30 ON THE DISPOSAL OF SEWAGE. 



portion of this organic matter is removed therefrom partly by 

 natural subsidence and partly by the precipitates formed by the 

 ferrous sulphate. The filtration reixioves all the suspended 

 matter, and a further precipitation is effected by the porous 

 magnetic oxide, which, on absorption of organic matter in its 

 pores, oxidises it by help of the dissolved oxygen existing in the 

 liquid." 



This method has also been lately adopted at Hendon, and then 

 the sludge, air-dried, is said to be very valuable as a manure — 

 Dr. Angell estimating the value of the sludge from one million 

 of gallons of sewage to be worth £27. At this rate a ton of 

 sewage would be worth l^d. lUit Dr. Angell estimates the 

 quantity of sludge, air-dried to 12^ per cent, of moisture, in 

 one million gallons to be sixteen tons, so that each ton of sludge 

 would be worth about 84s. and the cost of production 3s. 9d. 

 If this is so, it is by far the most valuable product which has yet 

 been obtained, and treatment of sewage by this process would be 

 made decidedly payable. 



The treatment of sewage by electrolysis, termed the Webster 

 Process, has from all accounts been very encouraging. Sir H. 

 Roscoe says, respecting the sewage operated : — " The reduction 

 of organic matter in solution is the crucial test of the value of a 

 precipitating agent, for unless the organic matter is reduced the 

 effluent will putrefy and rapidly l)ecome offensive. I have not 

 observed in any of the unfiltered effluents from this process, 

 which I have examined, any signs of putrefaction, but, on the 

 contrary, a tendency to oxidise. The absence of sulphuretted 

 hydrogen in samples of unfiltered effluents, which have been 

 kept in stoppered bottles for three weeks, is also a fact of 

 importance. By this process the soluble organic matter is 

 reduced to a condition favourable to the further precipitation of 

 natural agencies." 



In the matter of cost this system is said to compare favour- 

 ably with all others. 



In dealing with the subject of sewage disposal the idea of 

 its manurial value should not be allowed to influence one to any 

 great degree. The difficulty has always l)een elsewhere, and I 



