1906.] 



Nitrification of Sewage. 



63 



stage, proportionate to the albuminoid ammonia reduction, and the rapidity of 

 the change in this case also is equally marked. 



5. With reference to the oxidised nitrogen the figures are very startling, 

 for not only are they indicative of extremely healthy and active biological 

 conditions, but, considered in relation to the other figures, they also 

 demonstrate conclusively, in the case of the sewage in question at any rate, 

 that the work of purification is practically completed within a few inches 

 of the surface of the filter. Within 12 minutes a foul and offensive liquid 

 is rendered not only clear and non-odorous, but also absolutely stable as 

 regards putrescible qualities, as is proved by the oxygen absorbed figures 

 before and after incubation. I must say that this result surprised me very 

 much. Theoretically, as I have said, one expected to find that the process 

 was a more gradual and progressive one, nitrites being formed chiefly in the 

 superficial layers of the filter and nitrates in the deeper layers, but this 

 did not prove to be the case. At the same time I do not suggest that the 

 change is not gradational, but rather that the two sets of organisms are 

 at work side by side, and in this case the absence of more than a trace of 

 nitrous acid in the 1-foot effluent, although the tests were applied at the 

 moment of collection, may be accounted for by the preliminary stage of the 

 oxidation process being an extremely evanescent one owing to the highly 

 efficient working conditions. 



6. The column headed " depth of column necessary to obscure test lines " 

 requires explanation. It is a standard opacity test which I adopted some 

 years ago for Staffordshire, and which has since been adopted by some 

 other river authorities. The lines referred to are engraved on a white 

 porcelain disc, which is attached by means of a socket and rubber washer to 

 a 2-foot glass tube, graduated in half-inches. The exact mesh and depth of 

 lines are as shown in the drawing, and in fixing the scale 

 as regards mesh and length of tube, in the first instance, 

 I was guided by my experience at the time as to the 

 opacity of well-clarified effluents. J have since found, 

 however, that the clarification effected by fine-grade 

 filters and efficient distribution is so perfect that the 

 lines are not usually obscured by a depth of 24 inches, and this was the 

 case in the present instance. It will be noticed that while the mean 

 depths of fluid necessary to obscure the lines in the case of the crude sewage 

 and the detritus and septic tank effluents were 0'5, 1*6, and 1*5 inches 

 respectively, the filter effluents, even those from the 1-foot tray, were so 

 highly clarified that the full 24-inch column did not suffice to obscure the 

 lines. It is true that the opacity test in itself is not a reliable index of purity ; 











































































































































































































































































