76 SEWAGE AND ITS PURIFICATION 



instance, in well aerated filtering trays the absence of nitrites 

 shows that the organisms producing these compounds from 

 ammonia are absent, unless the filtrate is so far free from 

 ammonia that they might be present, but inactive from want 

 of food-supply ; or it may be that the nitrite is transformed at 

 once to nitrate by the nitrohacter. 



Organisms in Oxidizing Filters. 



At this stage, if the working be efBcient and the aeration 

 thorough, the organisms which are exclusively anaerobic will 

 disappear, while others will be reduced in numbers and re- 

 placed by new varieties, including those which produce nitrites 

 and nitrates. 



Their action, which is similar to that occurring in soils, was 

 first studied by Miintz and Schloesing in 1877, who proved 

 that soil sterilized by heat or by antiseptics such as chloroform 

 would not nitrify, that the organic matter must be first con- 

 verted into ammonia, and that certain conditions were neces- 

 sary. Warington, Miiller, Marie Davy, Heraeus, Munro, and 

 others, elaborately investigated the subject, but failed to dis- 

 cover the specific organisms. These were first isolated and 

 described by Winogradsky and P. Frankland in 1890, the 

 former growing them in media almost absolutely free from 

 organic matter. Kuhne afterwards found that they were easily 

 cultivated on silica jelly (p. 67), and others have since suc- 

 ceeded in acclimatizing them on agar plates prepared by 

 Beyerinck's method. 



Nitrosomonas (Winogradsky) oxidizes ammonia to nitrite, 

 requiring no organic matter for its nutrition, as it assimilates 

 carbon from acid carbonates. It appears as circular corpuscles 

 less than i yit in diameter, and sometimes as oval cocci. ^ The 

 organisms from different parts of the world appear to be the 

 same.2 



Nitrohacter (Winogradsky) was isolated by P. Frankland, 

 by dilution from ammoniacal broth, as a bacillo-coccus which 

 refused to grow in gelatine ; but as it is unable to oxidize 

 ammonia, the best medium for its growth is an inorganic 

 solution containing potassium nitrite and an acid carbonate. 

 Omeliansky has confirmed the fact that both types of organisms 



^ Warington, Journal of the Chemical Society, 1891, p. 484. 

 2 Burri and Stutzer, Chem. Centr., 1896, ii., p. 113. 



