During sludge treatment at plants with biological phosphorus elimination, part of the stored phosphate can be released into the liquid phase of the process water and recycled back with the return water to the wastewater treatment part, resulting in an increase of the phosphorus load. From a theoretical examination it can be derived that the phosphate feedback from sludge thickening processes is lower than 5 %. A nearly complete phosphate release can be expected during sludge stabilization. Dependent on the wastewater conditions, a chemical fixation of the released phosphate in form of calcium-phosphate-precipitates or ammonium-magnesium-phosphate is possible. Therefore, the concentration of soluble phosphate in the supernatant of digester water at large biological phosphorus removal plants is often very low. The paper also describes techniques to prevent a phosphate feedback. Emphasis is placed on the chemical conditioning of the stabilized sludge or precipitation of the process water from sludge treatment.

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