The potential for PHB (poly-beta-hydroxybutyrate) to serve as the electron donor for effective simultaneous nitrification and denitrification (SND) was investigated in a 2-L sequencing batch reactor (SBR) using a mixed culture and acetate as the organic substrate. During the feast period (i.e., acetate present), heterotrophic respiration activity was high and nitrification was prevented due to the inability of nitrifying bacteria to compete with heterotrophs for oxygen. Once acetate was depleted the oxidation rate of PHB was up to 6 times slower than that of soluble acetate and nitrification could proceed due to the decreased competition for oxygen. The slow nature of PHB degradation meant that it was an effective substrate for SND, as it was oxidised at a similar rate to ammonium and was therefore available for SND throughout the entire aerobic period. The percentage of nitrogen removed via SND increased at lower DO concentrations during the famine period, with up to 78% SND achieved at a DO concentration of 0.5 mg L-1. However, the increased percentage of SND at a low DO concentration was compromised by a 2-times slower rate of nitrogen removal. A moderate DO concentration of 1 mg L-1 was optimal for both SND efficiency (61%) and rate (4.4 mmol N. Cmol X-1 . h(-1)). Electron flux analysis showed that the period of highest SND activity occurred during the first hour of the aerobic famine period, when the specific oxygen uptake rate (SOUR) was highest. It is postulated that a high SOUR due to NH4+ and PHB oxidation decreases oxygen penetration into the floc, creating larger zones for anoxic denitrification. The accumulation of nitrate towards the end of the SND period showed that SND was finally limited by the rate of denitrification. As PHB degradation was found to follow first-order kinetics (df(PHB)/dt = -0.19 . f(PHB)), higher PHB concentrations would be expected to drive SND faster by increasing the availability rate of reducing power and reducing penetration of oxygen into the floc, due to the corresponding increased SOUR. Process control techniques to accumulate higher internal PHB concentrations to improve PHB-driven SND are discussed. (C) 2003 Wiley Periodicals, Inc.