Integrated biological, chemical and physical processes kinetic modelling Part 1 – Anoxic-aerobic C and N removal in the activated sludge system

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. (1997, 2000a,b,c) 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 CO2 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 kLA value for the aeration system, which affects the pH in the anoxic and aerobic reactors through CO2 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 CO2. The performance of a ND activated sludge system with low influent alkalinity is evaluated. Keywords: Activated sludge, weak acid/base chemistry, integrated modelling, N removal Water SA Vol. 31(4) 2005: 529-54

Origin, causes and effects of increased nitrite concentrations in aquatic environments

Literature frequently mentions increased nitrite concentrations along with its inhibitory effect towards bacteria and aquatic life. Nitrite accumulation has been studied for decades, and although numerous causal factors have already been commented on in literature, the mechanism of nitrite accumulation is not always clear. From the broad range of parameters and environmental factors reviewed in this paper, it is obvious that the causes and consequences of nitrite accumulation are not yet completely understood. Among others, pH, dissolved oxygen, volatile fatty acids, phosphate and reactor operation have been found to play a role in nitrite accumulation, which results from differential inhibition or disruption of the linkage of the different steps in both nitrification and denitrification. In the case of nitrification, this differential inhibition could lead to the displacement or unlinking of the ammonia oxidisers and nitrite oxidisers. In this paper, the idea is formulated that the nitrifier population forms a role model for the total microbial community. Increased nitrite concentrations would in this aspect not only signal a disruption of nitrifiers, but possibly also of the total configuration of the microbial community. [KEYWORDS: denitrification, nitrification, nitrite accumulation, nitrite toxicity]