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



such filters, to be an extremely rapid biological process, requiring for its 

 completion only the time taken for the liquid to pass through the filter 

 (approximately 2 to 3 hours, possibly a little more). The rapidity of the 

 process is probably to be explained by the very great number of nitrifying 

 bacteria present and the very efficient aeration which obtains. In such 

 filters also, the general conditions are ideal for quick action, as the continuous 

 trickle secures rapidity of diffusion, and forms a great contrast to the much 

 slower effect in stationary fluids. 



6. Temperature has a marked influence upon the oxidation of sewage, a 

 higher temperature being noticeably more favourable. This indicates that 

 the efficiency of sewage filters in practice would be much increased if at a 

 reasonable cost they could be artificially maintained at a warm temperature 

 during the winter.* 



7. The previous conclusions are chiefly drawn from experiments with 

 continuous filters, but filters working as contact beds were also investigated 

 and the two methods compared. On the " ammonia adsorption theory," the 

 contact method should have proved the most efficient. This, however, was 

 not found to be the case. The advantages of the continuous method would 

 seem to lie in the much more complete aeration and efficient diffusion, and 

 also in the stratified distribution in the filter of the different stages of the 

 sewage purification. Some of the present experiments were quite comparable 

 with practical installations as regards quantity of liquid treated, con- 

 centration of nitrogen, etc., and the results were much more satisfactory than 

 those usually obtained in practice. The obvious difficulty in practical 

 employment of continuous filters is with regard to the solids in suspension, 

 which can only be permitted upon the filter to a small extent without risk of 

 clogging. The present experiments were ail made with roughly filtered 

 solutions, but the difficulty could be met in practice by a previous screening 

 of the sewage or by passing it through a septic tank. Should clogging occur, 

 it will probably take place in the superficial layers and could be remedied by 

 simple mechanical treatment. In the case of contact beds, however, clogging 

 necessitates the cleansing of the whole bed, an exceedingly costly process. 

 From these considerations, and as a result of the present experimental study, 

 the method of continuous filtration would appear to be a most advantageous 

 method of purifying sewage. 



* Ducat filters, which are artificially warmed in cold weather, perform an amount of 

 nitrification which is well above the average. 



