212 MICRO-ORaANISMS IN WATER 



' This question of the quality of the organic matter 

 left by precipitation has not been sufficiently investi- 

 gated, but the indications are, that it is more objection- 

 able than the same amount in the effluents from sewage 

 filtration through sand, but less objectionable than that 

 in sewage. 



' Wlien untreated sewage is put into a small stream 

 or pond, it often happens that the suspended matters 

 settle out, forming considerable deposits, which, putre- 

 fying out of contact of the air, give rise to very offensive 

 gases. It is hardly probable that well-precipitated 

 sewage would do this, for almost no suspended organic 

 matter is present when it leaves the settling tank, and 

 very little soluble matter is precipitated on exposure to 

 the air. 



'Another nuisance which might be caused by putting 

 precipitated sewage into a stream or pond is the growth 

 of algae — green plants fed by the ammonia of the sew- 

 age. It may be said, however, that this growth would 

 be no greater than that caused by the crude sewage, 

 and probably not much greater than that caused by 

 filtered sewage ; for, in the latter case, while the 

 ammonia is removed, nearly an equivalent of nitrate is 

 formed, and this serves as food for algae almost as 

 readily as ammonia. A number of fishes w^ere put into 

 precipitated sewage. In each case the fish died within 

 five minutes. This sudden death cannot be due to the 

 chemicals used, for it was found that the fishes lived 

 for a considerable time in solutions of the chemicals 

 much stronger than those present in the sewage. The 

 fishes died for want of air. Sewage contains no dis- 

 solved oxygen, and, if any is absorbed from the air, it 

 is quickly taken up by the organic matter. The pre- 

 cipitated sewage also contains no oxygen. 



' Using lime as a precipitant, we have found that 



