1 86 SCIENCE PROGRESS 



yield of crude grease is 1*4 cwt. per ton of pressed sludge 

 (fifty per cent, water), or in other words gs. g\d. per £y ton. 

 Therefore the products obtained are of the total value of 

 135. 6\d. from every ton of sludge containing fifty per cent, 

 water, or £1 ys. id. per ton of dry sludge. The working 

 charges are put at 6s. 11W. per ton of sludge (fifty per cent, 

 water), or 13s. nd. per ton of dry sludge. The difference 

 leaves a margin of 135. 2d. per ton of dry sludge, which in 

 the case of a town of 100,000 inhabitants should result in a 

 profit of at least £2,500 per annum. However much opinions 

 may differ as to the practicability of Dr. Grossmann's scheme, 

 all will agree that it is a step in the right direction, and the 

 further results of his experiments on a large scale will be 

 watched with very great interest and with every wish for 

 their complete success. 



The method of sewage treatment by precipitation is thus by 

 no means obsolete. The Royal Commission in its Fifth Report 

 (1908) stated that : " In the case of some sewages which contain 

 trade wastes, it is almost essential to subject the sewage to some 

 form of chemical treatment before attempting to oxidise the 

 organic matter contained in it, and in the case of domestic 

 sewages, chemical precipitation materially aids the deposition 

 of the suspended solids, and facilitates subsequent filtration." 

 Brewery waste might be indicated as an instance of the class of 

 trade waste where chemical precipitation is found to serve a 

 useful purpose. 



(3) Biological Purification. — All biological methods of puri- 

 fication are said to have two features in common, viz. : 



(a) The destruction of the injurious and putrescible sub- 

 stances in the sewage. There is a destruction of sewage as 

 sewage and a building up of new substances in its place. 



(b) The production of this desired effect, not by adding 

 anything to the sewage, but by the action of the organisms 

 normally present in the sewage or the medium (land or artificial 

 filter) upon which the sewage is treated. 



"All biological processes depend upon the presence of bacteria 

 in some shape or form." In addition to this, it is claimed that 

 the bacterial treatment of sewage is under our control, can be 

 regulated at our will, and, lastly, that " the processes of decom- 

 position and nitrification ultimately destroy the pabulum upon 

 which the organisms in question depend for their existence, 



