268 



SEWAGE AND ITS PURIFICATION 



Separate Zones. 

 It has been already pointed out how in ordinary bacteria beds 

 the natural reactions are somewhat fortuitously reversed and 

 confused, according to the periods of filling or rest, the fault 

 being caused by mixing all the different bacteria in one or two 

 large filters. 



L M 



^ZZI-2 





A, Sewage inlet ; B, C, Grit chamber and Sump ; D, Cultivation tank ; E, 

 grating ; F, G, underspace ; H, Outlets to channel J ; K, L, valves ; M, N, 

 Tippers ; P, Nitrifying trays ; Q, exit. 



Fig. 29. — AsHTEAD Cultivation Bed and Zonal Tray Filters. 



By using a series of smaller, separate areas, and passing the 

 effluent continuously and progressively through them, with 

 ample opportunity for the access of the air where it is required, 



