CONTINUOUS FILTERS AND THEIR CONSTRUCTION.
Filtration of water consists in passing it through some substance
which retains or removes some of its impurities. In its simplest form
filtration is a straining process, and the results obtained depend upon
the fineness of the strainer, and this in turn is regulated by the
character of the water and the uses to which it is to be put. Thus in
the manufacture of paper an enormous volume of water is required free
from particles which, if they should become imbedded in the paper,
would injure its appearance or texture. Obviously for this purpose the
removal of the smaller particles separately invisible to the unaided
eye, and thus not affecting the appearance of the paper, and the
removal of which would require the use of a finer filter at increased
expense, would be a simple waste of money. When, however, a water is
to be used for a domestic water supply and transparency is an object,
the still finer particles which would not show themselves in paper, but
which are still able, in bulk, to render a water turbid, should be as
far as possible removed, thus necessitating a finer filter; and, when
there is reason to think that the water contains the germs of disease,
the filter must be fine enough to remove with certainty those organisms
so extraordinarily small that millions of them may exist in a glass of
water without imparting a visible turbidity.
It is now something over half a century since the first successful
attempts were made to filter public water-supplies, and there are
now hundreds of cities supplied with clear, healthy, filtered water.
(Appendix IV.) While the details of the filters used in different
places present considerable variations, the general form is, in
Europe at least, everywhere the same. The most important parts of a
filter are shown by the accompanying sketch, in which the dimensions
are much exaggerated. The raw water is taken from the river into a
settling-basin, where the heaviest mud is allowed to settle. In the
case of lake and pond waters the settling-tank is dispensed with, but
it is essential for turbid river-water, as otherwise the mud clogs
the filter too rapidly. The partially clarified water then passes to
the filter, which consists of a horizontal layer of rather fine sand
supported by gravel and underdrained, the whole being enclosed in a
suitable basin or tank. The water in passing through the sand leaves
behind upon the sand grains the extremely small particles which were
too fine to settle out in the settling-basin, and is quite clear as it
goes from the gravel to the drains and the pumps, which forward it to
the reservoir or city.
[Illustration: FIG. 1.—SKETCH SHOWING GENERAL ARRANGEMENT OF FILTER
PLANTS.]
The passages between the grains of sand through which the water must
pass are extremely small. If the sand grains were spherical and 1/50 of
an inch in diameter, the openings would only allow the passage of other
spheres 1/320 of an inch in diameter, and with actual irregular sands
much finer particles are held back. As a result the coarser matters
in the water are retained on the surface of the sand, where they
quickly form a layer of sediment, which itself becomes a filter much
finer than the sand alone, and which is capable of holding back under
suitable conditions even the bacteria of the passing water. The water
which passes before this takes place may be less perfectly filtered,
but even then, the filter may be so operated that nearly all of the
bacteria will be deposited in the sand and not allowed to pass through
into the effluent.
As the sediment layer increases in thickness with continued filtration,
increased pressure is required to drive the desired volume of water
through its pores, which are ever becoming smaller and reduced in
number. When the required quantity of water will no longer pass with
the maximum pressure allowed, it is necessary to remove, by scraping,
the sediment layer, which should not be more than an inch deep. This
layer contains most of the sediment, and the remaining sand will then
act almost as new sand would do. The sand removed may be washed for use
again, and eventually replaced when the sand layer becomes too thin
by repeated scrapings. These operations require that the filter shall
be temporarily out of use, and as water must in general be supplied
without intermission, a number of filters are built together, so that
any of them can be shut out without interfering with the action of the
others.
The arrangement of filters in relation to the pumps varies with local
conditions. With gravity supplies the filters are usually located below
the storage reservoir, and, properly placed, involve only a few feet
loss of head.
In the case of tidal rivers, as at Antwerp and Rotterdam, the quality
of the raw water varies with the tide, and there is a great advantage
in having the settling-basins low enough so that a whole day’s supply
can be rapidly let in when the water is at its best, without pumping.
At Antwerp the filters are higher, and the water is pumped from the
settling basins to them, and again from the reservoir receiving the
effluents from the filters to the city. In several of the London
works (East London, Grand Junction, Southwark and Vauxhall, etc.) the
settling-basins are lower than the river, and the filters are still
lower, so that a single pumping suffices, that coming between the
filter and the city, or elevated distributing reservoir.
In many other English filters and in most German works the
settling-basins and filters are placed together a little higher than
the river, thus avoiding at once trouble from floods and cost for
excavation. The water requires to be pumped twice, once before and once
after filtration. At Altona the settling-basins and filters are placed
upon a hill, to which the raw Elbe water is pumped, and from which it
is supplied to the city after filtration by gravity without further
pumping. The location of the works in this case is said to have been
determined by the location of a bed of sand suitable for filtration on
the spot where the filters were built.
When two pumpings are required they are frequently done, especially in
the smaller places, in the same pumping-station, with but one set of
boilers and engines, the two pumps being connected to the same engine.
The cost is said to be only slightly greater than that of a single
lift of the same total height. In very large works, as at Berlin and
Hamburg and some of the London companies, two separate sets of pumping
machinery involve less extra cost relatively than would be the case
with smaller works.
SEDIMENTATION-BASINS.
Kirkwood[2] found in 1866 that sedimentation-basins were essential
to the successful treatment of turbid river-waters, and subsequent
experience has not in any way shaken his conclusion. The German works
visited by him, Berlin (Stralau) and Altona, were both built by English
engineers, and their settling-basins did not differ materially from
those of corresponding works in England. Since that time, however,
there has been a well-marked tendency on the part of the German
engineers to use smaller, while the English engineers have used much
larger sedimentation-basins, so that the practices of the two countries
are now widely separated, the difference no doubt being in part at
least due to local causes.
Kirkwood found sedimentation-basins at Altona with a capacity of
2-1/4 times the daily supply. In 1894 the same basins were in use,
although the filtering area had been increased from 0.82 acre to 2.20
acres, and still more filters were in course of construction, and
the average daily quantity of water had increased from 600,000 to
4,150,000 gallons in 1891-2, or more than three times the capacity of
the sedimentation-basins. In 1890 the depth of mud deposited in these
basins was reported to be two feet deep in three months. At Stralau in
Berlin, also, in the same time the filtering area was nearly doubled
without increasing the size of the sedimentation-basins, but the Spree
at this point has such a slow current that it forms itself a natural
sedimentation-basin. At Magdeburg on the Elbe works were built in 1876
with a filtering area of 1.92 acres, and a sedimentation-basin capacity
of 11,300,000 gallons, but in 1894 half of the latter had been built
over into filters, which with two other filters gave a total filtering
surface of 3.90 acres, with a sedimentation-basin capacity of only
5,650,000 gallons. The daily quantity of water pumped for 1891-2 was
5,000,000 gallons, so that the present sedimentation-basin capacity is
about equal to one day’s supply, or relatively less than a third of the
original provision. The idea followed is that most of the particles
which will settle at all will do so within twenty-four hours, and that
a greater storage capacity may allow the growth of algæ, and that the
water may deteriorate rather than improve in larger tanks.
[Illustration: PAVED EMBANKMENT BETWEEN TWO FILTERS, EAST LONDON.]
[Illustration: FILTERS AND CHANNELS FOR RAW WATER, ANTWERP.]
[_To face page 10._]
At London, on the other hand, the authorities consider a large storage
capacity for unfiltered water as one of the most important conditions
of successful filtration, the object however, being perhaps as much to
secure storage as to allow sedimentation. In 1893 thirty-nine places
were reported upon the Thames and the Lea which were giving their
sewage systematic treatment before discharging it into the streams
from which London’s water is drawn. These sewage treatments are, with
hardly an exception, dry-weather treatments, and as soon as there is
a considerable storm crude sewage is discharged into the rivers at
every point. The rivers are both short, and are quickly flooded, and
afterwards are soon back in their usual condition. At these times of
flood, the raw water is both very turbid and more polluted by sewage
than at other times, and it is the aim of the authorities to have the
water companies provide reservoir capacity enough to carry them through
times of flood without drawing any water whatever from the rivers. This
obviously involves much more extensive reservoirs than those used in
Germany, and the companies actually have large basins and are still
adding to them. The storage capacities of the various companies vary
from 3 to 18 times the respective average daily supplies, and together
equal 9 times the total supply.
In case the raw water is taken from a lake or a river at a point where
there is but little current, as in a natural or artificial pond,
sedimentation-basins are unnecessary. This is the case at Zürich (lake
water), at Berlin when the rivers Havel and Spree spread into lakes, at
Tegel and Müggel, and at numerous other works.
SIZE OF FILTER-BEDS.
The total area of filters required in any case is calculated from the
quantity of water required, the rate of filtration, and an allowance
for filters out of use while being cleaned. To prevent interruptions
of the supply at times of cleaning, the filtering area is divided into
beds which are operated separately, the number and size of the beds
depending upon local conditions. The cost per acre is decreased with
large beds on account of there being less wall or embankment required,
while, on the other hand, the convenience of operation may suffer,
especially in small works. It is also frequently urged that with large
filters it is difficult or impossible to get an even rate of filtration
over the entire area owing to the frictional resistance of the
underdrains for the more distant parts of the filter. A discussion of
this point is given in Chapter III, page 41. At Hamburg, where the size
of the single beds, 1.88 acres each, is larger than at any other place,
it is shown that there is no serious cause for anxiety; and even if
there were, the objectionable resistance could be still farther reduced
by a few changes in the under-drains. The sizes of filter-beds used at
a large number of places are given in Appendix IV.
At a number of places having severe winters, filters are vaulted over
as a protection from cold, and in the most important of these, Berlin,
Warsaw, and St. Petersburg, the areas of the single beds are nearly
the same, namely, from 0.52 to 0.59 acre. The works with open filters
at London (seven companies), Amsterdam, and Breslau have filter-beds
from 0.82 to 1.50 acres each. Liverpool and Hamburg alone use filters
with somewhat larger areas. Large numbers of works with both covered
and open filters have much smaller beds than these sizes, but generally
this is to avoid too small a number of divisions in a small total area,
although such works have sometimes been extended with the growth of the
cities until they now have a considerable number of very small basins.
FORM OF FILTER-BEDS.
The form and construction of the filter-beds depend upon local
conditions, the foundations, and building materials available, the
principles governing these points being in general the same as for the
construction of ordinary reservoirs. The bottoms require to be made
water-tight, either by a thin layer of concrete or by a pavement upon
a puddle layer. For the sides either masonry walls or embankments are
used, the former saving space, but being in general more expensive in
construction. Embankments must, of course, be substantially paved near
the water-line to withstand the action of ice, and must not be injured
by rapid fluctuations in the water-levels in the filters.
Failure to make the bottoms water-tight has perhaps caused more
annoyance than any other single point. With a leaky bottom there
is either a loss of water when the water in the filters is higher
than the ground-water, or under reverse conditions, the ground-water
comes in and mixes with the filtered water, and the latter is rarely
improved and may be seriously damaged by the admixture. And with very
bad conditions water may pass from one filter to another, with the
differences in pressure always existing in neighboring filters, with
most unsatisfactory results.
COVERS FOR FILTERS.
The filters in England and Holland are built open, without protection
from the weather. In Germany the filters first built were also open,
but in the colder climates more or less difficulty was experienced
in keeping the filters in operation in cold weather. An addition to
the Berlin filters, built in 1874, was covered with masonry vaulting,
over which several feet of earth were placed, affording a complete
protection against frost. The filters at Magdeburg built two years
later were covered in the same way, and since that time covered filters
have been built at perhaps a dozen different places.
[Illustration: INTERIOR VIEW OF COVERED FILTER, ASHLAND, WIS.
When in use the water rises nearly to the springing line of the arches.]
[Illustration: COVERED FILTER IN COURSE OF CONSTRUCTION, SHOWING WOODEN
CENTERS FOR MASONRY VAULTING, SOMERSWORTH, N. H.]
[_To face page 12._]
It was found at Berlin that, owing to the difficulty of properly
cleaning the open filters in winter, it was impossible to keep the
usual proportion of the area in effective service, and as a result
portions of the filters were greatly overtaxed during prolonged
periods of cold weather. This resulted in greatly decreased bacterial
efficiency, the bacteria in March, 1889, reaching 3000 to 4000 per
cc. (with 100,000 in the raw water), although ordinarily the effluent
contained less than 100. An epidemic of typhoid fever followed, and was
confined to that part of the city supplied from the Stralau works,
the wards supplied from the covered Tegel filters remaining free from
fever. Open filters have since been abandoned in Berlin.
At Altona also, where the water is taken from an excessively polluted
source, decreased bacterial efficiency has repeatedly resulted in
winter, and the occasional epidemics of typhoid fever in that city,
which have invariably come in winter, appear to have been directly due
to the effect of cold upon the open filters. The city has just extended
the open filters, and hopes with an increased reserve area to avoid
the difficulty in future without resource to covered filters. (See
Appendices II and VII.)
Brunswick, Lübeck, and Frankfort on Oder with cold winters have open
filters, but draw their water-supplies from less polluted sources, and
have thus far escaped the fate of Berlin and Altona. The new filters
at Hamburg also are open. At Zürich, where open and covered filters
were long used side by side, the covered filters were much more
satisfactory, and the old open filters have recently been vaulted over.
Königsberg originally built open filters, but was afterward obliged to
cover them, on account of the severe winters; and at Breslau, where
open filters have long been used, the recent additions are vaulted over.
The fact that inferior efficiency of filtration results with open
filters during prolonged and severe winter weather is generally
admitted, although there is some doubt as to the exact way in which
the disturbance is caused. In some works I am informed that in cutting
the ice around the edges of the filter and repeatedly piling the
loose pieces upon the floating cake, the latter eventually becomes so
thickened at the sides that the projecting lower corners actually touch
the sand, with the fluctuating levels which often prevail in these
works, and that in this way the sediment layer upon the top of the sand
is broken and the water rapidly passes without adequate purification at
the points of disturbance.
This theory is, however, inadequate to account for many cases where
such an accumulation of ice is not allowed. In these cases the poor
work is not obtained until after the filters have been scraped. The
sand apparently freezes slightly while the water is off, and when water
is brought back and filtration resumed, normal results are for some
reason not again obtained for a time.
In addition to the poorer work from open filters in cold weather, the
cost of removing the ice adds materially to the operating expenses, and
in very cold climates would in itself make covers advisable.
I have arranged the European filter plants, in regard to which I have
sufficient information, in the table on page 15, in the order of the
normal mean January temperatures of the respective places. This may not
be an ideal criterion of the necessity of covering filters, but it is
at least approximate, and in the absence of more detailed comparisons
it will serve to give a good general idea of the case. I have not
found a single case where covered filters are used where the January
temperature is 32° F. or above. In some of these places some trouble is
experienced in unusually cold weather, but I have not heard of any very
serious difficulty or of any talk of covering filters at these places
except at Rotterdam, where a project for covering was being discussed.
Those places having January temperatures below 30° experience a great
deal of difficulty with open filters; so much so, that covered filters
may be regarded as necessary for them, although it is possible to keep
open filters running with decreased efficiency and increased expense by
freely removing the ice, with January temperatures some degrees lower.
Where the mean January temperature is 30° to 32° F. there is room for
doubt as to the necessity of covering filters, but, judging from the
experience of Berlin and Altona, the covered filters are much safer at
this temperature.
TABLE OF PLACES HAVING OPEN AND COVERED FILTERS.
ARRANGED ACCORDING TO THE MEAN JANUARY TEMPERATURES.
------------+------------------+--------------------------------------
Normal Mean | |
January | Place. | Kind of Filters and Results.
Temperature.| |
Degrees F. | |
------------+------------------+--------------------------------------
37-40° |All English cities|Open filters only are used, and no
| | great difficulty with ice is
| | experienced.
33-35° |Cities in Holland |All filters are open, and there is
| | little serious trouble with ice;
| | but at Amsterdam and Rotterdam
| | the bacteria in effluents are said
| | to be higher in winter than at
| | other times.
32° | Bremen |Open filters.
31° | Altona |Much difficulty with ice in open
| | filters (see Appendices II and VII).
31° | Brunswick |Open filters.
31° | Hamburg |Open filters.
31° | Lübeck |Open filters.
31° | Berlin |Open filters were formerly used, but
| | owing to decreased efficiency in
| | cold weather they have been
| | abandoned for covered ones.
31° | Magdeburg |Covered filters, but a recent addition
| | is not covered.
30° |Frankfort on Oder |Open filters.
30° | Stuttgart |Part of the filters are covered.
30° | Stettin |Part of the filters are covered.
29° | Zürich |Covered filters were much the most
| | satisfactory, and the open ones were
| | covered in 1894. The raw water has a
| | temperature of 35°.
29° | Liegnitz |Open filters.
29° | Breslau |Open filters have been used, but
| | recent additions are covered.
29° | Budapest |Covered filters only.
29° | Posen |Covered filters only.
26° | Königsberg |The original filters were open, but it
| | was found necessary to cover them.
24° | Warsaw |Covered filters only.
16° | St. Petersburg |Covered filters only.
------------+------------------+--------------------------------------
In case the raw water was drawn from a lake at a depth where its
minimum temperature was above 32°, which is the temperature which must
ordinarily be expected in surface-waters in winter, open filters might
be successfully used in slightly colder places.
The covers are usually of brick or concrete vaulting supported by
pillars at distances of 11 to 15 feet in each direction, the whole
being covered by 2 or 3 feet of earth; and the top can be laid out as
a garden if desired. Small holes for the admission of air and light
are usually left at intervals. The thickness of the masonry and the
sizes of the pillars used in some of the earlier German vaultings are
unnecessarily great, and some of the newer works are much lighter. For
American use, vaulting like that used for the Newton, Mass., covered
reservoir[3] should be amply strong.
Roofs have been used at Königsberg, Posen, and Budapest instead of
the masonry vaulting. They are cheaper, but do not afford as good
protection against frost, and even with great care some ice will form
under them.
Provision must be made for entering the filters freely to introduce and
remove sand. This is usually accomplished by raising one section of
vaulting and building a permanent incline under it from the sand line
to a door above the high-water line in the filter.
The cost of building covered filters is said to average fully one half
more than open filters.
Among the incidental advantages of covered filters is that with the
comparative darkness there is no tendency to algæ growths on the
filters in summer, and the frequency of scraping is therefore somewhat
reduced. At Zürich, in 1892, where both covered and open filters were
in use side by side, the periods between scrapings averaged a third
longer in the covered than in the open filters.
It has been supposed that covered filters kept the water cool in summer
and warm in winter, but owing to the large volume of water passing, the
change in temperature in any case is very slight; Frühling found that
even in extreme cases a change of over 3° F. in either direction is
rarely observed.
[Illustration: REMOVING ICE FROM A FILTER, EAST LONDON.
This represents the greatest accumulation of ice in the history of the
works.
[_To face page 16._]]
At Berlin, where open and covered filters were used side by side at
Stralau for twenty years, it was found that, bacterially, the open
filters were, except in severe winter weather, more efficient. It was
long supposed that this was caused by the sterilizing action of the
sunlight upon the water in the open filters. This result, however, was
not confirmed elsewhere, and it was finally discovered, in 1893, that
the higher numbers were due to the existence of passages in corners
on the columns of the vaulted roof and around the ventilators for the
underdrains, through which, practically, unfiltered water found its
way into the effluent. This at once removes the evidence in favor of
the superior bacterial efficiency of open filters and suggests the
necessity of preventing such passages. The construction of a ledge all
around the walls and pillars four inches wide and a little above the
gravel, as shown in the sketch, might be useful in this way, and the
slight lateral movement of the water in the sand above would be of no
consequence. The sand would evidently make a closer joint with the
horizontal ledge than with the vertical wall.
[Illustration: FIG. 2.]
In regard to the probable requirement or advisability of covers for
filters in the United States, I judge, from the European experience,
that places having January temperatures below the freezing-point will
have considerable trouble from open filters, and would best have
covered filters. Places having higher winter temperatures will be
able to get along with the ice which may form on open filters, and
the construction of covers would hardly be advisable except under
exceptional local conditions, as, for instance, with a water with an
unusual tendency to algæ growths.
I have drawn a line across a map of the United States on this basis
(shown by the accompanying plate) and it would appear that places far
north of the line would require covered filters, and that those south
of it would not, while for the places in the immediate vicinity of the
line (comparable to Hamburg and Altona) there is room for discussion.
In the United States covered filters have been constructed at St.
Johnsbury, Vt., Somersworth, N. H., Albany, N. Y., Ashland, Wis., and
Grand Forks, N. Dak., all of these places being considerably north of
the above-mentioned line.
The filter at Lawrence, Mass., with a mean January temperature of
about 25°, is not covered, but serious difficulty and expense have
been experienced at times from the ice, so much so that it has been
repeatedly recommended to cover it. Open filters have also been in use
for many years at Hudson and Poughkeepsie, N. Y., with mean January
temperatures about 24°; and although considerable difficulty has been
experienced from ice at times, these filters, particularly the ones
at Poughkeepsie, have been kept in very serviceable condition at all
times, notwithstanding the ice.
At Mount Vernon, N. Y., with a mean January temperature of about
31°, and with a reservoir water, no serious difficulty has been
experienced with ice; and at Far Rockaway, L. I., with a slightly
higher temperature and well-water, no difficulty whatever has been
experienced with open filters. Filters at Ilion, N. Y., with a mean
January temperature of about 23°, are not covered, and are fed from a
reservoir. No serious difficulty has been experienced with ice, which
is probably due to the fact that the water applied to them is taken
from near the bottom of the reservoir, and ordinarily has a temperature
somewhat above the freezing-point throughout the winter.
[Illustration: Map showing
Normal Mean January Temperatures
IN THE UNITED STATES
and the Area in which Filters should be covered]
The cost of removing ice from filters depends, among other things,
upon the amount of reserve filter area. When this reserve is small
the filters must be kept constantly at work nearly up to their rated
capacity; the ice must be removed promptly whenever the filters
require cleaning, and under some conditions the expense of doing this
may be considerable. If, on the other hand, there is a considerable
reserve area, so that when a filter becomes clogged in severe weather,
the work can be turned upon other filters and the clogged filter
allowed to remain until more moderate weather, or until a thaw, the
expense of ice removal may be kept at a materially lower figure.
In case open filters are built near or north of this line, I would
suggest that plenty of space between and around the filters for piling
up ice in case of necessity may be found advantageous, and that a
greater reserve of filtering area for use in emergencies should be
provided than would be considered necessary with vaulted filters or
with open filters in a warmer climate.