602 



ftEPORT— 1901. 



Table 1, the Leeds Report on Sewage Disposal, Decemter 1898, Table 1, and 

 partly from the table (p. 68) given in Dibdin's book on the ' Purification of Sew- 

 age and Water.' 



It is quite evident, therefore, that a considerable portion, and in most cases the 

 greater part, of the unoxidised nitrogen which disappears must be got rid of in 

 some other form, and the question arises as to how this may occur. In all prob- 

 ability there are two — and only two — alternative ways in which the nitrogen can 

 be lost, viz. — 



(1) It may escape in the gaseous state as free nitrogen, or possibly as oxides* 

 of nitrogen. 



(2) It may pass into the tissues of animals or vegetables, the former of which 

 may escape from the bacteria beds, and the latter (and possibly the former also) 

 may remain permanently in the beds. 



In other words there may be either a chemical or a biological explanation, ot 

 both together. 



Chc7nical Explanation. — In an investigation ou the effects of double contact 

 with bacteria beds on screened and settled sewage the authors made analyses of 

 the dissolved gases present, both in the original sewage and in the effluent from 

 both beds, the samples being collected in such a manner that they did not come 

 into contact with the air. 



The general results of these analyses were as follows : — (1) Practically no 

 oxygen was present, either in the sewage or effluents. (2) Tile effluent from 

 first contact always contained considerably more carbonic anhydride than the 

 original sewage, and with two exceptions the effluent from second contact also 

 contained an excess of that gas. (3) In eleven out of twelve series of analyses the 

 quantit}' of nitrogen in the effluent was in excess of that present in the original 

 sewage, and generally speaking it was in larger excess in the effluent from double 

 contact than in that from single contact. 



As the first six series of analyses only were made under exactly the same con- 

 ditions, the authors find that, taking them as the basis of calculation, on the average 

 the excess of nitrogen in the effluent from second contact over that present in the 

 sewage amounted in weight to 0-272 part per 100,000, while the loss of un- 

 oxidised nitrogen which had occurred in the sewage (by Kjeldahl's process) 

 amounted to2'2 parts, or that 12 per cent, of the nitrogen lost from the sewage 

 during purification was thus accounted for, while in one particular case it amounted 

 to 31 per cent. 



In all probability only a fraction of the free nitrogen actually evolved would 

 be retained by the effluent, the rest escaping into the air. 



Biological Explanation, — As regards the possibility that nitrogen is lost bio- 

 logically, i.e., is absorbed into the tissues of animals or plants which feed on the 

 sewage, there can be no doubt that a portion does escape in that way. The bacteria 

 beds at Belfast and elsewhere swarm with minute insects [Podura aquatica). 

 These, escaping in myriads, often form a thick layer on the surface of the effluent, 

 which looks like soot. There can be no question that in thus escaping these 



