TREATMENT OF WATERS.
Having now reviewed the most important methods in use for the treatment
of waters, we may take a general view of their application to various
classes of waters. Different raw waters vary so much, and the
requirements of filtration are so different, that it is not possible
to outline any general procedure or combination of procedures, but
each problem must be taken up by itself. Nevertheless, some general
suggestions may be of service.
In the first place, we may consider the case of waters containing very
large quantities of oxidizable organic matter. Such waters are obtained
from some reservoirs containing very active vegetable and animal
growths, or from rivers receiving large amounts of sewage. Waters
of both of these classes are, if possible, to be avoided for public
water-supplies. When circumstances require their use, they can best be
treated by intermittent filtration, this process being best adapted to
the destruction by oxygen of excessive quantities of organic matter.
Where the pollution is less, so that the dissolved oxygen contained
in the raw water is sufficient for the oxidation of the organic
matters, continuous filtration will give substantially as good results
as intermittent filtration, and in other respects it has important
advantages. The application of intermittent filtration for the
treatment of public water-supplies is thus somewhat limited, and, as a
matter of fact, it has been used in only a few cases.
For the treatment of very highly polluted waters double filtration has
been used in a number of cases, notably by the Grand Junction Company
at London, at Schiedam in Holland, and at Bremen and Altona in Germany.
At the two first-mentioned places two separate systems of filters are
provided differing somewhat in construction, the first filters being at
a higher level than the after filters. The first filters supply water
of comparative purity, and very constant composition, to the after
filters, which are able to treat it with great efficiency and at very
low operating cost.
This procedure is probably the most perfect which has been used for the
removal of disease-producing qualities from highly polluted waters; and
the cost of the process may not be as much greater than that of simple
filtration as would at first appear, because the cost of cleaning the
after filters is merely nominal, and the attendance, pumping, etc.,
are practically common to both sets of filters, and are not materially
greater than they would be for a single set.
For very bad waters the first filters might appropriately be
intermittent, while the after filters should be continuous. This was
the procedure originally intended for Lawrence, but the intermittent
filter first constructed yielded such very good results that it has not
been considered necessary to complete the plant as originally projected.
At Bremen and at Altona a different procedure has been adopted. The
filters are all upon the same level, and of the same construction.
When a filter is put in service the effluent from it, instead of being
taken to the pure-water reservoir, is taken to another filter which
has already been some time in service. After the first filter has been
in operation for some time its effluent is taken to the pure-water
reservoir, and in turn it is supplied with the effluent from a filter
more recently cleaned. The loss of head of water passing a freshly
cleaned filter is comparatively slight, and the water of the second
filter is allowed to fall a few inches below the high-water mark,
at which level it will take the effluent from the other filter. The
connections between the filters are made by siphons of large pipe, the
summits of which are considerably above the high-water line. These
siphons are filled by exhausting the air, and when opened to the air
there is no possibility of a flow of water through them. The process
has given extremely good results in practice, yielding effluents of the
very greatest purity and at a quite moderate cost of operation.
An objection to the method is the possible filling of a siphon some
time when the water standing upon the after-filter is higher than that
in the pure-water well of the fore-filter, and while the fore-filter is
connected with the pure-water reservoir. Such a connection would send
unfiltered water into the pure-water reservoir direct. I do not know
that any trouble of this kind has ever been experienced at Bremen or at
Altona; and the objection to this system is perhaps not well founded
where the management is careful and conscientious. The fact that an
unscrupulous attendant can make the connection at any time to help out
a deficiency of supply, or simply through carelessness, is certainly
objectionable.
For the treatment of river-waters and lake-waters containing only
a small quantity of sediment, and where the removal of bacteria or
disease-producing qualities is the most important object of filtration,
sand filters can be used. Where the rivers are subject to floods and
moderate amounts of muddy water, sedimentation-basins or storage
reservoirs for raw water will often be found advantageous.
For the treatment of extremely muddy waters, and waters which are
continuously muddy for long periods of time, and for the removal
of color from very highly colored waters, resource must be had to
coagulants. The coagulants which are necessary in each special case and
which can be used without injury to the water must be determined by
most careful investigation of the raw water.
For the filtration of these waters after coagulation either sand or
mechanical filters can be employed. As the principal work in this
case is done by the coagulant, the kind of filtration employed is
of less consequence than where filtration alone is relied upon,
and the cheapest form of filter will naturally be employed. Under
present conditions mechanical filters will usually be cheaper than
sand filters for use in this way; but where waters, in addition to the
mud, carry bacteria in such large numbers as to make high bacterial
efficiency a matter of importance, sand filters may be selected, as the
bacterial efficiency obtained with them is not dependent upon the use
of coagulant; and is therefore less subject to interruptions from the
failure to apply coagulant in the right proportion.
Mechanical filters have also been used for the treatment of
comparatively clear waters where bacterial efficiency was the principal
object of filtration. For this purpose the efficiencies obtained with
them are usually inferior to those obtained with sand filters, while
the cost of coagulants is so great as to make their use often more
expensive than that of sand filters.
In the case of many streams which are comparatively clear for a part of
the year, but occasionally are quite turbid, the use of sand filters
has this advantage, that the use of coagulants can be stopped and the
cost of operation reduced whenever the water is clear enough to allow
of satisfactory treatment by them; and that coagulant can be employed
on those days when otherwise insufficient clarification would be
obtained.
In this case the high bacterial efficiency is secured at all times,
while the cost of coagulant is saved during the greater part of the
time. In such cases, also, the preliminary process of sedimentation and
storage should be developed as far as possible.
The application of other processes of filtration to special problems
are not sufficiently well understood to allow general discussion, and
must be taken up separately with reference to the requirements of each
special situation.
COST OF FILTRATION.
The cost of filtration of water depends upon the character of the raw
water, upon the nature of the plant employed, upon its size, and
upon the skill and economy of manipulation. These conditions affect
the cost to such an extent as to make any accurate general estimate
quite impossible. Nevertheless a little consideration of the subject,
although not leading to exact results, may be helpful as furnishing a
rough idea of the probable cost before estimates for local conditions
are made.
Open sand filters, with masonry walls, with reasonably favorable
conditions of construction, and not too small in area, have averaged
to cost in the United States within the last few years perhaps about
thirty thousand dollars per acre. The relative cost of small plants is
somewhat greater, and with embankments instead of masonry walls, the
cost is somewhat reduced. The cost is less where natural deposits of
sand can be made use of practically in their original condition, and is
increased where the filtering materials have to be transported by rail
for long distances, or where the sites are difficult to build upon.
Covered filters cost about a half more than open filters. Mechanical
filters at current prices cost about $20 per square foot of filtering
area, to which must be added the cost of foundations and buildings,
which perhaps average to cost half as much more, but are dependent upon
local conditions and the character of the buildings.
To these figures must be added the costs of pumps, reservoirs,
sedimentation-basins, and pipe-connections, which are often greater
than the costs of the filters, but which differ so widely in different
cases as to make any general estimate impossible.
Filters must be provided sufficient to meet the maximum and not the
average consumption. The excess of maximum over average requirements
varies greatly in different cities, and depends largely upon reservoir
capacities and arrangements.
As a result of a considerable number of estimates made by the author
for average American conditions, the cost of installing filters may
be taken very roughly as five dollars per inhabitant, but the amounts
differ widely in various cases.
The cost of operation of sand filters in England probably averages
about one dollar per million gallons of water filtered. The following
table shows the costs of operation of the filters of the seven London
companies for fifteen years, compiled in the office of Mr. W. B. Bryan,
Chief Engineer of the East London Water Company. The results have been
computed to dollars per million U. S. gallons, and include the cost of
all labor, sand, and supplies for the filters, but do not include any
pumping or interest costs.
COST OF FILTRATION, LONDON WATER COMPANIES.
(Computed from data furnished Wm. B. Bryan, C.E., East London Water
Works.)
Dollars per Million U. S. Gallons.
--------+-------+------+--------+-------+-----+---------+---------+--------
| | East | Grand | | New |Southwark| West |
|Chelsea|London|Junction|Lambeth|River| & |Middlesex|Average.
| Co. | Co. | Co. | Co. | Co. |Vauxhall | Co. |
| | | | | | Co. | |
--------+-------+------+--------+-------+-----+---------+---------+--------
1880-1 | 1.16 | 1.16 | 1.00 | 0.83 |1.34 | 1.16 | 1.67 | 1.19
1881-2 | 1.19 | 1.39 | 0.95 | 0.82 |1.15 | 1.37 | 1.54 | 1.20
1882-3 | 1.10 | 1.23 | 1.39 | 0.96 |1.40 | 1.47 | 1.74 | 1.33
1883-4 | 1.00 | 1.06 | 1.73 | 0.92 |1.11 | 1.62 | 1.67 | 1.30
1884-5 | 1.06 | 1.06 | 1.82 | 0.90 |1.02 | 1.40 | 1.30 | 1.22
1885-6 | 1.15 | 1.16 | 1.35 | 0.90 |1.00 | 1.15 | 1.07 | 1.11
1886-7 | 0.80 | 0.96 | 1.39 | 0.87 |0.98 | 1.43 | 1.70 | 1.16
1887-8 | 1.07 | 1.22 | 1.74 | 0.90 |0.92 | 1.28 | 1.00 | 1.16
1888-9 | 0.83 | 1.28 | 1.55 | 0.95 |0.98 | 1.52 | 0.83 | 1.13
1889-90 | 0.66 | 1.50 | 1.22 | 0.88 |0.90 | 1.70 | 3.56 | 1.49
1890-1 | 0.72 | 1.42 | 1.32 | 0.85 |1.02 | 1.16 | 1.00 | 1.07
1891-2 | 0.75 | 1.54 | 1.23 | 1.00 |0.92 | 1.15 | 0.96 | 1.08
1892-3 | 0.67 | 1.42 | 1.30 | 1.19 |1.16 | 1.26 | 1.42 | 1.20
1893-4 | 1.15 | 2.63 | 2.00 | 1.46 |1.43 | 1.52 | 0.95 | 1.59
1894-5 | 0.60 | 1.68 | 1.67 | 2.53 |1.03 | 1.34 | 0.96 | 1.40
--------+-------+------+--------+-------+-----+---------+---------+--------
Average | 0.93 | 1.38 | 1.44 | 1.06 |1.09 | 1.37 | 1.43 | 1.24
--------+-------+------+--------+-------+-----+---------+---------+--------
Average of seven companies for 15 years, $1.24 per million gallons.
Variations from year to year are caused by differences in the amounts
of ice, and in the quantities of new sand purchased. Wages average
about $1.00 per day. At Liverpool for 1896 the cost was $1.08 per
million U. S. gallons.
In Germany, with more turbid river-waters, the costs of operation are
somewhat higher than the London figures, while at Zürich, where the
water is very clear, they are lower.
In the United States the data regarding the cost of operation of sand
filters are less complete. At Mt. Vernon, N. Y., with reservoir-water,
the cost has averaged about two dollars per million gallons. At
Poughkeepsie, N. Y., with the Hudson River water, which is occasionally
moderately turbid, the cost for twenty years has averaged three dollars
per million gallons. This cost includes the cost of handling ice, and
as the average winter temperature is considerably below that suggested
for open filters, the expense of this work has been considerable, and
has increased considerably the total cost of operation.
At Far Rockaway, L. I., and Red Bank N. J., for iron-removal plants,
the cost of operation has hardly been appreciable. The plants are both
close to the pumping-stations, and it has been possible to operate
them with the labor necessarily engaged at the pumping-station without
additional cost, except a very small amount of labor on the sand at Far
Rockaway. No computation has been made in these cases of the additional
coal required for pumping.
At Lawrence, Mass., the cost of operation for 1895 was as follows:
Cost of scraping and replacing sand $3,467
Cost of care of ice 2,903
------
Total cost of operation $6,370
Water filtered, millions of gallons 1,097
Cost per million gallons $5.80
The cost of care of ice has been excessive at Lawrence, and it has
been repeatedly recommended to cover the filter to avoid this expense.
The cost of handling sand has been very greatly increased, because the
filter is built in one bed, and all work upon it has to be done during
the comparatively short intervals when the filter is not in use, an
arrangement which is not at all economical in the use of labor. The
cost of operation is thus much higher than it would be had the plant
been constructed in several units, each of which could be disconnected
for the purpose of being cleaned in the ordinary manner. As against
this the first cost of construction was extremely low, and the saving
in interest charges should be credited against the increased cost of
labor in cleaning.
The cost of operating filters at Ashland, Wis., has been estimated by
Mr. William Wheeler at $2.26 per million gallons. This estimate is
based upon the performance for the first year that they were in service.
In the operation of mechanical filters one of the largest items of
expense is for the coagulant, and the amount of this depends entirely
upon the character of the raw water and the thoroughness of the
treatment required. The data regarding the other or general costs of
operation of mechanical filters are few and unsatisfactory.
I recently made some estimates of cost of clarifying waters of various
degrees of turbidity by sand and mechanical filters. These estimates
were made for a special set of conditions, and I do not know that
they will fit others, but they have at least a suggestive value. The
results shown by Fig. 26 include only the cost of operation, and not
interest and depreciation charges. These figures, when used for plants
in connection with which preliminary treatments are used, should be
applied to the turbidity of the water as applied to the filters, and
not to the raw water, and the costs of the preliminary processes should
be added.
With sand filters the frequency of scraping is nearly proportional
to the turbidity; and as scraping represents most of the expenses,
the costs of operation are proportional to the turbidity, except
the general costs, and the cost of the amount of scraping, which is
necessary with even the clearest waters.
With mechanical filters the amount of sulphate of alumina required for
clarification increases with the turbidity, and most of the costs of
operation increase in the same ratio. The diagram shows the amount of
sulphate of alumina in grains per gallon necessary for clarification
with different degrees of turbidity.
With the clearest waters the costs of operation on the two systems are
substantially equal. With muddy waters, the expense of operating sand
filters increases more rapidly than the expense of operating mechanical
filters.
[Illustration: TURBIDITY
FIG. 26.—COST OF OPERATION OF FILTERS.]
There is another element which often comes into the comparison, namely,
the question of purification from the effects of sewage-pollution.
Nearly all rivers used for public water-supplies receive more or less
sewage, and in filtering such waters it is regarded as necessary to
remove as completely as possible the bacteria.
The quantities of sulphate of alumina required for the clarification
of the least turbid waters are not sufficient to give even tolerably
good bacterial efficiencies. To secure a reasonably complete removal of
bacteria with mechanical filters, the use of a considerable quantity
of sulphate of alumina is required. Let us assume that 98 per cent
bacterial efficiency is required, and that to produce this efficiency
it is necessary to use one grain of coagulant to the gallon. With water
requiring less than this quantity of coagulant for clarification this
quantity must nevertheless be used, and the costs will be controlled
by it, and not by the lower quantities which would suffice for
clarification, but would not give the required bacterial efficiency.
I have added this line to the diagram, and this, combined with the
upper portion of the line showing cost of clarification, represents the
cost of treating waters with mechanical filters, where both bacterial
efficiency and clarification are required.
This line, considered as a whole, increases much less rapidly with
increasing turbidity than does the corresponding line for sand filters,
and the two lines cross each other. With the clearest waters sand
filters are cheaper than mechanical filters, and for the muddiest
waters they are more expensive. It does not appear from the diagram,
but it is also true in each case, that the cheaper system is also the
more efficient. Sand filters are more efficient in removing bacteria
from clear waters than are mechanical filters, and mechanical filters
are more efficient in clarifying very muddy waters than are sand
filters.
WHAT WATERS REQUIRE FILTRATION?
From the nature of the case a satisfactory general answer to this
question cannot be given, but a few suggestions may be useful.
In the first place, ground-waters obviously do not require filtration:
they have already in most cases been thoroughly filtered in the ground
through which they have passed, and in the exceptional cases, as, for
instance, an artesian well drawing water through fissures in a ledge
from a polluted origin, a new supply will generally be chosen rather
than to attempt to improve so doubtful a raw material.
River-waters should be filtered. It cannot be asserted that there
are no rivers in mountainous districts in which the water is at once
clear and free from pollution, and suitable in its natural state
for water-supply; but if so, they are not common, least of all in
the regions where water-supplies are usually required. The use of
river-waters in their natural state or after sedimentation only,
drawn from such rivers as the Merrimac, Hudson, Potomac, Delaware,
Schuylkill, Ohio, and Mississippi, is a filthy as well as an unhealthy
practice, which ought to be abandoned.
The question is more difficult in the case of supplies drawn from
lakes or storage reservoirs. Many such supplies are grossly polluted
and should be either abandoned or filtered. Others are subject to algæ
growths, or are muddy, and would be much improved by filtration. Still
others are drawn either from unpolluted water-sheds, or the pollution
is so greatly diluted and reduced by storage that no known disadvantage
results from their use.
In measuring the effects of the pollution of water-supplies, the
typhoid-fever death-rate is a most important aid. Not that typhoid
fever is the sole evil resulting from polluted water, but because it
is also a very useful index of other evils for which corresponding
statistics cannot be obtained, as, for instance, the causation of
diarrhœal diseases or the danger from invasion by cholera.
I think we shall not go far wrong at the start to confine our attention
to those cities where there are over 25 deaths from typhoid fever per
100,000 of population. This will at once throw out of consideration
a large number of relatively good supplies, including those of New
York and Brooklyn. It is not my idea that none of these supplies
cause disease. Many of them, as for instance that of New York, are
known to receive sewage, and it is an interesting question worthy of
most careful study whether there are cases of sickness resulting from
this pollution. The point that I wish to make now is simply that in
those cases the death-rate itself is evidence that, with existing
conditions of dilution and storage, the resulting damage of which we
have knowledge is not great enough to justify the expense involved by
filtration.
In this connection it should not be forgotten that, especially with
very small watersheds, there may be a danger as distinct from present
damage which requires consideration. Thus a single house or groups of
houses draining into a supply may not appreciably affect it for years,
until an outbreak of fever on the water-shed results in infecting the
water with the germs of disease and in an epidemic in the city below.
This danger decreases with increasing size of the water-shed and volume
of the water with which any such pollution would be mixed, and also
with the population draining into the water, as there is a probability
that the amount of infection continually added from a considerable town
will not be subject to as violent fluctuation as that from only a few
houses.
Thus in Plymouth, Pa., in 1885, there were 1104 cases of typhoid
fever and 114 deaths among a population of 8000, as the result of the
discharge of the dejecta from a single typhoid patient into the water
of a relatively small impounding reservoir. The cost of this epidemic
was calculated with unusual care. The care of the sick cost in cash
$67,100.17, and the loss of wages for those who recovered amounted to
$30,020.08. The 114 persons who died were earning before their sickness
at the rate of $18,419.52 annually.
Such an outbreak would hardly be possible with the Croton water-shed
of the New York water-supply, on account of the great dilution and
delay in the reservoirs, but it must be guarded against in small
supplies.
Of the cities having more than 25 deaths per 100,000 from typhoid
fever, some will no doubt be found where milk epidemics or other
special circumstances were the cause; but I believe in a majority
of them, and in nearly all cases where the rate is year after year
considerably above that figure, the cause will be found in the
water-supply. Investigation should be made of this point; and if the
water is not at fault, the responsibility should be located. If the
water is guilty, it should be either purified or a new supply obtained.