SEWAGE PURIFICATION 303 



is a greater tendency for incompletely oxidised nitrogenous 

 matter to pass away from them, either in solution, in the 

 colloidal state, as granular residual ' humus,' or as debris of 

 growths formed in the filter. In the contact bed, as has been 

 shown, the oxygen of the nitrate interacts with the undecom- 

 posed oxidisable matter during the period of standing full. 



It is necessary, in the case of trickling filters, that means 

 should in all cases be provided for arresting suspended matter 

 which continuously passes away from them. For this purpose 

 either so-called ' humus tanks ' or sand strainers may be 

 employed. Especially are these necessary after the filter 

 has had a period of rest. The colloidal matter deposited on 

 the filtering medium suffers oxidation during such times, and 

 is rendered granular, and readily detaches itself in conse- 

 quence from the filtering material. At such times, therefore, 

 the effluent will contain abnormal quantities of solid matter. 

 This is also the case in spring time, when the organic life in 

 the filter is particularly active. The material which has 

 been stored during the previous months is then to a large 

 extent ejected from the filter. 



This phenomenon is further instructive, as showing that 

 the changes taking place in these filters are by no means 

 instantaneous, but take place over a prolonged period of time. 



CONDITIONS GOVERNING CHOICE OP FILTRATION METHODS 



In designing works for the purification of sewage, the 

 choice of method for the final purification of the effluent 

 must depend on a number of factors, more particularly the 

 following : (a) area of land and fall available ; (6) strength 

 of sewage ; (c) methods of preliminary treatment. The full 

 discussion of this aspect of the question involves engineering 

 considerations, which are outside the scope of this work, but 

 it is obvious that deep trickling filters can only be made use 

 of economically when there is sufficient fall between the 



