BACTERIAL PURIFICATION 251 



the one under anaerobic conditions, and the second with free 

 admission of air but not of light, when the distinctly nitrifying 

 bacteria should be free to work under the most favourable con- 

 ditions. Even in very strong sewages there seems almost no 

 limit to the capacity of the hydrolytic ferments to break down 

 nitrogenous matter into ammonia. Thus Marchal found that 

 a common organism exercising this function, B. mycoides, could 

 thrive in a medium containing 0*2 per cent, of caustic potash, 

 equivalent to 660 parts per 100,000 of free ammonia, and in 

 septic effluents in the first stage I have found as much as 30 to 

 40 parts per 100,000 of NH3. 



But there is a limit to the amount of anaerobic change if 

 nitrification in the further stage is to be successful. The pre- 

 judicial influence of ammonia on the nitrifying bacteria was 

 pointed out by Warington in 189 1 {ante, p. 124). Winogradsky 

 and Omeliansky^ investigated the influence of different sub- 

 stances, and found that : 



1. Sodium carbonate is essential for the growth of both 

 nitrous and nitric organisms. There would always be sufficient 

 in sewage from the presence of washing soda, also from the 

 action of ammonium carbonate on the sodium chloride of 

 urine. 



2. Various organic infusions, such as hay, peptone, sugar, 

 broth, etc., have little effect, or are even favourable, except in 

 amounts unlikely to be present in sewage. 



3. Urea is without effect when the amount is only 0*05 per 

 cent., but 0*5 to o*8 per cent, hinders nitrification. Addition of 

 2 per cent, of urine results in the time required for oxidation 

 being increased five times, and this result is due to the large 

 excess of ammonia. 



4. Iron salts seem to assist the nitrification. 



Withers and Frap's results indicate (i) that there are 

 organisms which nitrify organic matters in preference to am- 

 monium salts, (2) that the presence of the latter may hinder 

 the activity of the nitrifying bacteria, and (3) that acids formed 

 in the fermentations may also be a hindrance when insufficient 

 base is present to neutralize them.^ 



By the agency of sewage bacteria in denitrification, nitrates 

 directly oxidize organic matter, producing carbonates and 

 nitrites without any gas, and rendering the liquid strongly 



1 Chem. Centralblatt, iSgg, ii. , 132, 217, and 264. 

 '^ /. Amer. Chem. Soc, xxiii., No. 5. 



