196 Elements of Water Bacteriology. 



material from the point of view of the epidemiologist " 

 (Houston, i902 a ). 



Experimental studies with specific bacteria have con- 

 firmed these conclusions. Houston (1904^ found that 

 B. pyocyaneus appeared in the effluent of a trickling bed 

 ten minutes after application to the top and continued to 

 be discharged for ten days. In septic tanks and contact 

 beds, the same germ persisted for ten days. Rideal 

 (1906) quotes experiments by Pickard at Exeter, which 

 show that typhoid bacilli may persist for two weeks in a 

 septic tank and that contact bed treatment only effects a 

 90 per cent removal of these organisms. 



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 millions 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 



