estimates
the amount of sulphate of alumina required for the clarification of the
Ohio River at Louisville at 3.00 grains per gallon of water filtered
if all applied at one point, or at 1.75 grains by taking advantage of
subsidence to its economical limit prior to the final coagulation. The
saving to be effected in this way is sufficient to justify the works
necessary to allow it to be carried out. With less turbid waters, or
waters highly turbid for only short intervals, the advantages of double
coagulation would be less apparent.
THE AMOUNT OF COAGULANT WHICH VARIOUS WATERS WILL RECEIVE.
The amount of coagulant which can be safely used is dependent upon
the alkalinity of the raw water. When sulphate of alumina is added
to water it is decomposed, as explained above, with the formation
of alumina, which is alone useful in the work of purification, and
sulphuric acid, which combines with the calcium carbonate or lime
present in the water. There should always be an excess of alkalinity or
lime in the raw water. If for any reason there is not, there is nothing
to combine with the liberated sulphuric acid, and the decomposition of
the coagulant is not complete, and a portion of it goes undecomposed
into the effluent. The effluent then has an acid reaction, and is unfit
for domestic supply. When distributed through iron pipes, it attacks
the iron, rusting the pipes, and giving rise to all the disagreeable
consequences of an iron containing water.
The amount of lime in a water available to combine with the sulphuric
acid can be determined by a very simple chemical operation, namely,
by titration with standard acid with a suitable indicator. The amount
of coagulant corresponding to a given quantity of lime can be readily
and accurately calculated, but it is not regarded safe to use as
much sulphate of alumina as corresponds to the lime. The quantity of
coagulant used is not susceptible to exact control, but fluctuates
somewhat, and if the exact theoretical quantity should be employed
during 24 hours, there would surely be an excess during some portion of
that time from which bad results would be experienced. It is therefore
considered only prudent to use three quarters as much sulphate of
alumina as corresponds to the lime in the water. With sulphate of
alumina containing 17 per cent of soluble aluminum oxide and the
corresponding amount of sulphuric acid, the amount which can be applied
to a water in grains per gallon is slightly less than the alkalinity
expressed in terms of parts in 100,000 of calcium carbonate.
Many waters contain sufficient lime to combine with the acid of all
the coagulant which is necessary for their coagulation. Others will
not, and it thus becomes an important matter to determine whether a
given water is capable of decomposing sufficient coagulant for its
treatment. It is usually the flood-flows of rivers which control in
this respect. The water at such times requires much larger quantities
of coagulant for its clarification, and it also usually contains much
less lime than the low-water flows. The reason for this is obviously
that the water of the flood-flows is largely rain-water which has come
over the surface without coming into very intimate contact with the
soil, and consequently without having taken from it much lime, while
the low-water flows contain a considerable proportion of water which
has percolated through the soil and has thus become charged with lime.
In some parts of the country, as, for instance, in New England, the
soil and underlying rock are almost entirely free from lime, and
rivers from such watersheds are capable of receiving only very small
quantities of coagulant without injurious results.
The deficiency of alkalinity in raw water can be corrected by the
addition to it of lime or of soda-ash. Lime has been used for this
purpose in many cases. When used only in moderate amounts it hardens
the water, and is thus seriously objectionable. The use of so large a
quantity as would precipitate out, as in Clark’s process, has not been
employed in practice. If it should be attempted, the amount of lime
would require to be very accurately controlled, and the effluent would
have to be treated with carbonic acid to make it suitable for supply.
Waters so hard as to require the use of the Clark process almost always
have sufficient alkalinity, and do not require to be treated with lime
in connection with the use of sulphate of alumina.
The use of soda-ash is free from the objections to the use of lime,
but is more expensive, and would require to be used with caution. Its
use has often been suggested, but I do not know that it has ever been
employed in practice. In small works the use of a filtering material
containing marble-dust, or other calcareous matter, would seem to have
some advantages in case of deficiency of alkalinity, although it would
harden the water so treated.
The alkalinities of a number of waters computed as parts in 100,000 of
calcium carbonate (approximately equal to the safe doses of sulphate to
alumina in grains per gallon) are as follows:
-------------------------------------+--------+--------+--------
|Maximum.|Minimum.|Average.
-------------------------------------+--------+--------+--------
Boston water, 1898 | 2.87 | 0.33 | 1.08
Conestoga Creek, Lancaster, Penn. | 12.20 | 3.70 | 6.80
Allegheny River, Pittsburg | 8.00 | 1.02 | 2.90
Mahoning River and tributaries, 1897 | 20.00 | 2.20 | 10.00
Scioto River and tributaries, 1897 | 35.00 | 10.00 | 20.00
Ohio River, Cincinnati, 1898 | 7.00 | 2.00 | 4.50
Ohio River, Louisville | 10.87 | 2.12 | 6.70
Lake Erie, Lorain, Ohio | | | 9.50
Lake Michigan, Chicago | | | 11.50
-------------------------------------+--------+--------+--------