54 



SEWAGE AND ITS PURIFICATION 



80° F. for five days. The three minutes absorption of O is 

 then again determined. If any putrefaction has taken place 

 the oxygen absorbed in three minutes will increase in amount, 

 owing to the more ready oxidizability of the products of putre- 

 faction. On the other hand, if the sample keeps sweet there 

 will be a slight decrease in the three minutes absorption after 

 incubation, owing to the slight oxidation of the impurities 

 which has taken place during the five days by means of air 

 dissolved in the sample." Any change of odour or appearance 

 of putridity is also carefully noticed. 



This test, although extremely useful, is arbitrary in character, 

 as an effluent is not intended to be stored by itself, but when 

 finished to be discharged at once into water which is moving 

 and aerated. If an effluent passes the incubator test it can be 

 discharged into a dry ditch without fear of subsequent putrefac- 

 tion. At Manchester they determined the behaviour of 

 mixtures of equal volumes of bacterial filtrate and Ship Canal 

 water, and have found that almost invariably such mixtures 

 remain sweet. The Ship Canal water alone, when tested in 

 this way, frequently gives an unsatisfactory result, so that the 

 effluent actually improves the waters of the canal. 



The Mersey and Irwell Joint Board incubate at 65° F. for 

 seven days, but by working at the higher temperature of 80° F., 

 as above, results are obtained in a shorter time. 



Weights of Excreta discharged per Day per A verage Person. 

 These have been given as follows •} 



^ Frankland, Rivers Pollution Commission, 1890. 



