THE USE OF UNFILTERED SURFACE-WATERS.
The use of surface-water without filtration in Europe is comparatively limited. In Germany this use is now prohibited by the Imperial Board of Health. In Great Britain, Glasgow draws its supply unfiltered from Loch Katrine; and Manchester and some other towns use unfiltered waters from lakes or impounding reservoirs the watersheds of which are entirely free from population. The best English practice, however, as in Germany, requires the filtration of such waters even if they are not known to receive sewage, and the unpolluted supplies of Liverpool, Bradford, Dublin, and many other cities are filtered before use.
THE USE OF GROUND-WATER.[65]
Ground-waters are extensively used in Europe, and apparently in some localities the geological formations are unusually favorable to this kind of supply. Paris derives all the water it now uses for domestic purposes from springs, but has a supplementary supply from the river for other purposes. Vienna and Munich also obtain their entire supplies from springs, while Budapest, Cologne, Leipzig, Dresden, Frankfurt, many of the great French cities, Brussels, a part of London, and many other English cities derive their supplies from wells or filter-galleries, and among the smaller cities all over Europe ground-water supplies are more numerous than other kinds.
APPENDIX X.
LITERATURE OF FILTRATION.
The following is a list of a number of articles on filtration. The list is not complete, but it is believed that it contains the greater part of articles upon slow sand-filtration, and that it will prove serviceable to those who wish to study the subject more in detail.
Anklamm. Glasers Annalen, 1886, p. 48.
A description of the Tegel filters at Berlin, with excellent plans.
Baker. Engineering News.
Water purification in America: a series of descriptions of filters, as follows: Aug. 3, 1893, Lawrence filter and description of apparatus of screening sand and gravel; Apr. 26, 1894, filter at Nantucket, Mass.; June 7, 1894, filters at Ilion, N. Y., plans; June 14, 1894, filters at Hudson, N. Y.; July 12, 1894, filters at Zürich, Switzerland, plans; Aug. 23, 1894, filters at Mt. Vernon, N. Y., plans.