248 Dr. H. Chick. The Process of Nitrification [Apr. 1, 



opportunity was afforded for studying the maturing process, and nitrification 

 generally. 



Filters like those previously described were again erected, and three of 

 them treated as formerly with a continuous trickle of diluted liquid manure. 

 Another similar set of three filters was treated, for contrast, as contact 

 filters by the procedure mentioned on p. 242. Eight litres of sewage were 

 treated every 48 hours by each of the filters. The capacities of the tall,* 

 medium and short filters were respectively, after wetting, 6, 4 and 2 litres, so 

 that each filling remained in contact at least four hours in the two taller filters 

 and two hours in the short one. For 38 out of the 48 hours of the cycle the 

 filters were empty or emptying. These contact filters were cone-shaped below, 

 with a narrow opening that could be closed with a cork for the purpose of 

 filling them. 



In the case of the continuous filters, oxidised nitrogen again made its first 

 appearance in the filtrates four weeks after starting (compare Tables Ia and 

 1b). The fact that all three Munich filters behaved alike in this respect 

 (forming a contrast to the Vienna ones) has already been explained as a 

 temperature effect, see p. 246. It was noticed that during the maturing 

 period the two different stages of nitrogen oxidation merged one into the 

 other, and were not so clearly separated as was the case in the Vienna filters 

 (Tables Ia and Ib) ; noticeably there was here no long period in which nitrites 

 were formed in quantity without any accompanying nitrates. This difference 

 has already been discussed (p. 247), and explanation is doubtless to be found 

 in the much less ammoniacal nature of the sewage here employed. 



The contact filters did not yield nearly ■such good results as the continuous 

 filters (Table Ib). The period which elapsed before nitrogen oxidation was 

 apparent was, in the former, more than half as long again as in the latter. 

 Again, when the short continuous filter showed complete oxidation of its 

 nitrogen, the tall contact filter still showed presence of nitrites in its filtrate. 



The Munich continuous filters had completely matured in about ten weeks 

 from the time of starting, but they were yielding a very satisfactory effluent 

 much earlier. After three months the sewage was changed for a much 

 more strongly ammoniacal liquid (cows' urine, diluted 1 in 100, containing 

 14 to 17 parts ammoniacal nitrogen per 100,000), in order to test the 

 capabilities of the filters as regards nitrogen oxidation. Most satisfactory 

 results were obtained (Table II, analyses 33 to 36, and Table IV), the 

 filtrates contained, as a rule, only traces of ammonia and nitrites, but 

 abundance of nitrates. Attempt to further tax the capabilities of the filters 



* In these Munich experiments the tall filters, both contact and continuous, were only 

 150 cm. high, instead of 200 cm., as in Vienna. 



