200 



SEWAGE AND ITS PURIFICATION 



this imperfectly-burned clinker to take fire again after being 

 removed. 



2. A supply of sufficient oxygen to maintain steady combus- 

 tion without over-cooling the gases produced. 



3. A suitable site. If this can be central to the district, it 

 will greatly save cost of cartage, etc., and facilitate disposal. 

 Refuse properly cremated, with a high chimney, creates no 

 nuisance, even in populous neighbourhoods. The prevailing 

 winds should be studied, with regard to the carriage of the gases. 



4. Carriage to the works without offence. Improved covered 

 -carts are now constructed. The supply should be as regular as 

 can be managed. 



5. Where used for steam-raising, the boilers must be so 

 placed as not to cool down too much the evolved gases. 



Street sweepings, which furnish a large portion of the matter 

 to be treated in a dust destructor, vary very much in com- 

 position. 



The following analyses are recorded : 



With regard to the fuel value of dry refuse, authorities agree 

 that it is about one-seventh to one-ninth that of coal. The 

 best conditions, both as to prevention of smoke and fume, and 

 the concentration of heat for utilization, being a slow, steady 

 combustion, quick-burning materials do not prove to have any 

 advantage. It is in the drying stage that the main risk of 

 nuisance occurs ; in many old processes it has simply amounted 

 to a distillation in which strongly odorous substances have 

 escaped with the water vapour. Therefore the aim has been 

 (i) to raise the drying gases to as high a temperature as possible; 

 (2) to subject the evolved vapours to a secondary cremation by 

 passing through ignited material. 



