244 SEWAGE AND ITS PURIFICATION 



secondary contact bed, the effluent passing straight through the 

 material to the under drains, the construction may be as in 

 Method I, even if used without clayey ground or puddled sides. 

 Choice of methods to a certain extent depends on the quality of 

 the sewage and local factors. 



Continuous Filtration. 



The intermittent system recommended by the Rivers Pollu- 

 tion Commission of 1868, and enjoined by the second Royal 

 Commission on Metropolitan Sewage Discharge in the words 

 "the intermittency of applications is a sine qua non," was 

 almost universall}^ adopted. But apart from the labour of regu- 

 lation and supervision, another fault of the method was that the 

 oscillations of functions between anaerobic and aerobic actions, 

 involved in " resting full " and " resting empty " periods, created 

 disturbance, and was detrimental to the consistent action of 

 bacteria. In Ducat's and Whittaker-Bryant's filters, and for- 

 merly in Lowcock's, the working was continuous, but rests at 

 longer intervals were usually found necessary. 



At Salford Sewage Works Mr. Joseph Corbett made a series 

 of experiments from 1893 to 1898,^ on the basis of a continuous 

 passage of both sewage and air simultaneously through filters 

 of gravel and sand, of coke breeze, and of cinders ; all were 

 " very good, but the latter the best." The filtrates after fifteen 

 months' continuous working showed an average reduction to 

 free ammonia 0*829, and albuminoid 0*13, in parts per 100,000. 

 The principle of dividing into three or four heights of 20 inches 

 thick with ventilating spaces between was also tried, but the 

 analyses seemed to show a slight difference in favour of the filter 

 in one mass. The distribution was effected at first by wooden 

 troughs with holes, afterwards by spray jets,^ with a head of 

 4 feet pressure, the amount delivered being 500 to 1,000 gallons 

 per square yard. The chief requirement was that the filters 

 should be thoroughly open. The Salford Corporation decided 

 to lay down these filters for treating the whole of the sewage 

 at the rate of 500 gallons per square yard on beds 5 feet deep, 

 and on the refusal of the Local Government Board to sanction 

 the scheme unless it included the purchase of land over which 

 the filter effluent should be turned, the Corporation arranged 

 to borrow ^80,000 for the works under Section 35 of the Public 



^ Public Health Engineer, February 3, 1900. 

 ■- See Chapter XII. 



I 



