SEWAGE PUEIFICATION 299 



period of standing empty, the greater will be the amount of 

 nitrate found, but if nitrates are present at all, it is evident that 

 the conditions are still mainly aerobic, and therefore suited 

 to the maintenance of organic life. If a contact bed becomes 

 clogged and waterlogged, not only will nitrates be absent 

 in the effluent, but very often crowds of worms will emerge 

 at the surface of the bed, seeking their necessary air 

 supply. 



An interesting apphcation of de-nitrification has been made 

 by Letts at Belfast, whose object was to produce an effluent 

 containing as little nitrogen as possible, either in the form of 

 ammonia or nitrate, in order to minimise the growth of Ulva 

 htissima, which was found to derive its nitrogen equally well 

 from either of these sources. Letts purified a portion of his 

 effluent by means of trickling filters in order to obtain as high 

 a yield of nitrate as possible ; this nitrified effluent was then 

 mixed with the remainder of the unfiltered effluent, and the 

 mixture treated on a de-nitrifying bed. The nitrates in the one 

 portion interacted with the organic matter in the other, with 

 ehmination of nitrogen, and production of a purified effluent, 

 containing a minimum of nutriment for the Viva. 



(b) Trickling or Percolating Filters. — The operation of a 

 trickhng filter differs from that of a contact bed, in that the 

 liquid is applied to it in such a way that it flows over the frag- 

 ments of filtering medium in a thin film, and the oxidation pro- 

 cess is consequently continuously proceeding. It is in this 

 sense that the trickhng filter may be spoken of as a continuous 

 filter, as distinguished from a contact bed, whose operation 

 is intermittent, and clearly divisible, as we have seen, into 

 two distinct processes. Mechanically speaking, it is doubtful 

 whether a really continuous filter has yet been constructed. 

 When a sewage effluent, e.g., is sprinkled upon a trickhng 

 filter by an ordinary rotary distributor, the operation is 

 rcaUy, of course, a discontinuous one, each element of surface 



