Chlorination of Water Part 3

Rideal[6] was the first to note the strong germicidal power of chloramine and attributed the persistent germicidal activity of hypochlorites in sewage to the formation of chloramine and chloramine derivatives.

Further evidence against the nascent oxygen theory of chlorine disinfection is to be found in the fact that such active oxidising agents as sodium, pota.s.sium, and hydrogen peroxides have a much lower germicidal activity than chlorine when compared on the basis of their oxygen equivalents. Table III shows chlorine to be approximately five times as active as pota.s.sium permanganate when compared on this basis.

TABLE III.[A]--COMPARISON OF BLEACH AND POTa.s.sIUM PERMANGANATE

-----------+------------------+------------------- BLEACH Available Chlorine POTa.s.sIUM Contact 0.35 p.p.m. PERMANGANATE.

Period. +------------------+------------------- Oxygen Equivalent. Parts Per Million.

+--------+---------+---------+--------- 0.08 0.133 0.266 0.400 -----------+--------+---------+---------+--------- Nil 140 ... ... ...

30 mins 90 122 115 110 1 hour 68 115 100 80 1-1/2 hours 63 108 95 75 4 hours 50 95 80 50 -----------+--------+---------+---------+---------

[A] Results are _B. coli_ per 10 c.cms.

The germicidal activity of oxidising agents has been shown by Novey and others to be somewhat proportional to the energy liberated during the reaction but even when this factor is taken into consideration chlorine compounds are more active than other oxidising agents. Hypochlorous acid is far superior to hydrogen peroxide as a germicidal agent and is as active as ozone, which liberates a greater amount of energy.

2HClO = 2HCl + O_{2} + 18,770 calories

2H_{2}O_{2} = 2H_{2}O + O_{2} + 46,120 calories

2O_{3} = 3O_{2} + 60,000 calories.

Again, solutions of chlorine gas and hypochlorites having the same oxidising activity, as determined by t.i.tration with thiosulphate after the addition of pota.s.sium iodide and acid, i.e. contain equal amounts of available chlorine, show approximately the same germicidal activity in water. On the addition of ammonia, the hypochlorite solutions retain their ability to liberate iodine from pota.s.sium iodide (Wagner test) but the property of oxidising such dyestuffs as indigo is destroyed and the germicidal activity is increased. Ammonia, when added to solutions of chlorine gas, diminishes the property of liberating iodine from pota.s.sium iodide, the bleaching effect on dyestuffs, and the germicidal action. It is often a.s.sumed that chlorine forms hypochlorous acid on solution in water Cl_{2} + H_{2}O = HClO + HCl but the results obtained on the addition of ammonia indicate that either very little hypochlorous acid is formed or that ammonia and hypochlorous acid do not form chloramine in the presence of hydrochloric acid.

When chlorine gas was treated with a 0.5 per cent solution of ammonia in the proportion of 1 molecule of chlorine to 1.90-1.95 molecules of ammonia, Noyes and Lyon[8] found that nitrogen and nitrogen-trichloride were formed in equimolar quant.i.ties.

12NH_{3} + 6Cl_{2} = N_{2} + NCl_{3} + 9NH_{4}Cl.

Bray and Dowell[9] showed that this reaction depended upon the hydrogen ion concentration and proceeded in accordance with the following equations:

(i). Acid solution 4NH_{3} + 3Cl_{2} = NCl_{3} + 3NH_{4}Cl

(ii). Alkaline solution 8NH_{3} + 3Cl_{2} = N_{2} + 6NH_{4}Cl.

In (i) with a ratio of chlorine to ammonia of 3:1 by weight, one-half of the chlorine is lost as ammonium chloride and one-half forms nitrogen trichloride, concerning which comparatively little is known; in (ii) the whole of the chlorine forms ammonium chloride, which has no germicidal value.

The effect of ammonia on the germicidal action of a solution of chlorine gas is shown in the Table IV.

TABLE IV.[A]--EFFECT OF AMMONIA ON CHLORINE GAS SOLUTION

_Conditions._ Colour of water 40 p.p.m. Turbidity, 5 p.p.m.

---------+---------------------------------------- Available Chlorine 0.20 p.p.m., Ammonia.

Contact Parts Per Million.

Period. +---------+---------+----------+--------- Nil. 0.05 0.10 0.20 ---------+---------+---------+----------+--------- Nil. 130 ... ... ...

10 mins. 135 140 130 135 1 hour 130 130 128 120 4 hours 120 112 110 105 24 hours 120 145 160 170 ---------+---------+---------+----------+---------

[A] Results are _B. coli_ per 10 c.cms.

Even when the ratio of Cl:NH_{3} was 4:1 by weight, practically the same as was used in the experiments of Noyes and Lyon, and Bray and Dowell, quoted above, the germicidal action was totally destroyed and the 24-hour results showed aftergrowths which were somewhat proportional to the amount of ammonia added. This was probably due to the formation of ammonium chloride, which provided additional nutriment for the organisms.

It has often been a.s.sumed that hypochlorite solutions are decomposed on addition to water containing free or half-bound carbonic acid with the production of free chlorine, but no evidence has been adduced in support. Free chlorine can be separated from hypochlorous acid in aqueous solution by extraction with carbon tetrachloride and when this solvent is shaken with a carbonated hypochlorite solution it is found that only traces of chlorine are removed.

Hypochlorous acid reacts with hydrochloric acid with the evolution of free chlorine HClO + HCl = Cl_{2} + H_{2}O but in very dilute solution the amount of free chlorine formed is exceedingly minute. Jakowkin[10]

has shown that this reaction does not proceed to completion and that the concentration of free chlorine can be calculated from the equation HClO H^{.} Cl' = 320Cl_{2} in which the reactions are expressed in gram molecules per litre. The hydrogen ions and chlorine ions are obtained from the dissociation of carbonic acid (H_{2}CO_{3} <=> H^{.} + HCO_{3}') and chlorides (NaCl <=> Na^{.} + Cl') and also by the dissociation of hydrochloric acid produced by the interaction of hypochlorous acid and organic matter. HClO = O + HCl <=> H^{.} + Cl'. If the formula of Jakowkin can be correctly applied to solutions containing fractions of a part per million of hypochlorous acid the free chlorine liberated by the addition of 1 p.p.m. of bleach to a water low in chlorides would be of the order 10^{-7}-10^{-8} p.p.m.

_Sodium hypochlorite_ is probably hydrolysed in dilute solution in a manner similar to that of bleach.

2NaOCl = NaCl + NaOH + HClO.

For solutions containing equal amounts of available chlorine, electrolytic sodium hypochlorite is more dissociated than bleach because of the absence of an excess of base, and this, together with the presence of sodium chloride, accounts for the slightly higher germicidal velocity obtained. The experience of pulp mills, with bleach and electrolytic hypochlorites, confirms this: the latter is a much quicker bleaching agent than bleach and it is often so rapid as to make it desirable to reduce the velocity by the addition of soda ash.

Regarding hypochlorite solutions a phenomenon of more scientific interest than of practical importance has been noted by Breteau[12] who found that alkaline solutions of sodium hypochlorite containing 0.94 per cent of available chlorine lost 3.6 per cent of their t.i.ter on dilution with 80 volumes of water; also that this loss was increased by the addition of small quant.i.ties of salt (sodium chloride) and more so by carbonates and bicarbonates. The author has noted similar losses on diluting bleach solutions and that the loss increased on standing. The loss can be explained by the decomposition of hypochlorous acid, in the presence of light, into hydrochloric acid and oxygen. 2HClO = 2HCl + O_{2}

CHLORINE WATER. When a solution of chlorine in water is used as a germicide the chemical reactions that occur differ materially from those of hypochlorite solutions. On solution in water, hydration or solvation probably takes place with the production of heat. Cl_{2}Aq. = 2,600 calories. Chlorine water is comparatively stable but decomposes under the influence of light in accordance with the equation Cl_{2} + H_{2}O = 2HCl + O; a similar reaction occurs in the presence of organic matter or any substance capable of oxidation. Chlorine water contains only minute traces of hypochlorous acid and there is no evidence that the endothermic reaction

Cl_{2}Aq + H_{2}O = HClOAq + HClAq -2600 - 68,460 = -29,930 - 39,315 - 1815

occurs in a measurable degree.

From thermochemical considerations hypochlorous acid and chlorine water should be about equally active as oxidising agents.

2HClOAq = 2HCl + O_{2} + 18,770 calories

2Cl_{2}Aq + 2H_{2}O = 2HCl + O_{2} + 15,340 calories

2Cl_{2} + Aq + 2H_{2}O = 2HCl + O_{2} + 20,540 calories

When a solution of chlorine or hypochlorite is added to water as a germicidal agent, a variety of reactions occur the character of which is determined by the nature of the mineral and organic matter in the water and the type of chlorine compound added. The general reactions are of three types (1) oxidation of the organic matter, (2) direct chlorination of the organic matter, and (3) a bactericidal action.

In the treatment of waters that contain appreciable amounts of organic matter almost all the chlorine is consumed in reaction (1) and even with filter effluents it is probably true that oxidation accounts for the greater portion of the chlorine consumed. The author has found that a dosage of 0.02 part per million of available chlorine was more effective in destroying _B. coli_ in distilled water than 0.40 p.p.m. in a water absorbing 9.5 p.p.m. of oxygen (30 mins. at 100 C.).

Reaction (1) can be adequately explained by the nascent oxygen hypothesis and it is this reaction that determines the dosage required for effective sterilisation. (See Chap. III.)

Very little information is available regarding reaction (2) but there is little doubt that a direct chlorination of the organic matter does occur and it is more than probable that these chlorinated derivatives are largely responsible for the obnoxious tastes and odours produced in some waters. It has been suggested that these were due to the formation of chloramines. This view was formerly supported by the author but the chloramine treatment at Ottawa and other places has demonstrated the inadequacy of this explanation. It is true that the odour of chloramine is stronger and more pungent than that of chlorine, but chloramine in the Ottawa supply, even with doses as high as 0.5 part per million of available chlorine, has caused no complaints.

The odour of some of the organo-chloro compounds is more penetrating and obnoxious than those of chlorine and chloramine, and it is quite possible that some of the higher h.o.m.ologues of chloramine are in this cla.s.s. It should be noted, however, that some of the chloro-amido compounds prepared by Dakin are white, odourless, crystalline substances.

Practically nothing is known regarding the specific nature of the mechanism involved in reaction (3). The hypothesis that chlorine, and chlorine compounds, exert a direct toxic action on the micro-organisms marks an advance in the science of water treatment but does not indicate the physiological processes involved. Cross and Bevan[11] have shown that chloro-amines have a tendency to combine with nitrogenous molecules and to become fixed on cellulose; it is therefore possible that reaction is a cytolytic one in which the chlorine attacks and partially or wholly destroys the membranous envelope of the organisms. A portion of the chlorine or chlorine-compound may also penetrate the membrane and produce changes that result in the death of the organism.

BIBLIOGRAPHY

[1] Fischer and Proskauer, Rev. d'Hyg., 1884, 6, 515.

[2] Warouzoff, Winogradoff, and Kolessnikoff. Russkaia medicina, 1886, Nos. 3 and 32.

[3] Race. Jour. Amer. Water Works a.s.soc., 1918, 5, 63.