title: Effects of ionic strength and ion pairing on (plant-wide) modelling of anaerobic digestion.
abstract: Plant-wide models of wastewater treatment (such as the Benchmark Simulation Model No. 2 or BSM2) are gaining popularity for use in holistic virtual studies of treatment plant control and operations. The objective of this study is to show the influence of ionic strength (as activity corrections) and ion pairing on modelling of anaerobic digestion processes in such plant-wide models of wastewater treatment. Using the BSM2 as a case study with a number of model variants and cationic load scenarios, this paper presents the effects of an improved physico-chemical description on model predictions and overall plant performance indicators, namely effluent quality index (EQI) and operational cost index (OCI). The acid-base equilibria implemented in the Anaerobic Digestion Model No. 1 (ADM1) are modified to account for non-ideal aqueous-phase chemistry. The model corrects for ionic strength via the Davies approach to consider chemical activities instead of molar concentrations. A speciation sub-routine based on a multi-dimensional Newton-Raphson (NR) iteration method is developed to address algebraic interdependencies. The model also includes ion pairs that play an important role in wastewater treatment. The paper describes: 1) how the anaerobic digester performance is affected by physico-chemical corrections; 2) the effect on pH and the anaerobic digestion products (CO2, CH4 and H2); and, 3) how these variations are propagated from the sludge treatment to the water line. Results at high ionic strength demonstrate that corrections to account for non-ideal conditions lead to significant differences in predicted process performance (up to 18% for effluent quality and 7% for operational cost) but that for pH prediction, activity corrections are more important than ion pairing effects. Both are likely to be required when precipitation is to be modelled.
authors.affiliation: Division of Industrial Electrical Engineering and Automation (IEA), Department of Biomedical Engineering, Lund University, Box 118, SE-221 00 Lund, Sweden.
authors.forename: Kimberly
authors.initials: K
authors.lastname: Solon
authors.affiliation: CAPEC-PROCESS, Department of Chemical and Biochemical Engineering, Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark.
authors.forename: Xavier
authors.initials: X
authors.lastname: Flores-Alsina
authors.affiliation: Advanced Water Management Centre (AWMC), University of Queensland, St Lucia, Brisbane, Queensland 4072, Australia.
authors.forename: Christian Kazadi
authors.initials: CK
authors.lastname: Mbamba
authors.affiliation: Department of Biosystems Engineering, Ghent University, Coupure Links 653, B-9000 Gent, Belgium.
authors.forename: Eveline I P
authors.initials: EI
authors.lastname: Volcke
authors.affiliation: Advanced Water Management Centre (AWMC), University of Queensland, St Lucia, Brisbane, Queensland 4072, Australia.
authors.forename: Stephan
authors.initials: S
authors.lastname: Tait
authors.affiliation: Advanced Water Management Centre (AWMC), University of Queensland, St Lucia, Brisbane, Queensland 4072, Australia.
authors.forename: Damien
authors.initials: D
authors.lastname: Batstone
authors.affiliation: CAPEC-PROCESS, Department of Chemical and Biochemical Engineering, Technical University of Denmark, Building 229, DK-2800 Lyngby, Denmark.
authors.forename: Krist V
authors.initials: KV
authors.lastname: Gernaey
authors.affiliation: Division of Industrial Electrical Engineering and Automation (IEA), Department of Biomedical Engineering, Lund University, Box 118, SE-221 00 Lund, Sweden. Electronic address: ulf.jeppsson@iea.lth.se.
authors.forename: Ulf
authors.initials: U
authors.lastname: Jeppsson
mesh_terms: D000693:Anaerobiosis; D008962:Models, Theoretical; D009994:Osmolar Concentration; D014865:Waste Disposal, Fluid
keywords: ADM1; BSM2; Non-ideality; Physico-chemical framework; Wastewater plant-wide modelling