| Percentage Reduction, 15 Minutes’ Contact | ||||||||
| Available Chlorine p.p.m. | Nature of Test Organism. | |||||||
| B. cloacæ. | B. fæcalis alkali- genes. | B. para- typho- sus. | Proteus mira- bilis. | B. enter- itidis. | B. lactis aero- genes. | B. cholerœ- suis. | ||
| 0.1 | ..... | 99.98 | ..... | 27.3 | ..... | ..... | ..... | |
| 0.2 | 99.69 | 99.99 | 99.97 | 45.5 | 99.83 | 99.17 | 95.8 | |
| 0.3 | 99.75 | 100.00 | 100.00 | 63.7 | 99.98 | 99.98 | 100.0 | |
| 0.5 | 100.00 | ..... | ..... | 72.7 | 100.00 | 100.00 | ..... | |
| 0.7 | ..... | ..... | ..... | 63.7 | ..... | ..... | ..... | |
| 1.0 | ..... | ..... | ..... | 63.7 | ..... | ..... | ..... | |
| 3.0 | ..... | ..... | ..... | 90.9 | ..... | ..... | ..... | |
| 5.0 | ..... | ..... | ..... | 90.0 | ..... | ..... | ..... | |
| Original number of organisms per c.cm. | } | 160,000 | 9,500 | 3,000 | 8,000 | 180,000 | 180,000 | 500 |
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
| TEMPERATURE 63° F. | |||||
| Contact Period. | Water “A” Colour 3 Available Chlorine p.p.m. | Water “B” Colour 40 Available Chlorine p.p.m. | |||
| 0.2 | 0.2 | 0.4 | 0.5 | ||
| Nil | 194 | 194 | 194 | 194 | |
| 5 | minutes | 121 | 165 | 129 | 66 |
| 1 | hour | 7 | 95 | 20 | 1 |
| 5 | hours | 0 | 4 | 0 | 0 |
| 24 | hours | 0 | 1 | 1 | 0 |
| 48 | hours | 0 | 0 | 0 | 0 |
| [A]Results are B. coli per 10 c.cms. of water. | |||||
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.