THE FIRST GERMAN MUNICIPAL EXPOSITION 629
The disadvantages of this system, and the danger of serious con- tamination, have brought the municipality to realize the need of the use of ground water. Work has been begun, upon an entire new system, with this end in view. Hamburg uses water from the river Elbe, filtered, as above, through sand. Apparently no Ger- man municipality is reckless enough to use the contaminated water of a navigable river without first filtering it through sand filters. The impurities are all caught before they have passed through the fine sand, and, indeed most of them settle in a thin layer upon the top. A certain amount of impure matter, filling the interstices between the grains of sand, makes the filter more effective ; but an excessive amount impedes the rapid filtration of the water. Hence the necessity for cleaning the filters. Konigsberg exhibited a model showing how the top of the layer of sand, containing the impurities, is removed in wheelbarrows, cleaned, and replaced, In this manner twelve men are occupied for eight hours in clean- ing each one of the four filters a surface of about 1,650 square yards (the filters in Hamburg are each about 9,000 square yards in extent).
As in Breslau, so also in Berlin and in Hamburg, the dis- advantages of using river water for drinking purposes, especially when it is drawn from rivers devoted to navigation, have been recognized. In both Berlin and Hamburg, therefore, experi- ments and preparations have been undertaken, looking to the supplying of those cities with ground water.
In the last decades the storing of water, by damming mountain valleys, has commended itself to certain municipalities. Chem- nitz, Solingen, and Barmen are examples. Barmen, though for a long time supplied exclusively with spring water, now obtains water from mountain valleys. The natural gravity pressure of the water carries it to the old water-works, where it is filtered. Plauen i. V., at present supplied with spring water from six sources, is planning to erect a dam and filter works in the Geigenbach valley. The water will be distributed, by gravity pressure, through the existing water-pipe system.
Statistics presented by Dortmund population, 140,000 are
