CHAPTER II 



CHEMICAL ANALYSIS OF SEWAGE AND EFFLUENTS 



Methods of collecting samples — Gauging the flow — Samples should 

 be collected proportional to the flow and corresponding to one 

 another — Weirs — Floats — V notch — Meters — Hourly variation 

 of sewage — Oflicial methods of collection and analysis — Recom- 

 mendations of the British Association — Determinations of total 

 solids — Chlorine — Free and albuminoid ammonia — Oxygen 

 consumed — Mineral constituents. 



Samples for analysis should be taken proportionately to the 

 flow, and not in equal quantities, as is often done. The method 

 of working that I have adopted is as follows : 



A sample is collected and a gauging taken every hour, or, if 

 possible, every half-hour, by any one of the following methods. 

 The fluid may be passed over a sharp-edged horizontal weir, 

 and the depth of liquid flowing over the weir measured in 

 inches by means of a post placed behind. By Hawksley's 

 formula, if A = this depth, / the length of the weir in feet, 

 then Q, the flow in gallons per second, will be obtained by 



From these data tables are calculated giving the flow in 

 gallons corresponding to decimals of an inch. 



A practical difficulty in measuring the volume of raw sewage 

 is that weirs or constrictions of any kind cause an obstruction 

 to the flow, and therefore clogging or deposits. Where the 

 channel admits of access at several points an estimate of the 

 discharge can be obtained without retardation by the float 

 method. A piece of wood is released at one point, and the 

 time required for its appearance at another point is registered ; 

 the distance being known, the surface velocity of the stream is 

 ascertained in feet per second. The surface velocity in the 

 centre of a current is greater than the mean velocity of the 

 whole, and correction for this variability is somewhat uncertain. 

 A nearer approach to the average can be attained by using as 

 the traveller a piece of glass tubing closed at the lower end 



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