1905.] with reference to the Purification of Sewage. 



255 



the liquid to pass through the filter, and that the nitrites and nitrates 

 appearing at any particular time in the filtrates are the result of a slower 

 change which has been effected by the nitrifying bacteria upon ammonia 

 previously absorbed in some physical manner upon the surface of the filtering 

 material. The procedure for the purification of sewage used in "contact 

 beds " has been held to assist successively the processes of adsorption of 

 ammonia and nitrification. Dunbar and Thumm* consider that, in the 

 " filling " and " full " stages, putrescible and oxidisable substances are 

 retained upon the surface of the filtering material, and are subsequently 

 oxidised at times when the bed is full of air, the oxidation being the work of 

 bacteria in the bed, among which the nitrifying bacteria rank high in order 

 of importance. 



As regards the complex oxidisable putrescible substances of high molecular 

 weight, the solid suspended matter will be retained, of course, by mechanical 

 filtration, while the soluble constituents may, doubtless, be supposed to 

 undergo some physical adsorption.fi But the greater part of the nitrogen 

 present in sewage is there in the form of free and saline ammonia, and these 

 are the compounds most markedly retained as the sewage passes through 

 the filter ; yet for such simple compounds as these, adsorption by solids has 

 been shown to take place only to a small degree or not at all.§ To attempt 

 to explain removal of ammonia by adsorption then, would appear inadequate. 



Special experiments were therefore made to investigate the behaviour of 

 filtering; materials with ammonia and its salts ; also during the investigations 

 in Section I, careful note was also made of any facts which should tend 

 to confirm or refute the theory of nitrification quoted above. 



If the theory of a previous ammonia, absorption and a subsequent oxidation 

 were true, then contact beds should be much more efficient nitrifiers than 

 continuous filters, but the contrary proved to be the case, the latter doubtless 

 owing their greater efficiency to their more perfect aeration. Again, while 

 complicated organic substances appeared to be absorbed in the top layer of 

 the filter (Table EH, 3), the disappearance of free and saline ammonia was 

 shown usually to take place lower down in the filter (Table III, 1), and to be 

 always associated with the appearance of oxidised nitrogen (Table III, 2, etc.). 

 Moreover, during the maturing of the filters, before oxidation of nitrogen 

 had occurred, no absorption of ammonia could be detected, although this was 



* Dunbar and Thumm, ' Beit, zur Abwasserreinigungsfrage,' 1902. 



f Soyka, ' Archiv f. Hygiene,' voL 2, 1884. 



| Kattein and Liibbert, ' Gesundheitsingenieur,' vol. 25, 1903. 



§ "Weppen, ' Ann. d. Chem. u. Pharni.,' vol. 55, 1845, and A. Mayer, ' Lelirbuch d 

 Agrikulturchemie, : 1871. 



