66 Dr. G. Reid. [May 7, 



It will be seen that the improvement effected by apparently so trivial an 

 alteration is remarkable. The improved quality of the effluents from the 

 altered section of the filter at a depth of 3 feet compared with those from 

 the same section and the unaltered section at a depth of 4 feet 6 inches is 

 most striking. 



With reference to the work done at different depths in filters, Scott- 

 Moncrieff has added very much to our knowledge of the subject by his 

 well-known experiments : on a small scale at Ashtead some eight years ago, 

 and, later, on a larger scale, at Caterham Barracks. The filters were built up 

 of nine trays, each 7 inches deep, with perforated bottoms, and supported 

 vertically over one another with about 3 -inch interspaces. The depth of the 

 filtering medium in each tray was 6 inches, giving a total effective depth of 

 4 feet 6 inches. The medium consisted of coke, broken to pass through 

 a ring 1 inch in diameter, and the rate of delivery was about 200 gallons 

 per square yard (1,000,000 gallons per acre). 



The object Scott-Moncrieff had in view was a two-fold one : first, to secure 

 the freest possible aeration of the filter ; and, secondly, to separate, as far as 

 possible, the various stages of the process by providing facilities for the 

 differentiation of the colonies of nitrifying organisms. Incidentally the 

 arrangement also allowed of the collecting of samples of effluents for 

 analysis from different depths, for comparative purposes as regards the 

 oxidising changes. 



Without going into detail, I may mention that while the nitrification 

 effected by the tray filters was exceptionally high, the changes, notwith- 

 standing the free aeration which the arrangement provided for, did not take 

 place so rapidly, nor at so shallow a depth, as in the case of the Hanley 

 experiments. The nitrous change was more marked, and was continued to 

 a greater depth, and the reduction in the free ammonia, as well as in the 

 oxygen absorbed, was far more gradual. It is true that the sewage was 

 a much stronger one, but, making allowance for this, the work done per foot 

 deep of filtering medium was relatively considerably less than in the case of 

 Hanley. I feel pretty confident that had a much finer filter medium been 

 used by Scott-Moncrieff the high oxidising changes would have been effected 

 at a shallower depth, and practically the whole of the suspended solids 

 would have been liquefied and nitrified within the filter. 



Dr. Harriette Chick, in a paper published in the ' Proceedings of the Eoyal 

 Society/ 1906, B, vol. 77, gives an account of certain laboratory experiments 

 she conducted in 1901, and again in 1903, having reference to the nitrifi- 

 cation of sewage. Three cylindrical filters were employed, each 4f inches in 

 diameter, and approximately 1 J, 3£, and 6^ feet in depth respectively. The 



