The stripping of phosphorus, potassium and magnesium, and uptake of organic substrate and calcium was studied in the anaerobic reactors of two biological phosphorus and nitrogen removal plants. Most of the experiments were performed in batch systems simulating the function of the anaerobic reactor. Acetate-, propionate- and lactate addition to the sludge under anaerobic conditions resulted in a significant P-release. The P-release with glucose was significantly smaller. Approximate values for the ∆ P/∆COD ratios were estimated. Average ∆ K/ ∆P and ∆Mg/ ∆P values were estimated to about 0.29 and 0.25 mg/mg, respectively.

Phosphate appears to be precipitated simultaneously with the biological P-stripping. In the plant under detailed investigations, about 60% of the total phosphate is precipitated, irrespective of the total P-content in the sludge. Calcium and magnesium are the dominating counterions, the (Ca+Mg)/P molar ratio being about 1.1. Some iron appears also to be associated with the phosphate, 0.6% of the dry sludge.

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