The most important advantage of the process is the elimination of the aftergrowth problem. At Denver, where the aftergrowth trouble is possibly more acute than at any other city on the continent, it was effectively banished by the use of chloramine. At Ottawa, the sanitary significance of _B. coli_ aftergrowths is no longer of practical interest because such aftergrowths have ceased to occur. Whatever may be their opinion as to the sanitary significance of aftergrowths, all water sanitarians will agree that the better policy is to prevent their occurrence.
_Operation of Chloramine Process._ For the successful operation of the chloramine process, the essential factors are low concentrations of the hypochlorite and ammonia solutions. The author has found that hypochlorite containing 0.3-0.5 per cent of available chlorine and ammonia containing 0.3-0.5 per cent of anhydrous ammonia can be mixed in a 4:1 or 8:1 ratio without appreciable loss in titre. Solutions of these concentrations mixed in 4:1 ratio lost only 2-3 per cent of available chlorine in fifteen minutes and less than 10 per cent in five hours. The effect of mixing solutions containing 4.35 per cent of available chlorine and 2.2 per cent of ammonia is shown in Table XXX.
The stability of chloramine is a function of the concentration and the temperature and in practice it will be found advisable to determine in the laboratory the maximum concentrations that can be used at the maximum temperature attained by the water to be treated (cf. Muspratt and Smith[6]).
According to Raschig[1] two competing reactions occur when ammonia is in excess.
(1) NH_{ 2}Cl + NH_{ 3} = N_{ 2}H_{ 4}HCl hydrazine hydrochloride and (2) 3NH_{ 2}Cl + 2NH_{ 3} = N_{ 2} + 3NH_{ 4}Cl.
When the excess of ammonia is large, as on the addition of ammonia fort, the second reaction predominates and the yield of nitrogen gas is almost quantitatively proportional to the quantity of available chlorine present. As ammonium chloride has no germicidal action, and hydrazine a carbolic coefficient of only 0.24 (Rideal), the formation of these compounds should be avoided.
TABLE XXX.--LOSS ON MIXING HYPOCHLORITE AND AMMONIA
Hypochlorite containing 4.35 per cent available chlorine. Ammonia contained 2.2 per cent NH_{ 3}