Method for eliminating phosphorous contained in effluents

Process for the treatment of sewage by a biological route and a physicochemical route which employs biological dephosphatation, this process comprising stages of anaerobic, anoxic and aerobic treatments, followed by a stage of secondary clarification, the sewage being mixed with biological sludge recycled from the secondary clarification stage in a first anaerobic stage, in order to bring about release of phosphorus into a soluble phase by the poly-P bacteria, the mixture subsequently being subjected to the conventional stages of anoxic and aerobic treatments, activated sludge being separated from the treated water in the secondary clarification stage and subsequently being partially recycled in the anaerobic stage, at the head of a treatment line, and partially extracted from the system wherein:

• the concentration of soluble phosphorus in the treated water is measured at at least two different points on the treatment line:
• at the inlet of the secondary clarification stage, in order to obtain a measurement P1, and at the outlet of the treatment line, in order to obtain a measurement P2, the measurement P1 determining the doses of the product which precipitates soluble phosphorus by the physicochemical route in the liquor of at least one of the tanks where the anoxic or aerobic stages are carried out upstream of the secondary clarification;
• the measurement P2 is continuously compared with the measurement P1, in order to detect possible release of phosphorus in the secondary clarification;
• and
• when the difference in the two measurements (.DELTA.P1-P2) is found, at least one of the two following measures is triggered:
• an additional injection of precipitation is carried out before the secondary clarification, in order for precipitation of the phosphorus released during the secondary clarification stage to be able to take place by virtue of the excess of the precipitant injected;
• the flow rate for recirculation of the sludge from the clarification stage to the anaerobic first stage is modified in order to reduce its residence time during the clarification stage.