History of the Water Supply of the World / arranged in a comprehensive form from eminent authorities, containing a description of the various methods of water supply, pollution and purification of waters, and sanitary effects, with analyses of potable waters, also geology and water strata of Hamilton county, Ohio, statistics of the Ohio river, proposed water supply of Cincinnati.

by Thomas J. Bell

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:

“Subsidence does not completely clarify the water even in a fortnight or three weeks’ time.”

London has 262 acres of subsiding reservoirs for removing the turbidity of the Thames and Lea Rivers, and used as storage at times of sudden freshets.

Mere exposure to the air, even if accompanied by violent agitation, is comparatively powerless for the removal of polluting organic matter from water. Although, however, the flow of a river has thus but little effect in purifying the water by the oxidation of the dissolved organic matters, it has a most material influence in the removal by subsidence of a large proportion of the suspended impurities both organic and mineral, especially if the flow be sluggish in places.

In passing through still pools, the turbid streams let fall its load of grosser mechanically suspended particles, and thus the water becomes clearer, although the dissolved impurity remains nearly as great as ever. It is, doubtless, this clarification by subsidence which has led to the very general but erroneous belief in the rapid self-purifying power of running water.