Modelling of ionic interactions with organic components in wastewater.
In current biological wastewater treatment models, physico-chemical processes (ionic speciation reactions, gas-liquid exchange, and liquid-solid interactions such as precipitation and adsorption) either are not explicitly considered, or are incorporated as simplified descriptions. This may result in an inaccurate prediction of digester behaviour. Specifically, the ionic behaviour of biomass is not explicitly included in standard models. The objectives of this study were to develop a model component that describes ionic behaviour of biomass, use this to predict the overall solution pH buffering capacity and determine its impact in an anaerobic digester’s operating range (pH 6-8). The study hypothesises that the ionic behaviour of biomass can be described in terms of glycine equivalence; alternatively, it can be described by a model component consisting of functional groups characterised by concentration per unit mass of sludge and pKₐ value for each group, either at equilibrium conditions, or considering kinetic effects. The methodology involved constructing a mass balance / ionic speciation model capable of simulating alkaline and acidimetric experimental titrations with modifications for each hypothesis. Varying concentrations of glycine or suspensions of biomass (particulate organic matter) in background salt solutions were titrated and the model was fitted to the data by changing the parameters associated with the biomass description and, (where appropriate) associated kinetic terms, with associated estimation of parameter uncertainty. A model component, UKZiNe was developed consisting of 4 functional groups; 2 carboxyl groups, 1 phosphate group and 1 amine group. Kinetic effects including carbon dioxide exchange and pH probe lag were explored. The hypothesis that glycine could represent the ionic behavior of biomass was not supported. The alternate hypothesis, considering UKZiNe at equilibrium conditions, required further testing to evaluate the effects of kinetic reactions; the second alternate hypothesis that non-equilibrium effects significantly influence the measured experimental pH value, was supported. All model formulations predicted that the biomass contribution to the overall buffer capacity in the operating region of an anaerobic digester was insignificant. The study implies that the inclusion of an ionic description of biomass does not considerably improve the pH prediction in digester simulations and can be excluded in future model development.