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Experiments have been performed to investigate the hypothesis that the alternation of anoxic and aerobic conditions is the major factor influencing filamentous bulking in low F/M systems. The results provided strong supporting evidence for the hypothesis; nitrite, rather than nitrate, appears to play a dominant role in causing the bulking.

The biological kinetic Activated Sludge Model No. 1 (ASM1, Henze et al., 1987; Dold et al., 1991) for carbon (C) and nitrogen (N) removal is integrated with the mixed weak acid/base model of Musvoto et al. (1997Musvoto et al. ( , 2000a to extend application of ASM1 to situations where an estimate for pH is important. Because chemical precipitation is generally not significant when treating municipal wastewaters for C and N removal, only gas and liquid phase processes were considered for this integrated model. The biological processes in ASM1 were modified to take into account the effect of the interaction of the weak acid/ base species of the ammonia, carbonate and phosphate systems and pH on heterotrophic and autotrophic organism behaviour, which includes generation and utilisation CO 2 in metabolism, use of specific weak acid/base species for organism growth and generation and utilisation of H + . With these modifications, simulations with the model were compared with those of ASM1 and experimental data in the literature; a good correlation was obtained. However, these comparisons are only a preliminary validation, because, despite their inclusion, the weak acid/bases and pH do not have a significant effect on the biological processes in the cases considered (i.e. well buffered wastewater). A difficulty in calibrating this model is selection of the k LA value for the aeration system, which affects the pH in the anoxic and aerobic reactors through CO 2 gas exchange. Aerobic reactor outflows from two full-scale wastewater treatment plants with fine bubble aeration systems were found to be around 20% supersaturated with CO 2 . The performance of a ND activated sludge system with low influent alkalinity is evaluated.