SEWAGE PURIFICATION 297 



microscope, after the lapse of some days, or even weeks, 

 numerous forms of life are generally visible. This life, 

 potentially present in the sewage, is probably an important 

 initial source of population of the sewage filter beds. The 

 effect of the different methods of preliminary treatment, above 

 referred to, upon this organic life, has been only imperfectly 

 studied as yet. We should expect, a priori, that effluents 

 from simple sedimentation, or from the aerobic tank, would 

 be more favourable to the existence of aerobic organisms 

 of this sort than either anaerobic treatment, which might 

 destroy them owing to the absence of oxygen, or chemical 

 precipitation, which would tend mechanically to remove them. 

 It is not unlikely that the organic life of sewage will vary 

 according to the amount of subsoil and surface water drainage 

 entering the sewers. The author has indeed found, in investi- 

 gating the conditions of purification of sewage obtained in an 

 absolutely fresh condition, without admixture of surface 

 water, that decomposition and nitrification take place with 

 extreme slowness, when the sewage is allowed spontaneously 

 to oxidise in a bottle. Inoculation, by means of medium 

 from a filter, greatly accelerated the rate of oxidation. He 

 has further found that the effluent from an aerobic tank 

 oxidised spontaneously more quickly than the effluent from 

 chemical precipitation, containing an equivalent amount of 

 oxidisable matter. 



Whatever the primary source of the population of a sewage 

 filter bed may be, whether derived from the original sewage 

 or from the bacteria naturally present in all unsterilised 

 material, such as is likely to be used for the construction of 

 such filters, there is no doubt that, in course of time, countless 

 numbers of bacteria, and other organisms of the nature 

 specified above, establish themselves in the filter. During 

 the period when the contact bed is empty, and when conse- 

 quently its interstitial spaces are full of air, these organisms 

 act upon the suspended and dissolved impurities retained 



