NOVELTY - The apparatus (1) has a permeation unit (5) that is comprised so that the moisture content of fell freshwater is moved to a side of a seawater (S) through forward osmosis membrane (5a). A fresh purified water (A) is dropped in a fall portion (2) so as to purify the sewage. A separation unit (6) is configured to obtain the purified water by decreasing solid substance contained in bioreaction portion into which the organic substance in sewage is decomposed by the bioreaction of microorganisms, and a sewage (Z) after a bioreaction by solid-liquid separation.    USE - Apparatus for generating electric power using water such as seawater, fresh pond water, purified water, river water, lake water, underground water.    ADVANTAGE - The separation unit is configured to obtain the purified water by decreasing solid substance contained in bioreaction portion into which the organic substance in sewage is decomposed by the bioreaction of microorganisms, and a sewage after a bioreaction by solid-liquid separation. Thus, the electric power is produced effectively by dropping the freshwater existing in the place where above sea level is comparatively low with sufficient elevation difference. Since the freshwater is dropped to a position lower than sea surface by fall portion, the elevation difference for dropping the portion to drop freshwater to a position lower than sea surface and freshwater is enlarged. Thus, freshwater is dropped with comparatively sufficient elevation difference. The purified water in the water-storage tank in which the difference of osmotic pressure with seawater is small is discharged to the sea according to the pump for discharge when the purified water in which the osmotic pressure increased by the permeation portion is returned by the water-storage tank so as to suppress the difference of osmotic pressure of purified water and seawater in the portion. Thus, the purified water in the water-storage tank with small difference of osmotic pressure with the seawater is reduced effectively. Hence, the forward osmosis phenomenon is fully maintained by the permeation unit.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method for generating electric power.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the apparatus for generating electric power.    Apparatus for generating electric power (1)    Fall portion (2)    Permeation unit (5)    Forward osmosis membrane (5a)    Purified water (A)