130 SEWAGE AND ITS PURIFICATION 



inducing or commencing this process if sufficient oxygen be 

 present. Those mentioned on p. 68 had been separated by 

 plate cultivation in gelatine ; therefore the ordinary nitrifying 

 organism, which will not grow in gelatine, could not have been 

 concerned. 



The Massachusetts Report of 1890, p. 788, states that " an 

 effluent from a sewage filter, where nitrification is complete, con- 

 taining 2 per cent, of the total organic matter of the sewage, 

 will not serve as food for bacteria, because it has been worked 

 over already by bacteria in the filter, nearly everything 

 available having been removed." This is true of most species, 

 but we have seen that denitrifying organisms in presence of 

 nitrates can freely attack this residual organic matter, and that 

 after partial nitrification in a filter the action of these bacteria, 

 which absolutely require a certain amount of organic food, 

 converts it into carbonic acid and harmless gases, taking the 

 requisite oxygen from the nitrates dissolved in the water. I 

 refer later to the COo evolved in the " resting empty " stage of 

 intermittent filtration. 



The weight of dissolved oxygen in well-aerated river water 

 being approximately I part per 100,000, the oxygen-consumed 

 figure in a sewage or effluent indicates the minimum quantity 

 of such water required to destroy the organic matter by means 

 of the free dissolved oxygen alone. In raw sewages this may 

 amount to as much as 20 volumes. In the raw sewages 

 yielding the effluents referred to in the table on p. 131, the 

 oxygen-consumed figure was as follows in parts per 100,000 : 



Exeter, 6-56. Sutton, 294. Caterham, i4'97. 



The " available oxygen " is that present as nitrate or nitrite, 

 and the amount of carbonaceous matter requiring destruction 

 is measured by the ordinary figure of " oxygen consumed " as 

 determined by permanganate, since after four hours' heating 

 with permanganate no dangerous matter can be left. The 

 table shows that the available oxygen as nitrates and nitrites 

 is in good effluents quite sufficient to deal with the organic 

 matter, even without help from the oxygen dissolved in river 

 water. A large number of the published analyses of effluents 

 are vitiated by the fact that the samples have not been analysed 

 until some days after collection, frequently at the end of a 

 long transit by rail or other conveyance, during which the 

 agitation and inevitable contact with air will have considerably 



