258 Dr. H. Chick. The Process of Nitrification [Apr. 1, 



the disappearance and oxidation of the ammonia to be parts of one process, 

 which is carried out by the nitrifying bacteria in the time taken by the sewage 

 to pass through the filters. 



For these experimental filters the time taken for the passage was measured 

 directly.* For the Vienna continuous filters on July 1, 1901, the time was 

 approximately 3| hours for the tall and medium filters and only 5 minutes 

 for the short filter (nitrogen oxidation in this filter had not then progressed 

 beyond formation of nitrites). For the Munich continuous filters on March 17, 

 1903, the time was 2 hours for the tall filter, and hour for the short one. 

 At this date, a too concentrated sewage was being employed, and the filters 

 were not at their best, but still four-fifths of the ammonia in the sewage was 

 oxidised while passing through the tall filter. 



Section IV. — General Conclusions. 



1. Nitrification of ammonia during sewage purification occurs in two 

 stages which may be referred to the activity of two classes of bacteria, one 

 producing nitrites, and the second oxidising the nitrites to nitrates. These 

 bacteria exist not only in the substance of the filter, but are also carried 

 away in large quantities in the filtrates. 



2. These organisms belong to the same group as those concerned with 

 nitrification in the soil, isolated by Winogradsky. It is, at first, difficult to 

 understand how organisms so susceptible to the presence of organic matter 

 are able to live and do their work in sewage filters. The following, one or 

 all, form possible explanations. 



(a) The nitrifying bacteria may be, to a certain extent, protected by the 

 presence of other organisms, and this view is strengthened by the results of 

 certain experiments with the nitrate-producer; in symbiosis with such 

 organisms, made in the course of the present investigation. 



(b) It has been shown that porous materials, such as coke, are able to 

 retain upon their surface complicated organic substances of high molecular 

 weight, when these are presented in solution. We may suppose this 

 absorption (together with the mechanical separation of the suspended 



* 100 c.c. of a 2-per-cent. solution of sodium chloride were sprinkled over the top of the 

 filter (that being the volume of liquid usually delivered at each discharge of the siphon). 

 The filtrates were then continuously tested with silver nitrate until a copious precipitation 

 was obtained ; the ordinary sewage filtrate yielded only a slight reaction with silver 

 nitrate. The passage of liquids through such filters is a very complicated process and one 

 not yet thoroughly investigated, and though, doubtless, the times thus determined may be 

 considered to apply to the majority of the liquid going on at any particular time, they 

 must still, strictly speaking, be regarded as approximate and indeed minimal values. 



