354 APPLIED BACTERIOLOGY 



paper, vegetable debris, etc.). — These are almost entirely 

 removable by any of the precipitation processes in use. 



4 Organic Matter in Solution. — None of the chemical 

 precipitation processes in use can have much effect on the 

 removal of organic matter in solution, while some of them 

 in which much lime is employed actually increase it. 



It is the organic matter in Solution that is the cause of 

 many effluents, although bright and clear and compara- 

 tively free from smAl, subsequently becoTning putrid. 



The natural purification of sewage may be regarded as 

 taking place in two stages — -though these may to some 

 extent occur simultaneously — first, the digestion or lique- 

 faction of the solid matters in suspension, and secondly, the 

 oxidation of these with the organic matter originally in 

 solution. 



The study of these changes has now led to practical 

 results in the introduction of new methods for the treat- 

 ment of sewage, and we are now able to produce effluents 

 of a far greater purity and uniformity than has been 

 possible before. 



It has been known for a considerable time that sewage 

 from which the suspended matter has been removed may 

 be very much improved by passing through land, but that 

 to obtain good results the land must be prepared by under- 

 draining, and that different soils vary very much in their 

 power of producing purification. 



The liability of land to become water-logged . has led to 

 the construction of artificial filter-beds of coke breeze, 

 gravel, or furnace clinker, using the materials of such a 

 size as to ensure speedy draining of the bed when the 

 water is turned off. 



Up to the present time the Local Government Board has 

 insisted on the provision of land for the final purification of 

 sewage effluents, but now that there is indisputable evidence 



