Hitherto it has been found that of all the appliances and methods for
separating the liquid from the solid matters, whether it is in the case
of effluents from tanneries and other manufactories, or the ocherous and
muddy sludges taken from the settling tanks in mines, some of which
contain from 90 to 95 per cent. of water, the filter press is the best
and the most economical, and it is to this particular process that
Messrs. Johnson's exhibits at the Health Exhibition, London, chiefly
relate. Our engravings are from _The Engineer_. A filter press consists
of a number of narrow cells of cast iron, shown in Figs. 3 and 4, held
together in a suitable frame, the interior frames being provided with
drainage surfaces communicating with outlets at the bottom, and covered
with a filtering medium, which is generally cloth or paper. The interior
of the cells so built up are in direct communication with each other, or
with a common channel for the introduction of the matter to be filtered,
and as the only exit is through the cloth or paper, the solid portion is
kept back while the liquid passes through and escapes by the drainage
surfaces to the outlets. The cells are subjected to pressure, which
increases as the operation goes on, from the growing resistance offered
by the increasing deposit of solid matter on the cloths; and it is
therefore necessary that they should be provided with a jointing strip
around the outside, and be pressed together sufficiently to prevent any
escape of liquid. In ordinary working both sides of the cell are exposed
to the same pressure, but in some cases the feed passages become choked,
and destroy the equilibrium. This, in the earlier machines, gave rise to
considerable annoyance, as the diaphragms, being thin, readily collapsed
at even moderate pressures; but recently all trouble on this head has
been obviated by introducing the three projections near the center, as
shown in the cuts, which bear upon each other and form a series of stays
from one end of the cells to the other, supporting the plates until the
obstruction is forced away. We give an illustration below showing the
arrangement of a pair of filter presses with pneumatic pressure
apparatus, which has been successfully applied for dealing with sludge
containing a large amount of fibrous matter and rubbish, which could not
be conveniently treated with by pumps in the ordinary way. The sludge is
allowed to gravitate into wrought iron receivers placed below the floor,
and of sufficient size to receive one charge. From these vessels it is
forced into the presses by means of air compressed to from 100 lb. to
120 lb. per square inch, the air being supplied by the horizontal pump
shown in the engraving. The press is thus almost instantaneously filled,
and the whole operation is completed in about an hour, the result being
a hard pressed cake containing about 45 per cent. of water, which can be
easily handled and disposed of as required. The same arrangement is in
use for dealing with sewage sludge, and the advantages of the compressed
air system over the ordinary pumps, as well as the ready and cleanly
method of separating the liquid, will probably commend itself to many of
our readers. We understand that from careful experiments on a large
scale, extending over a period of two years, the cost of filtration,
including all expenses, has been found to be not more than about 6d. per
ton of wet sludge. A number of specimens of waste liquors from factories
with the residual matters pressed into cakes, and also of the purified
effluents, are exhibited. These will prove of interest to many, all the
more so since in some instances the waste products are converted into
materials of value, which, it is stated, will more than repay for the
outlay incurred.
[Illustration: Fig. 3. Fig 4.]
Another application of the filter press is in the Porter-Clark process
of softening water, which is shown in operation. We may briefly state
that the chief object is to precipitate the bicarbonates of lime and
magnesia held in solution by the water, and so get rid of what is known
as the temporary hardness. To accomplish this, strong lime water is
introduced in a clear state to the water to be softened, the quantity
being regulated according to the amount of bicarbonates in solution. The
immediate effect of this is that a proportion of the carbonic acid of
the latter combines with the invisible lime of the clear lime water,
forming a chalky precipitate, while the loss of this proportion of
carbonic acid also reduces the invisible bicarbonates into visible
carbonates. The precipitates thus formed are in the state of an
impalpable powder, and in the original Clark process many hours were
required for their subsidence in large settling tanks, which had to be
in duplicate in order to permit of continuous working. By Mr. Porter's
process, however, this is obviated by the use of filter presses, through
which the chalky water is passed, the precipitate being left behind,
while, by means of a special arrangement of cells, the softened and
purified water is discharged under pressure to the service tanks. Large
quantities can thus be dealt with, within small space, and in many cases
no pumping is required, as the resistance of the filtering medium being
small, the ordinary pressure in the main is but little reduced. One of
the apparatus exhibited is designed for use in private mansions, and
will soften and filter 750 gallons a day. In such a case, where it would
probably be inconvenient to apply the usual agitating machinery, special
arrangements have been made by which all the milk of lime for a day's
working is made at one time in a special vessel agitated by hand, on the
evening previous to the day on which it is to be used. Time is thus
given for the particles of lime to settle during the night. The clear
lime water is introduced into the mixing vessel by means of a charge of
air compressed in the top of a receiver, by the action of water from the
main, the air being admitted to the milk of lime vessel through a
suitable regulating valve. A very small filter suffices for removing the
precipitate, and the clear, softened water can either be used at once,
or stored in the usual way. The advantages which would accrue to the
community at large from the general adoption of some cheap method of
reducing the hardness of water are too well known to need much comment
from us.
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