302 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



the exact operation of mechanical devices ; in other words, a 

 large factor of safety should be provided. 



The physical conditions governing the rate of passage of 

 hquid through trickUng filters have been studied by W. Clifford 

 in the researches referred to on p. 227. 



We are now in a position to follow the changes which take 

 place in such a filter. In the first place, as in the contact 

 bed, a purely mechanical effect is exerted, and the suspended 

 and colloidal matters deposit themselves on the surface of 

 the medium. This will take place obviously to a greater 

 extent in the upper layers of the filter, and there is conse- 

 quently a limit to the depth of such filters, owing to the con- 

 centration of deposited matter in the upper layers, which will 

 take place if such effluent is poured upon them at a very 

 high rate. For this reason also, trickhng filters are better 

 adapted to deal with large volumes of dilute effluent, rather 

 than with a more concentrated Kquid, the application of 

 which results in a rapid accumulation of undigested organic 

 matter in the upper layers of the filter. In course of time 

 forms of Hfe estabhsh themselves in these filters, worms, 

 larvse, infusoria and bacteria, which maintain the cycle of 

 changes. Albuminoid substances are broken down to amino 

 compounds, and finally oxidised to nitrates. The trickhng 

 filter differs from the contact bed primarily in the predomin- 

 ance of nitrification, owing to the constant presence of oxygen 

 in its interstices. No doubt some de-nitrification takes place 

 in the interstices of the medium, but speaking generally, a 

 greater proportion of the nitrogen is recovered as nitrate than 

 in the case of a contact bed. A further great advantage 

 possessed by the trickhng fflter is that the effluent passing 

 away from it is constantly saturated with dissolved oxygen, 

 and consequently the effluents from these filters contain in 

 general a greater reserve of oxygen, available for further 

 purification in the stream into which the effluent may flow. 

 On the other hand, owing to their method of operation there 



