^s 



CHEMICAL CHANGES 129 



s in sewage) and a high temperature. ^ Th. Pfeiffer ^ showed 

 that denitrification did not generally take place in the absence 

 of particles of straw, faeces, or vegetable tissue which act 

 as food to the denitrifying organisms, and considered their 

 chief food substance to be xylane, or wood-gum (CfiHjoOg), 

 isomeric with cellulose, but soluble in alkalies, therefore re- 

 moved by the first alkaline fermentation. This is an additional 

 fact, explaining why the sewage should be properly fermented 

 before entering the final nitrifying filters (see further, the 

 London results. Chapter IX.). 



Denitrifying bacteria are of three classes : (i) Those which 

 destroy nitrites, but not nitrates — namely, Bacterium denitri- 

 ficans I. of Burri and Stutzer. (2) Those destroying nitrates, 

 but not nitrites — B. pyocyaneus and Bacterium denitrificans V. 

 (and also many of those already quoted from Percy Frankland). 

 (3) Other denitrifying bacteria which destroy both nitrites 

 and nitrates. From Adeney's and other researches, the last 

 are not common, though among them sometimes appear B. 

 fluorescens liquefaciens,^ B. pyocyaneus, and Vibrio denitrificans J^ 

 They rapidly produce N, and perhaps N oxides, in presence of 

 much CO2, but are antagonized by abundant aeration. Jensen 

 describes six others.^ 



Houston, in the London County Council Report, i8gg, 

 records : B. Coli Communis. — In twenty-four hours at 37° C, 

 reduction of nitrates to nitrites well marked (broth, 5 per 

 cent. ; potassium nitrate, o"i per cent. ; water, 94*9 per cent.). 

 B.Mesentericus. — Sewage variety E.: Great reduction of nitrates 

 to nitrites in twenty-four hours at 2y° C. Sewage variety I.: 

 No reduction of nitrates (showing the value of the nitrite test for 

 diagnosis). He also gives B, frondosus fusiformis as negative, 

 B memhraneus patulus and B. capillar eus as active, in formation 

 of nitrites from nitrates. 



A large number of organisms found in sewage exert a distinct 

 influence in bringing about nitrification, besides the species 

 specially described as "nitrifying," since many which grow 

 rapidly and break up sewage material have the power of 



^ Landw. Jahrb., 1899, 217. Dr. Hugo Weissenberg has also some elaborate 

 studies on " Denitrification," in the Archiv f. Hygiene, 1897, xxx., 3. 



- Dent. Landiv. Prcsse, 1897, 911 ; also J. Stocklasa, Bied, Centr., 1899, xxvii., 

 707; Stutzer and Hartleb, ibid., 1900, xxix., 126 ; Jensen, Centy. Bakt., iv., 401. 



'■'' Hygien. Rundsch., 1899., ix., 538; Chem. Centr., 1900, i., 52. 



^ Bied. Centr.^ 1899, xxv., 854. 



^ Centr. /. Bakt., 1898, iv., 401. 



