88 SEWAGE AND ITS PURIFICATION 



able chlorine per 100,000, although about half the amount 

 immediately combines with any organic matter present, if the 

 aerating filter has not worked efficiently, the micro-organisms 

 by contact with the remainder are gradually killed, so that 

 plate cultivations of such sewage taken after fourteen minutes 

 showed no growth with three and a half days' incubation. 



Later, in 1904, an experimental plant was installed at the 

 Guildford Sewage Works for treating the sewage at its various 

 stages of purification with an electrolytic chlorine solution, in 

 order to determine the amounts of available chlorine required 

 to deodorize and sterilize the various effluents. It was found 

 that chlorine had very little precipitating or coagulating effect, 

 the only visible change being a slight bleaching, the deposited 

 sludge becoming dense and more amenable to treatment. The 

 destruction of odours was complete and very rapid, provided 

 the addition of available chlorine was equivalent to the imme- 

 diate oxygen - consumed figure of the effluent. Absolute 

 sterilization was not found to be practicable ; in a cubic centi- 

 metre of sewage spores are present which will resist several 

 minutes' steaming, and the quantity of available chlorine 

 required to destroy these would of itself be objectionable, 

 irrespective of the high cost. But it was found that the addi- 

 tion of available chlorine in excess of that immediately taken 

 up by the organic matter, as indicated by an oxygen-consumed 

 determination, rapidly destroyed the vast majority of organisms, 

 so that after a few hours' contact the bacteriological condition 

 of the effluent was almost comparable with standards for 

 potable waters. 



In Chapter VIII. further details will be found. 



In the report of the London County Council, October, 1899, 



Dr. Houston specially studied the possibility of the survival of 



pathogenic organisms after passage through bacterial filter beds, 



and from his investigation of the intermittent filters under 



experiment, he summarizes his opinion as follows : 



" It is to be noted, in the first place, that the biological treatment 

 of sewage is conducted under control; secondly, that the process 

 always gradually secures the destruction of the pabulum on which 

 bacteria feed, and hence leads to their death ; thirdly, that the 

 balance of evidence points to the probability that some, at all events, 

 of the pathogenic organisms are crowded out in the struggle for 

 existence in a nutritive medium containing a mixed bacterial flora, 

 their vitality being weakened or destroyed by the enzymes of the 

 saprophytic species ; fourthly, that while it is true that bacteria 

 produce poisonous substances in their growth, it also is true that 



