SEWAGE PURIFICATION 293 



that some form of catchment tank is necessary to retain the 

 suspended matter coming away from the slates. 



It was at one time thought that the clarification of sewage 

 by means of chemicals must give place entirely to biological 

 treatment of one sort or another. The findings of the Koyal 

 Commission on Sewage Disposal have, quite rightly, in the 

 author's opinion, suggested that many cases still exist where 

 this method of purification is to be preferred. Where the 

 large amount of sludge produced by chemical precipitation 

 can be easily and cheaply disposed of, and where the space 

 available for the final filtration process is limited, the total 

 expense involved will probably be less by this method than 

 by any other, owing to the longer life of the filter beds in con- 

 sequence of the small amount of suspended solids passing on 

 to them. A typical case for the application of chemicals is 

 afforded by the conditions of the sewage works at Salford. 

 Here the available area of filters is necessarily very restricted, 

 owing to the site of the works, and to maintain the high rate 

 of filtration necessary if the sewage is to be dealt with 

 thoroughly, preliminary treatment is called for. On the 

 other hand, as the sludge is sent to sea in a steamer, the 

 standing charges of which have always to be maintained, an 

 increase in the sludge production does not necessarily mean 

 a proportional increase in the cost. 



There are definite limits to the economic use of chemicals. 

 It has been shown that beyond a certain point an increase in 

 the amount of chemicals added does not produce a proportional 

 reduction in the amount of suspended matter. Further, with 

 very dilute sewages, the colloidal matter to be removed is 

 disseminated through a large volume of water, so that very 

 considerable quantities of chemicals have to be added in 

 order to precipitate it, and here again the cost is out of 

 proportion to the purifying effect obtained. Speaking gener- 

 ally, therefore, the use of chemical clarification may be 

 recommended where the sewage is concentrated, where the 



