Relationship between phosphate stress, effluent quality, and observed cell yield in a pure‐oxygen activated‐sludge plant

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Abstract
The effect of continuous and discontinuous phosphate stress on the observed cell yield coefficient was investigated in a petrochemical pure‐oxygen wastewater treatment plant. A continuous phosphate stress in the activated‐sludge system resulted in a reduction of sludge production by 36%, but no improvement of the effluent quality was noticed compared with that of nonstress treatment. However, discontinuous phosphate stress produced a high‐quality effluent due to the excellent setting settling properties of the sludge flocs. In addition, the observed cell yield coefficient and concomitantly the sludge production remained 45% below normal. The introduction of the discontinuous phosphate stress in the pure‐oxygen activated‐sludge plant at the BP Chemicals works in Antwerp resulted in a 24% reduction of the total wastewater treatment operating costs.