----------------+--------------------+--------------------- | Water "A" Colour 3 | Water "B" Colour 40 | Available Chlorine | Available Chlorine Contact Period. | p.p.m. | 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. CHLORINE TREATMENT AT MONTREAL