692 Manurial Value of Sewage Sludge. [dec., 



the nitrate from a sewage effluent otherwise than through the 

 agency of plants, and there must of necessity be a considerable 

 loss of valuable nitrogenous manure from water-borne sewage, 

 even in the case of land purification. In the case of artificial 

 filtration, supposing the effluent to be subsequently discharged 

 into a stream, the loss is, of course, much greater, all the manurial 

 constituents of the sewage, apart from those in the sludge, being 

 wasted. 



The only available manurial product, therefore, is the sewage 

 sludge, which is produced by settlement after chemical precipi- 

 tation, settlement without chemicals, or septic tank treatment. 



Commercial Manure produced from Sewage Sludge. — The 

 Commissioners report on two processes for converting sludge 

 into a marketable manure, which appear to have had a limited 

 success, but as regards one method they doubt whether the 

 process could be worked as a financial success if it were adopted 

 at a large number of places, as a greatly increased production 

 would be likely to lower the price considerably. As regards the 

 other method, the sale of the product is diminishing, and owing 

 to difficulties in manufacture it is found more convenient to 

 sell the sludge as pressed cake. 



Production of Pressed Sewage Sludge. — The plan of pressing 

 sludge is adopted at a large number of places. It has the effect 

 of converting the sludge into a form in which it can either be sold 

 to farmers or can be tipped or buried without serious expense. 



The plan is as follows : — The sludge produced by the settle- 

 ment of the sewage, with or without the addition of chemicals, 

 is first run, or pushed by hand labour, from the settling tanks 

 into a sludge well or tank. In this well it is allowed to settle 

 for some little time, and, after the supernatant liquor has been 

 drawn off, the thick sludge is mixed with from o -5 to 1 per cent, 

 of lime (or even more) in the form of milk of lime and then 

 forced by means of compressed air into filter presses. In this 

 way, on the average, something like 30 or 35 per cent, of the 

 water in the wet sludge (which may be taken at approximately 

 90 per cent, on the average) is removed, leaving a moist but 

 solid cake which may contain from 50 to 65 per cent, of water. 

 The liquid pressed from the sludge is run back into the settling 

 tanks for re-treatment. It is strongly alkaline, owing to the 

 lime which has been added to the wet sludge. The pressed 



