26 LIMNOLOGY, WATEE SUPPLY AND WASTE DISPOSAL 



obtained from carriers in India, reported that even as much as 10 ppm 

 of residual chlorine failed to destroy all cysts in water of relatively low 

 organic content with a contact time of 60 minutes. These authors used 

 bromthymol blue as an indicator in regulating the pH of the chlorinated 

 water to neutral. Since organic dyes like bromthymol blue react rapidly 

 with active chlorine to give false colors, it is apparent that they were 

 working at unknown pH values. In view of the fact that these authors 

 used calcium hypochlorite for active chlorine and NaOH for regulating 

 the pH, it is probable that these authors were working at high pH values. 



In two recent reports, the present author (Chang 1944a; 1944b) brought 

 out the fact that in chlorination studies, the chemistry of chlorine and 

 chlorine compounds must be thoroughly understood. The effect of pH on 

 the cysticidal efficiency of chlorine is profound and lies in the alteration 

 of the proportions of chlorine present as hypochlorous acid and hypochlorite 

 ions. In the pH range up to 9.0, the cysticidal activity seems to be en- 

 tirely due to the HOCl concentration. At 25-28° C and with a contact 

 time of 10 minutes, about 2 ppm of residual chlorine as HOCl destroyed 

 all cysts when a cyst concentration of 30-60 per ml was used. More 

 recently, we showed that the cysticidal efficiency of hypochlorite ions is 

 only about 1/300 of that of HOCl (Chang, Fair and Morris 1945; 1946). 

 In another report (Fair, Chang and Morris 1947), it has been shown that 

 the cysticidal efficiency of HOCl can be expressed by the cysticidal con- 

 centration-time relationship which is represented by the equation : tC" = k% 

 where fc' is the disinfection constant, t the disinfection time in minutes, 

 C the concentration of disinfectant in ppm, and n the concentration coeffi- 

 cient of the disinfectant. Substituting the values for HOCl into the equa- 

 tion, it was found that, 



i = 205C-iat 3° C. 

 t = 39C-1 at 23° C. 



From the value of n, it is seen that the concentration affects the cysticidal 

 efficiency of HOCl just as much as the contact time. In the same report 

 (Fair, Chang and Morris 1946), it has been shown that the temperature 

 affected the cysticidal efficiency of HOCl to give a Qjo oi about 2.2. 



The chloramines and chloramides are definitely less cysticidal than 

 HOCl. Among the monochloramines and chloramides, ammonia-mono- 

 chloramine is more cysticidal than the organic compounds or hypochlorite 

 ion. This is believed to be due to the small molecular size of ammonia- 

 monochloramine, which penetrates more readily into the cysts than the 

 larger molecules of organic chloramines and chloramides and negatively 

 charged hypochlorite ions. Among the organic monochloramines and 

 chloramides, succinchlorimid seemed to be the most cysticidal. At 23° C 

 and with a 10-minute contact time, 10 ppm of residual chlorine as ammonia- 

 monochloramine destroyed all cysts, while the same efficiency was reached 

 by 80 ppm of residual chlorine as succinchlorimid, or 140 ppm of residual 

 as monochloramine-T or azochlorimid. It must be pointed out that the 



