STERILIZATION 187 



Aas reduced by 3 parts of av. CI from several millions to 50,000, 

 [by 5 parts to 20, and by 7 parts to 10, per c.c. 



Incubation tests by dilution with g volumes of river water 

 md keeping at 20° C. : 3 parts av. CI, very slightly foul after 

 :hree days ; 5 and 7 parts av. CI, inoffensive after four days' 

 Bncubation. 



In another case 3*7 parts per 100,000 of av. CI added to raw 



jewage reduced the coli from over 1,000,000 per c.c, so that 



[none were found in i c.c. ; the enteritidis spores from over 



[1,000 to less than 10 ; and the total organisms from 23,200,000 



to 540 per c.c. 



Experience showed that considerably smaller quantities of 

 :he solution could be used with sewages of average strength, 

 ind within limits a longer period of treatment allows of reduc- 

 tion in the amount of available chlorine. Taking the raw 

 sewage as it entered the works, it was found practicable to 

 treat it direct with an electrolysed salt solution in place of 

 ;hemical precipitants in settling tanks. Much less sludge was 

 >roduced than by ordinary chemical treatment, and neither 

 sludge nor effluent readily decomposed. The expense of 

 :hemicals, power for the mixers, and labour in slaking lime 

 tnd in other operations, must thus be balanced against the 

 :ost of the sterilizing solution. The effluent from the latter 

 ^as fit for discharging direct on to land, and the final effluent 

 From the underdrains was suitable for passing into any body 

 )f water. The sludge when spread upon land remained sweet 

 IS compared with that from ordinary chemical treatment. 



It was further found that nearly a constant proportion, about 

 )0 per cent., of the available chlorine was taken up almost 

 immediately, while about 40 per cent, remained and declined 



lore gradually, pointing distinctly to the chlorine existing in 

 the reagent in two forms — one which acts at once on readily 

 )utrescible matter, the other remaining to attack bacteria and 

 resistant organic substances. 



Septic Tank Effluents. — It was found better in most cases to 

 ^deal with the sewage after it had undergone a preliminary septic 

 treatment, since thereby it was carried a stage further in its 

 resolution. But the most important reason was that vegetable 

 masses (and, still more, solid excreta) are, as is well known, very 

 difficult to sterilize by any means, because there is little pene- 

 tration ; and the difficulty is encountered in the disinfection of 

 the stools of fever patients. A fault of the Hermite process as 



