Reduction of nutrient emission by sludge hydrolysis

Biological sludge hydrolysis was demonstrated in full scale at three Danish wastewater treatment plants. For primary sludge the hydrolysis yield expressed in terms of filtrable COD varied from 9-16% of the total COD in the sludge (WTPs 1 & 2) and for the hydrolysis of activated sludge a yield of 2.5% of the total COD was found. The addition of hydrolysate was demonstrated to improve the biological P removal considerably. No effect on the nitrogen removal could be identified due to a favourable wastewater composition during the demonstration phase. A cost-benefit analysis showed that biological sludge hydrolysis may be a cost efficient process that should be considered in connection with the upgrading of wastewater treatment plants to nutrient removal. (C) 1997 IAWQ. Published by Elsevier Science Ltd.