SEWAGE PURIFICATION 295 



on the plan devised by the late Colonel Ducat, has been in use 

 for many years, and has been reported upon by the Royal 

 Commission. 



In the majority of cases it will probably be necessary, or 

 at any rate preferable, to adopt some form of preliminary 

 treatment for the sewage, before its final purification on 

 filter beds, and we may now consider the changes which take 

 place when such partially treated sewage is applied to filters. 



The artificial niters in general use are of two types, which 

 may be broadly divided, according as the sewage is applied 

 intermittently or continuously, into 



(a) Contact beds ; 



(6) Percolating or trickling filters. 



Primary 

 Contact Bed 



a,,.- 



Valve 



Fia. 27. CONTACT FILTER BED. 



(a) Contact Beds. The general design of a contact bed 

 is seen in Fig. 27. It consists of a water-tight tank, generally 

 of concrete, filled with filtering material carefully screened 

 and graded to a definite size. The essentials of such a 

 material are that it should be durable, that is, not likely to 

 crumble on use, and should expose as large a surface as 

 possible. Hard well-fused clinkers fulfil this condition most 

 perfectly, but other materials may also be used, if good 

 clinker is not available. Effluent from the preliminary process 

 is passed on to such a bed, and allowed to remain in contact, 

 generally for about two hours, and then run off ; and if not 

 sufficiently purified, submitted to similar treatment on another 

 bed, at a lower level. The material in the second contact 



