SEWAGE PURIFICATION 305 



The methods of sewage and of water analysis are closely 

 allied, the chief difference being in matters of detail, necessi- 

 tated by the different quantities of oxidisable or organic 

 matter which have to be determined in a given volume of the 

 respective liquids. As a matter of fact, a sewage effluent of 

 high quality may contain no more organic matter than a low- 

 grade drinking water. 



The main difference between the two branches of analysis 

 lies in the significance of the presence of nitrate in the two 

 cases, and the importance attaching to the determination of 

 the number and character of the bacteria present. 



A good sewage effluent, as we have seen, is generally 

 characterised by the presence of an abundant proportion of 

 nitrates. The presence of nitrates in a water supply may often, 

 on the other hand, give rise to suspicion, as pointing to the 

 oxidation of previously present organic matter. An excep- 

 tion to this rule is met with in the case of deep well waters, 

 where the nitrates may arise from long past deposits. In such 

 cases, as a rule, the nitrates will be unaccompanied by nitrites ; 

 the presence of the latter, which are unstable intermediate sub- 

 stances, point to an oxidation process in actual operation, or 

 possibly, of course, de-nitrification changes which may be 

 equally due to organic matter. 



In regard to the presence of bacteria, these are of compara- 

 tively little significance in the case of an ordinary sewage 

 effluent, as none of the processes of sewage purification in 

 common use, short of sterilisation or slow sand filtration, do 

 more than reduce the number of organisms present. For this 

 reason the detection of Bacillus coli in a drinking water is 

 presumptive evidence of sewage pollution. This test is one of 

 extreme delicacy and it is, therefore, quite possible for a sample 

 of water to pass the usual chemical tests, and yet to be placed 

 under suspicion, when examined bacteriologically. 



The Analysis 01 Sewage. Bearing in mind the importance 



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