SUBSIDENCE 



153 



[of stones, pebbles, and coarse gravel. Although it is claimed 

 that the "function of the strainers is merely mechanical sedi- 



lentation," they also perform a bacterial office, as can be 

 fudged from his report. All materials used — stones, broken 



>ricks, coal, ballast, or large coke — exert at first simply a 



lechanical action, but after a time develop coatings of organ- 

 isms which greatly extend their effect. The Massachusetts 



Report stated that "with the gravels and sands, from the 

 ;oarsest to the finest, we find that nitrification takes place in 

 ill, when the quantity of sewage is adapted to their ability, and 

 ihe surface is not allowed to become clogged by organic matter 

 to the exclusion of air." 



Subsidence and Mechanical Clarification. 



After any method of straining, sewage remains turbid from a 

 large quantity of suspended matter, which, as shown in the 



icond chapter (p. 28), contains about one-third of the organic 



litrogen and half the carbonaceous matter of the sewage. 



jSettling basins were once almost the only means of clearing a 



jtrained sewage, the deposit being at intervals cleaned out and 



thrown on land, or even into the nearest ditch or watercourse. 



'he deposition was sometimes supplemented by adding clay, 

 Lshes, slag, shale, peat, or charcoal, so that these in settling 

 lown should entangle the solid impurities. But except with 



^eak liquids the result was not good, as the fermentation kept 

 the organic matter in suspension, and a nuisance was also 

 )ccasional. 



P. F. Frankland's experiments in 1885 on clarification with 

 :halk, animal charcoal, coke, spongy iron, china clay, brick 

 lust, plaster of Paris, oxide of manganese, etc., with special 

 reference to the removal of organisms, showed that although 

 luspended matters were at first carried dow^n, they rose again 

 lubsequently, and the organisms, particularly those which were 



lotile, multiplied in the liquid.^ Kriiger, in i88g, confirmed 

 these conclusions.'^ Therefore, at a time when it was sought to 

 remove all micro-organisms from sewage, mechanical clarifica- 

 tion was proved to be unsatisfactory, and it had little or no 

 effect on organic matter in solution. 

 Moreover, any such system results in the formation of 



^ Proceeding's of the Royal Society, 1885 ; Proceedings of the Institute of Civil 

 'iigineers, 1886. 

 2 Zeits.f. Hygiene, vii., 86. 



