1905.] with reference to the Purification of Sewage. 259 



materials in sewage, also largely of organic origin) to take place principally 

 in the upper layers of the filter. The nitrifying organisms will then be able 

 to live and multiply lower down in the filter where the amount of organic 

 matter present will be comparatively small, and this view has been experi- 

 mentally confirmed in the present work. 



(c) It has been lately shown by Wimmer* in the case of the nitrate 

 organism, that a porous medium has a markedly mitigating effect when 

 organic matter is present, and the coke and other materials of which sewage 

 filters are made, are selected mainly on account of their porosity. It is only 

 fair, however, to state that Wimmer's experiments were not made with 

 absolutely pure cultures, and part of the beneficial effect observed may have 

 been due to a symbiosis, though, from the nature of his experiments, it 

 would seem unlikely. 



(d) The nitrifying bacteria are doubtless present in very great numbers in 

 the filters, and this may assist them in withstanding the effect of organic 

 matter. This view is based upon certain observations of Winogradshy and 

 Omeliansky* in which nitrifying organisms, if present in sufficient quantity, 

 were shown to withstand amounts of organic matter otherwise inhibiting 

 them. 



3. In the maturing of sewage filters, the two stages of nitrification may be 

 markedly separate in time (Vienna experiments), or may be both developed 

 together (Munich experiments). This difference is correlated with the 

 greater or less ammoniacal content of the sewage. In the stronger sewage 

 used for the Vienna filters, the well known inhibitory action of abundance of 

 ammoniacal compounds (especially of free ammonia and carbonate of ammonia, f 

 which are so largely represented in the sewage), presumably retarded the 

 development of the nitrate-producer, until the nitrite-producer was sufficiently 

 well established to be converting most of the ammonia into nitrites. 



4. As a result of special experiments with coke, and of analyses of the 

 filtrates at different depths of the filters, and at different stages during the 

 maturing period, it would appear that there is no evidence of absorption of 

 free and saline ammonia without contemporaneous nitrification. Further 

 research is necessary, but the theory of a previous physical " adsorption " of 

 ammonia and subsequent slower nitrification would appear, at present, to be 

 without experimental foundation. 



5. One is therefore inclined, in the present state of our knowledge, to 

 consider the process of nitrification, during the filtration of sewage through 



* See footnote, p. 242. 



t Lohais, ' Centralbl. f. Bakt.,' 2 Abt., 13, 1904, and Boulanger and Massol, ' Comptes 

 Eendus; vol. 140, 1905. 



