F 



INTRODUCTION 17 



Phelps remarks -} " Rideal's formula serves to distinguish three 

 possible cases. If C be negative, the effluent not only will not 

 putrefy by itself, but by virtue of its excess of available oxygen 

 will tend to improve the condition of the stream, if the latter 

 be already polluted. If C be greater than unity, the effluent 

 will draw upon the oxidizing power of the stream, diminishing 

 the power of the latter to deal successfully with further pollution; 

 but in this case the stream will not itself become foul from the 

 effluent. If C be positive, but less than unity, the stream will 

 be overburdened by the addition of the effluent, and will 

 become foul." 



In the favourable cases where bacteria and algae are active, 

 and the oxygen of the river is able, by their help, to deal 

 rapidly with the incoming residues, the minimum ratio between 

 the volume of the stream and the volume of effluent that could 

 be allowed to be discharged into it would be indicated by the 

 value of C in the above equation, which would also approxi- 

 mately denote how far the population might increase before 

 the proportion could be seriously disturbed. The minimum 

 figure will be reduced by the nitrites or nitrates of the river 

 water itself, or the free oxygen which may be present in the 

 effluent. River water often contains about 90 per cent, of its 

 nitrogen in the oxidized form, and when saturated holds about 

 700 c.c, or, approximately, i gramme of dissolved free oxygen 

 per 100 litres. These materials for purification require to be 

 supplemented by the agency of the natural bacteria, which, 

 with the almost unlimited exposure and admixture in a flowing 

 river, we may assume as certain to be present. Hence, in 

 theory, comparatively few volumes of a river water will supply 

 the requisite oxygen, which explains the well-known fact that 

 in the lower reaches of a river the dissolved impurity is only 

 a fraction of what has entered in its upper course. Dupre 

 states that, on an average, dilution with thirty volumes of fully 

 aerated water prevents sewage from fouling, and ultimately 

 purifies it. Even a less proportion, in my experience, has been 

 effectual. 



For one town, then, on the banks of a large river, or even 

 several towns, if they are sufficiently separated to allow natural 

 recovery and aeration of the stream, the elementary method of 

 discharging the sewage into the water direct has been successful 



^ Earle B. Phelps, American Teclitwlogv Quarterly, vol. xxviii., June, 1905, 

 p. 127. 



