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 291/2 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.

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.

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.

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.

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 H2SO4, 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 hexamethyltripara-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.

Fig. 5.—Rideal-Evans Chlorometer.