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.