Abstract: A method of improving the phosphorus elimination capacity of a lake (11). Flexible wall (16) is extended from the bottom (21) of the lake a substantial distance toward the surface (23) of the lake at the beginning of the vegetation period of the lake to separate the lower parts of a minor portion of the lake from the lower parts of the rest of the lake. Sheets (18) are positioned between an inlet channel (12) and the flexible wall (16) as growth areas for algae. The flexible wall (16) is lowered around the end of the vegetation period to allow the flushing away of sedimented algae from the bottom of the minor portion of the lake.

Abstract: A plant for treatment of urban run off water where a body of water (50) is separated from a water recipient (60) by means of a flexible curtain (13) extending between the bottom (80) and the surface (90) of the water recipient. The bottom end of the curtain is anchored (14) to the bottom while the surface end is provided with float means (11) being movable across the surface to change the volume of the body of water.

Abstract: A device for increasing the oxygen concentration in a subsurface stratum of a thermally stratified body of water. The device comprises a flexible tube (13), which is surrounded by a flexible shell (12). Compressed oxygen-containing gas is introduced through first nozzle means (29) at the lower end of the tube and through second nozzle means (31) into channel means (34) between the shell (12) and the tube (13). There is an upward flow of water in the tube and a downward flow in the channel means (34) around the tube.

Abstract: The invention relates to a method of producing an aqueous solution rich in phosphorous and recovering metal compounds in the purification of sewage-water or raw-water, in a plant which includes at least one chemical treatment stage in which an iron compound is supplied to the water to precipitate phosphorous present therein. The method is characterized in that the sludge from the chemical treatment stage is treated with hydrogen sulphide under anaerobic conditions, phosphorous compounds being dissolved from the sludge and metal sulphide being formed. The metal sulphides are dissolved to form metal salt solutions, of which at least the iron salt solution is separated and recycled to the chemical treatment stage. It is also possible prior to separating the iron salt solutions to recycle the metal salt solutions one or more times. The dissolved phosphorous can be removed in the form of an aqueous solution and treated in accordance with the so-called Donnert-process, to form calcium phosphate.