SEWAGE PURIFICATION 307 



The other analytical figures, in the absence of trade 

 effluents, will probably vary in proportion. 



It is, of course, necessary in determining the strength of 

 sewage that an average be taken if possible over several days, 

 samples being taken hourly and mixed in proportion to the rate 

 of flow. 



2. In determining the efficiency of purification works, the 

 analysis will show the progressive reduction in impurity 

 attained in the various stages of the process. The oxygen 

 absorption, the ammoniacal and albuminoid nitrogen and 

 the suspended matters should decrease. A considerable 

 proportion of the nitrogen should reappear as nitrate. The 

 resultant effluent should have lost its putrescibility, that 

 is to say, when kept in a closed and full bottle for a few 

 days, at a temperature, say, of 80 F., it should not 

 become offensive. 



The chlorine figure, which is due to the sodium chloride 

 present in the sewage, is unaltered by the purification process, 

 and therefore serves as a useful index to show whether the 

 effluent really represents the sewage from which it is produced. 

 In a true comparison the chlorine number should be the same 

 in both cases. 



3. The effect of an effluent upon a body of water depends 

 essentially, as we have seen, on the amount of dissolved 

 oxygen it is capable of abstracting from a body of water, 

 and the Royal Commission have therefore summarised, as 

 it were, the various methods of sewage analysis, and have 

 sought to define a good effluent in terms of its power of con- 

 suming dissolved oxygen. The importance of the absence 

 of suspended solids, which may form troublesome deposits, 

 is also recognised, and they suggest that an effluent would 

 generally be satisfactory if it complied with the following 

 conditions : 



(1) ' That it should not contain more than three parts 

 per 100,000 of suspended matter; and 



x2 



