70 Dr. G. Eeid. [May 7, 



depth, when it was found that the dark discoloration from deposit was 

 confined to the surface, as was evident from the untarnished appearance 

 of the light-coloured filtering medium below the top 14 to 18 inches. Also, 

 the relative amount of organic solids in the interstices at different depths, 

 ascertained by drying and igniting 10 grammes of the filter particles in each 

 case, supports this contention, as the following figures show : — 



Percentage Loss on Ignition of Filter Particles at Different Depths. 

 6 in. 1 ft. 2 ft. 3 ft. 4 ft. 



3*25 0-99 0-65 0'53 053 



I may mention that on ignition there was practically no smell, except in the 

 case of the samples collected at the 6-inch and 1-foot depths, and that in the 

 case of these only an odour such as that of burning soil could be detected. 



Apart from all theory, however, the fact has been established beyond 

 all doubt that, in the case of the sewage in question at any rate, the lower 



or 3 feet of filter medium is absolutely unnecessary, and, so far as the 

 cost of construction of the filter is concerned, the expenditure might be 

 reduced by about one-half. Again from the point of view of cost, another 

 important consideration comes in. It frequently happens that the absence of 

 2 or 3 feet of available fall is the determining factor between a gravitation 

 and pumping scheme, and in this respect the reduced depth of filter might 

 lead to further economy, not only in capital outlay, but in maintenance 

 charges. 



Now, the question may be asked whether this experience acquired at 

 Hanley may be applied in other cases where the sewage may be of a stronger 

 character ? I am not at present in a position to give a positive answer 

 to that question, but if, as is probable, such should prove not to be the case, 

 the observations I have recorded clearly point to the conclusion that the 

 extra filtering capacity should be provided for by increasing the area rather 

 than the depth of the beds. The three factors in the nitrifying process being 

 a given time, a given volume of air, and a given bacterial population, all these 

 would be supplemented by extending the area in accordance with the 

 combustion which has to be effected, and thereby diminishing the delivery per 

 square yard of filter. Of course, I do not suggest that the depth of filters 

 may be reduced to 1 foot, because we must allow for the effluent drains, 

 and the few inches of large material immediately on the top of them, but 

 I do suggest the practicability, if a fine medium is used, of reducing the 

 total depth to, say, 2 feet 6 inches. 



The tables appended give the details of the analyses and other determina- 

 tions made during the investigations which have been described. 



