The amount of chlorine required for efficient treatment is very largely determined by the amount required to satisfy the oxidisable matter present in the water. Many experimenters have reported results that would indicate that appreciable concentrations of chlorine are required for bactericidal action but the details of the technique, as published, show that the effect of the organic matter added with the test organism was not thoroughly appreciated. One cubic centimetre of a culture in ordinary peptone water, added to one litre of water, would increase the organic content by approximately 10 parts per million, an amount that would absorb appreciable amounts of chlorine.Other conditions also make it very difficult to compare the results obtained in the past: one of these is the degree of purity set as the objective. German bacteriologists added enormous numbers of the test organism and endeavoured to obtain the complete removal of the organism from such quantities as one litre of water with a contact period often as short as 10 minutes. Nissen,[1] of the Hygienic Institute of Berlin, found that a 1:800 dilution of bleach (420 p.p.m. of chlorine) was required to destroy B. typhosus in one minute and a 1:1600 dilution (210 p.p.m. of chlorine) in 10 minutes. DelÉpine[2] obtained somewhat similar results by means of the thread method for testing disinfectants. Phelps,[3] using gelatine plates for enumeration of the bacteria, obtained a 90 per cent reduction of B. typhosus in twenty minutes with 5 p.p.m. of available chlorine; over 99 per cent reduction in one hour, and over 99.99 per cent reduction in 18 hours. Wesbrook, Whittaker, and Mohler[4] tested bleach solutions with various strains of B. typhosus by means of the plate method and found that the most resistant one was reduced from 20,000 per c.cm. to sterility (in 1 c.cm.) by 3 p.p.m. of available chlorine in fifty minutes and that the least resistant one only required 1.0 p.p.m. with a thirty minutes’ contact.Lederer and Bachmann[5] have reported the following results:

TABLE V

With the exception of P. mirabilis, which forms endospores, all the organisms were killed (less than 1 per c.cm.) by 0.5 p.p.m. of available chlorine in fifteen minutes.

All these observers found that B. coli, the organism usually employed as an index of contamination, had approximately the same degree of resistance to chlorine as B. typhosus, though Wesbrook et al. directed attention to the varying viability of organisms derived from different sources.

These experiments merely indicate the dosage required for exceptional conditions such as it is inconceivable would ever occur in water-works practice. No information is available regarding the actual B. typhosus content of waters that have caused epidemics of typhoid fever, but for the present purpose it may be assumed that the extreme condition would be a pollution by fresh sewage giving a B. coli content of 1,000 per c.cm. or 200 times worse than the average condition that can be satisfactorily purified without overloading a filter plant (500 B. coli per 100 c.cms.). Experiments made by the author indicate that a suspension of 1,000 B. coli per c.cm. in water, in the absence of organic matter, can be reduced to a 2 B. coli per 100 c.cms. standard (the U.S. Treasury Standard) by 0.1 p.p.m. of available chlorine in ten minutes at 65° F. This experiment indicates the amount of chlorine that is required for the bactericidal action only; such a dosage could never be used in practice to meet a pollution of this degree because of the accompanying organic matter. In actual practice the author has experienced the above condition but once, and on that occasion the B. coli were derived from soil washings and not from fresh sewage.

The amount of chlorine required for germicidal action is small, and the main factors that determine the dosage necessary to obtain this action are (1) the content of readily oxidisable organic matter, (2) the temperature of the water, (3) the method of application of the chlorine and (4) the contact period.Oxidisable Matter. The oxidisable matter may be divided into two classes (a) inorganic and (b) organic. The inorganic constituents naturally found in water, that are readily oxidisable, are ferrous salts (usually carbonates), nitrites, and sulphuretted hydrogen, and these react quantitatively with chlorine until fully oxidised. The oxygen value of chlorine is approximately one-quarter (actually 16:71) the available chlorine content in accordance with the equation Cl2/71 + H2O = 2HCl + O/16. One part per million of available chlorine will oxidise 1.58 p.p.m. of ferrous iron; 0.197 p.p.m. of nitrous nitrogen; and 0.479 p.p.m. of sulphuretted hydrogen.

TABLE VI.[A]—EFFECT OF COLOUR

The organic matter found in water may be derived from various substances such as urea, amido compounds, and cellulose; humus bodies derived from soil washings and swamps may also be present. The humus compounds of swamps and muskeg are usually associated with the characteristic colour of the water derived from these sources. The effect of this coloured organic matter upon the chlorine dosage is well illustrated in Table VI. In this experiment B. coli was used as the test organism and the only varying factor was the organic matter. To obtain the same result with a contact period of one hour at 63° F. it was necessary to use about two and one-half times the amount of chlorine with a water containing 40 p.p.m. of colour as with one practically free from colour. It will be noted that water “A,” in which the colour had been reduced to 3 p.p.m. by coagulation with aluminium sulphate, required a greater dosage of chlorine than was necessary for bactericidal action only. This was due to a residual organic content which produced none or but a trace of colour, for although the colour had been reduced by 92 per cent the organic matter, as measured by the oxygen absorbed test, had only been reduced by 70 per cent.

The results obtained by Harrington[6] at Montreal are in the same direction. During the greater part of the year the water is obtained from the St. Lawrence river, which is colourless and low in organic matter; in the spring months the flood waters of the Ottawa, a highly coloured river, enter the intake and necessitated a much higher dosage.

Chlorine Treatment at Montreal

Ellms[7] obtained similar results and reported “that the rate at which sterilisation proceeds varies, in a general way, directly with the concentration of the applied available chlorine and the temperature, and inversely as the amount of easily oxidisable matter present.”

Experience with filter plants shows the same facts, the amount of chlorine required for the sterilisation of a filter effluent being invariably less than that necessary to purify the raw water to the same extent.The effect of coloured organic matter upon the absorption of chlorine, in the form of hypochlorite, is shown on Diagram I.

DIAGRAM I
EFFECT OF COLOUR ON ABSORPTION OF CHLORINE BY WATER

The shape of the curve obtained with a colour of 40 p.p.m. somewhat resembled that of a mono-molecular reaction and the results were calculated accordingly. The mathematical expression of this law is dN/dt = KN where N is the concentration of the available chlorine in parts per million. Integrating between t1 and t2 the formula K = log(N1/N2)/(t2 - t1) is obtained. If the compound absorbing the chlorine were simple in character, and the chlorine were present in large excess, the value of K would be constant. In the experiments recorded, K constantly decreases, due to the decreasing concentrations of the reacting substances and the complex nature of the organic matter.The results show the effect of organic matter on the reduction of the chlorine concentration available for germicidal action and also the importance of avoiding a local excess of chlorine (vide p. 41).

An effort has been made by some observers to find a quantitative relation between the organic matter, expressed as oxygen absorbed in parts per million, and the chlorine required for oxidation, but without definite result. Some of the results obtained are given in Table VII.

TABLE VII.—OXYGEN TO CHLORINE RATIO

The value of 0.4 (0.39) obtained by the author is the average of over one hundred determinations covering a period of two years. The experiments of Zaleski and Elmanovitsch were made with the water of the Neva River.

The divergence in the ratios affords additional evidence in favor of reaction (2) mentioned on page 28 and also shows that the chlorinated compounds are less readily oxidized than those from which they are produced. Heise[8] has found that the amount of chlorine consumed is usually proportional to the concentration in which it is added though not necessarily a function of the concentration of the organic matter.Temperature. The evidence regarding the effect of temperature upon the dosage required is somewhat conflicting. Ellms (vide supra) found that the velocity of the germicidal action varied directly with the temperature and this has also been the author’s experience with laboratory experiments. Typical examples of these are given in Tables VIII and IX.

TABLE VIII.[B]—EFFECT OF TEMPERATURE

TABLE IX.[C]— EFFECT OF TEMPERATURE

The reaction velocity of a germicide is proportional to the temperature[9] and the influence of temperature may be mathematically expressed by the formula K1/K2 = ?(T2 - T1), in which K1 and K2 are the constants of the reaction at temperatures T2 and T1, respectively, and ? is the temperature coefficient. From the value of ?, the velocity constant of a germicide for any temperature may be calculated from the equation KT = K20° × ?(T - T20°). K1 and K2 are obtained from the formula KT = log(N1/N2)/(t2 - t1) in which N1 - N2 is the number of bacteria destroyed in the interval t2 - t1.

A reduction of temperature also lowers the oxidizing activity of the chlorine so that a greater concentration is available for germicidal action. This is shown by the results plotted in Diagram II.

DIAGRAM II
EFFECT OF TEMPERATURE ON ABSORPTION OF CHLORINE BY WATER

Tables VIII and IX, however, show that the temperature coefficient of the germicidal action has a greater effect than the reduction in the amount of chlorine absorbed and removed from the reaction.

The results obtained on the works scale with these waters are very different to the laboratory ones and show that more chlorine is required during the summer season than in winter. The results with bleach and liquid chlorine are in the same direction (vide Diagrams III and IV). The bleach was regulated so as to maintain a constant purity, whilst in the other case the dosage was constant with a varying B. coli content. In Diagram IV the B. coli is plotted; this does not represent all the factors involved as the B. coli content of the treated water is also a function of that of the raw water, but in the example given this factor is of no moment because it was comparatively constant during the period plotted (extreme variation 80 per cent).

The discrepancies between the laboratory and works results cannot be easily explained. The only difference in the conditions is the nature of the containing vessel. Glass is practically inert at all temperatures but the iron pipes, through which the water passed before the samples were taken, may exert an absorptive influence on the chlorine at the higher temperatures experienced during the summer months.

Waters containing organic matter that differs much in quantity from the examples above may yield very different results and no generalisation can be made that will cover all cases. An increase of temperature increases the germicidal velocity and also the rate of absorption of chlorine by the organic matter; other factors determine which of these competitive actions predominates.Method of Application (admixture). A thorough admixture of the water and chlorine is a sine qua non for successful operation. This should, if possible, be attained by natural means, but if there is any doubt as to the efficiency of the mixing process, mechanical appliances should be utilised. Pumps, especially centrifugal pumps, constitute a very convenient and efficacious method of mixing the germicide and the water, and the solutions should never be injected into the discharge pipes when it is possible to make connections with the suctions.