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PROCEEDINGS OF SZCTION H. 683 
Further tests were made with increased quantities of lime alone 
in the form of milk of lime. The precipitating action was good, 
but the quantity used to obtain this result was so large in propor- 
tion to the sewage dealt with, that the cost of the material and 
machinery for mixing and agitating together, with the increased 
bulk of the sludge, w vould far outw reigh any advantage obtained 
in clarification or purification. 
A further trial was made with milk of lime and sulphate of 
iron in the proportion of two of the former to one of the latter. 
In this case the quantity of chemicals used was such as would 
make it expensive, and preclude its use ona large scale. The 
result, however, as to a high standard of purification and clarifica- 
tion was disappointing. It was found that the effluent could not 
be let into a river, especially one from which a water supply 
would be drawn, without danger. 
In vol. XIII. of the Proceedings of the Association of 
Municipal and Sanitary Engineers, Sir Robert Rawlinson, in 
speaking on the chemical treatment of sewage, stated that “to 
purify sewage by any known chemical process you must leave 
the word purify out. When, again, as regards precipitation, 
whatever action the precipitants may have in throwing down the 
floating and floculent matter in sewage, seven-eighths of the 
salts of sewage still remain. You may clarify sewage until 
it is as bright as spring-water, and yet seven-eighths of the salts 
which act injuriously when turned out into a water-course 
remain in the clarified effluent.” 
“ At Coventry, England, where £60,000 had been lost in 
attempting to purify sewage, it was found, after a second company 
had been tormed, with a subsidy of £1200 per annum from the 
corporation, that it would have failed if they had not procured a 
portion of valuable land over which the clarified sewage could 
pass before it passed into the watercourse.” 
In all the towns that have, in the past, attempted to purify 
the sewage by chemical treatment, the greater number have 
abandoned the process for one of disposal and filtration over land, 
and in the light of experience which has been obtained in other 
lands, as well as the successful issue of the experiments at 
Adelaide and Sydney, the question of purification of sewage 
should be easily solved. Where, however, suitable land is not 
available, the question becomes narrowed down to the choice of 
chemical treatment— Aeration process and by electrolysis. 
Ill. Filtration through land is carried out by disposing of the 
sewage, after screening off coarser matters, directly over land 
specially prepared for it, or in combination with chemical or 
mechanical precipitation. Except in special cases the latter 
system is not resorted to. 
In the case of the Sydney main drainage works the sewage has 
to be screened, and the suspended matter precipitated in specially- 
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