54 



and one of the most interesting experiments is now being tried 

 by Mr. W. Webster, F.C.S., who by means of electricity only, 

 has on a small scale, succeeded in throwing ii]) the solids to the 

 smface in a few minutes and leaving a clear water below. He, 

 however, is asking permission to try his process on the London 

 Sewage at Crossness, and it will be interesting to see the result. 



The treatment of Seivage naturally divides itself into three parts. 

 (1) The separation of solid and suspended impurities from the 

 liquid portion. (2) The purification of the effluent water. (3) The 

 treatment and disposal of the semi-solid residue, called " Sludge." 

 These processes require large impervious tanks to receive the 

 Sewage at the outfall works, and machinery to carry on pumping 

 and other operations. 



Subsidence is the simplest form of separation during a period 

 of rest in the tanks, but it is found that much of the lighter 

 suspended matter and all the matters in solution passed into the 

 river and rendered it highly putrid, besides wasting most of the 

 valuable fertilizing ingredients. 



Strainind or Mechanical Filtration has been tried. Gravel was 

 used at Ludlow and East Barnet ; gorse and broken stones at 

 Ulverston ; charcoal, coke and ashes at Bacup, and other places ; 

 an iron slag filter at Merthyr Tydvil ; straw, cocoa-nut matting, 

 peat, unglazed porcelain, &c., have been used, but they very soon 

 choked up and all of them permitted the escape of a foul effluent 

 which could not be tolerated by the Local Government Board. 



Chemical Precipitation has greatly assisted in the separation of 

 the solid portions and part of the matters in suspense and 

 solution ; and is now generally practised. The action is partially 

 mechanical and partially chemical, forming insoluble compounds 

 which sink with the solids. A curious result, however, often 

 occurs, namely, that the chemical introduced has actually in- 

 creased the impurity of the liquid j)ortion by dissolving sub- 

 stances that were before only in suspension. Some substances 

 such as oils, fats, essences, salt, and gas-works refuse, resist 

 precipitation, and it is greatly to be regretted that the ammoniacal 

 salts and alkaline nitrates and phosphates are not precipitable, as 

 their retention would greatly increase the value of the sludge as 

 manure. 



The precipitants most generally used are lime, iron and 

 alumina, besides these, permanganate of potash, ferrous carbon, 

 charcoal, ashes, clay, black ash, tar, &c., have been employed. 

 Lime at present seems to be the sheet-anchor of nearly all 

 Sewage Works, either alone or with some of the salts of iron 

 or alumina ; the proportion used varying from 5 to 10 or 15 

 grains per gallon. 



The effluent water, after the best chemical treatment, may be 

 clarified, but it is not purified, and contains three-fourths of the 



