NORTH SYDNEY AND DOUBLE BAY SEWERAGE SCHEMES. XXL 



used. The rate of flow would therefore be 120 gallons per square 

 yard per hour, or say 13,939,200 gallons per acre, when it was 

 found a very small quantity of sand passed through the coke 

 into the pipes. The ^ in. and T 3 g in. had given slightly the best 

 results. 



The experiments were continued to 28th December, 1897, and 

 quantities varying at the rate of from 4,250,000 gallons per acre 

 per day, to 14,907,200 gallons, when practically no sand was 

 carried into the pipes. 



As, therefore, not more than one-tenth of the smaller quantity, 

 4,250,000 gallons per acre per day, would ever be put into the 

 filter beds at Willoughby, it was, the Author thinks, safely 

 assumed that (a) the perforations and coke surrounding the 

 pipes would permit of the effluent entering the pipes faster than 

 required, and (b) that with the maximum flow of sewage at the 

 rate of say 400,000 gallons per acre per day, no sand would be 

 carried into the effluent pipe. 



The Author finally decided to adopt the T 3 g in. perforations, as 

 no more sand passed through these than the ^ in. perforations. 

 It is also thought that the presence of the coke round the pipes 

 will have the effect of preventing the growth of fungi in the 

 perforations. 



The principles which underlie the purification of sewage by 

 intermittent downward filtration need no justification on the part 

 of the Author. The highest scientific and practical evidence has 

 firmly established the efficiency of this method based, as it is, on 

 natural processes. 



It is only, therefore, necessary to mention that in the North 

 Shore works the main features of the most modern system of 

 filtration have been adopted. 



Each filter bed is in reality a bacterial hot bed, where, through 

 chemical changes, brought about by micro-organisms, the organic 

 impurities of the sewage, which constitute its chief danger, are 

 broken up into harmless forms. 



About two-thirds of the suspended solids, organic and other- 

 wise, are left behind as sludge in the settling tanks, and the 



