360 Decomposition and nitrification of sewage, etc. 
were in distilled water alone. But the conditions for a large 
increase of nitrifying bacteria were obviously not so good as in 
artificial filter beds, and therefore a very large production of nitrates 
was hardly to be expected. It is of considerable interest to note 
that the action of the soda was in the same direction as the salt 
solutions and the sea-water, in that it interfered with the produc- 
tion of nitrates and nitrites. The incubations produced only a 
gradual breakdown of the various complex compounds in sewage, 
accompanied by very little nitrification : whilst in the salt and the 
sea-water incubations even this small quantity of nitrification was 
not produced. It is possible that if the incubations had been 
carried on long enough, the figures for the total ammonias might 
have disappeared, as there was a continual and gradual decrease. 
And, in view of this gradual decrease, and the absence of nitrates 
and nitrites, the only explanation to account for the disappearance 
of the nitrogen is that the decomposition was slowly carried on to 
the farthest point, and that nitrogen was given off as a gas. This 
explanation will need further investigations, for no complete 
experiments have hitherto been made in this direction, although 
Professor Letts has suggested the possibility of the production of 
free nitrogen gas in his evidence before the Royal Commission on 
the Disposal of Sewage. He states that in the Belfast experi- 
mental filter beds there was practically no nitrate in the effluent 
at all, although there was a great disappearance of free ammonia : 
and that in some of his experiments he could account for 20 per 
cent, of the total nitrogen which disappears as nitrogen in the 
free state*. 
There can be little doubt, therefore, that the salts in sea-water 
do interfere with the development of useful nitrifying organisms 
whose normal action under proper conditions is to convert sewage 
into substances like nitrates. The smell often noticed when 
sewage is poured directly into the sea or an estuary appears to be 
caused by the presence of complex and varied organic compounds, 
which are only slowly and incompletely oxidised. But the final 
proof of the inhibition and disappearance of the nitrifying 
organisms is a bacterial one, and is a subject for future research. 
The above investigation was carried on in the University 
Chemical Laboratory. 
* Rogal Commission Report on Sewage Disposal, Vol. u. Evidence, pp. 482, 483, 
pars. 8605 — 8610. 
