CHEMICAL PRECIPITATION 



163 



Calcium carbonate ... 



Calcium hydrate (" free lime ") 



Silica (sandy matter) 



Ferric oxide 



Alumina (from clay) ... 



Phosphoric acid 



Magnesia ... 



7'94 

 2-45 

 8-o8 

 0-97 



3'39 

 0-66 

 trace 



[the total amount of wet sludge being 30 tons per million 



[gallons. 



With regard to the effluent, Mr. Dibdin estimates that the 



(organic matter put into the Thames estuary in the course of 



[twelve months is equal to only yV grain per ton of water, " but 

 Ls the organic matter probably does not last in any form longer 

 than a week, there would be at no time more than, on an 



[average, 5 J^ grain per ton of water."^ 



At Wimbledon in 1893 8*2 tons of pressed sludge cake were 

 )btained per million gallons sewage, the average for a number 



[of towns where filter-presses are used being 9*28 tons per 



[million gallons. In pressing sludge, lime is generally added to 

 lake the substance more manageable ; as much as 2 per cent. 



Bs often used. The result, as we have indicated, is a dissolving 

 )f the organic matters and an extra foulness of the pressed 

 liquid, besides the additional bulk. At Ealing, Bolton, and 

 some other places, the sludge has been mixed with town ashes 

 .nd burnt in a refuse destructor. James Ashton reports that 

 jludge cake cannot produce a clinker, but when burned either 

 falls to dust and is blown over the grate into the fines, or falls 

 >ut on breaking the hard clinker obtained from the ashpit 

 'efuse. At Birmingham the sludge was mixed with the general 

 refuse and offered as manure. 

 Although free lime tends to inhibit the multiplication of 

 licro-organisms, we have already noticed that large quantities 

 >f it, added either to the raw sewage or mixed with the sludge 

 to assist consolidation, increase the amount of organic matter 



in the effluent or in the sludge-press water, so that these 



liquids, after their alkalinity has been diminished by dilution 

 )r absorption of carbonic acid from the air, readily putrefy. 



Sludge includes the greater proportion of the organisms of 

 the original sewage, and when fresh may contain, according 



to Professor Boyce, 150 millions per c.c, but on standing the 



^ Journal of Preventive Medicine, July, 1905 ; see also Royal Commission on 

 _ Jevvage, vol. iii., 1904, pp. 47, 75, etc., as to the volume of the Thames and of the 

 "sludge discharged. 



