i6 SEWAGE AND ITS PURIFICATION 



found necessary in England. The usual water-supply in the 

 United States is given by Mr. Fuller in 1904/ as from 50 to 

 300 gallons per head, averaging 135 gallons. The mean rain- 

 fall is about twice as great as in England. 



It is possible, however, to form an estimate as to the amount 

 of sewage which can be dealt with by a flowing stream if one 

 remembers that the bacteria, always naturally abundant in 

 river water, are able by^ the aid of the oxygen dissolved from 

 the air to oxidize more or less rapidly any ammonia or organic 

 matter that may be present. That the volume of the sewage 

 and the oxygen required by the organic matter in it as measured 

 by permanganate — i.e.^ the standard figure of ''oxygen con- 

 sumed " — should bear some relation to the free dissolved 

 oxygen in the river and the flow of the river is obvious. But 

 it is also desirable to take into account the amount of available 

 oxygen as nitrate and nitrite, since it has been proved that, 

 always with the help of bacteria, the oxygen of nitrates and 

 nitrites is available for the burning up of organic matter. 



From these factors the following formulae may be deduced. 

 Where X is the flow of the stream, O the amount of dissolved 

 oxygen, S the volume of effluent, M the '* oxygen consumed" 

 by the latter, N the available oxygen as nitrate and nitrite, 

 C the ratio between the amount of oxygen in the stream and 

 that which is required to oxidize the organic matter in the 

 effluent, then the equivalent will be — 



XO = C(M-N)S. 



Where the sewage is fresh, and no nitrates have been formed, 



XO = CMS. 



If N be less than M, M — N = the deficit of oxygen in the 

 effluent, requiring to be supplemented by the free oxygen in the 

 river ; such an eflluent will throw a burden on the river, and 

 cannot be considered in a satisfactory state, and it will be a 

 question of volume and other circumstances whether it can be 

 permitted to be discharged at all. This may be determined by 

 the consideration that if the available oxygen of the river, XO, 

 be greater than the demand (M — N) S, there will be a chance 

 of the stream dealing with the inflowing liquid; but if the 

 reverse be the case, foulness will necessarily accrue. Or, as 



^ Transactions of the Ameiican Society of Civil Engineers, liv., part E, 1905. 



