The available water of many villages and small towns is that of the
chalk beds, but it is invariably very hard, and should be softened. We
have received so many inquiries respecting a simple means of carrying
out Clarke's water-softening process, that the following description
of a set of apparatus devised for this purpose by Messrs. Law and
Chatterton, MM.I.C.E., may interest many besides those who contemplate
the construction of small waterworks supplied by the chalk springs.
The apparatus, as made in various sizes by Messrs. Bowes, Scott,
and Read, of Broadway-chambers, Westminster, we illustrate by the
accompanying engravings.
_Softening hard water_.--The disadvantages attending the use of hard
water either for drinking purposes, steam generation, lavatory purposes,
and for many manufacturing purposes, are well known, but as there are
several methods of softening waters which are hard in different degrees
by different substances, we may be pardoned if we here reproduce, for
the convenience of some of our readers, a few passages from the sixth
report of the River Pollution Commission, 1874, pages 21 and 201-16,
which give some very valuable information on the relative merits of
hard and soft waters in domestic and trade uses. "Some of the mineral
substances which occur in solution in potable waters communicate to the
latter the quality of hardness. Hard water decomposes soap, and cannot
be efficiently used for washing. The chief hardening ingredients are
salts of lime and magnesia. In the decomposition of soap these salts
form curdy and insoluble compounds containing the fatty acids of
the soap and the lime and magnesia of the salts. So long as this
decomposition goes on the soap is useless as a detergent, and it is
only after all the lime and magnesia salts have been decomposed at the
expense of the soap, that the latter begins to exert a useful effect.
As soon as this is the case, however, the slightest further addition of
soap produces a lather when the water is agitated, but this lather is
again destroyed by the addition of a further quantity of hard water.
Thus the addition of hard water to a solution of soap, or the converse
of this operation, causes the production of the insoluble curdy matter
before mentioned. These facts render intelligible the process of washing
the skin with soap and hard water. The skin is first wetted with the
water and then soap is applied; the latter decomposes the hardening
salts contained in the small quantity of water with which the skin
is covered, and there is then formed a strong solution of soap which
penetrates into the pores, and now the lather and impurities which it
has imbibed require to be removed from the skin by wiping the lather off
with a towel or by rinsing it away with water. In the former case the
pores of the skin are left filled with soap solution; in the latter they
become clogged with the greasy, curdy matter which results from the
action of the hard water upon the soap solution which had previously
gained possession of the pores of the cuticle. As the latter process of
removing the lather is the one universally adopted, the operation of
washing with soap and hard water is analogous to that used by the dyer
and calico printer for fixing pigments in calico, woolen, or silk
tissues. The pores of the skin are filled with insoluble greasy and
curdy salts of the fatty acids contained in the soap, and it is only
because the insoluble pigment produced is white, or nearly so, that so
repulsive an operation is tolerated. To those, however, who have been
accustomed to wash in soft water, the abnormal condition of skin thus
induced is for a long time extremely unpleasant.
Of the hardening salts present in potable water, carbonate of lime is
the one most generally met with, and to obtain a numerical expression
for this quality of hardness a sample of water containing 1 lb. of
carbonate of lime, or its equivalent of other hardening salts, in
100,000 lb.--10,000 gallons--is said to have 1° of hardness. Each degree
of hardness indicates the destruction and waste of 12 lb. of the best
hard soap by 10,000 gallons of water when used for washing. Hard water
frequently becomes softer after it has been boiled for some time. When
this is the case, a portion at least of the original hardening effect is
due to the bicarbonate of lime and magnesia. These salts are decomposed
by boiling into free carbonic acid, which escapes as gas, leaving
carbonates of lime and magnesia; the latter being nearly insoluble in
water, ceases to exert more than a very slight hardening effect, and
produces a precipitate. As the hardness resulting from the carbonates
of lime and magnesia is thus removable by boiling the water, it is
designated temporary hardness, while the hardening effect which is due
chiefly to the sulphates of lime and magnesia, and cannot be got rid of
by boiling, is termed permanent hardness. The total hardness of water
is therefore commonly made up of temporary and permanent hardness.
A constant supply of hot water is now almost a necessity in every
household, but great difficulties are thrown in the way of its
attainment by the supply of hard water to towns forming thick calcareous
crusts in the heating apparatus.
Waters with much temporary hardness are most objectionable in this
respect, and the evil is so great where the heating is effected in a
coil of pipe, as practically to prevent, in towns with hard water, the
use of this most convenient method of heating water. The property of
being softened by boiling which temporarily hard water possesses is not
of much domestic use, for water is, as a rule, either not raised to a
sufficiently high temperature or not kept at it for a long enough time.
Seeing then the disadvantages attendant on the use of hard water, it
remains to be considered how best to soften it. Four processes are known
to the arts. They are: Distillation, carbonate of soda, boiling, lime.
Of these processes the first and second are the most effective, but
owing to their expense are not applicable on a large scale. The third
and fourth processes are efficient only with certain classes of water,
rendered hard by the presence of the bicarbonate of lime, magnesia,
or iron. The fourth is, however, a very cheap process, and is easily
applicable to the vast volumes of water supplied to large cities,
provided the hardening ingredients are of the character described.
_Softening by distillation_.--By evaporation, water is completely
separated from all fixed saline matters, and consequently from all
hardening matters. Distilled water, however, has a vapid and unpleasant
taste, due partly to deficient aeration and partly to the presence of
traces of volatile organic matter; and though filtration through animal
charcoal will remove this, and the aeration can begin chemically, the
process is too expensive, except in certain cases, as on board ship, or
at military or naval stations where no potable water exists.
_Softening by carbonate of soda_.--The hardness of water, as already
explained, being principally due to the presence in solution of
bicarbonates and sulphates of lime and magnesia, can be reduced by
addition of carbonate of soda, which decomposes these salts slowly in
cold water but quickly in hot, forming insoluble compounds of lime and
magnesia, which are slowly precipitated as a fine mud, leaving the water
charged, however, with a solution of bicarbonate and sulphate of soda.
This process, on account of expense, is only applicable on a small scale
to the water for laundry purposes, as the water acquires an unpleasant
taste from the presence of the soda salts. For laundry purposes it is,
however, valuable, as it effects a great saving of soap.
_The softening of water by boiling_.--That portion of the hardness of
water due to the presence of bicarbonate of lime, magnesia, or iron, is
corrected by boiling the water for half an hour. During ebullition the
bicarbonates, which are soluble, become carbonates, which are insoluble,
giving off their carbonic acid as gas, rendering--by the precipitate
produced, but not allowed in a boiler time to settle--the water muddy,
but incapable of decomposing soap. To raise the temperature of 1,000
gallons of water to the boiling point and to maintain it for half an
hour requires the consumption of about 2½ cwt. of coal, or by the
wasteful appliances found in households, probably three times that
amount. Softened by boiling, then, 1,000 gallons of water would cost
about 7s. 6d., while the cost of softening the same amount by soap is
9s., at £2 6s. 6d. per cwt.
_The softening of water by lime_.--The economy which carbonate of
soda exhibits in comparison with soap as a softening material is far
surpassed by the use of lime. Lime costs about 8d. per cwt., and this
weight of lime will soften the same volume of water as would require the
use of 20¼ cwt. of soap. From the above it is evident--so soon as it is
conceded that there is an advantage in using soft water--that the lime
process is by far the most economical. Besides the chemical action
affecting the hardness, it has another most important mechanical action,
in consequence of the weight of each particle composing the precipitate
produced by it. These particles during subsidence become attached to the
almost microscopical organic impurities present in all river water, and
drag them down to the bottom of the settling tank, whereby the water is
rendered, after some eight hours, clear as crystal. The average cost of
the water supplied by the leading metropolitan water companies is £10
10s. 9¾d. per million gallons. The charge made by the companies to
consumers is about 6d. per 1,000 gallons, or £25 per million gallons.
It has been found that water can on a large scale be softened from 14°
hardness to 5° at a cost of 20s. per million gallons--that is, 10 per
cent. on the cost of the water to the companies, or 4 per cent. as the
price charged to consumers. This estimate does not take into account the
value of the precipitated chalk, which has a market price, and is used
for many purposes, being, in fact, whiting of the purest quality. The
operations necessary in Clarke's process are four in number: (1) The
preparation of milk of lime; (2) the preparation of a saturated solution
of lime; (3) the mixture of this solution with the water to be softened;
(4) the classification of the softened water by the separation of
the precipitated substances Messrs. Law and Chatterton effect these
processes by simple mechanical means which are so far automatic that
they only require the presence of a person, without technical knowledge,
once in each twenty-four hours. No filtering medium whatever is
required, which is a great advantage for the following reasons: (1)
Filtering materials require periodical cleaning and renewal, which
not only occasion much trouble and mess, but are also frequently
inefficiently performed. (2) Experience has shown that the filtering
material, whether cloth, charcoal, or other substance, is extremely
liable to become mouldy or musty, which makes the wafer both unwholesome
and unpalatable. This system is especially adapted for small water
supplies and for use in country houses, there being no operation to
perform requiring either technical, chemical, or mechanical knowledge,
nor producing dust or dirt.
[Illustration: Fig. 1.--LAW AND CHATTERTON'S WATER-SOFTENING APPARTUS.]
The following is a description of this apparatus as fitted at the
Hoo, Luton, Bedfordshire, for the supply of Mr. Gerard Leigh's house,
grounds, and home farm. The mixing of the lime and the subsequent
stirring of the water is effected by water power obtained from a
turbine. The whole of the apparatus and tanks occupy a space 60 ft.
square, 3,600 ft. area, and soften a daily supply of 50,000 gallons.
[Illustration: Fig. 2]
A pump driven from the turbine forces the water to a reservoir in the
park and on to the house, an ingenious automatic arrangement worked by
the overflow from the cistern throwing the pump out of gear when the
tank is full. A, B, and C. Figs. 1 to 6 herewith, are three tanks in
which the water remains to be softened, each capable of holding one
day's supply. D and E are two smaller tanks in which the lime water is
prepared; X is the automatic valve apparatus by which the connections
between the several tanks are effected in the order and at the times
required; H and H show the positions in which two pumps should be
placed, the former for pumping unsoftened water into the tanks, the
latter to pump the softened water into the supply cistern. J is the pipe
from the well or other source of supply--in case the supply is at a
higher level, one pump can be dispensed with. The operation consists in
adding to the water to be softened a certain quantity of lime water,
depending upon the degree of hardness, and in then allowing the mixture
to rest in a state of perfect quiescence until the whole of the lime has
been deposited and the water has become perfectly clear. The tank, A,
has been filled with unsoftened water. Tank B contains the water
and lime in process of clarification by subsidence after mechanical
agitation by the screw. Tank C contains the softened water--and the
precipitate--in process of removal for consumption. The mode of working
is as follows: The milk of lime, prepared by slaking new lime in a
"Michele mixer"--not shown. One of the tanks, D, having been filled with
softened water, run by gravity from one of the tanks, A, B, or C, the
requisite amount of milk of lime is allowed to flow into it from the
lining machine, and the whole having been thoroughly mixed by the patent
agitator, G, is left in a quiescent state for some hours, when the
superabundant lime falls to the bottom, and the tank contains a
perfectly clear and saturated solution of lime. The requisite quantity
of lime water is then suffered to flow by gravity into whichever of the
three tanks is empty. In the mean while, the softened water is being
withdrawn by pumping or gravitation, as the case may be, from the tank
C, until, upon the water being lowered to within a certain distance of
the bottom, an automatic arrangement shifts the valve, X, so that the
supply then commences from B, the unsoftened water flows into C, and
the water is in process of clarification in A, and thus the operation
proceeds continuously. Where the water can be supplied by gravitation,
and the tanks can be placed at a sufficient elevation to command the
service cistern, no pumps are required, the softening process, in fact,
in no way necessitating pumping. The space occupied by the whole of the
tanks and apparatus is 60 ft. square, 3,600 ft. area, and softens 50,000
gallons per day. For the daily softening of quantities less than 1,000
gallons, the tanks are made of galvanized sheet iron, and the whole
apparatus and tanks are self-contained, so as only to require the making
of the necessary connections with the existing supply and delivery
pipes, and fixing in place. No expensive foundations are required, and
the entire cost of an apparatus--see Figs. 2, 3, 4, 5, and 6--capable of
softening 500 gallons per day is about £75. Annexed is a more detailed
description of the manner of fixing and working the smaller apparatus.
[Illustration: Fig. 3]
The tank must, of course, be set up perfectly level. The pipe from the
source of supply--in the present case from the hydraulic ram--must
be attached to the upper three way cock at A, on the accompanying
engravings, and the pipe to supply softened water is to be connected
to the lower three-way cock at B, and should be led into the elevated
cistern with a ball cock so as to keep it always filled. The three ball
cocks in C, D, and E should be adjusted to allow the tanks to fill to
within 3 in. of the top. The nuts at the upper extremity of the three
rods, F, G, and H, should be so adjusted that when the water in the
several tanks has been drawn down to within 15 in. of the bottom the
rocking shaft, I I, is drawn down and the vertical rod, J, lifted so as
to allow the wheel, K, and spindle, L, to revolve by the action of the
weight, M. The length of the chain is such that when the weight, M,
rests upon the floor the face of the raised rim on the wheel, K, should
not quite touch the rod, J, and if necessary, a thin packing should be
put for the weight to drop upon. The lime to be used should be pure
chalk lime free from clay, mixed with water to a smooth, creamy
consistency, and then poured into the small tank, N. This tank should
then be filled with water to within 3 in. of the top, and the small air
pump worked until the lime has become thoroughly mixed and diffused
throughout the water. Care must be taken that previous to filling the
tank the float, O, is raised up, as shown by the dotted lines in Fig. 3.
After the lime has been thoroughly mixed it should be left for at
least eight hours for the superabundant lime to subside, leaving the
supernatant fluid a perfectly clear saturated solution of lime. At the
end of this time the float, O, should be lowered, so that it may float
upon the lime water, and the three-way cock, P, should be turned in such
a position as to allow the contents of the tank, N, run into the
tank, Q, until the necessary quantity has been supplied, the mode of
determining which is hereinafter described.
[Illustration: Fig. 4]
The spindle, L, should then be turned into the position which allows the
water from the source of supply to be discharged into the tank, Q, the
float, R, having first been raised into the position shown in Figs. 2
and 5. A second quantity of the lime should now be added to the tank, N,
mixed with water, and after agitation, another eight hours allowed for
the contents of both the tanks, Q and N, to subside. At the end of
this time the three-way cock, P, should be turned through a third of
a circle, so as to discharge the lime water into the tank, S; and the
spindle, L, should be turned in the contrary direction to the hands of a
watch through the third of a circle, so as to allow the water from the
source of supply to be discharged into the tank, S, care being taken as
before to raise the float, T, out of the water. A third quantity of lime
must be added to the tank, N, and now mixed with water to be drawn from
the tank, Q, by the tap, U, and after agitation again left for eight
hours to subside. The float, R, may now be lowered into the water in the
tank, Q, when it will be found that the clear softened water contained
in the tank, Q, will be discharged through the pipe attached to the
bottom of the three way tap, B. The weight, M, must now be lifted about
5 in., so as to allow the ring at the end of the chain to be moved back
to the next stud on the wheel, K. The lime water in the tank, N, must
next be discharged into the tank, V, and then another quantity of lime
must be added to the tank, N, and filled up with softened water from the
tank, S, by means of the tap, W, and after being duly agitated and left
to subside. As soon as the softened water from the tank, Q, has been
drawn down to within 15 in. of the bottom, the rod, H, will move the
rocking shaft, I, and lift the rod, J, so releasing the wheel, K, and
allowing the weight, M, to descend and turn the spindle, L, and the
upper and lower three-way cocks through a third of a circle; the effect
of which movement will be to continue the supply of softened water from
the tank, S, and to fill up the tank, V, with water from the source of
supply.
[Illustration: Fig. 5]
The apparatus will now be in the condition to afford a regular supply
of softened water; all that will be necessary to insure its continuous
action will be that at certain stated intervals dependent upon the
rapidity with which the water is used--but which interval should not
be less than eight hours--the following things should be done: (1) The
float must be raised out of the tank last emptied. (2) The float must be
lowered into the tank last filled. (3) The weight, M, must be raised,
and the ring of the chain shifted to the next stud on the wheel, K. (4)
The clear lime water found in the tank, N, must be turned into the tank
last emptied. (5) The requisite quantity of lime must be put into the
tank, N. (6) The requisite quantity of water must be drawn off from the
tank last filled into the tank, N. (7) The contents of tank, N, must be
thoroughly mixed by means of the air pump. The quantity of lime to be
used for each tankful of water must depend upon the hardness of the
water, ¾ oz. being required for each tankful for each degree of
hardness. It is desirable, however, always to have an excess of lime in
the tank, N, so as to insure obtaining a saturated solution of lime.
When first mixed the contents of the tank, N, will have a creamy
appearance, but when the superabundant lime has subsided the supernatant
liquid will be a perfectly clear saturated solution of lime. Therefore,
in the first instance, 3 lb. of lime should be put into the tank, N, and
subsequently each time such a quantity of lime should be added as is
found to be necessary by the method hereinafter described. The quantity
of the saturated lime water to be run into each of the softening tanks,
Q, S, and V, will depend upon the hardness of the water. For every
degree of temporary hardness a depth of 1-6/10 in. of the contents of
the tank, N, will be required; so that if the water has 14 deg. of
temporary hardness, then 22½ in. in depth of lime water must be run off
into each of the tanks, Q, S, and V. In the first instance an excess of
lime may be used, and the softened water tested by means of nitrate of
silver in the following manner: A solution of 1 oz. of nitrate of silver
in a pint of twice distilled water should be obtained. Having let two
or three drops of this solution fall on the bottom of a white tea cup,
slowly add the softened water; then if there be any excess of lime, a
yellow color will show itself, and the quantity of lime water used must
be reduced until only the faintest trace of color is perceptible.--_The
Engineer_.
[Illustration: Fig. 6]
* * * * *