Chlorination of Water Part 16

Ca(OCl)_{2} + 2NH_{3} = 2NH_{2}Cl + Ca(OH)_{2}.

The marked activity of chloramine as a chlorinating agent could be predicated from its heat of formation, which is 8,230 calories. The other possible chloramines should be even more active as the heat of formation of these compounds are:

Dichloramine NHCl_{2} -- 36,780 calories.

Nitrogen trichloride NCl_{3} -- 65,330 calories.

Dichloramine is unknown but nitrogen chloride has been prepared and is a highly explosive yellow oil that decomposes slowly when kept under water in the ice box. NCl_{3} can be easily prepared by pa.s.sing chlorine gas into a solution of ammonium chloride and this process would suggest that a method might be found of utilising chlorine and ammonia as gases for the production of nitrogen trichloride as a germicide for water chlorination. NH_{4}Cl + 3Cl_{2} = NCl_{3} + 4HCl.

The "available" chlorine content of the chloramines is double the actual chlorine content as each atom of chlorine will liberate two atoms of iodine from hydriodic acid.

NH_{2}Cl + 2HI = I_{2} + NH_{4}Cl.

NCl_{3} + 6HI = 3I_{2} + NH_{4}Cl + 2HCl.

HALAZONE

For the sterilisation of small individual quant.i.ties of water such as are required by cavalry and other mobile troops bleach and acid sulphate tablets have been usually employed. Such tablets have given fairly satisfactory results but certain difficulties inherent to these chemicals have made it desirable to seek other methods.

The subject was investigated by Dakin and Dunham,[7] who first tried chloramine-T (sodium toluene-_p_-sulphochloramide). It was found that heavily contaminated waters, and particularly those containing much carbonates, required a comparatively high concentration of the disinfectant: 40 parts per million of chloramine-T were necessary in some cases and such an amount was distinctly unpalatable. By adding tartaric acid or citric acid the effective concentration could be reduced to 4 p.p.m. but the mixture could not be made into a tablet without decomposition and a two-tablet system was deemed undesirable.

Toluene sulphodichloramines were next tried. Excellent bacteriological results were obtained but the manufacture of tablets again presented difficulties. When the necessary quant.i.ty of dichloramine was mixed with what were a.s.sumed to be inert salts--sodium chloride for example--the normal slow rate of decomposition was accelerated. The dichloramine, in tablet form, was also found to be too insoluble to effect prompt sterilisation.

The most suitable substance found by Dakin and Dunham was "halazone" or _p_-sulphodichloramin.o.benzoic acid (Cl_{2}NO_{2}SC_{6}H_{4}COOH).

This compound is easily prepared from cheap readily available materials and was found to be effective and reasonably stable.

The starting point in the preparation of halazone is _p_-toluenesulphonic chloride, a cheap waste product in the manufacture of saccharine. By the action of ammonia, _p_-toluene sulphonamide is produced and is subsequently oxidised by bichromate and sulphuric acid to _p_-sulphonamidobenzoic acid. This acid, on chlorination at low temperatures, yields _p_-sulphondichloramin.o.benzoic acid (halazone). The reactions may be expressed as follows:

CH_{3} COOH COOH / / / | | --> | | --> | | / / / SO_{2}NH_{3} SO_{2}NH_{2} SO_{2}NCl_{2}

Halazone is a white crystalline solid, sparingly soluble in water and chloroform, and insoluble in petroleum. It readily dissolves in glacial acetic acid from which it crystallizes in prisms (M.P. 213 C.).

The purity of the compound can be ascertained by dissolving in glacial acetic acid, adding pota.s.sium iodide, and t.i.trating with thiosulphate; 0.1 gram should require 14.8 to 14.9 c.cms. of N/10 sodium thiosulphate.

Each chlorine atom in halazone is equivalent to 1 molecule of hypochlorous acid and the "available" chlorine content is consequently 52.5 per cent or double the actual chlorine content.

>SO_{2}NCl_{2} + 4HI = >SO_{2}NH_{2} + 2HCl + 2I_{2}.

From the bacteriological results given by Dakin and Dunham it would appear that 3 parts per million of halazone (1.5 p.p.m. available chlorine) are sufficient to sterilise heavily polluted waters in thirty minutes and that this concentration can be relied upon to remove pathogenic organisms.

The formula recommended for the preparation of tablets is halazone 4 per cent, sodium carbonate, 4 per cent (or dried borax 8 per cent), and sodium chloride (pure) 92 per cent.

Halazone and halazone tablets, when tested in the author's laboratory on the coloured Ottawa River water seeded with _B. coli_, have given rather inferior results. With 1 tablet per quart, over six hours were required to reduce a _B. coli_ content of 100 per 10 c.cms. to less than 1 per 10 c.cms. Clear well waters gave excellent results and large numbers of _B.

coli_ were reduced to less than 1 per 10 c.cms. in less than thirty minutes. McCrady[A] has also obtained excellent results with various strains of _B. coli_ seeded into the colourless St. Lawrence water.

[A] Private communication.

BIBLIOGRAPHY

[1] Raschig. Chem. Zeit., 1907, =31=, 926.

[2] Rideal. S. J. Roy. San. Inst., 1910, =31=, 33-45.

[3] Race. J. Amer. Waterworks a.s.soc., 1918, =5=, 63.

[4] Race. Eng. and Contr., 1917, =47=, 251.

[5] Contract Record. Aug. 15, 1917, 696.

[6] Muspratt and Smith. J. Soc. Chem. Ind., 1898, =17=, 529.

[7] Dakin and Dunham. Brit. Med. Jour., 1917, No. 2943, 682.

CHAPTER X

RESULTS OBTAINED

The object of adding chlorine or chlorine compounds to water is for the purpose of destroying any pathogenic organisms that may be present. In a few instances some collateral advantages are also obtained but, in general, no other object is aimed at or secured.

Chlorination does not change the physical appearance of water; it does not reduce or increase the turbidity nor does it decrease the colour in an appreciable degree.

The chemical composition is also practically unaltered. When bleach is used there is a proportionate increase in the hardness but the amount is usually trifling and is without significance. During 1916 when the Ottawa supply was entirely treated with bleach at the rate of 2.7 parts per million (0.92 p.p.m. of available chlorine) the average increase in the total hardness as determined by the soap method was 2.5 parts per million.

When chlorine is added to prefiltered water, as an adjunct to filtration, an increase in the number of gallons filtered per run has been noted at some plants. This increase is not so great with rapid as with slow sand filters but in some instances it has led to appreciable economies.

Walden and Powell[1] of Baltimore, found that the addition of a quant.i.ty of bleach equal to approximately 0.50 p.p.m. of available chlorine enabled the alum to be reduced from 0.87 to 0.58 grain per gallon. The percentage of water used in washing the filters was also reduced, from 4.1 per cent to 2.9 per cent, whilst the filter runs were increased on the average by one hour and ten minutes. The net saving in coagulant alone amounted to 30 cents per million gallons.

Clark and De Gage[2] found that the use of smaller amounts of coagulant during the period of combined disinfection and coagulation resulted in an increase of nearly 25 per cent in the quant.i.ty of water pa.s.sed through the filter between washings, and also in a material reduction of the cost of chemicals, which averaged $2.62 per million gallons for combined disinfection and coagulation as against $4.86 for coagulation alone. The water used in these experiments was obtained from the Merrimac River at Lawrence.

The effect of hypochlorite on the reduction of algae growths on slow sand filters was first noticed by Houston during the treatment of the Lincoln supply in 1905. Two open service reservoirs were fed with treated water and were themselves dosed from time to time. "Previous to 1905 they developed seasonally most abundant growths, but during the hypochlorite treatment it was noticed that they remained bright, clear, and remarkably free from growths" (Houston[3]).

Ellms,[4] of Cincinnati, has also noted the effect of hypochlorite on algae. When the bleach was added to the coagulated water the destruction of the plankton was not as satisfactory as had been antic.i.p.ated and it was found that large doses destroyed the coating of the sand particles and rendered the filters less efficient. The use of bleach in the filtered water basin was more successful and cleared it of troublesome growths.

In 1916, during the treatment of the London Supply with bleach (dosage 0.5 p.p.m. of available chlorine), Houston made further observations on this point. The Thames water, taken at Staines, had previously been stored for considerable periods in reservoirs, but this necessitated lifting the water by pumps which consumed large quant.i.ties of coal that were urgently needed for national purposes. As a war measure, the storage was eliminated and the water treated with hypochlorite at Staines and allowed to flow by gravitation to the various works where the slow sand filters are situated. The treatment resulted in a marked reduction in the growths of algae, the reduction in the area of filters cleaned in 1916 (June to September) as compared with 1915 being as follows:

Percentage Filter Works. Reduction (Approximate).

Grand Junction (Hampton) 6 Grand Junction (Kew) 43 East London (Sunbury) 30 Kempton Park 33 West Middles.e.x 56

A portion of this reduction can probably be attributed to the elimination of storage.

Chlorination, by decreasing the load on filter beds, has enabled the rate of filtration to be increased in some cases. This increased capacity, which would otherwise have necessitated additional filter units, has been obtained without any further capital outlay. At Pittsburg (Johnson[5]) the rate of filtration, after cleaning, was increased 250,000 gallons each hour until the normal rate was reached; restored beds were maintained at a 250,000 gallon rate for one week.

After the introduction of chlorination it was found possible to increase the rates more rapidly without adversely affecting the purity of the mixed filter affluents.

_Hygienic Results._ Evidence as to the actual reduction of the number of such pathogenic germs as _B. typhosus_ in water supplies by chlorination is most readily found in the death rates from typhoid fever in cities that have no other means of water purification. In some cases this evidence is necessarily of a circ.u.mstantial nature; in others it is definite and conclusive.