cleansing may be washed in hoppers admitting the water
from below, or in troughs through which water is constantly
flowing. Deep filter beds keep the water cooler in summer
and retard freezing in winter, the latter being the more
important, since freezing not only interferes with the efficiency
of the filtration, but may damage the walls of the filter
beds, by the expansion of the surface water in the act of
freezing.
In many places water is obtained from galleries or
trenches sunk along the edge of lakes or running streams, the
general impression being that the water so obtained is
derived from the lake or stream, and that it undergoes a
process of natural purification and filtration in its passage
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240 WATER SUPPLIES
through the intervening soil. In many cases, however, this is
really ground water which is intercepted on its way to its
natural outlet. Such water is usually very free from organic
matter, and contains but few bacteria. Where the ground
water falls below the level of the water in the stream or lake,
doubtless a certain quantity of the water which passes into
the galleries is derived from the latter sources, and is not so
likely to be of good quality, since it only passes through soil
which is constantly saturated with water, and therefore neVer
aerated, and destitute of any oxidising powers. In such cases
also the filtration is liable to be inefficient, and to allow
of bacteria and other particulate matters passing into the
collecting channels.
Many attempts have been made to filter water on the
large scale without employing filter beds, which are expensive
not only on account of the space required, but of the con-
stant labour and attention required to keep them in a state
of efficiency. One of the best-known processes is that of the
Atkins Filter and. Engineering Company, which is in use
by the Henley-on-Thames Water Company, and has been
adopted by many large institutions. The filtering apparatus,
technically known as the " Scrubber," consists of a perforated
metal cylinder to contain the sand or other filtering material,
fitted into a tank and so arranged as to revolve easily by
turning a handle. The cylinder is only partly filled with
the filtering material, and the collecting tubes, which convey
away the filtered water, lie as nearly as possible in the centre
of this as it lies in the cylinder. To clean the filter it is
only necessary to turn the handle, when the cylinder revolves,
agitating the filtering material with the water, and the latter,
together with the impurities washed out, are run off" through
a by -pass. Several such "scrubbers" can be connected
together. By another arrangement the sand is put into a
number of discs fitted on a revolving centre collecting-tube.
The water filters through the flat surface of each disc, so that
the area of filtering surface is much increased. More perfect
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PURIFICATION OF WATER ON LARGE SCALE 241
filtration can be secured by passing the water through two
"scrubbers" in succession, and affords, naturally, safer results
for drinking water. The Company claims that, with an area
of only 600 square feet, their machines will filter as much
water as an acre of filter bed (3,000,000 gallons per day).
Under the latter system the cost of cleansing is said to be
from 5s. to 10s. per million gallons, whereas it is only about
half the amount with the Atkin "scrubbers," with "the
great sanitary improvement of daily cleansing in addition."
Such machines for rapid filtration do not appear to be regarded
with much favour in this country, and there are no records of
the bacteriological examination of waters which have passed
through these filters. The conditions laid down by the
Massachusetts Board as being necessary for perfect filtration
not being observed, experimental evidence of efficiency is much
to be desired. Other machines of a similar character — the
"Loomis," the "Duplex," the "Hyatt," the "Bowden," etc.
— are, however, in use in the United States, chiefly for filter-
ing turbid river-water, and Dr. P. S. Wales, Medical Director,
United States Navy, states that, even with this rapid
filtration, 98 per cent of the micro-organisms can be
removed, but that "spores readily passed through the
filtering material." (The typhoid and cholera bacilli are
not known to form spores.) The four machines above
referred to have been used for experimental purposes at the
Museum of Hygiene, Washington, D.C., and gave very satis-
factory "results. The system of rapid filtration is successfully
pursued, amongst other places, at —
Oakland, Cal., capacity for 24 hours
Atlanta, Ga. „ ,,
Loug Branch, N.Y. ,, ,,
Ottnmwa, Iowa ,, ,,
Athol, Mass. ,, ,,
4,000,000 gallons.
3,000,000 „
2,000,000 „
1,500,000 „
1,000,000 „
The city of Alleghany, Pa., was contemplating erecting a
plant for filtering 30,000,000 gallons per day, when Dr.
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242 WA TER SUPPLIES
Wales's paper was published. 1 These filters appear to be
especially applicable for the waters of muddy, rapid rivers,
which speedily clog the ordinary sand filter, and arrest the
flow of water. To expedite the process of sedimentation so
as to remove more of the suspended matter before passing the
water into the filters, alum is largely used. The addition of
about half a grain per gallon, on the average, is sufficient.
At the Atlanta Waterworks, during 1890, 253 lbs. of alum
were used per day, corresponding to 617 grains per 1000
gallons. Some waters, such as that of the Potomac, cannot
be clarified without a coagulant. In this country the water
supply to the village of Ingatestone (Essex), previously referred
to, derived from a fine sandy clay, for years resisted all
our efforts to clarify it Alum, or rather Spence's Alumino-
ferric, was used as a coagulant, and the water then filtered
through vertical sheets of flannel. This not proving satis-
factory, various recently-introduced filtering and purifying
materials were experimented with. Finally, at my recom-
mendation, a filter bed was made of sand and polarite mixed
in equal proportions, and with a few inches of fine sand on
the top. This filter has now been in use for nearly two
years, and has answered admirably. The use of the alum
was discontinued, as it was found quite unnecessary. Two
beds were prepared, so that one could be used whilst the
other was cleansed and allowed to rest for re-aeration.
At the Antwerp Waterworks, " spongy iron," together with
gravel, was used as filtering material, but the beds choked up
gradually and the iron became almost inactive. For three
years, however, the results were satisfactory, so far as regards
the purification of the water. To meet the difficulties just
referred to, Dr. W. Anderson, F.R.S., invented the " Revolving
Purifier," which has been in use at Antwerp since 1885, and
has also been adopted at Boulogne-sur-Seine, Agra, Monte
Video, and other places. The apparatus is described by the
1 Transactions of International Congress oj Hygiene, London, 1891.
vol. vii.
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PURIFICATION OF WATER ON LARGE SCALE 243
inventor as a " cylinder supported horizontally on two hollow
trunnions, of which one serves for the entrance and the other
for the exit of the water. The cylinder contains a certain
quantity of metallic iron, in the form either of cast-iron
borings, or, preferably, of scrap iron, such as punchings from
boiler plates. The cylinder is kept in continuous but slow
rotation by any suitable means, the iron being continually
lifted up and showered down through the passing water by a
series of shelves or scoops fixed inside the shell of the cylinder.
By this means the water, as it flows through, is brought
thoroughly into contact with the charge of iron, which, in
addition, by its constant motion and rubbing against itself
and the sides of the cylinder, is kept always clean and active."
During its passage through the apparatus the water takes up
from ^ to \ of a grain of iron per gallon, which is got
rid of either by blowing in air or by allowing it to flow
along shallow open troughs. The oxide thus formed may
settle in subsidence reservoirs, or may be filtered out by rapid
passage through a thin layer of sand. At Boulogne the
average amount of organic matter removed by this process
from the Seine water was 63 per cent, and the microbes,
which in the unfiltered water ranged from 800 to over 7000
per cubic centimetre, were reduced to an average of about 40.
The bacteriological results are admittedly only approximate,
and on one occasion, at least, a large number of bacteria were
found in the filtrate. It seems probable that, compared with
sand filtration as usually conducted, the revolving purifiers
may destroy a larger proportion of the dissolved organic
matter ; but unless supplemented by careful sand filtration it
would be unsafe to assume that a specifically-polluted water
could be rendered safe for drinking purposes by passing
through one of these cylinders.
Whilst sand is almost universally employed for the filtra-
tion of water on the large scale, and usually is the sole
effective filtering medium, in a few instances other materials
have been used, together with the sand, either mixed there-
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244 WATER SUPPLIES
with, or in layers. A carbide of iron (Spence's Magnetic Car-
bide) was in use for a large number of years for filtering the
excessively-pol luted Calder water for the domestic supply to
Wakefield. This water was not only fouled by sewage, but
also deeply discoloured by the refuse from dye-works ; yet the
filters converted it into a colourless, palatable water. The
layer of carbide was in use for nearly thirty years, and was
never renewed ; all that was found to be required was the
cleansing of the surface sand. The filtration was intermittent,
to allow of the aeration of the filter. The magnetic carbide
is also in use at Calcutta for filtering the turbid and polluted
waters of the Hooghly, and at Cape Town, Demerara, and
other places. Its use was discontinued at Wakefield because
a purer supply has been obtained from another source.
Spongy iron, polarite, and other insoluble iron compounds are
used for similar purposes, and are useful in special cases, as
in the examples given. Now that the removal of dissolved
organic matter is considered to be of much less practical
importance than the removal of the living organisms, less
importance is being attached to the use of such materials, and
it can only be under exceptional conditions that these aids to
sand filtration are necessary. It is upon the proper use of
sand that the real efficiency of filtration must depend, though
where desirable this may be supplemented by the use of other
filters, or the introduction of a layer or layers of other
materials; and the substances above enumerated, yielding
nothing to the water, yet exerting an oxidising action upon
the organic matter, are probably the best which have yet been
discovered.
At Reading Waterworks polarite is now largely used for
filtering the water of the Kennet, a polluted, navigable stream.
The following description of the filters is taken from an
excellent paper read by Mr. Walter, the Waterworks Engineer,
at a recent meeting of the County Association of Municipal
Engineers held in Reading : —
" The process of purifying the river Kennet water is by
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PURIFICATION OF WATER ON LARGE SCALE 245
natural percolation, through a series of filters or chambers,
the first chamber containing coke, and the second and third
chambers * polarite,' granulated in two sizes ; there are also
intermediate or regulating water chambers for facilitating
cleaning out, the water passing from the last polarite chamber
into a distributing channel, and on to sand filters, as it has
been said, to make doubly sure of filtered water; but sub-
sequent experience has proved that perfect purification can be
obtained by polarite chambers without the aid of sand. The
first two sets of these chambers were started in work in
November 1892. Each polarite chamber measures 40 feet by
9 feet, and has a depth of 2^ feet of polarite, giving an area
of 40 yards super each chamber, or a total of 160 yards super
for the two sets. By adding the 2£ feet of polarite in each
set it gives a depth or thickness of 5 feet to each set of
chambers, and an area of 80 yards super per set. From
December 1892 to August 1893 there had passed through
these two sets a total quantity of 409,880,000 gallons of
water, giving an average of 18,848 gallons per yard super per
day. Two additional sets were started in August last, 1893,
of the same dimensions as the above, giving a total area of
160 yards super, with a depth of 5 feet for each set of
chambers, which have passed on an average 12,500 gallons
per yard super per day. From 1st January of
the present year to the 31st of March last, 190,218,319
gallons of water have passed through these chambers, giving
an average of 13,215 gallons per yard super per day, or at
the rate of 550*6 gallons per yard super per hour. The water
has been such that no complaints (which previously were an
everyday occurrence) have been made since purification by
polarite came into full working order. It has had a most
severe test during the past and previous autumn and winter
seasons, but like many a good engineer it has often been
overworked, but has stood it well. From experience gained
in connection with the treatment of the river Kennet water,
there is no hesitation in stating the opinion that ' polarite/
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246 WATER SUPPLIES
as applied here, is capable of effectually purifying a river water-
supply for all purposes, and the system can be carried out at
less cost of construction and maintenance than filtration by
large areas of sand beds."
The eflluent from the polarite filters is afterwards passed
through four sand filters, each having an area of 10,000
square feet. As these filters pass about 2,000,000 gallons
l>er day, this is at the rate of over 8 gallons per square
foot per hour, or four times the average of the London Water
Companies.
In connection with these works also there is an improved
system of sand-washing, which was invented by Mr. Walker.
Cone-shaped hoppers, mounted on trunnions, and connected at
the bottom of the inverted cone with the water supply under
pressure, are filled with the sand scrapings to be washed.
The water is then turned on, and the upward rush keeps it
in a continuous state of agitation, and the impurities are
carried off by an outlet at the rim of the hopper. By this
process sand -washing is not only less laborious, but less
expensive than by the older methods. One man can wheel,
tip, and wash 9 to 10 cubic yards of sand per day at a cost of
3fd. to 5d. per cubic yard. By the older processes the cost
was from Is. 6d. to 3s. per cubic yard.
Water, when softened by the addition of lime, also under-
goes an improvement in quality, the precipitate of carbonate
of lime carrying down with it a very large proportion of the
microbes previously suspended in the water. The filtration
through sand which follows, to remove the last trace of
carbonate, still further purifies the water, so that the soften-
ing process has a double advantage. As this process is
primarily conducted for removing the carbonate of lime, and
not for the removal of organic matter, and is of very
considerable importance, it will be fully considered in a
later chapter.
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CHAPTER XIV
DOMESTIC PURIFICATION
The water supplied by a public company can scarcely be
considered wholesome if it requires filtration by the consumer,
yet in many towns unfiltered surface water is distributed,
and as this often contains visible suspended impurities, some
form of filtration must be resorted to if the water is to have
a bright and pleasing appearance. The forms of filter
generally employed for purifying all the water consumed in a
dwelling may be classed under two heads — (a) low-pressure
filters, (b) high-pressure filters. The latter are directly in
communication with the service pipe, and the water is
filtered through under the pressure in the main ; whilst the
former are indirectly connected by means of a ball-cock, the
only pressure being the column of water in the filter above
the filtering material.
The high-pressure filters may contain any of the materials
ordinarily used for clarifying water, either in a granular
condition and tightly packed or in one porous mass. (Animal
charcoal, polarite, magnetic carbide, carferal, silicated carbon,
etc.) No doubt for a time such filters remove a considerable
portion of the suspended matter, but they can never be
trusted to remove more than a small portion of the bacteria,
the most dangerous of the constituents. The separated filth
accumulates, and to remove it there is usually an arrangement
permitting of water being forced through in the opposite
direction, whereby much of the dirt is washed away. All of
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248 WATER SUPPLIES
it cannot be thus removed; hence the efficiency of the filter is
more or less rapidly impaired, and the filtering material
requires constant renewal. Unfortunately, purchasers of such
filters are rarely aware of this fact, or, if they are, the trouble
and expense causes such renewals to take place at very long
intervals. The whole system is wrong, and should not be
Fig. 14.
encouraged. Even if carefully attended to such filters cannot
be depended upon for any length of time, and as they possess
few advantages over low-pressure filters their use should be
abandoned. The best filters of this class are Major Crease's,
the Berkefeld and Pasteur filters. The former consists of a
stout cylindrical vessel filled with carferal, a compound of
iron, alumina, and carbon. The water passes in from the
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DOMESTIC PURIFICATION 249
main at one end, and out to supply the house from the
opposite extremity. The filtering material within the
cylinder is packed between two perforated plates, one of
which can be screwed down upon the other so as to obtain
any required degree of compression. It can also be readily
unpacked for cleansing or for renewal of the carferal. The
"Berkefeld" is, strictly speaking, a bacteriological filter, its
object being, not the oxidation of dissolved organic matter,
but the removal of the whole of the suspended matter,
including the most minute organisms. The filtering cylinder
is composed of compressed fossil earth {Kieselgvhr), and the
water is purified by filtration through the side. The
suspended matters removed from the water remain upon the
surface, and can easily be washed or brushed away, and the
cylinders can be resterilised by being placed in warm water
and boiled for an hour. Fig. 14 is a section of a cistern
filter working with a pressure of 20 lbs. upwards. A 3-tube
filter of this kind will supply 50 gallons of water per hour.
A smaller, single-tube filter is shown in Fig. 15. It is
intended for attachment to the water supply either from a
constant main service, with a pressure of, say, 30 lbs. upwards,
or from a cistern not less than 20 feet above where the filter
is fixed.
The Pasteur or Chamberland-Pasteur filter is very similar
to the Berkefeld, but is made of china clay, is somewhat harder,
and therefore not so readily fractured. Both are efficacious
at first, but the latter is said to yield a more palatable filtrate.
To the use of the Pasteur filter by the French army during
recent years is attributed the great decrease in the mortality
from typhoid fever amongst the soldiers (50 per cent). In
other instances, when used for manufacturing purposes, their
use has been discontinued on account of the slowness of the
filtration, and because after prolonged use the filtered water
was no longer bacteriological ly satisfactory. In a series of
experiments made by Dr. Johnston, bacteria were found in
the water passing through a Berkefeld filter within from 3 to
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250 WATER SUPPLIES
10 days of continuous use. The Pasteur filtrate remained
sterile for six weeks. Recent experiments made by Dr. Sims
Woodhead (Brit. Med. Journal) confirm the superiority of
the Pasteur filter.
Fig. 15.
8. Water-inlet.
T. Outlet for filtered water.
U. Outlet for water used for washing cylinder.
A number of forms of these high-pressure filters are made
for fixing to taps, pumps, etc. They yield a water which
at first is absolutely free from micro-organisms, and as they
are extremely simple in construction and admit of being
very easily cleansed, no other filter can be compared with
them for high-pressure work.
Bailey Denton's self -supplying filter may be taken as
typical of the low-pressure service filter.
The upper compartment contains the filtering material,
which may be sand, charcoal, or any other of the substances
used for such a purpose, and is fed from the house cistern at
a higher level. When the filtered water in the tank below
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DOMESTIC PUR1FICA T10N 25 1
reaches a certain level the supply to the filter is cut off, and
the remaining water as it drains from the filtering material
is replaced by air, so that the filter is frequently aerated. If
fixed in an easily accessible situation, the material can be
examined and removed for cleansing as often as may be
required. The capacity of the lower compartment is made
suitable for the actual requirements of the household.
Bain water may be effectually filtered by some such
arrangement as the above, and if for any reason the reservoir
for the filtered water is below the level of the ground, the
water may be raised by a pump. Even with this system of
treatment the rain water should be collected by means of a
" separator," in order to prevent an unnecessary amount of
filth being passed into the storage cistern, which not only
fouls the water but causes the filter to require much more
frequent cleansing.
The number of domestic filters in the market is enormous,
and it may truthfully be asserted that the majority of
them are worthless. Some are intended merely to remove
a portion of the dissolved organic matter, and fail entirely to
remove any bacteria which may be present. Others, which
claim to remove the micro-organisms, only do this imperfectly
and for a short time, and after being in use for a period
the filtered water may actually contain more bacteria than
were present in the unfiltered water. The use of such filters
engenders a false feeling of security, and the users may fall
victims to their misplaced confidence. I have had occasion
to examine several much-vaunted filters, and found them
absolutely useless ; they were coarse strainers and nothing more.
The so-called " table filters " are usually the least reliable, since
the amount of filtering material is too small to purify the
water for any length of time, if at all ; and if the material be
made sufficiently compact to prevent the passage of micro-
organisms, the rate of filtration is excessively slow, and the
pores of the filter become rapidly choked. The Berkefeld and
Pasteur filters are probably the most reliable, but are very
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252 WATER SUPPLIES
slow in action. The tubes must be frequently removed,
washed, first with water, then with a dilute solution of
permanganate of potash, and finally sterilised by boiling or by
heating over a charcoal stove or Bunsen burner.
For ordinary domestic purposes an inexpensive sand filter,
which can be made by any person, is as good, or better, than
many of the high-priced filters in the market. The follow-
Fig. 16.
ing is a description of a cottage filter (Fig. 1 6) costing only a
few pence : — Take a large-sized earthenware flower-pot, and
plug the hole at the bottom with a cork, through which passes
a short piece of glass tube. Upon the bottom place a few
fragments of a broken flower-pot (pieces \ to \ inch square).
Upon these place a layer of small, clean- washed gravel, and upon
this 6 to 12 inches of well- washed, fine, sharp sand. Cover the
smooth surface of the sand with a circular piece of coarse filter
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DOMESTIC PURIFICATION 253
paper and sprinkle over this a few pieces of the small gravel.
Mount the pot on a tripod or other convenient stand, and it is
ready for use. The paper prevents the upper surface of the
sand being disturbed by pouring in the water, and can be