258 



SEWAGE AND ITS PURIFICATION 



then falls in a thin stream over an aerating weir, with a view to 

 restore aerobic conditions, by allowing products of fermentation 

 such as hydrogen and methane to escape, and introducing some 

 oxygen. It then flows through distributing channels on to 

 filters of coke breeze or clinker, similar to those at Barking 

 and Sutton, four of which are used at a time, and one kept in 

 reserve. Cameron's automatic gear^ regulates the cycles of 

 filling, resting full, emptying, and aeration, so that here again 

 no attention is required. The Local Government Board inquiry 

 of 1897 approved of the system being applied to the whole of 

 the city, of a population of 46,000, with the usual proviso as to 



Fig. 27.— Experimental Septic Tank and Filters, Belle Isle, Exeter. 

 The gas-lamp is lighted by the gases from the tank. 



land. For the daily flow of 1,064,610 gallons there are now 

 six tanks 181 x 35 X 7 ft. deep, with a capacity of 262,422 cubic 

 feet ; and eight filters, of 13,600 square feet each (total 2 J acres), 

 having a depth of 3J ft. of crushed furnace clinker on 6 in. of 

 coarse gravel, and a working capacity of 24 million gals, a day, 

 operating with alternating gear as before. 



Cameron, like Scott-Moncrieff, carefully differentiates be- 

 tween the hydrolytic or solution process, and the subsequent 

 oxidation required for final purification. Sims Woodhead has 

 shown that while the anaerobic organisms are more numerous 

 in the tank, a number of liquefying aerobic organisms are still 

 present, and increase on passing over the aerating weir. The 

 filters are, of course, intended for aerobic working, but are open 

 to the objections already urged against intermittent filters. 



1 See Chapter XII. 



