INTRODUCTION 15 



even in large towns, when sedimented in an estuary, commonly 

 forms a very thin film, and evidence of its formation is difficult 

 to find. When, on the other hand, suspended organic matter 

 is slowly removed to the river bed, and is attacked there, in the 

 absence of air and light, by the organisms naturally fitted to 

 the purpose, the products will dissolve and become available 

 for the water bacteria in the river. 



The standards of purity for sewage effluents have been fre- 

 quently modified,^ but it would seem that the conditions for 

 safe discharge into a flowing stream depend upon local circum- 

 stances now that methods have been found which, by natural 

 agencies, allow us to carry the purification to a rational and 

 harmless stage, when such factors as time, light, ^volume of 

 oxygen, and various life of a river will be more than sufficient 

 to deal with the effluent. 



Pettenkofer, from investigations on the river Isar, at Munich, 

 has concluded that if the sewage never amounts to more than 

 X5 of the river water, and the velocity of the latter is at least 

 equal to that of the former, the raw sewage may be poured into 

 the river without causing pollution. 



From observations of the Massachusetts Board of Health, 

 Rudolph Hering fixed a limit to the amount of free ammonia 

 permissible in a stream, and stated that if the flow is less than 

 2h cubic feet per second per 1,000 persons (or i gallon per 

 minute per person) "an offence is almost sure to arise," but when 

 it exceeds 7 cubic feet per second per 1,000 safety is assured. 

 " In other words, when the free ammonia is greater than 

 0'i2 part per 100,000, the conditions are probably objection- 

 able." These limits correspond to about 50 volumes of river 

 water to average sewage in England. Mr. Stearns, engineer 

 to the Board, concluded that if the average amounts to more 

 than -^Q, or 2*5 per cent., of the river water, it cannot be dis- 

 charged into the river in its raw state ; if less than jV and 

 more than -^1-^, it is doubtful ; if less than j-io) it may be admitted 

 without any doubt in its raw state into the river. These con- 

 clusions are, of course, empirical, and have not been generally 

 accepted ; they would be greatly afl"ected by the amount of 

 solid matter present in the discharge. The sewage in America 

 is much more dilute than in this country, the rivers have 

 greater volume, and the limit is much higher than we have 



1 Rideal, "Standards of Purity for Sewage Effluents," British Association 

 Reports, Bristol, 1898 ; also Chapter III. 



