Pilot plant studies for phosphorus removal with settled domestic waste water at low loadings resulted in 40–50 % P-removal. Addition of 100 mg acetic-acid-COD per liter waste water at low sludge loadings showed a P-removal of more than 97,5 %, even at long periods of rainfall. Instead of dosing acetic acid, experiments were carried out by prolongation of the sludge retention time in the anaerobic zone to promote production of low fatty acids. Half an hour stirring followed by three hours non-stirring of the anaerobic zone improved the P-uptake to 79 %. Seeing that the last-mentioned method did not lead to P-effluent values of less than 1 mg/1 biological P-removal experiments were carried out supported by partly P-stripping of the sludge. The P-effluent content dropped to a constant value of 0,4 mg P/l or more than 97,5 % P-removal. Also during long periods of rainfall the P-effluent value remained 0,4 mg P/l. The investigation is in full progress and focused on studying parameters to shorten the sludge retention time in the stripper tank.

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