246 PERUVIAN GUANO, ETC. [chap. 



treatment to get rid of suspended matter, can be 

 profitably utilised in raising crops. But light land, 

 permitting of free percolation, is necessary, and it must 

 not be overloaded with sewage but allowed intervals 

 for aeration and oxidation, or else the surface becomes 

 sealed with a layer of organic matter difficult to break 

 down and both percolation and purification cease. An 

 acre of land is not capable of dealing with the sewage 

 of many more than one hundred people. This is hardly 

 the occasion to discuss the various processes now in 

 vogue for the purification of sewage by bacterial action 

 or by land filtration, but at one point they do touch the 

 manure question by turning out " sewage sludges " 

 which possess a certain fertilising value. In many 

 of the processes the raw sewage is first submitted to 

 some process of chemical precipitation to effect the 

 removal of the suspended matter and obtain a clear 

 effluent, which can be purified by bacterial filter beds 

 or by application to the land. As precipitating agents, 

 lime, alum, and sulphate of iron are commonly employed, 

 alone or together, the object being to produce a bulky 

 colloidal precipitate which will entangle and drag down 

 the flocculent organic matter of the sewage. After 

 mixing with the precipitant, the sewage is left in tanks 

 to settle, the clear liquid is passed on for further treat- 

 ment, and the remaining sludge is freed from excess of 

 water by passing through some form of pressure filter. 

 The resulting press cakes are either disposed of locally 

 in the wet state or, in one case at least, dried and sold 

 as a manure under the name of " native guano." It is 

 obvious that any such precipitating process with either 

 lime or the sulphates of alumina and iron, can only take 

 out of the sewage such nitrogenous bodies as proteins, 

 leaving the greater part — the amides and ammonium 

 salts, still in solution. Thus the sludge will only con- 



