Biomass was enriched in a 5 l lab-scale sequencing batch reactor (SBR) seeded with sludge from the Caboolture Sewage Treatment Plant, Qld, Australia. The SBR was operated with a cycle time of 6 h consisting of a 117 min non-aerated and a 190 min aerated period, followed by 43 min settling and 10 min decant. The oxygen concentration during the aerobic period was measured with an on-line dissolved oxygen probe (YSI 5739, Yellow Springs Instruments, USA) and kept between 11 and 16 μmol l−1 by on/off control of air sparing at 300 ml min−1. Three litres of synthetic wastewater containing 3.84 mmol l−1 (230 mg l−1) acetic acid, 1.64 mmol l−1 (23 mg l−1 N), 0.58 mmol l−1 (18 mg/l P), and 0.525 ml micronutrient solution  were pumped into the reactor in the first 7 min of the anaerobic period, resulting in concentrations in the reactor at the start of the cycle of approximately 2.3 mmol l−1 acetate, 0.98 mmol , and 0.35 mmol . After the settling period, 3 l supernatant was removed, resulting in a hydraulic retention time (HRT) of 10 h. The solids retention time (SRT) was kept at about 15 days. The pH in the system was recorded but not controlled, and fluctuated between 7.0 and 7.5. Biomass concentration, biomass content of glycogen and PHA, effluent nitrogen and phosphorus, as well as phosphorus release at the end of the anaerobic period were monitored twice weekly. Cycle studies were carried out weekly by taking the samples every 20–30 min within a 6-h cycle. Ammonium, nitrate, nitrite, orthophosphate, glycogen, PHA, mixed liquor suspended solid (MLSS), and mixed liquor volatile suspended solids (MLVSS), were analysed as described in Zeng et al. . The on-line oxygen data were used to calculate the oxygen uptake rate (OUR) during the aerobic period. In the first 1.5 h of the aerobic period (from 2 to 3.5 h of the cycle), air was sparged continuously because the concentration did not reach the upper set point due the high oxygen uptake. The OUR during this period was calculated as OUR =KLa· (C*−C), where KLa is the oxygen transfer coefficient (estimated 3 h−1, R. Zeng, unpublished data), C* is the oxygen concentration at saturation (270 μmol l−1 in this study) and C is measured oxygen concentration. During the rest of the aerobic period (from 3.5 to 5 h of the cycle), oxygen sparging was turned off every time the concentration reached the upper set-point, and the OUR was calculated from the oxygen consumption rate measured when oxygen sparging was off: OUR = (Ct2−Ct1)/(t2−t1), where Ct is the oxygen concentration at the time (t).