

Nitrification of Sewage. 59 



(giving, with the subsidence tanks, a capacity for quiescence equal to a day's 

 flow). 



4. Two quarter-acre " percolating " filters 4 feet 9 inches deep, the one 

 circular and the other rectangular, and each having a different type of power- 

 driven distributor. I may here mention that I have long since discarded the 

 " contact " method of working bacteria beds in favour of what is termed 

 " percolating " filtration. 



The filtering medium in this case consisted of broken " saggars," the 

 discarded hard-burned clay vessels in which china is packed for firing, and 

 the filters were constructed in four separate sections, each having different 

 graded material. This material ranged (in separate sections) from 1-J inch to 

 J inch. I have elsewhere given a full description of the working of this 

 plant during a period of about eighteen months, and the results proved that 

 such filters, working at the rate of 200 gallons per square yard per 24 hours 

 (1,000,000 gallons per acre) continuously, produce a high-class effluent, the 

 best results being obtained from the section of the filter composed of the 

 finest grade particles (£ inch). . 



So much for the preliminary work which led up to the present investiga- 

 tions, the results of which seem to me to be of scientific interest and of 

 practical importance. 



It would seem to be established that the nitrification of hydrolysed sewage 

 takes place in stages, the nitrous change being effected by one set of organisms 

 as a preliminary to the nitric change brought about by another set, and the 

 theory is that the two sets of organisms are located in different layers of the 

 filter. With the view of verifying this, and, at the same time, ascertaining 

 the degree of purification effected at different depths, I determined to tap the 

 filter which had been used for the purpose of the experiment already referred 

 to, in such a manner that samples might be collected at different levels, 

 and I selected the finest grade section for this purpose (J-inch particles). 

 Accordingly, four longitudinal shallow trays, with perforated covers, were 

 placed in the body of the filter, the first at 1 foot from the surface, the second 

 and third at 1-foot intervals downwards, and the lowest 18 inches below the 

 third one, pipes being carried from the various collecting trays through the 

 wall of the filter, to allow of the collection of the respective samples. In 

 order that the results from the lower depths might not be affected by the 

 presence of the trays above, the trays were placed obliquely from above 

 downwards, so that no tray had another in the vertical line above it. As the 

 filter at this time had been constantly at work for two and a-half years, in 

 replacing the filtering material which had been disturbed in order to intro- 

 duce the trays care was taken to ensure as far as possible that each portion 



