During the period of nine months covered by the results in Table XXIX, only five cases of typhoid fever were reported in which the evidence did not clearly indicate that the infection had occurred outside the city. The reduction in the bleach consumed during the same period effected a saving of $3,200.
During one period of operation the hypochlorite dosage was gradually reduced to ascertain what factor of safety was maintained with a dosage of 0.5 p.p.m. of available chlorine and 0.06-0.08 p.p.m. of ammonia. The results are shown in Diagram VIII. The percentage of samples of treated water showing _B. coli_ in 50 c.cms. was calculated from the results of the examination of 4-7 samples daily.
The results showed that it was possible to reduce the chlorine dosage to 0.25 p.p.m. with 0.06 p.p.m. of ammonia without adversely affecting the bacteriological purity of the tap supply and fully confirmed the experimental results previously obtained.
The lowest ratio of available chlorine to ammonia used during this test was approximately 4:1. This is the ratio indicated by a consideration of the theory of the reaction, and not 2:1 as was formerly stated (Race[4]). If bleach is represented as Ca(OCl)_{ 2}, the equation
Ca(OCl)_{ 2} + 2NH_{ 3} = 2NH_{ 2}Cl + Ca(OH)_{ 2}
would indicate a ratio of 2:1; but only one molecule of Ca(OCl)_{ 2} is produced from two molecules of bleach and the theoretical ratio is therefore 4:1 (142:34),
2CaOCl_{ 2} = CaCl_{ 2} + Ca(OCl)_{ 2} and Ca(OCl)_{ 2} + 2NH_{ 3} = Cl = 142 34 = 2NH_{ 2}Cl + Ca(OH)_{ 2}.
The chlorine to ammonia ratio is very important because of its influence on the economics of the process (_vide_ p. 124).
All the laboratory and works results that have been obtained in Ottawa indicate the importance of an adequate contact period. The superiority of chloramine over other processes is due to the non-absorption of the germicidal agent and to obtain the same degree of efficiency the contact period must be increased as the concentration is decreased. For this reason the best results will be obtained by chlorinating at the entrance to reservoirs or under other conditions that will ensure several hours contact. At Ottawa the capacity of the pipes connecting the pumping station (point of chlorination) and the distribution mains provides a contact period of one and a quarter hours but even better results would be obtained if the contact period were increased.