Question

A treatment plant doing color removal by coagulation is using combined chlorine as a means of residual control. There have been complaints about chlorinous odors. The plant is operating with a chlorine-to-ammonia molar ratio of 5:1 and at a pH of 7. What precautions might be taken to reduce the odor complaints?

Answer 1

Since the pH is neutral (7), hence an excess of Chlorine as compared to ammonia (5:1) is causing the chlorinous smell. This basically means that the plant has lesser requirement of chlorine for the reaction of the relevant disposals. To control the smell, the molar ratio of Chlorine:Ammonia can be reduced from 5:1 to, say, 3:1 or to a value till the chlorinous smell is not complained about. Moreover, operating at a pH less than 7 can be useful, as it will facilitate the same acidic action on the disposals as a higher dosage of chlorine would have had.