i82 SEWAGE AND ITS PURIFICATION 



bleaching powder (i : 300 available chlorine) was used by Sims 

 Woodhead for sterilizing the Maidstone water-supply during 

 the 1897 typhoid epidemic. Schumacher, working at the 

 Hamburg Hygienic Institute, has also recently conducted a 

 long series of experiments which demonstrate the disinfectant 

 value of chloride of lime and its applicability for sewage treat- 

 ment/ 



Although powerful disinfectants, chlorine and the hypochlo- 

 rites have several disadvantages : 



1. Their own odour, and the persistent odours they create 

 and leave behind, lead often to their use irregularly, or in in- 

 effective quantities. 



2. The action on metals, wood, leather, and caoutchouc. 

 Lead even is corroded, so that in water-closets with leaden 

 siphons the pipe would be eaten through rapidly. Free 

 chlorine, or acidified chlorine mixtures, exert rapid action on 

 iron, cutting the fittings generally just at the level of the liquid, 

 and even, owing to evolved gas or spray, corroding the metal 

 some distance above. The hypochlorites, being alkaline, are 

 much less destructive, as shown by the fact that iron tanks are 

 largely used to store strong "bleach liquor" in factories. 



3. Their immediate destruction by amido-compounds like 

 urea or by ammonium salts, with loss of nitrogen, so that the 

 chlorine may be entirely used up in dealing with inodorous 

 and inoffensive matters, unless a large excess be employed. 

 One reaction between chlorine and ammonia has already been 

 given (p. 180). The complete decomposition would be — 



2NH3-H3Cl2 = N2 + 6HCl. 

 With hypochlorous acid : 



2NH3+ 3HCIO = N2 + 3HCI + 3H2O. 

 Urea and hypochlorous acid : 



COCNHg)^ -t- 3HCIO = N2 + 3HCI + CO, + 2H,0. 

 Urea and a solution of bleaching powder react thus : 



3Ca(C10)2 + 2CO(NH2)2 = 2N2 + 2CO2 + 3CaCl2 + 4H2O. 

 Soap and domestic slop waters rapidly exhaust chlorine 

 liquors, while paper, fibre, etc., absorb chlorine readily. 

 Although deodorization, and still more sterilization, can only 

 occur when the agent is in excess, an effluent containing free 

 chlorine or its oxides would not be allowed to be discharged 



1 Public Health Engineer, March and April, 1906. 



