SOIL AND SEWAGE 71 



as regards elimination of bacteria vary greatly with the method 

 of purification adopted. 



Houston 1 made a careful investigation for the Royal Com- 

 mission on Sewage Disposal of the biological qualities of the 

 effluents from sewage farms. 



Compared with the original sewages, all the effluents 

 exhibited a high percentage degree of purification ; but, apart 

 from a reduction in number, they showed no very appreciable 

 biological modification. Houston remarks that sometimes the 

 relative number of spores of B. enteritidis sporogenes was 

 reduced, and that there was some evidence, especially in the 

 better class of effluents, of a greater proportionate reduction, 

 as compared with crude sewage, in the number of microbes 

 growing at blood heat, over those growing at 20 C. Also on 

 several occasions he failed to isolate streptococci from the 

 effluent. 



In general, however, as Houston very definitely points out 

 (p. 169), "the results conclusively show that the treatment of 

 sewage on land cannot be relied on materially to modify the 

 potentially dangerous qualities of crude sewage. The actual 

 number of objectionable microbes persisting in the effluents is 

 too great to allow of much stress being laid on the great 

 percentage reduction effected in the total number of microbes 

 by the land treatment, or to insure any certainty that effluents 

 from land processes are * relatively safe.' " 



Houston 2 also extensively investigated the effluents from bio- 

 logical treatment processes. He sums up the general outcome of 

 the experiments as follows : " The effluents from septic tanks, 

 intermittent contact beds, continuous filtration beds, etc., contain 

 an enormous number of bacteria. In some cases the percentage 

 reduction of microbes in effluent as compared with raw sewage 

 is striking. But as an effluent must be judged by the actual 

 state it is in, and as the number of micro-organisms still 

 remaining is nearly always very large, percentage purification 

 would seem to be of minor importance. In not a few cases the 

 bacteria are practically as numerous in the effluent as in the 



1 Fourfh Report Royal Sewage Commission, vol. IV, part ill. 



2 Second Report Royal Sewage Commission^ 1902, p. 25. 



