There are several methods extant for the purpose of purifying and
softening water, and in the following brief account some of the chief
features of these methods are summarized. The Slack and Brownlow
apparatus we will deal with first. This purifier is one which is
intended to remove the matter in suspension in the water to be treated
by subsidence and not by filtration. The apparatus consists of a
vertical iron tank or cylinder, inside which are a series of plates
arranged in a spiral direction around a fixed center, and sloping at
an angle of 45° on both sides outward. The water to be dealt with
flows through a large inlet tube fixed to the bottom of the cylinder,
rises to the top by passing spirally round the whole circumference,
and depositing on the plates or shelves all solids and impurities at
the outer edges of the plates. Mud cocks are placed to remove the
solids deposited during the flow of the water upward to the outlet
pipe, placed close to the top of the cylinder. One of these tanks, a
square one, is at work purifying the Medlock water at Manchester, and
on drawing samples of water from nearly every plate, that from the
lower mud cock showed considerable deposit, which decreased in bulk
until the top mud cock was reached, when the water was quite free from
deposit. It is stated that one man would be sufficient to attend to 20
of these purifiers.
To filter or purify 2,000,000 gallons per 24 hours would require 40
tanks, 10 ft. by 7 ft. diameter, each doing 2,000 gallons per hour,
and would cost, with their fittings, £6,400, including all patent
rights, but exclusive of lime mixing tanks, agitators, lime water and
softening tanks, engine and boiler, and suitable buildings, the cost
of which would not be far short of £5,000, or a total of £11,400 to
soften 2,000,000 gallons per 24 hours. The labor and other working
expenses in connection with this plant would not be less than that
necessary to work the Porter-Clark process, which is given as O.55d.
per 1,000 gallons.
The Brock and Minton filter press system is another method. This
patent press is made of steel, perforated with ½ inch holes. On the
inside of the shell there is first laid a layer of fine wire netting,
then a layer of cloth, and lastly another layer of wire netting of a
larger mesh than the other. The matter treated is pumped into the body
of the cylinder, the liquid passing through the filtering material to
the outside, the solids being retained inside, and are got rid of by
partially revolving the upper half to relieve it from the knuckle
joint, and, after being raised, the lower half is turned over by
machinery, and the solid matter is simply allowed to fall out into
wagons or trucks run underneath for that purpose. Such, in brief, is
the manner of using this filter press for chemical works' purposes.
The cost of each filter press, including royalties, is from £250 to
£300, the size being 8 ft. by 4 ft. diameter. Having a filtering area
of 100 square feet, it would require 32 of these applied to softening
water to effectually deal with 2,000,000 gallons per 24 hours; this,
at the lowest estimate for filters alone, would be £8,000, and, using
the same figures, £5,000 for lime mixing tanks, etc., as referred to
in the "Slack and Brownlow" purifier, would bring the total cost up to
£13,000, and the working expense would not be less than that required
to work the Porter-Clark process, and would probably be very much
greater. This filter press is not in use anywhere for dealing with
large quantities of water in connection with a town water supply.
A process which has been working for a long time at Southampton is the
Atkins system, which also includes the use of filter presses. The
pumping station and softening works are situated at Otterbourne, eight
miles from Southampton, and were built together as one scheme. The
mixing room has two slaking lime tanks, with agitators driven by steam
power. The mixture is then run as cream of lime into a tank 20 ft.
square and is then pumped into the lower ends of two lime water
producing cylinders. The agitation is here obtained by pressure from a
small cistern placed above them with a 12 ft. head, the pipe from
which is attached to the lower ends of the cylinders. This has been
found by experiment to be the most satisfactory means of obtaining the
proper degree of agitation necessary; the clear lime water is then
drawn off at the top of the cylinders, and flows by gravity into a
mixer, where it comes in contact with the hard water. Both flow
together into a distributing trough, from which it overflows into a
small softening reservoir, having a capacity of one hour's supply, a
weir being placed along the lower end, over which the water flows to
13 filter presses. The clear water from the filters is then conveyed
to a small well, from which the permanent engines raise it to the
first of a series of high level covered service reservoirs.
In the filter press there are 20 hollow disks representing a filtering
area of 250 square feet, or a total of 3,250 square feet. The water to
be filtered passes into the body of the filter and then through a
filtering medium of cloth laid on a thin perforated zinc plate, into
the inner side of the disks, from whence it is conveyed through the
hollow shaft, to which the disks are attached, to the high level
pumps.
The filter cloths are cleaned three times every 24 hours, without
removal, by jets of softened water from the main, having a pressure of
60 pounds to the square inch. During cleaning operations the disks are
made to revolve slowly; this only occupies a space of five minutes for
each cleaning. The cloths last from six to eight months without being
renewed. They also occasionally use for further cleaning the cloths a
jet of steam injected upon the center of the disks in order to remove
by partial boiling the insoluble particles engrained in the cloths.
This has been found to make the cloths last longer. This cloth is
obtained from Porritt Bros. and Austen, Stubbing Vale, Ramsbottom, and
costs 13½d. per lineal yard of a width to suit the disks.
The quantity softened is 2¼ million gallons per 24 hours, but the
present plant can deal with 2½ million gallons, and the buildings are
erected for 3½ million gallons, additional filters and lime producing
tanks being only required to deal with the increased quantity. The
costs of the softening works was £10,394, of which £7,844 was for the
softening machinery and plant and £2,550 for the reservoir, buildings,
etc.
The working expenses, including lime, labor, cloths, general repairs,
and steam, is stated to be 0.225d. per 1,000 gallons, the labor
required being only two men, one on the day and the other on the night
shift, with an occasional man to assist.
The hardness of the Southampton water on Clark's scale is 18° of total
hardness, and this is reduced down to 6° or 8° by this process.--_Chem.
Tr. Jour._
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