3S8 BACTERIA IN THE TREATMENT OF SEWAGE 



the nitrate-forming bacteria had been estabUshed on the surface of the cHnker in a 

 manner which corresponds to what takes place in the "ripening" of a sand or trick- 

 ling filter. Apparently, ammonia inhibits the initial growth of nitrate-forming bac- 

 teria but their growth when once established is not destroyed by additions of am- 

 monia-bearing liquors. 



Denitrification has been studied by many observers. Mair^ claimed that the 

 denitrifying bacteria are essentially aerobic, but that they can grow under anaerobic 

 conditions in the presence of nitrate, the nitrate supplying the necessary oxygen for 

 their vital activities. 



It is well known that there is considerable loss of nitrogen, either as gaseous N2 

 or as oxides of nitrogen, in filters, especially in contact beds, and in activated-sludge 

 treatment when aeration is vigorous and prolonged. Denitrification probably does 

 not account for all of this loss, but as Clark, Letts, and recently Parisi^ have stated, 

 the loss must be explained by the theory that as nitrite is formed it reacts with the 

 amino acids as follows: 



R = CH = NH. = CO.H+HNO. = R = CH = OH = CO^H+H.O+Na . 



Parisi supports this theory by experiments. If sucrose in sufficient quantity to 

 prevent nitrification is added to a submerged soil containing an NH4 salt and the 

 mixture is aerated, the NH^ salt remains unaltered. Aeration of a soil impregnated 

 with sodium nitrite slowly converts the nitrite to nitrate, because insufficient amino 

 acids are present to react with the nitrite; but if sufficient asparagine is also added, 

 neither nitrites nor nitrates are found, interaction of asparagine and NaNOa result- 

 ing in the liberation of free nitrogen. 



In view of the vigorous denitrifying and nitrogen-liberating reactions in oxidiz- 

 ing processes of sewage treatment, it is apparent that considerable energy must be 

 expended and vigorous aerobic conditions maintained in order to build up nitrates 

 in the effluent. For this reason a recent tendency in activated-sludge development 

 has been to attempt to operate, if possible, without nitrate production, and with 

 minimum amounts of air. It is true that the oxygen demand may be greatly reduced 

 without appreciable amounts of nitrate in the effluent, but it is difficult to main- 

 tain well-activated sludge without nitrates. Also, the tendency toward "bulking" or 

 sudden increase of moisture content is intensified, and "filter-pressing" the sludge be- 

 comes more difficult. 



Very little bacteriological work has been done on the activated-sludge process, 

 mainly because of the difficulty of the problem. The question of fixation of atmos- 

 pheric nitrogen has been debatable, with claims of evidences of such fixation by 

 Fowler^ and Peck^, but these claims have been questioned by Richards and Sawyer,^ 



■ Mair, W.: "On the Role of Bacteria in the Biological Methods of Sewage Purification, with 

 Special Reference to the Process of Denitrification," /. Hyg., 8, 609-53. 1908. 



* Parisi, E.: "Denitrification in Oxidizing Media," Aim. chim. applic, 16,40-45. 1926. 



3 Fowler, G. J.: "The Nitrogen in Activated Sludge," J.Indian. Inst. Sc, 3, 256-64. Dec, 1920. 



■* Peck, C. L.: "Fixation of Atmospheric Nitrogen by Activated Sludge," Eng. Neu's-Rec, 90, 

 487. March 15, 1923. 



5 Richards, E. H., and Saw>'er, G. C: "Further Experiments with Activated Sludge," /. Soc. 

 Chan. Ind., 41, 62T. 1922. 



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