294 BACTERIOLOGICAL AND ENZYME CHEMISTEY 



available filtration area is limited, and where the sludge is 

 easily disposed of. 



A further case for chemical treatment may arise where 

 special trade effluents are present. Thus, at Bilston and 

 Wolverhampton, large quantities of lime have to be added, 

 to neutralise and precipitate the acid solution of ferrous 

 chloride, or 'iron pickle,' discharged into the sewers from 

 galvanising works. 



THE FINAL PURIFICATION OF SEWAGE 



In general, as has been explained, some form of preliminary 

 treatment is necessary before sewage can be finally minerahsed 

 in biological filter beds. It is possible, however, under special 

 conditions to treat crude sewage directly on filters. Where 

 the sewage is dilute, and where considerable faU is available, 

 the Hquid, after efficient screening and removal of the coarser 

 soUds, sand, etc., in catchpits, may be directly sprayed upon 

 coarse percolating filters of considerable depth. In this case 

 the oxidation of the suspended and colloidal matters takes 

 place by much the same agencies as are at work in the slate 

 bed, above described, and the resulting granular residue passes 

 out at the bottom of the filter, and can be retained in catchpits, 

 or on the surface of sand strainers. The works at Eothwell 

 in the West Riding of Yorkshire have been successfully 

 designed on these lines. The conditions dijEEer in such a filter 

 from those obtaining in the slate bed, in that the Hquid 

 portion of the sewage passes in a thin film over the filtering 

 medium, and its soluble impurities are therefore oxidised as 

 well as the matters in suspension. If the rate of filtration is 

 not too high, it is even possible completely to oxidise fairly 

 strong sewage in this way. A periodical renewal of a portion 

 of the filtering medium is, however, hkely to be caUed for in 

 such a case. A good instance of the adequate treatment of 

 strong sewage is to be seen at Little Drayton, where a filter 



