to occupy a high place as a biological filter, although previously,
owing to its chemical inactivity, it had been disregarded. Being
an inexpensive material, and easily renewed, it was destined to be
of great service in the purification of water. Experiments were
also made by the agitation of water with solid particles. It was
found that very porous substances, like coke, animal and vegetable
charcoal, were highly efficient in removing organised matter from
water when the latter came in contact with them in this manner. Also,
it was found that the well-known precipitation process, introduced by
Dr. Clark, for softening water with lime, had a most marked effect in
removing micro-organisms from water. In the case of water softened
by this process, it was found that a reduction of 98 per cent. in
the number of micro-organisms was effected, the chemical improvement
being comparatively insignificant.
Water which had been subjected to an exhaustive process of natural
filtration had been found to be almost free from micro-organisms.
Thus, the deep-well water obtained from the chalk near London
contained as few as eight organisms per cubic centimetre, whereas
samples of river water from the Thames, Lea, and Wey had been known
to contain as many thousands.
The same well-known authority on water published the following
statements in the _Nineteenth Century_. He described the subject of
domestic filtration as one which, in a town with a water supply like
that of London, possesses peculiar interest, and is of no little
importance. Most people imagine that by once going to the expense
of a filter they have secured for themselves a safeguard which will
endure throughout all time without further trouble. No mistake
could be greater, for without preserving constant watchfulness, and
bestowing great care upon domestic filtration, it is probable that
the process will not only entirely fail to purify the water, but will
actually render it more impure than before. For the accumulation of
putrescent organic matter upon and within the filtering material
furnishes a favourable nest for the development of minute worms
and other disgusting organisms, which not unfrequently pervade the
filtered water; whilst the proportion of organic matter in the
effluent water is often considerably greater than that present before
filtration.
Of the substances in general use for the household filtration of
water, spongy iron and animal charcoal take the first place. Both
these substances possess the property of removing a very large
proportion of the organic matter present in water. They both, in
the first instance, possess this purifying power to about an equal
extent; but whereas the animal charcoal very soon loses its power,
the spongy iron retains its efficacy unimpaired for a much longer
time. Indeed, in spongy iron we possess the most valuable of all
known materials for filtration, inasmuch as, besides removing such a
large proportion of organic matter from water, it has been found to
be absolutely fatal to bacterial life, and thus acts as an invaluable
safeguard against the propagation of disease through drinking-water.
It is satisfactory to learn that in countries where the results of
scientific research more rapidly receive practical application than
is unfortunately the case amongst us, spongy iron is actually being
employed on the large scale for filtration where only a very impure
source of water supply is procurable. This refers to the recent
introduction of spongy-iron filter beds at the Antwerp waterworks.
It would be very desirable that such filter beds should be adopted
by the London water companies until they shall abandon the present
impure source of supply.
Animal charcoal, on the other hand, far from being fatal to the lower
forms of life, is highly favourable to their development and growth;
in fact, in the water drawn from a charcoal filter which has not been
renewed sufficiently often, myriads of minute worms may frequently be
found.
Thus spongy iron enables those who can afford the expense to obtain
pure drinking-water even from an impure source; but this should
not deter those interested in the public health from using their
influence to obtain a water supply which requires no domestic
filtration, and shall be equally bright and healthful for both rich
and poor.
In a publication by Prof. Koch (_Med. Wochenschrift_, 1885, No.
37) on the scope of the bacteriological examination of water, it
is asserted that a large proportion of micro-organisms proves
that the water has received putrescent admixtures, charged with
micro-organisms, impure affluxes, &c., which may convey, along
with many harmless micro-organisms, also pathogenous kinds, i.e.
infectious matters. Further, that as far as present observations
extend, the number of micro-organisms in good waters ranges from 10
to 150 germs capable of development per c.c. As soon as the number
of germs decidedly exceeds this number the water may be suspected of
having received affluents. If the number reaches or exceeds 1000 per
c.c., such water should not be admitted for drinking, at least in
time of a cholera epidemic.
Dr. Link has lately examined a great number of the Dantzig
well-waters chemically and bacterioscopically. The results obtained
agree, however, very ill with the above opinions of Koch. On the
contrary, it appears very plainly that regular relations between
the chemical results and those of the bacterioscopic examination
do not obtain. Many well-waters, chemically good and not directly
or indirectly accessible to animal pollutions, often contained
considerable numbers of microbia, whilst other waters, chemically bad
and evidently contaminated by the influx of sewage, showed very small
numbers of bacteria undergoing development. If we further consider
that, by far the majority, indeed, as a rule the totality of the
bacteria contained in well-water, are indubitably of a harmless
nature, and that when a pollution of the water by pathogenous germs
has actually occurred, such germs will not in general find the
conditions necessary for their increase, especially a temperature
approximating to that of the body and a sufficient concentration
of nutritive matter, but that they will rather perish from the
overgrowth of the other bacteria inhabiting the water, we shall see
that a judgment on the quality of water--according to the results of
a bacterioscopic examination extending merely to a determination of
the number of germs capable of development--must lead to inaccurate
conclusions, which contradict the results of chemical analysis.
The attempt to put forward bacterioscopic examination as a decisive
criterion for the character of a water is therefore devoid of a
satisfactory basis. For the present, Dr. Link thinks the decision
must be left to chemical analysis.
At any rate it is doubtful whether the test of the number of
micro-organisms should determine the question whether a water is or
is not safe to drink. Dr. Koch’s gelatine peptone test has enabled
the analyst to recognise the absence or presence of microphytes;
but, as was stated at a recent meeting of the Society of Medical
Officers of Health, a sample of river water which might be marked
“very good” by this test would develop an enormous number of colonies
if kept for a few days, even in a “sterilised flask” protected from
aerial infection. Prof. G. Bischof says, in fact, that a sample
of New River water kept for six days in the above manner compares
unfavourably as regards the number of “colonies” with a sample taken
from the company’s main and polluted with one per cent. of sewage,
or with a sample of Thames water taken at London Bridge. It seems
certain too that the water stored on board ship must develop an
enormous number of “colonies”; but no special amount of disease is
attributable to them, and it would seem to follow that, unless the
number of microphytes can be shown to indicate, or to be a measure
of, pollution, Koch’s test is of little utility except as a guide
to waterworks’ engineers, by pointing out that the filters want
cleaning. In the laboratory the test is no doubt of considerable
value; but in analysing water it must be applied with discrimination,
and waters of a totally different character should not be compared
by the number of organisms. For instance, the water from Loch
Katrine might contain large numbers of micro-organisms, and yet be
perfectly safe as compared with a water in which few microphytes
could be found, but which had been accidentally polluted by some of
those pathogenous germs which undoubtedly exist, and which produce
disease when they find a suitable environment. Not until we are
able to discriminate between the harmless and the disease-producing
microphytes, shall we be able to test a water supply and declare it
practically pure.
The foregoing paragraphs will suffice to show what a very
unsatisfactory state our present knowledge of water is in. The
only useful fact to be deduced from all the argument is that every
household should filter its own drinking-water and take care that the
filters are always kept clean and in good working order. There is one
simple test for the purity of water, introduced by Dr. Hager in 1871,
consisting of a tannin solution, directions for which will be found
in the Housekeeper’s section. It remains to notice the chief kinds of
filter.
Filtration is destined to perform three distinct functions, at least
where the water is required for domestic use; these are (1) to remove
suspended impurities; (2) to remove a portion of the impurities in
solution, and (3) to destroy and remove low organic bodies.
The first step is efficiently performed by nature, in the case of
well and spring water, by subsidence and a long period of filtration
through the earth; in the case of river water supplied by the
various companies, it is carried out in immense settling ponds and
filter beds of sand and gravel. This suffices for water destined
for many purposes. The second and third steps are essential for
all drinking-water, and are the aim of every domestic filter. The
construction of water filters may now be discussed according to the
nature of the filtering medium.
Gravel and Sand.--The usual plan adopted by the water companies is
to build a series of tunnels with bricks without mortar; these are
covered with a layer of fine gravel 2 ft. thick, then a stratum of
fine gravel and coarse sand, and lastly a layer of 2 ft. of fine
sand. The water is first pumped into a reservoir, and after a time,
for the subsidence of the coarser impurities, the water flows through
the filter beds, which are slightly lower. For the benefit of those
desirous of filtering water on a large scale with sand filtering
beds, it may be stated that there should be 1½ yd. of filtering area
for each 1000 gal. per day. For effective work, the descent of the
water should not exceed 6 in. per hour.
This simple means of arresting solid impurities and an appreciable
portion of the matters in solution, may be applied on a domestic
scale, in the following manner.
Procure an ordinary wooden pail and bore a number of ¼ in. holes all
over the bottom. Next prepare a fine muslin bag, a little larger than
the bottom of the pail, and about 1 in. in height. The bag is now
filled with clean, well-washed sand, and placed in the pail. Water
is next poured in, and the edges of the bag are pressed against the
sides of the pail. Such a filter was tested by mixing a dry sienna
colour in a gallon of water, and, passing through, the colour was
so fine as to be an impalpable powder, rendering the water a deep
chocolate colour. On pouring this mixture on to the filter pad and
collecting the water, it was found free of all colouring matter.
This was a very satisfactory test for such a simple appliance, and
the latter cannot be too strongly recommended in cases where a
more complicated arrangement cannot be substituted. The finest and
cleanest sand is desirable, such as that to be purchased at glass
manufactories.
This filter, however, at its best, is but a good strainer, and will
only arrest the suspended particles. In a modern filter more perfect
work is required, and another effect produced, in order that water
containing objectionable matter in solution should be rendered fit
for drinking purposes. Many persons when they see a water quite clear
imagine that it must be in a good state for drinking. They should
remember, however, that many substances which entirely dissolve in
water do not diminish its clearness. Hence a clear, bright water may,
despite its clearness, be charged with a poison or substances more
or less injurious to health; such, for instance, as soluble animal
matter.
To make a perfect filter, which should have the double action of
arresting the finest suspended matter and removing the matters held
in solution, and the whole to cost but little and capable of being
made by any housewife, has long been an object of much attention,
and, after many experiments and testing various substances in many
combinations, the following plan is suggested as giving very perfect
results, and costing only about 8_s._
Purchase a common galvanised iron pail, which costs 2_s._ Take it to
a tin-shop and have a hole cut in the centre of the bottom about ¼
in. diameter, and direct the workman to solder around it a piece of
tin about ¾ in. deep, to form a spout to direct the flow of water
downward in a uniform direction. Obtain about 2 qt. of small stones,
and, after a good washing, place about 2 in. of these at bottom of
pail to form a drain.
On this lay a partition of horse-hair cloth or Canton flannel cut
to size of pail. On this spread a layer of animal charcoal, sold by
wholesale chemists as boneblack at about 5_d._ a lb. Select this
about the size of gunpowder grains, and not in powder. This layer
should be 3 or 4 in. A second partition having been placed, add 3
in. of sand, as clean and as fine as possible. Those within reach of
glassmakers should purchase the sand there, as it is only with that
quality of sand that the best results can be obtained. On this place
another partition, and add more fine stones or shingle--say for 2 or
3 in. This serves as a weight to keep the upper partition in place,
and completes the filter. By allowing the filtration to proceed in
an upward instead of a downward direction much better results are
obtained.
Charcoal, simple.--All kinds of charcoal, but especially animal
charcoal, are useful in the construction of filters, and have
consequently been much used for that purpose. Charcoal, as is
well known, is a powerful decolorising agent, and possesses the
property in a remarkable degree of abstracting organic matter,
organic colouring principles, and gaseous odours from water and
other liquids. It has been shown that it deprives liquids, for
example, of their bitter principles, of alkaloids, of resins, and
even of metallic salts, so that its usefulness as a medium through
which to pass any suspected water is undoubted. The one point to
be observed is that it does not retain its purifying power for any
great length of time, so that any filter depending upon it for its
purifying principle must either be renewed or the power of the
charcoal restored from time to time, and this the more frequently
in proportion to the amount of impurity present in the water. A
combination filter of sand or gravel and granulated charcoal is a
good one; but the physical, or chemico-physical, action of such
compound filters, or of the other well-known filter, composed of
a solid porous carbon mass, differ in no respect from that of the
simple substances composing them; that is to say, such combinations
or arrangements are much more a matter of fancy or convenience than
of increased efficiency.
Experiments on the filtration of water through animal charcoal were
made on the New River Company’s supply in the year 1866, and they
showed that a large proportion of the organic matter was removed
from the water. These experiments were afterwards repeated, in 1870,
with Thames water supplied in London, which contains a much larger
proportion of organic matter, and in this case also the animal
charcoal removed a large proportion of the impurity. In continuing
the use of the filter with Thames water, however, it became evident
that the polluting matter removed from the water was only stored up
in the pores of the charcoal, for, after the lapse of a few months,
it developed vast numbers of animalcula, which passed out of the
filter with the water, rendering the latter more impure than it
was before filtration. Prof. Frankland reported in 1874 on these
experiments as follows:--“Myriads of minute worms were developed
in the animal charcoal, and passed out with the water, when these
filters were used for Thames water, and when the charcoal was not
renewed at sufficiently short intervals. The property which animal
charcoal possesses in a high degree, of favouring the growth of the
low forms of organic life, is a serious drawback to its use as a
filtering medium for potable waters. Animal charcoal can only be used
with safety for waters of considerable initial purity; and even when
so used, it is essential that it should be renovated at frequent
intervals, not by mere washing, but by actual ignition in a close
vessel. Indeed, sufficiently frequent renovation of the filtering
medium is an absolutely essential condition in all filters.”
[Illustration: 9. 10. Atkins’s filters]
Fig. 9 shows Atkins’s filter, in which _a_ is the unfiltered and _b_
the filtered water, _c_ being a block of charcoal formed by mixing
powdered charcoal with pitch or resin, moulding and calcining. The
filter is capable of being taken to pieces and can thus be easily and
frequently cleaned. The block should on such occasions be scraped,
washed, boiled, and baked.
Fig. 10 illustrates another form of Atkins’s, in which powdered
charcoal is used, retained between movable perforated earthenware
plates.
[Illustration: 11. 12. Sawyer’s Filters.]
Figs. 11, 12 represent Sawyers filters, in which _a_ is unfiltered
water; _b_, filtered water; _c_, charcoal hollow cone; _d_, filtered
water tap; _e_, sediment tap; _f_, mass of granular charcoal. The
most important feature here is the _upward_ filtration.
Charcoal modified.--Several substances have been proposed for
combination with carbon to improve its filtering capacity or increase
its germ-destroying powers.
[Illustration: 13. Silicated Carbon. 14. Silicated Carbon.]
Silicated Carbon.--This was one of the earliest modifications of the
simple carbon block. Figs. 13, 14 show respectively the forms adopted
for downward and upward filtration. In the former, the stoneware
receptacle is divided into two parts by a diaphragm upon which there
is fixed, by a porcelain stay, a silicated carbon block, which
entirely closes the apertures in the diaphragm. The upper surface
and corners of the filtering block are non-porous, consequently the
water has to enter at the edges and follow the course indicated by
the arrows, before it can reach the clear water compartment below. In
cleaning the filter, it is only necessary to unscrew the nut, when
the block can be lifted out and soaked in boiling water, after which
the surface can be scrubbed.
The ‘Army Medical Report’ says of filters employing carbon in porous
blocks that “These are powerful filters at first, but they are apt to
clog, and require frequent scraping, especially with impure waters.
Water filtered through them and stored, shows signs of the formation
of low forms of life, but in a less degree than with the loose
charcoal. After a time, the purifying power becomes diminished in a
marked degree, and water left in contact with the filtering medium is
apt to take up impurity again, though perhaps in a less degree than
is the case with the loose charcoal.” The advantages of combining
silica with the carbon are not at first sight apparent.
[Illustration: 15. Maignen’s Filter.]
Maignen combines charcoal with lime to produce a compound which
he calls “carbo-calcis.” At the same time he employs an asbestos
filtering cloth. The arrangement of his filter is shown in Fig. 15.
The hollow, conical, perforated frame _a_ is covered with asbestos
cloth _b_; _c_ is a layer of finely powdered carbo-calcis, deposited
automatically by being mixed with the first water poured into the
filter; _d_ is granular carbo-calcis filling up the space between _c_
and the sides of the containing vessel; _e_, unfiltered water; _f_,
filtered water; _g_, tube for admitting air to aërate the water and
correct the usually vapid flavour of filtered water. This filter has
remarkable power; wine passed through it will come out colourless and
tasteless. Moreover the cleansing and renewal of the filtering media
are simple in the extreme.
Prof. Bernays, of St. Thomas’s Hospital, has taken out a patent
for a new filtering material, consisting of charcoal combined with
a reduced manganese oxide. The well-known purifying action of
charcoal (animal and vegetable), which in its ordinary state is
liable to certain difficulties and objections, is in this invention
supplemented and improved by heating it in covered crucibles with
5 to 15 per cent. or more of powdered manganese black oxide (the
mineral pyrolusite), together with a very small quantity of some
fixed oil, resin, or fat. Having ascertained that the simple
admixture of the manganese dioxide with the charcoal without previous
heating had no utility as a filtering medium, and was even injurious
by reason of the diminution of the porosity of the charcoal, Prof.
Bernays devised the above method with the object of oxidising the
hydrogen and other oxidisable impurities of the charcoal, and hence
approximating it to pure carbon in a state similar in efficacy to
platinum black rather than in its ordinary less powerful analogy to
spongy platinum. The heating is of course out of contact with air,
and the temperature sufficiently high to cause the reduction of
the manganese dioxide at least to manganous-manganic oxide, which
afterwards acts as a carrier of oxygen, and thereby much prolongs the
purifying action of the medium. Another method of obtaining charcoal
in combination with manganous-manganic oxide is to saturate charcoal
with manganous chloride (or even manganese residues) and afterwards
subject it to a strong heat in closed crucibles. The charcoal
prepared in the above manner may be employed in the filtration of
water in layers with sand and other filtering material in the usual
manner.
A filtering material which has all the properties of animal charcoal,
and is said to give higher results, is magnetic carbide, discovered
by Spencer, many years ago, and consists of iron protoxide in
chemical combination with carbon. It is considered that the purifying
effect is produced by its power of attracting oxygen to its surface
without the latter being acted on, the oxygen thus attracted being
changed to ozone, by which the organic matter in the water is
consumed.
There can be no doubt of the value of this filtering material. Its
manufacture is very simple, as it is obtained by roasting hematite
iron ore with granulated charcoal for 12 to 16 hours at a dull red
heat, and used in a granular form. Another form for making this
material is to heat the hematite (iron red oxide) with sawdust in a
close vessel. The product is magnetic, and never loses its activity
until the pores are choked up. The Southport Water Company formed
their filtering beds of this material, and after years of use it is
still giving satisfaction.
Iron.--From experiments made by allowing water to filter through
spongy iron on to meat, it has been found that after 6 weeks the meat
remained fresh. Another test was made by preparing a hay infusion,
which was kept till it showed abundance of organic life. The infusion
was filtered through spongy iron with layers of pyrolusite, sand,
and gravel, and then was kept in contact with meat for many weeks.
The meat showed no signs of putrescence. In some of the experiments
filtered air was supplied, which proves conclusively that bacteria
or their germs are not revived when supplied with oxygen after the
filtration; this is a result of importance, as it demonstrates that
by filtration through spongy iron, putrefaction of organic matter
is not only suspended for a time, but that it ceases entirely until
reinstated by some putrefactive agent foreign to the water. The
peculiar action of spongy iron is believed to be thus explained. If a
rod be inserted into a body of spongy iron which has been in contact
with water for some time, gas bubbles are seen to escape. These are
found to contain carbon and hydrogen, and experiments lead to the
conclusion that the carbon is due to the decomposition of organic
matter.