the amount of easily degradable COD in a sample can be determinedby the oxygen consumption resulting from the sample addition tothe sludge (Spanjers and Vanrolleghem, 1995; Xu and Hasselblad,1996). This procedure is not intended to provide information on theactual denitrification potential of the sample, but to estimate theamount of easily degradable COD present. Approximately 2.5 L ofsludge was harvested the day before each measurement, and incu-bated overnight in a water bath of 27◦C under constant aerationand stirring (250 rpm). Two hours before the test, N-allylthiourea(Sigma–Aldrich Chemie B.V., Zwijndrecht, The Netherlands) wasadded at a concentration of 20 mg/L to inhibit nitrification (Xu andHasselblad, 1996)