1905.] with reference to tJie Purification of Sewage. 



247 



This complete separation of a nitrite from a nitrate stage is doubtless due 

 to the comparatively strongly ammoniacal nature of the sewage employed. 

 Previous observers*! have shown the inhibitive effect of ammonia upon 

 nitrate-production, and it is probable that during the earlier stages of the 

 maturing period the nitrate bacteria were unable to become established in 

 the filter, and only later, when the ammonia of the sewage was being rapidly 

 oxidised to nitrites, was the environment suited to their growth and develop- 

 ment. The sewage employed frequently contained more than 15 parts free 

 and saline ammonia per 100,000, a concentration which has been shownf to 

 be sufficiently high to check completely the production of nitrates in pure 

 culture. An interesting confirmation of this explanation was obtained in the 

 maturing of the Munich filters, where nitrates appeared in the filtrates very 

 soon after the first appearance of nitrites. There was here no such " nitrite 

 stage," and the sewage was much less ammoniacal (2 to 4 parts ammoniacal 

 nitrogen per 100,000). 



Difference of Function in Different Strata of tJie Filters. — An attempt was 

 made to study the course of the oxidation at different depths in the tall and 

 medium filters (see Table III). In analysis 3 the " oxidisability " was taken as 

 a criterion and the decrease in the first 50 cm. of the tall filter was found to be 

 almost as great as in the whole length of the filter. One may therefore suppose 

 that the mechanical deposition of suspended particles as well as the absorption 

 of the more complicated organic matter in solution takes place principally in 

 the upper layers of the filter. It is also apparent from analysis 4 that the 

 formation of nitrites (these analyses were made during the nitrite stage) did 

 not at that date take place in quantity in this upper layer but lower down 

 for the most part ; this same fact is also shown in analysis 2. In the latter 

 case the free and saline ammonia was also estimated, and the decrease, which 

 is so marked as the sewage passes through the filter, was found not to begin 

 until after the first 50 cm. were passed. The same phenomenon appears in 

 analysis 8 and is a striking instance of the principle, already alluded to, and 

 discussed at length in the section devoted to absorption, that the dis- 

 appearance of ammonia and the oxidation of nitrogen are closely associated 

 both in time and space. 



Comparison of Contact and Continuous Filters — Munich Experiments. — 

 From the mature Vienna filters attempts were made to isolate the nitrifying 

 organisms, but before much progress had been made the work was dis- 

 continued and was not again resumed until after two years. This second 

 time, in Munich, in 1903, fresh filters had to be matured, and a second 

 * See footnote **, p. 242. 



t Warington, 'Chem. Soc. Joura.,' vol. 35, 1879, and vol. 59, 1891. 



