TURBIDITY AND COLOR, AND THE EFFECT OF MUD UPON SAND-FILTERS.
The ideal water in appearance is distilled water, which is perfectly
clear and limpid, and has a slight blue color. When other waters are
compared with it, the divergences in color from the color of distilled
water are measured, and not the absolute colors of the waters. Many
spring waters and filtered waters are indistinguishable in appearance
from distilled water.
Public water-supplies from surface sources contain two substances or
classes of substances which injure their appearance, namely, peaty
coloring matters, and mud. Waters discolored by peaty matters are most
common in New England and in certain parts of the Northwest, while
muddy waters are found almost everywhere, but of different degrees of
muddiness, according to the physical conditions of the water-sheds from
which they are obtained.
Muddy waters are often spoken of as colored waters, and in a sense this
is correct where the mud consists of clays or other materials having
distinct colors; but it is more convenient to classify impurities of
this kind as turbidities only, and to limit the term colored waters to
those waters containing in solution vegetable matters which color them.
The removal of either color or turbidity may be called clarification.
Colored waters are usually drawn from water-sheds where the underlying
rock is hard and does not rapidly disintegrate, and where the soils are
firm and sandy, and especially from swamps. The water here comes in
contact with peat or muck, which colors it, but is so firm as not to
be washed by flood flows, and so does not cause turbidity.
Large parts of the United States have for rock foundations shales or
other soft materials which readily disintegrate when exposed, and
which form clayey soils readily washed by hard rains. Waters from
such watersheds are generally turbid and very rarely colored. In fact
a water carrying much clay in suspension is usually found colorless
when the clay is removed, even if it were originally colored. It thus
happens that waters which are colored and turbid at the same time
hardly exist in nature.
Color-producing matters and turbidity-producing matters are different
in their natures, and the methods which must be used to remove them are
different.
THE MEASUREMENT OF COLOR.
The colors of waters are measured and recorded by comparing them with
colors of solutions or substances which are permanent, or which can be
reproduced at will. One of the earliest methods of measuring colors of
waters was to compare them with the colors of the Nessler standards
used for the estimation of ammonia in water analysis. The Nessler
standards were similar in appearance to yellow waters, and their colors
depended upon the amounts of ammonia which had been used in preparing
them, and a record was made of the standard which most closely
resembled the water under examination.
The method was open to the serious objections that the hues of the
standards did not match closely the hues of the waters; that the colors
produced with different lots of Nessler reagent differed considerably,
and therefore the exact values of results were more or less uncertain;
and further, that the numbers obtained for color were not even
approximately proportional to the amounts of coloring matter present.
Because of this peculiarity, in filtration the percentage of color
removal, as determined by the use of these standards, is not even
approximately correct, but is much above the truth.
In the Lovibond tintometer, which has been extensively used in England,
the standards of color are based upon the colors of certain glass
slips, which are in turn compared with standard originals kept for
that purpose. This process answers quite well, but is open to some
objections because of possible uncertainties in the standardization of
the units.
Another method of measuring colors is to compare them with dilute
solutions of platinum and cobalt. The ratio of cobalt to platinum can
be varied to make the hue correspond very closely with the hues of
natural waters, and the amount of platinum required to match a water
affords a measure of its color, one part of metallic platinum in 10,000
parts of water forming the unit of color.
This standard has the advantages that it can be readily prepared with
absolute accuracy in any laboratory, and that by varying the ratio of
platinum to cobalt the hues of various waters can be most perfectly
matched. It is important that the observations should not be made in
too great a depth, as the discrepancy in hues increases much more
rapidly than the depth of color.
For further information regarding colors the reader is referred to
articles in the American Chemical Journal, 1892, vol. xiv, page 300;
Journal of the American Chemical Society, vol. ii, page 8; vol. xviii,
1896, pp. 68, 264, and 484; Journal of the Franklin Institute, Dec.
1894, p. 402; Journal of the New England Water Works Association, vol.
xiii, 1898, p. 94.
AMOUNT OF COLOR IN AMERICAN WATERS.
New England surface-waters have colors ranging from almost nothing
up to 2.00. The colors of the public water-supplies of Massachusetts
cities have been recorded in the reports of the State Board of Health
for some ten years. The figures given were recorded first upon the
Nessler standard, and afterwards upon a modification of the same, known
as the natural water standard. The figures given are approximately
equal to those for the platinum color standard, the relations between
the two having been frequently determined by various observers and
published in the above-mentioned papers. The accompanying diagram shows
the colors in several Massachusetts supplies, as plotted from the
figures given in the published reports.
[Illustration: FIG. 18.—COLORS OF WATERS.
(Analyses of the Mass. State Board of Health.)]
In Connecticut also the colors of many public water-supplies have been
recorded in the reports of the State Board of Health on the platinum
color-standard.
The waters of the Middle States, with rare exceptions, are almost free
from color. In the Northwest waters are obtained often with very high
colors, even considerably higher than the New England waters, and some
of the Southern swamps also yield highly colored waters.
REMOVAL OF COLOR.
Peaty coloring-matter is almost perfectly in solution, and only a
portion of it is capable of being removed by any form of simple
filtration. In order to remove the coloring-matter it is necessary to
change it chemically, or to bring it into contact with some substance
capable of absorbing it. For this reason sand filtration with ordinary
sands, having no absorptive power for color, commonly removes only from
one fourth to one third of the color of the raw water.
MEASUREMENT OF TURBIDITY.
The amount of mud or turbidity in a water is often expressed as the
weight of the suspended matters in a given weight of the water. Most
of the data relating to turbidities of waters are stated in this
way, because this was the only method recognized by the earlier
investigators.
This method of statement has some disadvantages: it fails to take
into account the different sizes of particles which are carried in
suspension by different waters, and at different times. Thus the
Merrimac River in a great flood may carry 100 parts in 100,000 of
fine sand in suspension, and still it could hardly be called muddy;
while another stream carrying only a fraction of this amount of fine
clay would be extremely muddy. Further, an accurate determination of
suspended matters is a very troublesome and tedious operation, and
cannot be undertaken as frequently as is necessary for an adequate
study of the mud question.
Turbidity is principally important as it affects the appearance of
water, and it would seem that optical rather than gravimetric methods
should be used for its determination. Various optical methods of
measuring turbidity have been proposed. The general method employed
is to measure the thickness of the layer of water through which some
object can be seen under definite conditions of lighting. The most
accurate results can probably be obtained in closed receptacles and
with artificial light. Such a method has been used by Mr. G. W. Fuller
at Louisville and Cincinnati in connection with his experiments, and is
described by Parmelee and Ellms in the Technology Quarterly for June,
1899. This apparatus is called by Mr. Fuller a diaphanometer.
At the Lawrence Experiment Station of the Massachusetts State Board of
Health as early as 1889 it became necessary to express the turbidities
of various waters approximately, and the very simple device of
sticking a pin into a stick, and pushing it down into the water under
examination as far as it could be seen, was adopted. Afterwards a
platinum wire 0.04 of an inch in diameter was substituted for the pin,
and the stick was graduated so that the turbidities could be read from
it directly. The figures on the stick were inversely proportional to
their distances from the wire. When the wire could be seen one inch
below the surface, the turbidity was reported as 1.00; when the wire
could be seen two inches, the turbidity was 0.50, and when it could
be seen ten inches the turbidity was 0.10, etc. This scale is much
more convenient than a scale showing the depth at which the wire can
be seen; and within certain limits the figures obtained with it are
directly proportional to the amount of the elements which obstruct
light in the water. Thus, if a water having a turbidity of 1.00 is
mixed with an equal volume of clear water, the mixture will have a
turbidity of 0.50. Advantage is taken of this fact for the measurement
of turbidities so great that they cannot be accurately determined
by direct observation. For turbidities much above 1.00 it is very
difficult to read the depth of wire with sufficient accuracy, and such
waters are diluted with one, two, or more times their volume of clear
water in a pail or other receptacle, the turbidity of the diluted water
is taken, and multiplied by the appropriate factor.
For the greatest accuracy it is necessary that the observations should
be taken in the open air and not under a roof. They should preferably
be made in the middle of the day when the light is strongest, and in
case the sun is shining, the wire must be kept in shadow and not in
direct sunlight.
The turbidities of effluents are usually so slight that they cannot
be taken in this manner; in fact, turbidities of less than 0.02, with
the wire visible 50 inches below the surface, cannot be conveniently
read in this way. For the estimation of lower turbidities a water is
taken having a turbidity of 0.03 or 0.04 and as free as possible from
large suspended particles. The turbidity of this water is measured by
a platinum wire in the usual way, and the water is then diluted with
clear water to make standards for the lower turbidities.
The comparisons between standards and waters are best made in bottles
of perfectly clear glass, holding at least a gallon, and the comparison
is facilitated by surrounding the bottles with black cloth except at
the point of observation, and lighting the water by electric lights so
arranged that the light passes through the water but is hidden from
the observer. In case the water under examination is colored, the
comparison is rendered difficult, and it is often advisable to add a
small amount of methyl orange to the standards to make the colors equal.
Instead of diluting a water of known turbidity for the standards, a
standard can be made by precipitating a known amount of silver chloride
in the water. For this purpose about one per cent of common salt is
dissolved in clear water and small measured amounts of silver nitrate
added, until the turbidity produced is equal to that of the water under
examination. The relation of the amount of silver nitrate used to the
turbidity is entirely arbitrary, and is established by comparisons of
standards made in this way with waters having turbidities from 0.02 to
0.04, the turbidities of which are measured with the platinum wire, and
which afterwards serve to rate the standards. The silver chloride has a
slight color, which is an objection to its use, and perhaps some other
substance could be substituted for it with advantage. The standards
have to be made freshly each day.
One disadvantage of the platinum-wire method of observing turbidities
in the open air, as compared with the diaphanometric method using
artificial light, is that observations cannot be made in the night. To
get the general character of the water in a stream, daily observations
taken about noon will generally be sufficient; but for some purposes it
is important to know the turbidity at different hours of the day, and
in such cases the platinum-wire method is at a distinct disadvantage.
Variations in the amount of light, within reasonable limits, do not
affect the results materially, although extreme variations are to be
avoided. The size of the wire also influences the results somewhat. The
wire commonly used is 0.04 of an inch or one millimeter in diameter.
A wire only four tenths of this size in some experiments at Pittsburg
gave results 25 per cent higher; with a wire twice as large the
results were lower, but the differences were much less. Wire 0.04 of
an inch in diameter was adopted as being very well adapted to rather
turbid river-waters. For very clear lake or reservoir waters, usually
transparent to a great depth, a much larger object is preferable.
Within certain limits the results obtained with an object of any size
can be converted into corresponding figures for another object, or
another light, by the use of a constant factor. Thus the turbidities
obtained with a platinum wire always have approximately the same ratio
to the turbidities of the same waters determined by the diaphanometer.
The platinum-wire method has been used in many cases with most
satisfactory results. If it lacks something in theoretical accuracy
as compared with more elaborate methods, it more than makes up for it
by its simplicity; and reliable observations can be taken with it by
people who would be entirely incompetent to operate more elaborate
apparatus; and it can thus be used in many cases where other methods
would be impossible.
Upon this scale the most turbid waters which have come under the
observation of the author have turbidities of about 2.50, although
waters much more turbid than this undoubtedly exist. A water with
a turbidity of 1.00 is extremely muddy, and only one tenth of this
turbidity would cause remark and complaint among those who use it for
domestic purposes. In an ordinary pressed-glass tumbler a turbidity of
0.02 is just visible to an ordinary observer who looks at the water
closely, but it is not conspicuous, nor would it be likely to cause
general complaint; and this amount may be taken as approximately the
allowable limit of turbidity in a good public water-supply. In a
carefully polished, and perfectly transparent glass a turbidity of 0.01
will be visible, and in larger receptacles still lower turbidities may
be seen if the water is examined carefully. In gallon bottles of very
clear glass, under electric light and surrounded by black cloth, a
turbidity of 0.001 can be distinguished, but a turbidity even several
times as large as this could hardly be detected except by the use of
special appliances, or where water is seen in a depth of several feet.
RELATION OF PLATINUM-WIRE TURBIDITIES TO SUSPENDED MATTERS.
The relation of turbidity to the weight of suspended matters is
approximately constant for waters from which the coarser matters have
been entirely removed by sedimentation. For these waters the suspended
matters in parts per 100,000 are about 16 times the turbidity. For
river-waters the ratios are always larger. With very sluggish rivers
the ratio is only a little larger than for settled waters. For average
river-waters the ratio is considerably higher, and increases with the
turbidity, and for very rapid rivers and torrents the ratio is much
wider, as the suspended matters consist largely of particles which are
heavy but do not increase very much the turbidity.
The following table gives the amounts of suspended matters for various
classes of waters corresponding to the turbidities stated, which have
been deduced from the experience of the author. It is very likely that
ratios different from the above would be obtained with waters in which
the sediment was of different character.
--------------+-----------------------------------------------
| Suspended Matters: Parts in 100,000.
Turbidity, +-----------------------------------------------
Platinum-wire | | River | River | River
Standard. | Settled | Waters, | Waters, | Waters,
| Waters. | Finest | Average | Coarsest
| | Sediment. | Sediment. | Sediment.
--------------+-----------+-----------+-----------+-----------
0.01 | 0.16 | | |
0.05 | 0.80 | 0.85 | 1.30 | 2.40
0.10 | 1.60 | 1.75 | 2.60 | 4.90
0.20 | 3.20 | 3.60 | 5.50 | 10.00
0.30 | 4.80 | 5.70 | 8.50 | 15.00
0.40 | 6.40 | 7.80 | 11.60 | 21.00
0.50 | 8.00 | 10.00 | 15.00 | 26.00
1.00 | 16.00 | 23.00 | 36.00 | 59.00
1.50 | 24.00 | 40.00 | 62.00 | 97.00
2.00 | 32.00 | 61.00 | 94.00 | 140.00
3.00 | 48.00 | 110.00 | 175.00 | 250.00
--------------+-----------+-----------+-----------+-----------
SOURCE OF TURBIDITY.
Much turbidity originates in plowed fields of clayey soil, or in
fields upon which crops are growing. If it has not rained for some
days, and the surface-soil is comparatively dry, the first rain that
falls upon such land is absorbed by the pores of the soil until they
are filled. If the rain is not heavy, but little runs off over the
surface. If, however, the rain continues rapidly after the surface-soil
is saturated, the excess runs off over the surface to the nearest
watercourse. The impact of the rain-drops upon the soil loosens
the particles, and the water flowing off carries some of them in
suspension, and the water is said to be muddy.
The particles carried off in this way are extremely small. Mr. George
W. Fuller, in his report upon water purification at Louisville,
estimates that many of them are not more than a hundred thousandth of
an inch in diameter, and not more than a tenth as large as common water
bacteria.
The turbidity of the water flowing from a field of loose soil may be
2.00 or more; that is to say, the wire is hidden by a depth of half an
inch of water or less. When the water reaches the nearest watercourse
it meets with water from other kinds of land, such as woodlands and
grassed fields, and these waters are less turbid. The water in the
first little watercourse is thus a mixture and has a turbidity of
perhaps 1.00.
The conditions which control the turbidity of any brook are numerous
and complicated. The turbidity of a stream receiving various brooks
depends upon the turbidities of all the waters coming into it.
Generally speaking, the turbidity of a river depends directly upon the
turbidities of its feeders, and is not affected materially by erosion
of its bed or by sedimentation in it. There are, of course, some
streams which in times of great floods cut their banks, and all streams
pick up and move about from place to place more or less of the sand and
other coarse materials upon their bottoms. The materials thus moved,
however, have but little influence upon the turbidity.
After the rain is over some of the water held by the soil will find
its way to the watercourses by underground channels, and will prevent
the stream from drying up between rains, but the average volume of the
stream-flows between rains will be much less than the volumes during
the rains when the water is most turbid.
[Illustration: FIG. 19.—FLUCTUATIONS IN TURBIDITY OF THE WATER OF THE
ALLEGHENY RIVER AT PITTSBURG DURING 1898.]
These conditions are well illustrated by a few data upon the turbidity
of three Pennsylvania streams, recently collected by the author. One
of these streams is a small brook having a drainage area of less than
three square miles. The observations extended over a period of 47 days.
During this time there were five floods, or an average of one flood in
ten days. The duration of floods was less than twenty-four hours in
each case. Selecting the days when the turbidity was the highest, to
the number of one tenth of the whole number of days, the sum of the
turbidities for these days was 67 per cent of the aggregate turbidities
for the whole period. That is to say, 67 per cent of the whole amount
of mud was in the water of only a tenth of the days; the water of the
other nine tenths of the days contained only 33 per cent of the whole
amount of turbidity. The average turbidity of the water for the flood
days was eighteen times as great as the average turbidity for the
remaining days.
The next stream is a considerable creek having a drainage area of
350 square miles. The observations extended over 117 days, during
which time there were seven floods, or an average of one flood in 19
days. The floods lasted in each case one or two days, and the sum of
the turbidities for the one tenth of the whole number of days when
the water was muddiest was 55 per cent of the aggregate of all the
turbidities for the period.
The last case is that of a large river, with a drainage area of over
11,000 square miles. The observations extended over a full year. In
this period there were sixteen floods, each lasting from one to six
days, and the sum of the turbidities for the one tenth of the whole
number of days when the water was muddiest is 45 per cent of the
aggregate turbidities for the year. The floods occurred on an average
of once in 22 days, and the average duration was two and one half days.
The results are very striking as showing that a very large proportion
of the mud is carried by the water in flood flows of comparatively
short duration. They also show that in small streams the proportion of
mud in the flood-flows is greater, and the average duration of floods
is shorter, than in larger streams. In other words, the differences
between flood- and low-water flows are greatest in small streams, and
gradually become less as the size of the stream increases.
When a stream is used for water-works purposes in the usual way, a
certain quantity of water is taken from the stream each day, which
quantity is nearly constant, and is not dependent upon the condition
of the stream, or the volume of its flow. The proportions of the
total flows taken at high- and low-water stages are very different,
and it thus happens that the average quality of the water taken for
water-works purposes is different from the average quality of all the
water flowing in the stream.
Let us assume, for example, a stream having a watershed of such a
size that in times of moderate floods water from the most distant
points reaches the water-works intake in twenty-four hours. Let us
assume further that rainfalls of sufficient intensity to cause floods
and muddy water occur, on an average, once in ten days, and that the
turbidity of the water at these times reaches 1.00, and that for the
rest of the time the turbidity averages 0.10. Let us assume further
that at times of storms the average flow of the stream is 100 units
of volume, and for the nine days between storms the average flow is
10 units of volume. We shall then have in a ten days’ period, for one
day, 100 volumes of water with a turbidity of 1.00, and nine days with
10 volumes each, or a total of 90 volumes of water with a turbidity
of 0.10. The total discharge of the stream will then be 190 volumes,
and the average turbidity 0.57. The turbidity of 0.57 represents the
average turbidity all the water flowing in the stream, or, in other
words, the turbidity which would be found in a lake if all the water
for ten days should flow into it and become thoroughly mixed without
other change.
Now let us compute the average turbidity of the water taken from the
stream for water-works purposes. The water-works require, let us
say, one volume each day, and we have for the first day water with a
turbidity of 1.00, and then for nine days water with a turbidity of
0.10. The average turbidity of the water taken by the water-works for
the period is thus only 0.19 in place of 0.57, the average turbidity of
the whole run-off.
The average turbidity of all the water flowing in the stream is thus
three times as great as that of the water taken from the stream for
water-works purposes.
It is often noted that with long streams the water becomes muddier
farther down, and it may naturally be thought that it is because of the
added erosion of the stream upon its bed in its longer course. This, of
course, may be a cause, or the lower tributaries may be muddier than
the upper ones, but the fact that the water taken at the lower point is
more muddy than farther up is not an indication of this.
Let us take, for example, a watershed of twice the size of that assumed
above, that is, so long that 48 hours will be required for the water
from the most remote feeders to reach the water-works intake. Let us
divide this shed into two parts, which we will assume to be equal, one
of which furnishes water reaching the intake within 24 hours, and the
other water reaching the intake between 24 and 48 hours. Now suppose
a storm upon the watershed producing turbidities equal to those just
assumed for the smaller stream. On the first day the water from the
lower half of the shed, namely, 100 volumes having a turbidity of 1.00,
passes the intake, but this is mixed with 10 volumes of water from
the upper half of the watershed, having a turbidity of 0.10, and the
total flow is thus 110 volumes of water having a turbidity of 0.92.
On the second day the water from the lower half of the watershed has
returned to its normal condition, and the flood-flow of the upper half
of the watershed, 100 volumes with a turbidity of 1.00, is passing,
and mingles with the 10 volumes from the lower half with a turbidity
of 0.10, and the total flow is again 110 volumes having a turbidity of
0.92. The following eight days, until the next rain, will have flows
of 20 volumes each, with turbidities of 0.10. The average turbidity of
all of the water flowing off is 0.57 as before, but the water taken
for water-works purposes will consist of 2 volumes of water with
turbidities of 0.92, and 8 volumes with turbidities of 0.10 making 10
volumes with an average turbidity of 0.26.
By doubling the length of the watershed we have thus doubled the length
of time during which the water is turbid, and have increased the
average turbidity of the water taken for water-works purposes from 0.19
to 0.26, although the average turbidity of all the water running off
remains exactly the same.
If now we assume a watershed so long that three days are required
for the water from the most remote points to reach the intake, with
computations as above, water taken for water-works purposes will have
an average turbidity of 0.32; and with still longer watersheds this
amount will increase, until with a watershed so long that ten days,
or the interval between rains, are required for the water from the
upper portions to reach the intake, the average turbidity of the water
taken for water-works purposes will reach the average turbidity of the
run-off, namely, 0.57.
In the above computations the numbers taken are round ones, and of
course do not represent closely actual conditions. They do serve,
however, to illustrate clearly the principle that the larger the
watershed, other things being equal, the more muddy will be the water
obtained from it for water-works purposes, and the longer will be the
periods of muddy water, and the shorter the periods of clear water
between them.
It cannot be too strongly emphasized that the period of duration of
muddy water is, in general, dependent upon the length of time necessary
for the muddy water to run out of the stream system after it is once in
it, and be replaced by clear water; and that the settling out of the
mud in the river has very little to do with it.
Muddy waters result principally from the action of rains upon the
surface of ground capable of being washed, and the turbidities of the
stream at any point below will occur at the times when the muddy waters
reach it in the natural course of flow, and will disappear again when
the muddy waters present in the stream system at the end of the rain
have run out, and have been replaced with clear water from underground
sources, or from clearer surface sources.
THE AMOUNTS OF SUSPENDED MATTERS IN WATER.
There is a large class of waters, including most lake and reservoir
waters, and surface-waters from certain geological formations, which
are almost free from suspended matters and turbidities. That is to say,
the average turbidities are less than 0.10, and the average suspended
matters are less than 2 parts in 100,000, and are often only small
fractions of these figures. This class includes the raw waters of the
supplies of many English cities drawn from impounding reservoirs, and
also the waters of the rivers Thames and Lea at London, and the raw
waters used by both of the Berlin water-works, and in the United States
the waters of the great lakes except at special points near the mouths
of rivers, nearly all New England waters, and many other waters along
the Atlantic coast and elsewhere where the geological formations are
favorable.
Data regarding the suspended matters in these waters are extremely
meagre. The official examinations of the London waters contain no
records of suspended matters, although the clearness of filtered
waters is daily reported. Dibden, in his analytical investigations of
the London water-supply, mentioned in his book upon “The Purification
of Sewage and Water,” reports the average suspended matters in the
water of the Thames near the water-works intakes as 0.77 part in
100,000. No figures are available for the raw waters used by the Berlin
water-works, but both are taken from lakes, and are generally quite
clear. Even in times of floods of the rivers feeding the lakes, the
turbidities are not very high, because the gathering grounds for the
waters are almost entirely of a sandy nature, yielding waters with low
turbidities, and further, the streams flow through successions of lakes
before finally reaching the lakes from which the waters are taken. It
is safe to assume that the suspended matters and turbidities do not
exceed those of the London waters. Even at times when somewhat turbid
water is obtained, due to agitation by heavy winds, the suspended
matter is mainly of a sandy nature, readily removed by settling, and
it does not seriously interfere with filtration.
The examinations of the Massachusetts State Board of Health, with a
very few exceptions, contain no statements of suspended matters. This
is due to the fact that the suspended matters, in most of the waters,
are so small in amount as to make them hardly capable of determination
by the ordinary gravimetric processes, and the determinations if made
would have but little value. The Merrimac River at Lawrence, at the
time of the greatest flood in fifty years, carried silt to the amount
of about 111 parts in 100,000. This was for a very short time, and the
suspended matter consisted almost entirely of sand, which deposited
in banks, the deposited sand having an effective size of 0.04 or 0.05
millimeter. No clayey matter is ever carried in quantity by the river.
The reports of the Connecticut State Board of Health also contain no
records of suspended matters for the same reason. It may be safely said
that the average suspended matters of New England waters are almost
always less than 1 part in 100,000.
Lake waters are generally almost entirely free from sediment. At
Chicago the city water drawn from Lake Michigan has slightly more than
1 part in 100,000 of suspended matters, as determined by Professor Long
in 1888-9, and by Professor Palmer in 1896. The suspended matter in
this case is probably due to the nearness of the intake to the mouth of
the Chicago River, and to mud brought up from the bottom in times of
storms. The lake-water further away from the shore would probably give
much lower results.
Turning now to waters having considerable turbidities, at Pittsburg the
average suspended matters in the Allegheny River water, as shown by
the weekly or semi-weekly analyses of the Filtration Commission during
1897-8, were 4 parts in 100,000. During a large part of the time the
suspended matters were so small that it was not deemed worth while
to determine them, and the results are returned as zero. This is not
quite correct, and a recomputation of the amount of suspended matters,
based on the observed amounts, and the amounts calculated from the
turbidities when they were very low, leads to an average of a little
less than 5 parts in 100,000, which is probably more accurate than the
direct average. The average turbidity on the platinum-wire scale was
0.16.
At Cincinnati the suspended matters are about 23 parts in 100,000,
and at Louisville about 35 parts, both of these figures being from
Mr. Fuller’s reports. In all these cases the enormous and rapid
fluctuations in the turbidity of the water is a most striking feature
of the results.
Observations on the Mississippi River above the Ohio have been
made by Professor Long in 1888-9, and by Professor Palmer in 1896.
These results are not as full and systematic as could be desired,
but indicate averages of 20 to 30 parts in 100,000 at the different
points. Professor William Ripley Nichols, in his work on water-supply,
states the amount of suspended matter in the water of the Mississippi,
probably referring to the lower river, as 66.66 parts.
Investigations of Professor Long and Professor Palmer for numerous
interior Illinois streams extending over considerable periods give
average results ranging from 1 to 8 parts in 100,000. The very much
lower results for the interior streams as compared with the Mississippi
and Ohio rivers may be due to the relative sizes and lengths of the
streams, or in part to other causes.
Regarding muddy European rivers there are but few data. The Maas, used
for the water-supply of Rotterdam, is reported by Professor Nichols as
having from 1.40 to 47.61 and averaging 10 parts of suspended matters
in 100,000. More recent information is to the effect that the raw water
has at most 30 parts of suspended matters, and that that quantity is
very seldom reached.
At Bremen the Weser often becomes quite turbid. The turbidity of the
water is noted every day by taking the depth at which a black line on a
white surface can be seen. Assuming that this procedure is equivalent
to the platinum-wire procedure, the depths at which the wire can be
seen, namely, from 15 to 600 millimeters, correspond to turbidities of
from 0.04 to 1.70, a result not very different from the conditions at
Pittsburg.
At Hamburg and Altona the water is generally tolerably clear, but at
times of flood the Elbe becomes very turbid, and the amount of mud
deposited in the sedimentation-basins is considerable. At Dresden,
several hundred miles up the river, I have repeatedly seen the
river-water extremely turbid with clayey matter, the color of the clay
varying from day to day, corresponding to the color of the earth from
which it had been washed.
At Budapest, where filters were used temporarily, the Danube water
was excessively muddy with clayey material. At first very high rates
of filtration were employed and the results were not satisfactory.
Afterward the rate of filtration was limited to 1.07 million gallons
per acre daily, and good results were secured. There was no preliminary
sedimentation. Professor Nichols reports the average suspended matters
in the Danube at 32.68 parts in 100,000, but does not state at what
place.
Many of the French and German rivers drain prairie country not
different in its general aspect from the Mississippi basin, and the
soil is probably in many places similar. There is no reason to suppose
that the turbidities of these streams in general are materially
different from those of corresponding streams in the United States,
although it is true that, other things being equal, the average
turbidity of water taken for water-works purposes will increase with
the size of the stream; and it may be that some American streams,
especially the Ohio, Missouri, and Mississippi rivers, are of larger
size than European streams, and consequently that the turbidity of the
water taken from them for water-works purposes may be greater.
The following are the drainage areas of a number of European and
American streams yielding more or less muddy waters at points where
they are used for public water-supplies after filtration, with a few
other American points for comparison. The results are obtained in most
cases from measurements of the best available maps.
---------------------+------------------------+----------------
| | Drainage Area,
Place. | River. | Square Miles.
---------------------+------------------------+----------------
New Orleans, La. | Mississippi | 1,261,000
St. Louis, Mo. | Mississippi | 700,000
St. Petersburg | Neva | 108,000
Louisville, Ky. | Ohio | 90,000
Rock Island, Ill. | Mississippi | 88,000
Budapest | Danube | 79,000
Cincinnati, O. | Ohio | 75,700
Dordrecht | Maas | 68,000
Rotterdam | Maas | 68,000
Schiedam | Maas | 68,000
Altona | Elbe | 52,000
Hamburg | Elbe | 52,000
Stettin | Oder | 40,000
Magdeburg | Elbe | 36,000
Warsaw | Weichsel | 34,000
Odessa | Dneister | 26,000
Worms | Rhine | 25,000
Grand Forks, N. Dak. | Red River of the North | 22,000
Frankfort on Oder | Oder | 21,000
Bremen | Weser | 15,000
Suburbs of Paris | Seine | 12,000
Poughkeepsie, N. Y. | Hudson | 11,600
Pittsburg, Penn. | Allegheny | 11,400
Posen | Wartha | 9,400
Hudson, N. Y. | Hudson | 9,200
Albany, N. Y. | Hudson | 8,200
Breslau | Oder | 8,200
Brieg | Oder | 7,500
Lawrence, Mass. | Merrimac | 4,634
Stuttgart | Neckar | 1,660
Brunswick | Ocker | 650
Somersworth, N. H. | Salmon | 171
---------------------+------------------------+----------------
PRELIMINARY PROCESSES TO REMOVE MUD.
With both sand and mechanical filtration the difficulty and expense
of treatment of a water increase nearly in direct proportion to the
turbidity of the water as applied to the filter; and it is thus highly
important to secure a water for filtration with as little turbidity
as possible, and thus to develop to their economical limits the
preliminary processes for the removal of mud. One of the most important
of these processes is the use of reservoirs.
Reservoirs serve two purposes in connection with waters drawn
from streams: they allow sedimentation, and they afford storage.
If a water having a turbidity of 1.00 is allowed to remain in a
sedimentation-basin for 24 hours, its turbidity may be reduced by
as much as 40 per cent, or to 0.60. If it is held a second day the
additional reduction is much less.
If samples are taken of the water in the reservoir before and after
settling and sent to the chemist for analysis, he will probably report
that from 70 to 80 per cent of the suspended matters have been removed
by the process. The suspended matters are removed in much larger ratio
than the turbidity. This arises from the fact that there is a certain
proportion of comparatively coarse material in the water as it is
taken from the river. This coarse material increases the weight of the
suspended matters without increasing the turbidity in a corresponding
degree. In 24 hours the coarser materials are removed completely, and
at the end of that time only the clayey or finer particles remain in
suspension. It is these clayey particles, however, that constitute the
turbidity, which are most objectionable in appearance, and which are
most difficult of removal by filtration or otherwise.
Sedimentation thus removes the heavier matters from the water,
but it does not remove the finer matters which principally affect
the appearance of the water and are otherwise most troublesome. A
sedimentation of 24 hours removes practically all of the coarser
matters, and the clayey material remaining at the end of that time
can hardly be removed by further sedimentation. The economic limit of
sedimentation is about 24 hours.
Sedimentation has practically no effect upon the clearer waters between
flood periods.
Let us consider the effect of a sedimentation-basin, or reservoir
holding a 24-hours’ supply of water, into which water is constantly
pumped at one end, and from which an equal quantity is constantly
withdrawn from the other, upon the water of a stream of such size
that the time of passage of water from the feeders to the intake is
less than 24 hours. During the period between storms the water is
comparatively clear and passes through the sedimentation basin without
change. When a storm comes the water in the stream promptly becomes
muddy, and muddy water is supplied to the reservoir; but owing to
the time required for water to pass through it, the outflowing water
remains clear for some hours. There is a gradual mixing, however, and
long before the expiration of 24 hours somewhat muddy water appears
at the outlet. The turbid-water period rarely lasts in streams of
this size more than 24 hours, and at the expiration of that time the
water in the sedimentation-basin is as muddy or muddier than the water
flowing in the stream. After the height of the flood the stream clears
itself by the flowing away of the turbid water much more rapidly than
the water clears itself by sedimentation in the reservoir. That is to
say, if at the time of maximum turbidity we take a certain quantity of
water from the stream and put it aside to settle, at no time will the
improvement by settling equal the improvement which has taken place in
the stream from natural causes. Generally the improvement in the stream
is several times as rapid as in the sedimentation-basin, and the water
from it will at times have only a fraction of the turbidity of the
water in the basin.
Let us now consider what the sedimentation has done to improve the
water. During the period of clear water, that is for most of the
time, it has done nothing. For the first day of each flood period
very much clearer water has been obtained from it than was flowing
in the stream. For the first days following floods the water in the
sedimentation-basin has been more muddy than the water in the stream.
The only time when the sedimentation-basin has been of use is during
the first part of floods, that is, when the turbidity of the water in
the stream is increasing. During this period it has been of service
principally because of its storage capacity, yielding up water received
from the stream previously, when it was less muddy. Such sedimentation
as has been secured is merely incidental and generally not important in
amount.
It will be obvious from the above that for these conditions storage
is much more important than sedimentation. This brings us back to
the old English idea of having storage-reservoirs large enough to
carry water-works over flood periods without the use of flood-waters.
Reservoirs of this kind were, and still are, considered necessary for
the successful utilization of waters of many English rivers, although
these waters do not approach in turbidity the waters of some American
streams. This idea of storage has been but little used in the United
States.
In the above case, if we use our reservoir for storage instead of as a
sedimentation-basin, the average quality of the water can be greatly
improved. The reservoir should ordinarily be kept full, and pumping to
it should be stopped whenever the turbidity exceeds a certain limit,
to be determined by experience; and the reservoir is then to be drawn
upon for the supply until the turbidity again falls to the normal. In
the case assumed above, with a stream in which all of the water reaches
the intake in 24 hours, a reservoir holding a 24-hours’ supply, or in
practice, to be safe, a somewhat larger one, would yield a water having
a very much lower average turbidity than would be obtained with water
pumped constantly from the stream without a reservoir.
With a river having a watershed so long that 48 hours are required to
bring the water down from the most remote feeders, a reservoir twice as
large would be required, and would result in a still greater reduction
in the average turbidity.
As the stream becomes larger, and the turbid periods longer, the size
of a reservoir necessary to utilize this action rapidly becomes larger,
and the times during which it can be filled are shortened, and thus the
engineering difficulties of the problem are increased. For moderately
short streams, cost for cost, storage is far more effective than
sedimentation, and we must come back to the old English practice of
stopping our pumps during periods of maximum turbidity.
EFFECT OF MUD UPON SAND FILTERS.
There are two aspects of the effect of mud upon the operation of sand
filters which require particular consideration. The first relates to
the rapidity of clogging, and consequently the frequency of scraping
and the cost of operation; while the second relates to the ability of
the filters to yield well-clarified effluents.
EFFECT OF TURBIDITY UPON THE LENGTH OF PERIOD.
The amount of water which can be filtered between scrapings is directly
dependent upon the turbidity of the raw water. The greater the
turbidity, the more frequently will filters require to be scraped. In
the experiments of the Pittsburg Filtration Commission, with 4 feet of
sand of an effective size of about 0.30 millimeter, and with rates of
filtration of about three million gallons per acre daily, and with the
loss of head limited to 4 feet, sand filters were operated as follows:
For five periods the turbidities of the raw water ranged from 0.035 to
0.062, and averaged 0.051, and the corresponding periods ranged from
102 to 136, and averaged 113 million gallons per acre filtered between
scrapings. For ten periods the turbidities of the raw water ranged from
0.079 to 0.128, and averaged 0.102, and the periods averaged 78 million
gallons per acre between scrapings. For fifteen other periods the
turbidities of the raw water ranged from 0.134 to 0.269, and averaged
0.195, and the periods averaged 52 million gallons per acre between
scrapings. In two other periods the turbidities of the raw water
averaged 0.67, and the periods between scrapings averaged 16 million
gallons. In all cases the turbidity is taken as that of the water
applied to the filter. Usually this was the turbidity of the settled
water, but in some cases raw water was applied, and in these case the
turbidity of the raw water is taken. These results are approximately
represented by the formula
Period between scrapings, } = 12/(turbidity + 0.05).
million gallons per acre }
Except for very clear waters the amount of water passed between
scrapings is nearly inversely proportional to the turbidity. With twice
as great an amount of turbidity, filters will have to be cleaned twice
as often, the reserve area for cleaning will require to be twice as
great, and the cost of scraping filters and of washing and replacing
sand, which is the most important element in the cost of operation,
will be doubled.
With waters having turbidities of 0.20 upon this basis, the average
period will be about 51 million gallons per acre between scrapings.
This is about the average result obtained at the German works filtering
river waters, and there is no serious difficulty in operating filters
which require to be scraped with this frequency. With more turbid
waters the period is decreased. With an average turbidity of 0.50 the
average period is only 24 million gallons per acre between scrapings, a
condition which means very difficult operation and a very high cost of
cleaning. With much more turbid waters the difficulties are increased,
and if the duration of turbid water should be long-continued, the
operation of sand filters would clearly be impracticable, and the
expense, also, would be prohibitive.
In applying these figures to actual cases it must be borne in mind
that the turbidity is only one of the several factors which control
the length of period; and that the turbidity of a water of a given
stream is never constant, but fluctuates within wide limits; and that
raw water can be applied to filters for a short time without injurious
results, even though it is so turbid that its continued application
would be fatal.
It is very likely also that the suspended matters in different streams
differ in their natures to such an extent that equal turbidities would
give quite different periods, although the Pittsburg results were so
regular as to give confidence in their application to other conditions
within reasonable limits, and when so applied they afford a most
convenient method of computing the approximate cost of operation of
filters for waters of known or estimated turbidities.
POWER OF SAND FILTERS TO PRODUCE CLEAR EFFLUENTS FROM MUDDY WATER.
When the turbidity of the applied water is not too great it is entirely
removed in the course of filtration. With extremely muddy raw waters,
however, turbid effluents are often produced with sand filters.
The conditions which control the passage of the finest suspended
matters through filters have been studied by Mr. Fuller at Cincinnati
at considerable length. They are similar in a general way to the
conditions which control the removal of bacteria. That is to say, the
removal is more complete with fine filter sand than with coarse sand;
with a deep sand layer than with a shallow sand layer; and with low
rates of filtration than with high rates. The practicable limits to
the size of sand grain, depth of sand layer, and rate of filtration
are established by other conditions, and the question remains whether
within these limits a clear effluent can be produced.
At Pittsburg the turbidity of the effluent from a sand filter operated
as mentioned above, which received water which had passed through a
sedimentation-basin holding about a 24-hours’ supply, but without
taking any advantage of storage to avoid the use of muddy water, was
nearly always less than 0.02, which may be taken as the admissible
limit of turbidity in a public water-supply. This limit was exceeded on
less than 20 days out of 365, these days being during the winter and
spring freshets, and on these days the excess was not such as would be
likely to be particularly objectionable. For the water of the Allegheny
River, then, sand filtration with one day’s sedimentation is capable
of yielding a water not absolutely clear, but sufficiently clear to be
quite satisfactory for the purpose of municipal water-supply.
At Cincinnati, on the other hand, where the amount of suspended matters
was five times as great as at Pittsburg, the effluents which could be
obtained by sand filtration without recourse to the use of alum, even
under most favorable conditions, were very much more turbid than those
obtained at Pittsburg, and were, in fact, so turbid as to be seriously
objectionable for the purpose of public water-supply.
With rivers no more turbid than the Allegheny River at Pittsburg, and
rivers having floods of such short duration that the use of flood-flows
can be avoided by the use of reservoirs, sand filters are adequate for
clarification. For waters which are much muddier than the Allegheny,
as, for instance, the Ohio at Cincinnati and at Louisville, sand
filtration alone is inadequate. Mr. Fuller,[31] as a result of his
Cincinnati experiments, has stated the case as follows:
“For the sake of explicitness it is desired to show, with the data
of the fairly normal year of 1898, the proportion of the time when
English filters (that is, sand filters) would be inapplicable in the
purification of the unsubsided Ohio River water at Cincinnati. This
necessitates fixing an average limit of permissible suspended matter in
this river water, and is a difficult matter from present evidence.
“In part this is due to variations in the character and in the relative
amounts of the suspended silt, clay, and organic matter; and in part
it is due to different amounts of clay stored in the sand layer, which
affects materially the capacity of the filter to retain the clay of the
applied water. During these investigations the unsubsided river-water
was not regularly applied to filters; and, with the exception of
the results of tests for a few days only, it is necessary to depend
upon general information obtained with reference to this point. So
far as the information goes, it appears that an average of 125 parts
per million is a conservative estimate of the amount of suspended
matters in the unsubsided river-water, which could be regularly and
satisfactorily handled by English filters. But at times this estimated
average would be too low, and at other times too high....
“While English filters are able to remove satisfactorily on an
average about 125 parts of silt and clay of the unsubsided water,
actual experience shows that they can regularly handle suspended
clay in subsided water in amounts ranging only as high as from 30 to
70 parts (depending upon the amount of the clay stored in the sand
layer), and averaging about 50 parts per million. But it is true that
for two or three days on short rises in the river, or at the beginning
of long freshets, the retentive capacity of the sand layer allows of
satisfactory results with the clay in the applied water considerably in
excess of 70 parts. If this capacity is greatly overtaxed, however, the
advantage is merely temporary, as the stored clay is washed out later,
producing markedly turbid effluents.”
Translating Mr. Fuller’s results into terms of turbidity, the 125
parts per million of suspended matters in the raw water represent a
turbidity of about 0.40, and the 30 to 70 parts of suspended matters in
the settled water represent turbidities from 0.20 to 0.40, the average
of 50 parts of suspended matters corresponding to a turbidity of about
0.30.
Upon this basis, then, sand filters are capable of treating raw waters
with average turbidities up to 0.40, or settled waters with average
turbidities up to 0.30, but waters more turbid than this are incapable
of being successfully treated without the use of coagulants or other
aids to the process. These results are in general accordance with
the results of the experiments at Pittsburg, and demonstrate that
while sand filters as generally used in Europe are adequate for the
clarification of many, if not most, river waters in the United States,
there are other waters carrying mud in such quantities as to make the
process inapplicable to them.
EFFECT OF MUD UPON BACTERIAL EFFICIENCY OF FILTERS.
The question is naturally raised as to whether or not the presence of
large quantities of mud in the raw water will not seriously interfere
with the bacterial efficiency of filters. Experiments at Cincinnati
and Pittsburg have given most conclusive and satisfactory information
upon this point. Up to the point where the effluents become quite
turbid, the mud in the raw water has no influence upon the bacterial
efficiency; and even somewhat beyond this point, with effluents so
turbid that they would hardly be suitable for the purpose of a public
water-supply, the bacterial efficiency remains substantially equal to
that obtained with the clearest waters. Only in the case of excessive
quantities of mud, where, for other reasons, sand filters can hardly
be considered applicable, is there a moderate reduction in bacterial
efficiency. As mentioned above, particles constituting turbidity are
often much smaller than the bacteria, and in addition, the bacteria
probably have an adhesive power far in excess of that of the clay
particles. For these reasons clay particles are able to pass filters
under conditions which almost entirely prevent the passage of bacteria.
On the other hand, it does not necessarily follow that the removal
of turbidity is accompanied by high bacterial efficiency. Although
this is often the case, there are marked exceptions, particularly in
connection with the use of coagulants, where very good clarification is
obtained, and notwithstanding this, effluents are produced containing
comparatively large numbers of bacteria.
LIMITS TO THE USE OF SUBSIDENCE FOR THE PRELIMINARY TREATMENT OF MUDDY
WATERS.
When water is too muddy to be applied directly to filters, the most
obvious treatment is to remove as much of the sediment as possible by
sedimentation. Sedimentation-basins are considered as essential parts
of filtration plants for the treatment of muddy waters. The effect of
sedimentation, as noted above, is to remove principally the larger
particles in the raw water. By doing this the deposit upon the surface
of the filters and the cost of operation are greatly reduced.
These larger particles are mainly removed by a comparatively short
period of sedimentation, and the improvement effected after the first
24 hours is comparatively slight. The particles remaining in suspension
at the end of this time consist almost entirely of very fine clay, and
the rate of their settlement through the water is extremely slow; and
currents in the basin, due to temperature changes, winds, etc., almost
entirely offset the natural tendency of the sediment to fall to the
bottom.
There is thus a practical limit to the effect of sedimentation which is
soon reached, and it has not been found feasible to extend the process
so as to allow much more turbid waters to be brought within the range
which can be economically treated by sand filtration.