CHAPTER VII 



SUBSIDENCE AND CHEMICAL PRECIPITATION 



Screens — Settling tanks — Roughing filters — Clarification — Lime — 

 Aluminium sulphate — Ferric sulphate — Ferrous sulphate — • 

 Alumino-ferric — Sludge : its composition, volume, and disposal. 



Mechanical separation, used as an adjunct to many processes, 

 deals with suspended solids, inorganic or organic : 



(i) Grit and detritus, small stones and sand, carried down 

 largely by sewers of steep gradient, or in periods of storm, 

 under the combined system, will be always present, but even 

 under a separate system, intended to take only excretory and 

 household waste, cannot be entirely avoided. They are 

 removed by settlement without nuisance, since any entangled 

 organic matter rapidly disintegrates, as in gravel soil. They 

 collect in the street gullies, in sumps in the line of the sewers, 

 and the remainder in grit chambers at the sewage works. 

 Processes using mixing machinery require careful removal of 

 hard matters. 



(2) Organic residues — vegetable, faeces, paper, fibres, wood — 

 in great part float, owing to lightness, or to gases generated by 

 fermentation. Their inclusion or exclusion constitutes a main 

 difference, as we shall see further, between some modern 

 methods of ultimate treatment ; and the question as to whether 

 a sewage is dealt with strained, settled, or absolutely raw is a 

 matter of very great importance. 



Screening is often used to prevent the clogging of filters. The 

 screens are either cleared at intervals by hand labour or con- 

 tinuously by automatic contrivances, one of the most effective 

 being a revolving wire drum, rotated by a paddle wheel moved 

 by the current of sewage (Fig. 20). At Leeds the solids thus 

 removed, and requiring separate treatment, were estimated to 

 be thirty barrow-loads, or, say, 2 or 3 tons dail}^ mainly con- 

 sisting of faeces, paper, and vegetable residues. The screens 



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