Sillar’s process. The A. B. C. process, so called because composed of alum, blood, charcoal, and clay.
Hill’s process. Lime and tar are the precipitants. The effluent water is filtered through charcoal. The question now arises as to whether the sewer water after treatment with any of the above substances is in a fit condition to be poured into a stream or river. The Rivers Pollution Commissioners in their first and second reports give a number of analyses, from which it appears that on an average the chemical treatment removes
89·8 per cent. of the matters suspended in the sewage waters, but only 36·6 per cent. of the organic nitrogen is dissolved in them.
Of the A. B. C. process, Mr Crookes states that, when properly carried out, it removes all the phosphoric acid; and Professor Voelcker’s analysis of the effluent water from sewage treated by the acid phosphate of alumina process gives more ammonia than the original sewer water, less organic nitrogen by one half, and less phosphoric acid. Such a water is said by some authorities to be pure enough to be discharged into streams.
General Scott’s process. General Scott proposes to treat the sewer water with lime and clay, and instead of employing the precipitate obtained by this means as a manure, would, after burning it, use it as cement. He argues that the deposit contains so much combustible matter as to considerably reduce the quantity of coal usually expended in the manufacture of cement, and consequently the cement could be sold at a remunerative price.
This, like the ‘carbonisation’ process, possesses the merit of effectually destroying any noxious principles present in the deposit.
Commenting on the various precipitation processes Dr Parkes writes:—“When the sewer water is cleared by any of these plans is it fit to be discharged into streams? In the opinion of some authorities, if the precipitate is a good one it may be so, and it appears certain that in many cases it is chemically a tolerably pure water, and it will no longer silt up the bed nor cause a nuisance. But it still contains, in all cases, some organic matter, as well as ammonia, potash, and phosphoric acid. It has, therefore, fertilising powers certainly, and possibly it has also injurious powers. No proof of this has been given, but also no disproof at present, and when we consider how small the agencies of the specific diseases probably are, and how likely it is that they remain suspended, we do not seem to be in a position to expect that the water, after subsidence of the deposit, will be safe to drink.
(5) By irrigation and filtration. By this process is meant the passing of the sewer water over and through soil, with the object not only of effecting its purification to such an extent as to render it fit to be discharged into a river or stream, but also of employing it as a valuable manure. In the present article we shall treat only of the application of the process to the first of these purposes.
There is ample evidence to show that, if carried out with due attention to detail, no process for the treatment of effluent sewage water, so as to render it innocuous, is equal to that which subjects it to irrigation and filtration.
The Rivers Pollution Commissioners thus report on it:—“We are, therefore, justified in recommending irrigation as a safe as well as