68 Dr. G. Eeid. [May 7, 



It may now be convenient to sum up the arguments which may be 

 adduced, and the conclusions which may be drawn, from the recent observa- 

 tions at Hanley. In the first place, two important factors undoubtedly 

 contributed in no small degree to the high quality of the work done : (1) the 

 small amount of suspended matter in the effluent applied to the filter ; and 

 (2) the very efficient means of distribution provided. 



The oxidation of organic matter already in solution is a comparatively 

 simple process, but to bring about the liquefaction of suspended organic 

 solids takes longer time, hence the importance of providing, as far as possible, 

 for their removal by mechanical subsidence or other methods, and their lique- 

 faction by anaerobic organisms as a preliminary process. When septic tank 

 treatment was first introduced, its advocates, in their enthusiasm, predicted 

 that the difficult question of sludge disposal would thus be solved, but 

 experience does not quite corroborate this prediction. In the case in point I 

 attribute the satisfactory reduction of suspended solids in the septic tank 

 effluent to the large capacity of the detritus tanks. These were divided into 

 three, two of which were always in use, while the third was emptied for 

 cleansing weekly, and so on in rotation, the whole three being brought into 

 use during rain periods. 



As regards the distribution of the sewage on the filter, it is obvious that 

 uniformity is all-important, otherwise certain sections might be greatly over- 

 taxed, while other sections were working much below their capacity. The 

 distribution in this case was so perfect that frequent tests failed to establish 

 any appreciable difference between one square yard of filter and another. 



Now, as regards the rapidity of the oxidising changes, and the remarkable 

 purity of the effluent after filtration through 12 inches only. The three 

 essential factors in the final changes are time, air, and organisms, and, given 

 a sufficiency of air, the greater the number of organisms present the larger 

 the amount of work done, provided the organic matter both in solution and 

 suspension is brought into intimate contact with the organisms. The factor 

 which governs the bacterial population is the area available for growth, and 

 this may be increased by two methods, either by enlarging the cubic capacity 

 of the filter or by subdividing the filtering medium. In the case in point, 

 the latter was the expedient adopted, and the subdivision was carried as far 

 as it was thought possible to carry it without preventing the superficial 

 penetration of the suspended solids into the interstices. In view of the 

 results, it is needless to discuss whether the reduction in the size of particles 

 resulted in an inadequate air supply. 



The relative amount of carbonic acid in the air of the filter at different 

 depths also shows the highly active oxidising changes which take place in 



