BACTERIA OCCURRING IN SEWAGE 87 



Dr. Houston, with the Ducat filter, has shown that with 

 sewage containing 1,200,000 B. co// per c.c. a filtrate is obtained 

 which contained no colonies resembling the organism in this 

 quantity ; and that sewage containing between 1,000 to 

 10,000 spores of jB. enteritidis sporogenes per c.c. contained after 

 filtration less than 10 per c.c, whilst the aerobic bacteria 

 causing liquefaction of gelatine were likewise reduced from 

 22 to less than i per unit. 



Professor Boyce found that B. coli diminishes in the septic 

 tank, the liquid being inimical to it, and therefore to the other 

 more delicate pathogenic bacteria. In one series of experi- 

 ments the average number of coli per c.c. was — crude sewage, 

 5,011 ; open septic tank, 2,130 ; Cameron closed tank, 2,099. 

 In another series the average was — crude sewage, 45,600 ; 

 septic tank, 3,433 ; keeping the liquor for two days, the number 

 had gone down to 2,025.^ 



On p. 74 a table is given showing the maximum and 

 minimum numbers of coli organisms and spores of B. enteritidis 

 sporogenes obtained recently at the Guildford Sewage Works ; 

 it will be seen that coli organisms are in each effluent reduced 

 as the purification of the sewage proceeds, the total reduction 

 amounting to over 99*9 per cent, of those present in the original 

 crude sewage. As would be expected, the anaerobic B. enteri- 

 tidis does not show any reduction during the septic tank 

 treatment, but finally the spores are reduced to one-tenth the 

 original numbers. 



Before this evidence of the comparatively innocuous character 

 of the filtrates from bacterial systems was available, I pointed 

 out that subsequent chemical treatment could be used for 

 sterilizing the filtrate if necessary. Such reagents as may be 

 conveniently employed may be called " finishers," as when 

 employed the resulting purified sewage is satisfactory both from 

 the chemical and bacterial points of view. Chlorine is one of 

 such reagents, and the late Dr. Kanthack established the fact 

 that with I grain of free chlorine to 4 gallons of the tank 

 effluent or to 5 of filtrate, with a contact of about five minutes, 

 the number of bacteria can be reduced from any number (even 

 millions) that may be present to ten to fifty per cubic centimetre, 

 and that no pathogenic organisms were found in any of the 

 numerous samples of Maidenhead sewage finished in this way. 

 I found at the same inquiry that on adding 177 parts of avail- 



^ Royal Commission on Sewage, Second Report, 1902, p. 11. 



