6. Charcoal is, possibly, the best substance for removing organisms chemically; but its efficiency is destroyed by an insoluble precipitate of either lime or iron. Messrs. Adkins & Co., of London, have patented a method to overcome this objection, by use of charcoal plates that may be easily scraped.
Filtration through spongy iron (by Rivers Pollution Commission—Parts in 100,000 parts):
| ORGANIC | ORGANIC | PREVIOUS | |
| CARBON. | NITROGEN. | SEWAGE. | |
| Thames water, before | .120 | .013 | 1340 |
| Thames water, after | .025 | .004 | 10 |
Filtration through animal charcoal:
| ORGANIC | ORGANIC | PREVIOUS | |
| CARBON. | NITROGEN. | SEWAGE. | |
| Grand Junction Co.’s water, before | .164 | .030 | 320 |
| Grand Junction Co.’s water, after | .010 | .002 | 950 |
SUBSIDENCE
is the most popular method of clarification of water by the deposition of heavy matter, accomplished in large storage reservoirs.
“If the reservoir be very small and shallow, and containing not more than a day’s supply, for example, it is plain there can be but little opportunity for subsidence; but even in such cases, if the reservoir be kept full, or nearly full, the floating impurities might never enter the circulation. In the case of a large reservoir, holding many days’ supply, it is quite different. Time is then afforded for the heavier impurities to settle to the bottom; and, if the water is admitted at one end and taken out at the other end of the reservoir, very little, if any, of the heavier particles can pass into the circulation; and we can see no reason why any of the superficial impurities, such as remain on or near the surface, should ever be allowed to enter the circulation.”—(From Water Supply Commission of Engineers, Philadelphia, 1875.)
Fanning says: