40 



SEWAGE AND ITS PURIFICATION 



Mineral Constituents. — When there is time, much further 

 information can be gathered from an examination of the 

 inorganic mgredients of sewage. Where trade effluents are 

 concerned this often constitutes a principal part of the inquiry. 

 The processes are the same as those of ordinary analysis, but 

 on account of the changes that occur on incineration, many 

 of the estimations must be made on the original, and not on 

 the ash. 



Based on an average water-supply of 33 gallons, Wanklyn 

 gives the following amounts per head per day in grammes : 



Sulphates are often of great importance ; they are naturally 

 derived, with sulphides, from the breaking down of albuminous 

 matters, and also are artificially added in various forms of 

 chemical treatment. Here is an instance from my own ex- 

 perience, the river being a small one : 



Sulphates as SO5 in Parts per 100,000. 



It can be calculated from these figures that the effluents con- 

 taminated the river to the extent of one-eighth of its volume. 

 On an average the larger quantity of diluent water contributes 

 at least as much sulphuric acid as the smaller volume of urine ; 

 therefore in domestic sewages the measurement of sulphates is 

 of less value than the measurement of chlorine, Wanklyn 

 gives the following averages for sulphates as SO3 in various 

 waters and in what he names " typical sewage " — i.e., urine 

 diluted with pure water to 100 times its volume. This phrase 

 is based on the estimate of the average urine per person per 



