THE FIRST GERMAN MUNICIPAL EXPOSITION 817
tions of Frankfurt, the authorities decided upon rebuilding and enlarging their old plant without altering the system.
Chemical precipitation is similar to the mechanical, except that chemicals, such as lime, aluminium sulphate, and iron sulphate, are introduced to accelerate and further the precipitation. Leip- zig, Halle, and others presented this method.
Biological purification is accomplished by permitting the various organisms in the sewage to annihilate each other, and thus make the water harmless. The plan is still in its infancy, and in its present state of development is too expensive for the purification of great quantities of sewage. The soundness and practicability of the method were shown by the exhibit of a manufacturing firm, presenting a purifying plant in operation. Live gold fish were kept in the cleared fluid during the Exposition. Aachen showed plans for a new plant, costing 25,000 marks, where the method is to be tested. What future results will be brought forth cannot, of course, be foreseen.
Filtration is accomplished by using the sewage for irrigating (and fertilizing) specially prepared fields. The water sinks into the ground, leaving therein its foreign contents, which act as fertilizer. As the land selected is usually poor, sandy soil, the enrichment thereof is a distinct gain, and the water, which is drained off from the fields, has been purified by the process. Filtration by means of sewage farms affords the most complete available purification for sewage ; but certain conditions are neces- sary. A large tract of land is required ; according to German experience, the area should approximate that of the rather com- pact city to be served. The farms must be near the city, in order that the sewage need not be piped great distances ; and yet they must not be so near as to interfere with the spreading growth of the city. The land must be level, or only slightly sloping ; and the soil must be light above, and percolable below preferably, sandy topsoil and gravelly subsoil. The Rieselfelder of Charlot- tenburg (near Berlin) lie, relatively, so high that the drainage from them raises the ground water level and floods lower fields. To counteract this, Charlottenburg has to drain away the ground water. In spite of the many conditions necessary for sewage
