8l8 THE AMERICAN JOURNAL OF SOCIOLOGY
farms, they have been adopted by many of the German munici- palities. Berlin, Breslau, and Dortmund exhibited models of their Rie self elder, and plans and drawings were exhibited by Mag- deburg, Konigsberg, Liegnitz, and Munster.
In preparing the sewage farms, the land must be built into "rolling" fields. Large canals with smaller branches lead the sewage to various parts of the fields. The water led to the fields is a murky but inoffensive effluent ; it has already been somewhat purified by the settling and skimming processes, and sometimes has been purified in subsidence basins as, for example, in Dortmund, and in Cottbus, where it is treated by both the mechanical and the chemical processes. The sewage is led along the ridges in the fields. A drainage system of under- ground pipes collects the water after it has sunk into the soil, and leads it, now purified, to the ditches which alternate with the ridges. The ditches empty into some nearby stream. The greatest question with regard to the operation of the sewage farms presents itself in the rainy season. The fields are already saturated, and the draining streams are full ; yet an increased amount of fluid must be cared for. The adoption of the double- conduit system of sewers seems to be a solution of this problem. Frosty weather presents no insurmountable difficulties, for the freezing-point of the sewage is low, and there is no drainage water in the sewers. It should be remembered, for the sake of com- parison, however, that the climate of Germany is much milder than that of our northern states.
Usually the land is owned by the municipality, and the farms are rented. In Konigsberg the land is owned privately. Dort- mund established her sewage farms in 1898. She paid 1,500,000 marks for the land, 1,000,000 for pipes leading to the fields, and 4,000,000 for ditching and draining of the fields.
6. Utilization of the sludge. The material precipitated in the process of purification may be buried in trenches to serve as fer- tilizer, or dried, by heat or centrifugal process, pressed into cakes, and sold or given away. The cakes may be used for (a) fertilizer, () fuel, (c) humus for making gas (e. g., the Dietz process of mixing with brown coal, as exhibited) , or (d) the
