OTHER METHODS OF FILTRATION.
WORMS TILE SYSTEM.
This system, invented and patented by Director Fischer of the Worms
water-works, consists of the filtration of water through artificial
hollow sandstone tiles, made by heating a mixture of broken glass
and sand, sifted to determined sizes, to a point just below the
melting-point of the glass, in suitable moulds or forms. The glass
softens and adheres to the sand, forming a strong porous substance
through which water can be passed. These tiles are made hollow and
are immersed in the water to be treated, the effluent being removed
from the centre of each tile. They are connected together in groups
corresponding in size to the units of a sand-filtration plant. They
are washed by a reverse current of filtered water. These tiles have
been used for some years at Worms, Germany, and at a number of smaller
places, and were investigated experimentally at Pittsburg. Some
difficulty has been experienced in getting tiles with pores small
enough to yield an effluent of the desired purity, and at the same time
large enough to allow a reasonable quantity of water to pass. In fact,
with other than quite clear waters, it has not been found feasible to
accomplish both objects at the same time, and it has been necessary
to treat the water with coagulants and preliminary sedimentation or
filtration before applying it to the tiles. The problem of making the
joints between the tiles and the collection-pipes water-tight when
surrounded by the raw water also is a matter of some difficulty.
THE USE OF ASBESTOS.
It has been suggested by Mr. P. A. Maignen that the surface of sand
filters should be covered with a thin layer of asbestos, applied in
the form of a pulp, with the first water put onto the filter after
scraping. The asbestos forms a sort of a paper on the sand which
intercepts the sediment of the passing water. The advantage of the
process is in the cleaning. When dried to the right consistency this
asbestos can be rolled up like a carpet, and taken from the filter
without removing any of the sand.
This procedure is almost identical with that which has occurred
naturally in iron-removal plants, where algæ grow in the water upon the
filters, and form a fibrous substance with the ferric oxide removed
from the water, which can be rolled up and removed in the same way
as the asbestos. The advantages of the process, from an economical
standpoint, are less clear.
FILTERS USING HIGH RATES OF FILTRATION WITHOUT COAGULANTS.
Numerous filters have been suggested, and a few have been constructed
for the use of much higher rates of filtration than are usually
employed with sand filters, but without the use of coagulants. The
results obtained depend upon the requirements and upon the character
of the raw water. If a reservoir water contains an algæ growth, it can
often be removed by a coarse and rapid filter. The organisms in this
case are many times larger than the bacteria, and many times larger
than the clay particles which constitute turbidity. The requirements in
this case are rather in the nature of straining than of filtration.
The conditions necessary for the removal of bacteria and turbidity are
very well understood, and it can be stated with the utmost confidence
that no system of filtration through sand at rates many times as
high as are used in ordinary sand filtration, and without the use of
coagulants, will be satisfactory where either bacterial efficiency
or clarification is required. The application of such systems of
filtration would therefore seem to be somewhat limited.
[Illustration: REMOVING DIRTY ASBESTOS COVERING FROM AN EXPERIMENTAL
FILTER. MAIGNEN SYSTEM.
[_To face page 182._]
]
HOUSEHOLD FILTERS.
The subject of household filters is a somewhat broad one, as the
variety in these filters is even greater than in the larger filters,
and the range in the results to be expected from them is at least as
great. I shall only attempt to indicate here some of the leading points
in regard to them.
Household filters may be used to remove mud or iron rust from the tap
water, or to remove the bacteria in case the latter is sewage-polluted,
or to do both at once. Perhaps oftener they are used simply because
it is believed to be the proper thing, and without any clear
conception either of the desired result or the way in which it can
be accomplished. I shall consider them only in their relations to
the removal of bacteria, as I credit the people who employ them with
being sufficiently good judges of their efficiency in removing visible
sediment.
In the first place, as a general rule, which has very few if any
exceptions, we may say that all small filters which allow a good stream
of water to pass do not remove the bacteria. The reason for this is
simply that a material open enough to allow water to pass through it
rapidly is not fine enough to stop such small bodies as the bacteria.
The filters which are so often sold as “germ-proof,” consisting of
sand, animal charcoal, wire-cloth, filter-paper, etc., do not afford
protection against any unhealthy qualities which there may be in the
raw water. Animal charcoal removes color without retaining the far more
objectionable bacteria.
The other household filters have filtering materials of much finer
grain, unglazed porcelain and natural sandstone being the most
prominent materials, while infusorial earth is also used. The smaller
sizes of these filters allow water to pass only drop by drop, and when
a fair stream passes them the filters have considerable filtering
area (as a series of filter-tubes connected together). On account of
their slow action, filters of this class are, as a rule, provided with
storage reservoirs so that filtered water to the capacity of the
reservoir can be drawn rapidly (provided the calls do not come too
often). Some of these filters are nearly germ-proof, and are comparable
in their efficiency to large sand-filters. There is no sharp line
between the filters which stop and which do not stop the bacteria; but
in general the rule that a filter which works rapidly in proportion to
its size does not do so, and _vice versa_, will be found correct.
In thinking of the efficiency of household filters we must distinguish
between the filter carefully prepared for an award at an exhibition
and the filter of the same kind doing its average daily work in the
kitchen. If we could be sure in the latter case that an unbroken layer
of fine sandstone or porcelain was always between ourselves and the raw
tap-water we could feel comparatively safe. The manufacturers of the
filters claim that leaky joints, cracked tubes, etc., are impossible;
but I would urge upon the people using water filtered in this way that
they personally assure themselves that this is actually the case with
their own filters, for in case any such accident should happen the
consequences might be most unpleasant. The increased yield of a filter
due to a leaky joint is sure not to decrease it in favor with the cook,
who is probably quite out of patience with it because it works so
slowly, that is, in case it is good for anything.
The operation of household filters is necessarily, with rare
exceptions, left to the kitchen-girl and luck. Scientific supervision
is practically impossible. With a large filter, on the other hand,
concentrating all the filters for the city at a single point, a
competent man can be employed to run them in the best-known way; and
if desired, and as is actually done in very many places, an entirely
independent bacteriologist can be employed to determine the efficiency
of filtration. With the methods of examination now available, and
a little care in selecting the times and places of collecting the
samples, it is quite impossible for a filter-superintendent to
deliver a poor effluent very often or for any considerable length of
time without being caught. The safety of properly-conducted central
filtration is thus infinitely greater than that from even the best
household filters. Further, it may be doubted whether an infected water
can be sent into every house in the city to be used for washing and all
the purposes to which water is put except drinking, without causing
disease, although less than it would if it were also used for drinking.
The use of household filters must be regarded as a somewhat desperate
method of avoiding some of the bad consequences of a polluted
water-supply, and they are adopted for the most part by citizens who
in some measure realize the dangers from bad water, but who cannot
persuade their fellow-citizens to a more thorough and adequate solution
of the problem. Such citizens, by the use of the best filters, and by
carefully watching their action, or by having their drinking-water
boiled, can avoid the principal dangers from bad water, but their
vigilance does not protect their more careless neighbors.