BACTERIOLOGY OF SEWAGE 237 



Rideal (1906) quotes experiments by Pickard at Exeter, 

 which show that typhoid bacilli may persist for 2 weeks 

 in a septic tank and that contact bed treatment only 

 effects a 90 per cent removal of these organisms. 



Disinfection of Sewage Effluents. Where bacterial 

 purity is required, some special process of disinfection 

 must be combined with the contact bed or the trickling 

 filter. For this purpose treatment with chloride of 

 lime or other chemicals is rapidly gaining ground as an 

 important adjunct to bacterial disposal plants; and in 

 connection with this process bacteriological control is 

 an essential. 



Rideal (1906) first showed at Guildford that 30 parts 

 of available chlorine per million would reduce the 

 number of bacteria in crude sewage from several mil- 

 lions to 50,000, while 50 parts would reduce their 

 number to 20 per c.c. Colon bacilli were reduced from 

 one million per c.c. to less than one per c.c. by 30 parts 

 of chlorine. In septic effluent 25 to 44 parts of chlorine 

 per million reduced B. coli from two and a half to four 

 and a half million per c.c. to less than one per c.c. 

 With contact effluents smaller amounts of chlorine 

 proved efficient, The primary effluent required 20 

 parts per million, the secondary effluent 10.6 parts 

 per million and the tertiary effluent 2.5 parts per mil- 

 lion to reduce the number of B. coli so that this organism 

 could not be isolated in 5 c.c. 



In this country Phelps and Carpenter (1906) demon- 

 strated the practical usefulness of bleaching powder 

 disinfection, at the Sewage Experiment Station of the 

 Massachusetts Institute of Technology. As indicated 



