SEWAGE PURIFICATION 281 



domestic sewage only will be referred to, the question of the 

 treatment of trade effluents, or mixtures of trade effluents and 

 sewage, constituting a special problem. 



The main constituents of domestic sewage may be described 

 as follows : — 



(i.) Matters in Solution. (Mainly derived from urine.) 

 Nitrogenous substances, e.g., urea and kindred com- 

 pounds. 

 Mineral salts, chiefly sodium chloride together with 

 phosphates, 

 (ii.) Matters in Suspension, Emulsion, or Colloidal 

 Solution. — Nitrogenous substances of complex character 

 containing sulphur (mainly derived from fseces). 



Cellulose (disintegrated paper) and vegetable debris. 

 Soap and fat. 

 (iii.) Sedimentary Matters. — Silt, clay, sand, etc. 



There cannot, of course, be sharp lines drawn between these 

 various classes of substances. It will depend, e.g., on the 

 hardness of the water, how much, if any, of the soap is present 

 in solution or suspension ; substances in Class ii. will also 

 be partially carried down by the quickly sedimenting mineral 

 matters of Class iii. 



In general, about twenty gallons may be taken as the usual 

 water supply per head per day, in which the above constituents 

 are disseminated. 



The complete purification of the sewage, which is effected 

 by bacterial treatment, results finally in the production of 

 some or all of the following substances : — 



(i.) Gases : 



Methane (Marsh gas). 

 Hydrogen. 

 Nitrogen. 

 Carbon dioxide. 



