BLEACH TREATMENT
The treatment of water with bleach alone has been largely supplanted by
the liquid chlorine process but the following details will be of use on
meeting conditions for which liquid chlorine cannot be used and also for
the preparation of the hypochlorite solution required in the chloramine
process.
The essential features of a bleach installation are the solution or
mixing tanks, storage tanks, piping system, discharge orifice or weir,
and sludge drain.
Bleach is usually sent out by the manufacturers in sheet steel drums, 39
inches high and 29-1/2 inches in diameter, which contain about 14 cu.
ft. of bleach and weigh approximately 750 pounds gross and 690 pounds
net. It can be most economically purchased in car lots and if the
consumption warrants this procedure storage should be provided for about
70 drums or rather more than one car load. According to Hooker[1] bleach
loses 1 per cent of available chlorine per month in hot seasons and 0.3
per cent in cold ones so that it is advisable to carry as little stock
as possible during hot weather. Hot weather also causes a further loss
by accelerating the action of the bleach on the drum which rapidly
disintegrates and cannot be handled. Bleach can often be purchased more
cheaply in hot weather but such a policy is a short sighted one unless
it is required for immediate use.
The general design of a hypochlorite plant is largely determined by the
capacity but in all cases an effort should be made to avoid complicated
details which may appear advantageous in the drafting office but do not
stand up in actual practice. Many metals rapidly develop a protective
coating on immersion in bleach solution but if this is removed by
friction, rapid erosion ensues; bearing metallic surfaces should be
reduced to a minimum.
_Mixing Tanks._ All tanks, whether mixing or storage, should be
constructed of concrete and painted with two coats of asphalt.
Experience has shown that wooden tanks are not suitable. The author has
used pine, oak, and cypress tanks but all were rapidly leached by the
hypochlorite and ultimately had to be lined with concrete.
There is a considerable variation in the concentration of bleach
solution made in mixing tanks at various works. Some operators use about
one gallon of water per pound of bleach and mix the two to a cream by
wooden paddles, revolving on a central axis, for 1-2 hours; the paddles
are then stopped and the cream run out into the storage tanks and
diluted to the required strength by passing water through the mixing
tank. There are two objections to this method: (1) the addition of small
quantities of water to bleach tends to gelatinisation which may protect
lumps from the further action of water and (2) a stratification of the
solution occurs in the storage tank unless agitation is used.
Gelatinisation causes loss of available chlorine and stratification
causes irregular dosage unless corrected by agitation, which
necessitates power. Other operators mix the bleach and water to the
final concentration in the mixing tank and discharge the contents into
the storage tank, the intermittent process being repeated until the
storage tank is full. Gelatinisation is avoided by using a low original
concentration and as all batches are of equal density no stratification
is produced.
At Ottawa the bleach is crushed and, after weighing, dumped into a
circular concrete tank provided with a hinged wooden lid. The stirring
arrangement consists of a bronze shaft on which an aluminium impeller is
fixed which revolves in an iron tube set slightly above the bottom of
the tank (see Fig. 1). After the requisite amount of water has been
added the motor connected to the bronze shaft is started and the mixture
pumped for 15-20 minutes; without waiting for the sludge to settle the
contents are discharged into the storage tank and the operation repeated
until the tank is full. The piping between the mixing and storage tanks
is of galvanised iron of generous dimension so as to compensate for
incrustation. The pipes are straight and are provided with crosses at
every change of direction to enable excessive incrustation to be
removed. The valves should be made of hard rubber or special bronze; if
brass valves are used they will probably require renewing every twelve
months.
[Illustration: Fig. 1.--Mixing Tank for Bleach.]
The concentration of solution necessarily depends upon local conditions
but it is usually advisable to keep it below 2.5 per cent of bleach,
which is equivalent to 0.85 per cent of available chlorine.
_Storage Tanks._ These should be built of reinforced concrete and
painted inside with asphalt, which should be periodically renewed to
prevent the solution seeping through to the reinforcement. At least two
tanks should be provided so that one may be filled and allowed to settle
before being put in operation. The hypochlorite discharge pipe is
usually 6-9 inches from the bottom to permit the collection of sludge,
which is run off when it reaches the elevation of the hypochlorite
discharge. The sludge drain, which opens into the bottom of the tank, is
usually a 4- or 6-inch cast-iron pipe, with suitable gate valve, which
discharges into a common drain made of clay pipe.
The storage tanks should be provided with either glass gauges or float
indicators to enable the orifice discharge to be checked up at
periodical intervals.
_Regulation of Dosage._ The discharge of the hypochlorite solution is
usually regulated either by maintaining a constant head on an orifice of
variable dimension or by varying the head on an orifice of fixed
dimension. The weir principle may also be used but it is not so well
adapted for hypochlorite as for other chemicals.
In the constant head method, the head is maintained by a bronze valve
connected to a float made of glass or tinned copper. In many cases the
orifice is a rectangular slot in a brass plate and is adjusted by means
of a brass slide operated by a micrometer screw. Brass plates are not
very suitable as they become corroded and so reduce the size of the
orifice; if the incrustation is removed the orifice will discharge more
than the calibration indicates. Needle valves are unsuitable for similar
reasons.
An example of an orifice feed box of the constant head type is shown in
Fig. 2. A vertically arranged hard-rubber pipe passes though a hard
rubber stuffing box in the bottom of the tank and has one or more
orifices near its upper end. The area of the submerged portions of the
orifices is controlled by the hand wheel which is connected with the
threaded stem of the pipe. The stem has sixteen threads per inch, and
one revolution of the wheel will submerge the orifices one-sixteenth of
an inch. The extent to which the orifices are submerged is indicated on
the dial fixed to the side of the tank.
[Illustration: FIG. 2.--Dosage Tank.]
Fig. 3 shows the regulating mechanism of another apparatus of the
constant head type. The orifice consists of a circular slot in a hard
rubber disc and is regulated by means of a hand wheel which operates a
hard rubber slide.
[Illustration: FIG. 3.--Orifice Controlling Device.]
The general arrangement of one of the variable head types is shown in
Fig. 4. A constant head is maintained on the valve _V_ by a float and
cock operating in a lead- or porcelain-lined tank. The circular tapered
orifice _O_, cut in glass, is situated in the flanged end of the iron
casting _C_ and the head, indicated on the gauge glass, is regulated by
valve _V_. This arrangement is simple and reasonably accurate. The
orifice may show slight incrustation after being in service for some
time but it can be easily cleaned by means of a test-tube brush or a
small swab moistened with acid; a wire or rod tends to break the edge of
the conical orifice and should not be used.
The volume of solution discharged by orifices of various dimensions is
shown in Diagram XV, page 149. Diagram XVI, page 149, facilitates the
calculation of the number of pounds of bleach required for any dosage.
[Illustration: FIG. 4.--Variable Head Dosage Box.]
The solution discharged from the orifice box is carried to the point of
application either in galvanised iron pipes of generous dimension or in
rubber hose. Pumps may be used for raising the solution to a higher
elevation but unless special material is used in their construction they
corrode rapidly and cannot be kept in service. Whenever possible, a
water injector should be used as it does not corrode and assists in
maintaining the delivery pipes free from sludge. All delivery pipes
should be duplicated and blown out regularly by water under pressure;
they should also be protected from frost.
The adjustment of the hypochlorite dosage can be automatically regulated
in plants where the flow of the water to be treated is measured by a
Venturi meter or other suitable appliance. Various devices have been
suggested and used but, in general, they are not so successful as
automatic regulators for liquid chlorine on account of the presence of
sludge particles which tend to diminish the area of the orifice.
For small plants, barrels have often been used as solution and storage
vessels with, in some instances, fairly successful results. The bleach
process, however, cannot be recommended for small installations because
the chemical control necessary for successful operation is usually not
available. One drum of bleach may suffice for several months operation
and as the powder gradually loses strength, the dosage constantly
diminishes and may jeopardise the safety of the supply. Liquid chlorine
machines are much more suitable than hypochlorite installations for
supplies having no chemical control.
Bleach is being very extensively used for the sterilisation of the water
used by the allied troops in France. The water supplies on the British
front are all more or less subject to pollution and it is consequently
necessary, to ensure adequate protection, to chlorinate all supplies
with bleach. Other forms of chlorine have been tried but have not proved
successful near the firing lines. The details of the technique employed
cannot be given but it may be stated that the concentration of chlorine
employed is always more than sufficient and that residual tastes and
odours are regarded as secondary considerations. Treated water is always
tested by the starch-iodide method and a bacteriological examination is
frequently made by mobile laboratories.
=Control of Hypochlorite Plants.= If efficient operation and regular
dosage is to be obtained, it is necessary that hypochlorite plants
should be controlled by a trained chemist. Good results are occasionally
obtained without such control but in every plant circumstances arise at
some period or another which only a chemist is qualified to deal with.
The points that require consideration are (1) the composition of the
bleach; (2) concentration of available chlorine in the prepared
solutions; and (3) chemical tests for free chlorine in the treated
water.
(1) _Composition of Bleach._ Each drum of bleach should be sampled and
analysed before use. The sample is obtained by cutting out the head of
the drum and removing a vertical section by means of a special sampling
tube or a piece of half-inch iron pipe which is forced to the bottom of
the drum with a boring motion and then removed; the core is then forced
out by means of a rod, mixed, and quartered down to the required size.
For analysis weigh out 5 grms. on a balance sensitive to 0.01 grm. and
grind in a mortar with 50-70 c.cms. of water; wash into a 250 c.cm.
flask and make the volume up to 250 c.cms.; shake. After allowing the
sludge to settle remove 10 c.cms. by means of a pipette and titrate by
one of the following methods:
_Bunsen's Method._ Add 10 c.cms. of a 5 per cent solution of potassium
iodide and 0.5 c.cm. glacial acetic acid and titrate with sodium
thiosulphate (24.8 grms. of the C.P. crystalline salt and 1 c.cm. of
chloroform per litre) using a starch solution as indicator. Each cubic
centimetre of thiosulphate used = 1.755 per cent of available chlorine
(1 c.cm. N/10 sodium thiosulphate = 0.00355 grm. available chlorine).
_Penot's Method._ Dilute the hypochlorite solution with 15 c.cms. of
water and titrate with a solution of N/10 sodium arsenite using
starch-iodide paper as an external indicator. Each c.cm. of solution
used = 1.755 per cent of available chlorine (1 c.cm. = 0.00355 grm.
available chlorine). The use of an external indicator makes this process
a slow one and to overcome this objection Mohr proposed the addition of
an excess of sodium arsenite solution and then titrating with N/10
iodine solution after adding a few drops of starch solution.
Griffen and Hedallen[2] compared these three methods and found that
Penot's method and Mohr's modification of that method gave results
which were 0.6 per cent lower than those obtained by Bunsen's method.
For a separate estimation of the chlorine present as chloride, chlorate,
and hypochlorite the method given in Sutton's Volumetric Analysis, 10th
edition, page 178, should be followed.
_Storage Liquor._ This is tested by any of the above methods. It has
been proposed to determine the strength of the bleach solution by the
use of a hydrometer but the results are not sufficiently accurate and
the method cannot be recommended.
If bleach is properly broken up and thoroughly agitated in the mixing
tank at least 95 per cent of the available chlorine should be extracted.
The efficiency of the extraction process is checked by comparing the
tests of the storage liquor with those of the dry bleach and each batch
of liquor should be tested daily. It is sometimes advisable to take two
samples from each tank, one soon after a tank has been put into
operation, and a second sample at the end of the run. Considerable
differences are occasionally found between these samples and are due,
either to inadequate agitation of the liquor in the storage tank, or
inefficient mixing in the mixing tank. If the results are irregular the
former is the more probable cause but if the second sample is invariably
stronger the mixing tank operations should be investigated. The
increased concentration of the second sample is due to unextracted
bleach passing out of the mixing tank and gradually becoming leached as
the tank contents are run off. If the bleach is lumpy and is not
subsequently broken up, losses are almost inevitable.
Hale[3] found that during the period when the New York City supply was
being treated with bleach it was necessary to constantly check the
operations of the labourers by frequent samples. "During one week about
95 per cent of the chlorine added was actually applied, the second week
it dropped to 85 per cent. and the third week to 75 per cent. Whenever
a poor run is called to the attention of the labourers, results
improve."
By taking two samples daily from each tank discharged the author has
been able to obtain an average annual efficiency on the Ottawa plant of
94 per cent., i.e. the solutions contained 94 per cent. of the available
chlorine contained in the bleach. In making such checks it is necessary
to keep a careful account of the stock of bleach to prevent labourers
adding a few extra pounds of bleach to compensate for losses.
Sludge forms an appreciable but unavoidable source of loss of material.
When the sludge reaches the outlet of the hypochlorite pipe the sludge
must be run to waste; otherwise it will pass over and tend to choke the
dosage control apparatus. If the sludge is run into the same body of
water that forms the source of supply, it must be discharged very slowly
to prevent a possibility of over dosage and damage to fish life. With
proper control, sludge losses can easily be kept under 2 per cent. and
often under 1 per cent.
The greatest source of unavoidable loss in hypochlorite plants is from
deterioration of the bleach during storage; in warm climates this loss
may exceed 10 per cent. In Ottawa where high temperatures are only
experienced during the summer months the loss from this cause has
averaged from 7-8 per cent. on the bleach stored during that period.
_Detection and Estimation of Free Chlorine._ The oldest and probably the
best known test for free chlorine in water is the Wagner test, made by
adding a few drops of potassium iodide and starch; the presence of
chlorine is indicated by a deep rich blue colouration that is
proportional in intensity to the quantity of chlorine present. When this
test is used as a colorimetric method for the estimation of chlorine
several difficulties are encountered; the intensity of the colour
produced by the majority of treated waters gradually diminishes and the
loss is usually more rapid than in the standards made up with distilled
water; a different result is obtained if the solutions are acidified and
the results vary with different acids, acetic acid yielding a much lower
result than a mineral acid such as hydrochloric acid; in the presence of
acid the colouration usually intensifies on standing, whereas the
standard intensifies but little. The difference caused by the addition
of acid is imperfectly understood but it is obvious that the chlorine
set free by the acid cannot be present in the "free" state; it is
probably in a semi-labile condition loosely attached to organic
compounds. Whether this semi-labile chlorine is available for germicidal
action is at present not definitely known but it has been noted by
several observers that the germicidal action proceeds after the "free"
chlorine reaction has disappeared.
The method used by the author for the estimation of free chlorine is as
follows: place 500 c.cms. of the sample in a stoppered bottle, add 1
c.cm. of 5 per cent KI solution, 2 drops of conc. HCl and 1 c.cm. of
starch solution and titrate with N/1000 sodium thiosulphate until
colourless. The difficulty introduced by the opalescence of the liquid
is overcome by pouring portions of the liquid into two Nessler tubes and
adding a drop of thiosulphate solution to one and noting if any
reduction of colour occurs on shaking; if the intensity of the colour is
diminished, the contents of both tubes are poured back into the bottle
and titrated until no further colour removal, as shown by the tubes, can
be obtained. One c.cm. of N/1000 sodium thiosulphate = 0.07 p.p.m. of
available chlorine when 500 c.cms. of water are used.
Adams[4] has employed the colorimetric method of estimating the colour
obtained after the addition of dilute H_{2}SO_{4}, KI, and starch but
used standard solutions of dyes for comparison. The standards were
prepared from mixtures of Brilliant Mill Green "S" and Cardinal Red "J"
and were made up weekly.
Phelps found that ortho-tolidine in acetic acid solution produced an
intense yellow colouration with free chlorine and suggested the use of
this reagent as a qualitative test for chlorine. Ellms and Hauser[5]
developed this process into a quantitative one and substituted
hydrochloric acid for acetic acid as a solvent. One c.cm. of the reagent
(1 gram of pure _o_-tolidine dissolved in 1 litre of 10 per cent of
hydrochloric acid) is added to 100 c.cms. of the sample in a Nessler
tube and the colour compared after five minutes with permanent standards
made up with mixtures of potassium bichromate and copper sulphate. This
method was adopted as the official standard method of the American
Public Health Association; the details are given in the Appendix (p.
147).
The author has found that this method gives excellent results except for
coloured waters. The colouring matter in many waters diminishes in
intensity on the addition of acids and is somewhat similar in tint to
that produced by addition of _o_-tolidine. If the reaction is used
qualitatively on coloured treated water and a comparison made with the
untreated sample, a negative result, due to the reduction in colour
produced by the acid being greater than the increase caused by the
reagent, might be obtained when traces of free chlorine are present.
Similar difficulties are encountered when quantitative comparisons are
made against permanent standards.
Benzidine (Wallis[6]) has also been suggested for the detection of free
chlorine. On adding this reagent a blue colouration is produced but on
stirring it rapidly changes to a bright yellow which is proportional in
intensity to the amount of free chlorine present. Ellms and Hauser[5]
investigated benzidine in 1913 and found it to be inferior to
_o_-tolidine as a test reagent for free chlorine.
LeRoy[7] has proposed the use of
hexamethyltri_para_-aminotriphenylmethane for detecting and estimating
free chlorine. On the addition of a hydrochloric acid solution of this
compound to a sample containing free chlorine a violet colouration is
produced that can be matched in the usual way with standards. It is
stated that 0.03 p.p.m. of free chlorine gives a distinct colouration
and that the reagent reacts very slowly with nitrites and is quite
unaffected by hydrogen peroxide.
The starch-iodide and _o_-tolidine reactions are affected by oxidising
agents or reducible substances; nitrites and ferric salts are the
compounds that are most likely to interfere and Ellms and Hauser[5] have
found that these bodies do not affect the _o_-tolidine reaction to the
same extent as the starch-iodide reaction. Very small quantities of
nitrites (0.03 p.p.m. of N) and ferric salts (0.2 p.p.m. Fe) give a blue
colouration with the starch-iodide reagent and for this reason it is
always advisable, whenever possible, to make a control test on the
untreated water. Nitrites are oxidised by free chlorine and consequently
do not interfere with the estimation of it by the thiosulphate method;
the influence of ferric salts can be overcome by substituting 3 c.cms.
of 25 per cent phosphoric acid for hydrochloric acid (Winkler[8]).
An electrical instrument called a "chlorometer" has been devised by E.
K. Rideal and Evans[9] for the estimation of free chlorine. The
diagrammatic sketch, reproduced in Fig. 5, shows the general
construction of the apparatus. When water containing no free chlorine
passes through the copper tube, hydrogen is liberated on the platinum
rod by the electrolytic solution pressure of the copper and an electric
current is generated; a polarizing action follows and the flow of
current ceases. When free chlorine is present it combines with the
hydrogen as produced and so enables more copper to dissolve and produces
a permanent flow of current. The current produced is a function of the
depolarizing action, i.e. of the free chlorine, and is indicated by the
current meter which is graduated in parts per million of available
chlorine. The usual range of instrument is 5 p.p.m. and each division of
the scale is equal to one-tenth of one part per million.
Only strong oxidisers, such as chlorine, ozone, and permanganates, which
have a great affinity for hydrogen, are able to produce a permanent
current; ferric chloride and other weak oxidisers do not affect the
indicator.
[Illustration: FIG. 5.--Rideal-Evans Chlorometer.]
COSTS
_Cost of Construction._ According to the replies received by
the Committee on Water Supplies of the American Public Health
Association[10] the total cost of equipment for disinfection varies
widely and bears no apparent relation to the capacity of the equipment.
This is due to the temporary nature of the plants erected in many cities
and the necessity of erecting expensive structures in others. The cost
of construction varies also in different localities. The cost of
equipping hypochlorite plants with standard concrete tanks and dosage
regulators would be more uniform and for capacities between 10 and 50
million gallons per day would approximate $15 to $50 per million
gallons.
_The operating cost_ of bleach plants shows similar wide variations. In
some cases the labour required for mixing and supervision can be
obtained without extra cost whilst in others the labour charge exceeds
the cost of hypochlorite.
The price of bleach has shown violent fluctuations during the last three
years (see Diagram IX, page 125) but is now (1918) comparatively steady
at $2.25 to $2.75 per 100 pounds. Assuming that 33.3 per cent of
available chlorine can be extracted, each pound of chlorine costs
6.75-7.25 cents as compared with 15-25 cents for liquid chlorine. The
fixed charges on the capital expenditures together with the labour and
incidental charges almost invariably make the total cost of operation of
a straight bleach plant higher than that of a liquid chlorine plant. The
tendency during the last four years has been to substitute liquid
chlorine for hypochlorite and the majority of the plants are now of the
former type.
"ANTICHLORS"
Substances used for the removal of excess chlorine are usually known as
"antichlors" and those that have been most frequently employed are
sodium bisulphite, NaHSO_{3}, and sodium thiosulphate Na_{2}S_{2}O_{3}.
The reactions with chlorine are:
(i) NaHSO_{3} + Cl_{2} + H_{2}O = NaHSO_{4} + 2HCl.
(ii) Na_{2}S_{2}O_{3} + Cl_{2} = Na_{2}S_{4}O_{6} + 2NaCl.
Sodium bisulphite is a very efficient "antichlor," only 1.46 parts being
required to remove 1 part of chlorine, but owing to its instability the
action is uncertain. Sodium thiosulphate is a comparatively stable
cheap salt, containing 5 molecules of water of crystallization,
Na_{2}S_{2}O_{3} . 5H_{2}O but 7 parts are necessary to remove 1 part by
weight of chlorine.
"Antichlors" are used as aqueous solutions and the dosage controlled in
the same manner as for bleach solutions. The action is an instantaneous
one and it is consequently necessary that the germicidal action should
be complete before the "antichlor" is added.
Filters, containing solid materials capable of absorbing free chlorine,
have also been used for removing the excess of the germicidal reagent.
Iron borings and aluminium were used experimentally by Thresh[11] but
the process was not commercially developed. The "De Chlor" filter, in
which carbon is the active substance, has been installed at several
water works in England (Reading, Exeter, Aldershot) with apparently
successful results. The Reading experimental installation, described by
Walker,[12] consisted of a steel drum, 8 feet 3 inches in width, the top
and bottom being domed. In the upper portion, 10 feet 9 inches in depth,
provision was made for thorough admixture of the bleach solution and
water and a subsequent storage of thirty minutes. The lower section of
the filter was divided into three compartments, the first and last of
which contained graded silica; the middle compartment was filled with a
layer (20 inches deep) of specially prepared granulated charcoal or
carbon.
The filter was operated under pressure and passed an average of 192,000
Imp. gallons per day, the rate being 32,000 Imp. gallons per square yard
per day.
Water from the pre-filters (polarite and sand) was treated with bleach
to give a concentration of 1 p.p.m. of available chlorine and passed
through the De Chlor filter. The average bacteriological results
obtained during the first six months operation were as follows:
Bacteria Per c.cm. B. coli Index
Gelatine 3 Days at 20 deg. C. Per 100 c.cms.
Raw river water 6,775 600
Water from pre-filters 579 119
Water from De Chlor filter 33 Nil
Free chlorine could not be detected by chemical tests in the filtered
water which was also free from abnormal tastes and odours. It is stated
that the carbon has to be removed and revivified periodically. The
filter was washed about once per week, the wash water being only
one-tenth of one per cent.
The experimental filter was operated for nearly two years before being
removed to permit the erection of larger units having a total capacity
of one million Imp. gallons per day.
BIBLIOGRAPHY
[1] Hooker. Chloride of Lime in Sanitation, New York, 1913.
[2] Griffen and Hedallen. J. Soc. Chem. Ind., 1915, =34=, 530.
[3] Hale. Proc. N. J. San. Assoc., 1914.
[4] Adams. J. Amer. Pub. Health Assoc., 1916, =6=, 867.
[5] Ellms and Hauser. J. Ind. and Eng. Chem., 1913, =5=, 915 and 1030;
_ibid._, 1914, =6=, 553.
[6] Wallis. Ind. Jour. Med. Res., 1917, =4=, 797.
[7] Le Roy. Comptes rend., 1916, =163=, 226.
[8] Winkler. Zeit. angew. Chem., 1915, =28=, 22.
[9]: Rideal, E. K. and Evans. Analyst, 1913, =38=, 353.
[10] J. Amer. Pub. Health Assoc. 1915, =5=, 921.
[11] Thresh. Internat. Congress Appl. Chem., 1908.
[12] Walker. Jour. Roy. Inst. Pub. Health, Jan., 1911.