Chlorination of Water Part 10

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 c.o.c.k operating in a lead- or porcelain-lined tank. The circular tapered orifice _O_, cut in gla.s.s, is situated in the f.l.a.n.g.ed end of the iron casting _C_ and the head, indicated on the gauge gla.s.s, 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.

[Ill.u.s.tration: 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 a.s.sists 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 circ.u.mstances 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 a.n.a.lysed 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 a.n.a.lysis 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 t.i.trate by one of the following methods:

_Bunsen's Method._ Add 10 c.cms. of a 5 per cent solution of pota.s.sium iodide and 0.5 c.cm. glacial acetic acid and t.i.trate 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 t.i.trate with a solution of N/10 sodium a.r.s.enite 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 a.r.s.enite solution and then t.i.trating 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 a.n.a.lysis, 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 pa.s.sing 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 pa.s.s 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 pota.s.sium iodide and starch; the presence of chlorine is indicated by a deep rich blue colouration that is proportional in intensity to the quant.i.ty 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 t.i.trate 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 t.i.trated 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 quant.i.tative one and subst.i.tuted 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 pota.s.sium b.i.+.c.hromate and copper sulphate. This method was adopted as the official standard method of the American Public Health a.s.sociation; 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 quant.i.tative 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 quant.i.ties 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 subst.i.tuting 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 pa.s.ses 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.

[Ill.u.s.tration: 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 a.s.sociation[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. a.s.suming 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 subst.i.tute 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.