Chlorine Treatment at Montreal
| Source of Supply. | Alka- linity. | Colour. | Oxygen Absorbed (30 mins.) | Chlorine Required p.p.m. | Bacteria per c.cm. | Per Cent Removed. |
| Ottawa river | 15-20 | 50-70 | 14.0 | 1.50 | 3,000 | over 98 |
| St. Lawrence river | 90-100 | Nil. | 0.30 | 0.30 | 500 | over 99 |
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].