DISPOSAL OF EXCEETA 371 



organisms which in the presence of oxygen nitrify the 

 ammonia, converting it into nitrous and nitric acids. The 

 effluent is then passed on to the land and got rid of by 

 iiltration. This process has the great advantage of leaving 

 little or no sludge to be dealt with. A fuller account of 

 the above process of nitrification will be given under ' Soils.' 



Ventilation of Drains. — In speaking of the disadvantages 

 of subsoil drainage in stables, we referred to the fact that 

 it leads to the air of the building being placed in connection 

 with the sewer, and that the result of this might possibly 

 become serious. The object of ventilating drains is to 

 prevent this occurring, and to cut off the interior of a 

 building from the interior of the sewer, by means of an 

 opening outside the building which communicates freely 

 with the outside air. Under these circumstances any reflux 

 of sewer gas would escape outside and not inside the 

 building. 



There are other means of preventing sewer gas finding 

 its way into buildings, viz., by means of traps, but traps 

 can only be considered as auxiliaries to a good drainage 

 system, and without ventilation they cannot be relied on. 

 The chief means of preventing the sewer-gas pollution of 

 buildings is by disconnecting the drains by ventilation. 



The disconnection takes place outside the building where 

 the stable drain joins the sewer drain, and is accomplished 

 by the introduction of a disconnection syphon such as may 

 be seen in Eig. 158. The principle on which this works is 

 that under ordinary circumstances the water seal in the 

 syphon is sufficient to keep back the sewer air, but in the 

 event of it not doing so, the sewer gas escapes through the 

 fresh air inlet G. 



By means of this inlet the air in the stable drain up to 

 the stable trap is quite unpolluted, so that should this trap 

 fail to act no fear of sewer gas need be apprehended. 



It is usual in large buildings, where several drains join 

 before passing to the sewer, to have a disconnecting man- 

 hole based on much the same principle. 



Traps. — A trap is placed wherever the air of a building 



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