NOVELTY - The biological method comprises: subjecting wastewater to an anaerobic treatment so that phosphorus is released by bacteria, and then to an anoxic treatment and an aerobic treatment so that the phosphorus is reabsorbed by the bacteria; then subjecting to an operation of clarification; determining if the wastewater contains sulfides; performing a treatment of dephosphatation without preliminary treatment of sulfides if a sufficient presence of sulfides of 5 mg/l is detected; and adding sulfides to the waste water if an insufficient presence of sulfides of lower than 5 mg/l is detected.    USE - The biological method is useful for removing phosphorus from wastewater (claimed) such as urban or industrial wastewater.    ADVANTAGE - The biological method is capable of economically removing the phosphorus from the wastewater without generating odors, and reducing the consumption of additives such as metal salts and limiting dimensions of the treatment installation.    DETAILED DESCRIPTION - The biological method comprises: subjecting wastewater to an anaerobic treatment so that phosphorus is released by bacteria, and then to an anoxic treatment and an aerobic treatment so that the phosphorus is reabsorbed by the bacteria in quantity higher than that of previously released; then subjecting to an operation of clarification; determining if the wastewater contains sulfides; performing a treatment of dephosphatation without preliminary treatment of sulfides if a sufficient presence of sulfides of 5 mg/l is detected; adding sulfides to the waste water if an insufficient presence of sulfides of lower than 5 mg/l is detected; transforming sulfates present in the wastewater into sulfides before the anaerobic treatment and without preliminary treatment of sulfides; and mixing the wastewater ahead the anaerobic treatment and recirculating sludge from the clarification operation. A temperature of the wastewater during the treatment is maintained at 20-40 degrees C. The flow of recirculation of sludge is equal to half of the wastewater flow to be treated. The method further comprises recirculating liquor leaving the aerobic treatment between an output of the anaerobic treatment and an input of the anoxia treatment, and adjusting the content of sulfides by injecting sulfides in an anaerobic zone. The flow of the recirculation of liquor is least equal to half of the wastewater flow to be treated. A content of sulfides in the wastewater is 10 mg/l. A regulation of an external sulfide addition is carried out according to a phosphorus content in the treated water. The sulfides content is adjusted by an internal production of sulfides in the wastewater. The internal production of sulfides is performed in a sulfate reducing bioreactor upstream of the anaerobic treatment and fixed at a mobile bed or a bio disk, and by converting the sulfates into sulfides through an organic material in the wastewater. The internal production of sulfides is carried out by hydrolysis of primary mud upstream of the anaerobic treatment. The biogas rich in sulfide is injected in the anaerobic zone. An INDEPENDENT CLAIM is included for a biological Installation removing phosphorus from wastewater.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a biological installation for removing phosphorus from wastewater.    Anaerobic reactor (1)    Pre-anaerobic reactor (3)    Aerobic reactor (7)    Clarifier (9)    Loop of liquor recirculation. (11)