THE PURIFICATION OF WATER.
When a public water-supply is provided, it may reasonably be expected
to be furnished pure and fit for use; but this, occasionally is not so.
The reports, for instance, of the condition of the London Water Supply,
occasionally show that it is turbid and contains a slight excess of
organic matter. This is especially the case when, after heavy rainfall,
storm-water is brought into the reservoirs, and owing to deficient
storage, sufficient time is not allowed for deposit. Rain-water always
and other waters frequently require to be purified before use.
=Methods of Purification.=—The only certain way of obtaining pure water
is by =Distillation=; but this plan is scarcely applicable to water
on a large scale. Furthermore distilled water is not so palatable as
ordinary water. The distillation of water is more especially required
on board ship, during long voyages. It should be followed by the use of
some measure to secure efficient aeration.
2. =Boiling water= serves to remove the temporary hardness, and the
chalk carries down with it a large proportion of any organic matter
that may be present. Boiling deprives the water of its dissolved gases,
and renders it flat; it is desirable, therefore, to aerate it by
filtration or from a gazogene after boiling. All the microbes which are
known to produce disease are destroyed by efficient boiling. Certain
putrefactive microbes are more persistent of life, owing to the fact
that they form spores, which are not killed at the temperature of
boiling water. Tyndall showed that by boiling the liquid containing
these spore-forming microbes on three successive days, thus giving time
for the spores to develop into less resistant microbes, they could be
effectually destroyed. Boiled water will not cause enteric fever or
cholera, the two chief water-borne diseases.
3. The exposure of water in divided currents to the air by passing it
through a sieve has been proposed as a means of purifying water, but
it is inefficient when trusted to alone. Plants in reservoirs help to
absorb organic matter; and fish, by destroying small crustaceans, have
been found useful. Hard waters do not bear exposure to light, as a
thick green growth of chara occurs, which may block pipes, and give a
bitter taste to the water.
4. =The Addition of Chemical Substances.=—(1) _Clarke’s process_
consists in adding milk of lime, _i.e._ an emulsion of quicklime with
water, to the water in the reservoir on a large scale. By this means
calcium carbonate is precipitated, but no effect is produced on calcium
and magnesium sulphates and chlorides. The hardness of the Thames
water can thus be reduced from 16° to 3° or 4° (_Clarke’s scale_). The
calcium carbonate carries down with it suspended and possibly dissolved
organic matter. In the _Porter-Clarke process_ lime-water, _i.e._ milk
of lime diluted, and the excess of lime separated by settlement or
filtration, is mixed with the water to be purified, the water being
freed from the precipitated calcium carbonate either by subsidence or
by being forced through a filter of stretched canvas.
(2) _Carbonate of Soda_ added to boiling water throws down calcium
carbonate, and possibly lead if present. Much less is required when
added to boiling than to cold water. Maignen’s process consists in
adding _anti-calcaire_ powder, containing chiefly carbonate of soda,
lime, and alum.
(3) _Aluminous salts_ are very effectual in removing suspended organic
matter, if the water contains calcium carbonate. On the addition of
alum, calcium sulphate and aluminium hydrate are formed, both of which
fall to the bottom, carrying with them other impurities. The amount of
alum required is about 6 grains per gallon of water. If the water is
not hard, a little calcium chloride and carbonate of soda should be put
in before the alum is added, in order that a precipitable substance may
be formed.
(4) _Potassium permanganate_ readily removes the offensive smell of
stagnant water, but it gives a yellow tint to the water. The addition
of a little alum will help to carry down the decomposed permanganate.
(5) _Perchloride of Iron_, in the proportion of 2½ grains to a
gallon of water, has been found to completely purify water from finely
suspended organic matters and clay.
(6) More recently, other substances, such as _iodine_ and _hyposulphite
of soda_, have been recommended. These are supposed to act by
sterilizing the water, and iodine in suitable quantities undoubtedly
effects this.
Chemical processes for the purification of water, with the exception
of the softening process, are not to be recommended for general use.
Efficient filtration, or boiling, is safer than chemical treatment;
and it would only be justifiable to trust to the latter, when, as in
a military campaign, an attempt at purification was necessary, and no
means were available for filtering or boiling water.
7. =Filtration.=—The object of filtration is to remove the impurities
of water. The most dangerous impurities are suspended in it, especially
the microbes causing infectious diseases. Hence the most perfect filter
is the one which most completely prevents the passage through it of
microbes. If the water supply is pure, domestic filtration is not only
useless, but likely to do more harm than good. This is true for such
upland surface waters as those supplied to Liverpool, Glasgow, and
Manchester; for such deep well-water supplies as those of Brighton
(deep chalk), of Nottingham (new red sandstone), and others, when
pumped from wells remote from inhabited houses. For upland surface
waters known to attack lead pipes, filtration through charcoal or
spongy iron may be advisable; for river water, filtration through a
germ-proof filter is best.
Filtration =on a large scale= is generally carried on as follows:—A
preliminary step consists in collecting the water into settling
reservoirs, wherein the more bulky substances subside. The water is
then filtered through beds of gravel and sand, containing perforated
tubular drains below, into which the filtered water flows. The drains
are covered by a bed of gravel about 3 feet deep, over which is spread
a layer of sand about 1½ to 2 feet deep. Sharp angular particles
of sand are the best; and the gravel should gradually increase in its
coarseness as it descends.
The effect of this filtration is chiefly _mechanical_; it separates
any suspended matter, whether organic or inorganic. A certain amount
of _biological_ action possibly also takes place. Piefke found that a
perfectly cleaned and sterilised filter when first used, increases the
microbes in water, instead of decreasing them. Gradually a gelatinous
layer of slimy matter is formed on the top of the sand; the water now
filters through much more slowly, but it gradually becomes freer from
microbes, these being intercepted by the slimy layer. It is important
that this layer should not be disturbed by too rapid or forced
filtration, and that when the surface layer requires to be removed,
because the filter has become impervious, time should be allowed
for another thin film to form before the filtered water is again
utilised. Koch concluded that the rapidity of filtration should never
be allowed to exceed 100 millimetres (about 4 inches) per hour; and
that the number of microbes per c.c. in the filtered water should never
exceed 100. Some oxidation of organic matter, as well as detention
of microbes, may take place during the filtration of water, nitrates
being formed by the vital activity of certain “nitrifying” microbes in
the filter. (On nitrification, see pages 195 and 274.) P. Frankland’s
observations show that the number of microbes in Thames water is
reduced by filtration through sand and gravel beds, as practised by the
London Water Companies, so that only 3·4 per cent. of those originally
present remained. He also concludes that the majority of the microbes
present in filtered water are derived from post-filtration sources.
Thus the number is greater in tap-water than in water derived from near
the reservoirs.
Other materials besides sand have been used for filtration on a large
scale, but none with proved success.
=Domestic Filtration= ought, as already explained, not to be needed,
but circumstances often arise in which the public supply is open to
suspicion, and a second domestic line of defence against infection
through the water supply is desirable. When this is so, the form of
filter which will best protect the household is one attached to the
house-tap, so that all drinking-water is perforce filtered. When
filtering involves the transfer of water from the tap to the interior
of the filter, opportunity is left for carelessness or forgetfulness.
The one essential point of a domestic filter is that it will prevent
the passage through it of microbes. Every filter must be tested from
this standpoint.
On this point the experiments of Woodhead and Cartwright Wood are
conclusive. They first of all experimented on various filters with fine
artificial ultramarine containing particles 16 µ to 0·6 µ or even less
in diameter in suspension; and milk containing granules and globules of
fat 0·5 µ to 30 µ or more in diameter, freely diluted with water.
┌─────────────────────┬───────────────┬──────────────┬───────────────┐
│ │TIME IN MINUTES│PRESENCE OR │PRESENCE OR │
│ │REQUIRED FOR │ABSENCE OF │ABSENCE OF MILK│
│ │FILTRATION OF 1│ULTRAMARINE IN│IN FILTRATE │
│ │PINT OF WATER. │FILTRATE. │ │
├─────────────────────┼───────────────┼──────────────┼───────────────┤
│_Silicated carbon │ │ │ │
│ filter_ │ 68 │++ │+++ │
│_Carbon filter_ │ 18 │ + │+++ │
│_Maignen’s Filtre │ │ │ │
│ Rapide_ │ 4 │ 0 │ ++ │
│_Spongy iron filter_ │ 14 │ 0 │+++ │
│_Pasteur-Chamberland │ │ │ │
│ filter_ │420 │ 0 │ 0 │
│_Berkefeld filter_ │140 │ 0 │ 0 │
└─────────────────────┴───────────────┴──────────────┴───────────────┘
The number + indicates the relative amount of the experimental
substances that made their way through the filtering medium.
Experiments were then made with the actual microbes of certain
infectious diseases, and it was found that certain filters allow these
to pass. Thus a silicated carbon filter allowed 1,000 out of 15,000
typhoid bacilli suspended in water to pass through its substance;
a manganous carbon filter allowed 600 to 800 out of 10,000 cholera
vibrios to pass through; Maignen’s filter on the second day of
experiment allowed 150 out of 5,000 cholera vibrios to pass through;
Lipscombe’s charcoal filter experimentally only reduced typhoid bacilli
from 20,000 to 5,000; the magnetic carbide filter only reduced them
from 20,000 to 10,000; the spongy iron filter from 20,000 to 3,000;
while, on the contrary, the Pasteur-Chamberland and the Berkefeld
filter completely stopped all microbes and produced a sterile water.
(As to these two, see page 98.)
Of the materials enumerated =animal charcoal= was formerly regarded
as an excellent filtering medium. It is capable of oxidising organic
matter dissolved in water, but so far from sterilizing water, it
favours the growth of microbes in it. Water filtered through charcoal,
after the first few days of use of the charcoal, deteriorates, as the
charcoal yields up impurities to it.
=Manganous Carbon= consists of animal charcoal and black oxide of
manganese mixed with oil, and heated strongly together out of contact
with the air. The oxidising power of the carbon is said to be thus
greatly increased. It shares the objections to carbon.
=Silicated Carbon= consists of 75 per cent. of charcoal and 22 per
cent. of silica, with a little oxide of iron and alumina. It is not an
efficient filtering medium.
Spongy iron is prepared by the reduction of hæmatite ore with fusion,
so that the iron is obtained in a porous and finely-divided condition.
The Rivers Pollution Commissioners found spongy iron to be “a very
active agent, not only in removing organic matter from water, but
also in materially reducing its hardness, and otherwise altering its
character.” It is a powerful oxidising agent, some of the water being
decomposed, and hydrogen set free, and the oxygen acting upon any
organic matter present. It also removes lead from water. As already
seen, it does not, however, fulfil the primary object of water, by
depriving it of any microbes contained in it.
=Magnetic carbide of iron= is obtained by heating hæmatite ore with
sawdust. Its action is similar to that of spongy iron.
The =Pasteur-Chamberland filter= consists of a cylinder of unglazed
fine porcelain made from a well-baked Kaolin of a certain degree of
porosity and hardness. (Fig. 10.)
The water passes through the porcelain from without inwards, and with
the pressure of 1½ to 2½ atmospheres which is usually present in
the pipes of a water-service, passes through at the rate of about three
quarts per hour. The filter can easily be cleaned by brushing it in a
stream of hot water, or by subjecting to the heat of a Bunsen burner.
The filtration is entirely mechanical, the filtered water being quite
freed of microbes. No chemical action takes place.
[Illustration: FIG. 10.
PASTEUR-CHAMBERLAND FILTER.
A.—Outlet of filtered water. B.—Pasteur tube. C.—Metal tube containing
unfiltered water. D.—Unfiltered water delivered through tap. ]
The =Berkefeld filter= is cylindrical like the Pasteur-Chamberland
filter, and is used in the same way. It is made of infusorial earth,
which is soft and friable and liable to break. The cylinder becomes
gradually worn thin by cleaning, and it then ceases to filter
efficiently. Its sole advantage over the Pasteur-Chamberland filter is
the more rapid rate of filtration; and against this is to be set the
greater liability to fracture and the lack of continuance of efficient
filtration. Woodhead and Wood in the report already quoted, state:
“The Berkefeld filter appears to have the largest pores among the
efficient filters, as is evidenced by the fact that the water organisms
were not apparently weakened, that more species of organisms appeared
in its filtrate, and that lowering the temperature to 11° C. did not
prevent their appearance. The Pasteur-Chamberland filter, on the other
hand, at 11° C. was able to give an apparently sterile filtrate for a
prolonged period.” More recent experiments have shewn that pathogenic
(disease-producing) microbes contained in water after awhile grow
through the substance of a Berkefeld filter, and that this does not
happen with a Pasteur-Chamberland filter. The latter is therefore
preferable.
In determining the number of bougies required for any filter to secure
a given amount of pure water, it is necessary to calculate on the basis
of the output after several weeks’ use, not on the original output. If
this is done, pure water will be secured without disappointment as to
the amount supplied.