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A three phase (aqueous/solid/gas) mixed weak acid/base kinetic model developed by Musvoto et al. is applied to simulate the physical and chemical processes that occur on aeration of anaerobic digester liquors. Included in the model are the kinetic reactions for (i) weak acid/base dissocations (water, carbonate, ammonium, phosphate, and short-chain fatty acids), (ii) precipitation of struvite, newberyite, amorphous calcium phosphate, calcium and magnesium carbonate, (iii) ion pair formation and (iv) stripping of CO2 and NH3 gases. To generate data for model application, batch aeration tests were conducted on two anaerobic digester liquors from (i) a spent wine upflow anaerobic sludge bed (UASB) digester and (a) a sewage sludge anaerobic digester. In the batch tests pH, Ca, Mg, PO4-P, free and saline ammonia (FSA)H2CO3* alkalinity (from which inorganic carbon is calculated) were measured. After establishing from the lIterature values for (i) weak acid/base equilbrium constants (pKa), (ii) weak add/base kinetic rate constants (K(ra)), and (iii) ion pair stability constants (pK(ST)), and trial and error determination of (iv) mineral solubility products (pK(SP)) (within the range reported in the literature), (v) ion pair kinetic rate constants (K(rIP)), (vi) mineral precipitation rate constants (Kppt) and (vii) gas stripping rates (K(rG)), a good correlation between predicted and measured data was obtained for all the parameters for both liquors. The solubility product values for the minerals that precipitated were the same for both liquors and fall in the range of values quoted in the literature, but the specific precipitation rate constants of the minerals differed for the two liquors.

A three phase (aqueous/solid/gas) mixed weak acid/base kinetic model is developed to simulate the physical and chemical processes that occur on aeration of anaerobic digester supernatant. Included in the model are the kinetic reactions for (i) weak acid/base dissociations (water, carbonate, ammonium, phosphate, and short-chain fatty acids), (ii) precipitation of struvite, newberyite, amorphous calcium phosphate, calcium and magnesium carbonate and (iii) stripping of CO 2 and NH 3 gasses. A preliminary validation of the model is done using data available in the literature. The model was then applied to simulate batch aeration tests on two anaerobic digester supernatants from (i) a spent wine upflow anaerobic sludge bed digester and (ii) a sewage sludge anaerobic digester. In the batch tests pH, Ca, Mg, PO 4 -P, free and saline ammonia (FSA) and H 2 CO 3 ∗ Alkalinity (from which inorganic carbon is calculated) were measured. After establishing (i) the minerals most likely to precipitate viz. struvite (MgNH 4 PO 4 ), newberyite (MgHPO 4 ), amorphous calcium phosphate (ACP), CaCO 3 and MgCO 3 and trial and error determination of (ii) the solubility products, (iii) specific precipitation rate constants and (iv) the specific gas stripping rates, a good correlation was obtained for all the parameters for both supernatants. The solubility product values for the minerals that precipitated were the same in both supernatants, and fall in the range of values quoted in the literature but the specific precipitation rate constants of the minerals were different in the two supernatants. Compared to the equilibrium chemistry approach to modelling three phase mixed weak acid/base systems, the kinetic approach facilitates (i) integration of the weak acid/base model with biological kinetic models and (ii) determination of solubility products and precipitation rates in an integrated manner for a number of minerals simultaneously from a single batch test.

A kinetic model for mixed weak acid/base systems is described. In the model, the weak acid/base equilibria have been formulated in terms of the kinetics of the forward and reverse reactions for the dissociation of the weak acid/bases. The compound H + is explicitly included and pH is calculated from the H + concentration via pH=−log(H + )=−log f m [H + ]. For the conditions considered, the weak acid/bases included are the water, carbonate, ammonium, phosphate and short-chain fatty acids systems. However, the modelling approach is completely general and can be applied to include any other weak acid/base of importance. Ion pairing, precipitation of calcium carbonate and carbon dioxide gas exchange also are included. The model was validated by comparing model predictions with those obtained from conventional equilibrium chemistry based algorithms. A very good correlation was obtained. Compared to the equilibrium approach, the kinetic approach offers several advantages in that it facilitates seamless integration of (i) precipitation, gas stripping and dissolution kinetics of minerals and gases within the mixed weak acid/base chemistry structure and (ii) the biological kinetic models for wastewater treatment systems to extend application of these models to situations where the pH is not constant.

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: 529-544