Modelling phosphorus (P), sulfur (S) and iron (Fe) interactions for dynamic simulations of anaerobic digestion processes

This paper proposes a series of extensions to functionally upgrade the IWA Anaerobic Digestion Model No. 1 (ADM1) to allow for plant-wide phosphorus (P) simulation. The close interplay between the P, sulfur (S) and iron (Fe) cycles requires a substantial (and unavoidable) increase in model complexity due to the involved three-phase physico-chemical and biological transformations. The ADM1 version, implemented in the plant-wide context provided by the Benchmark Simulation Model No. 2 (BSM2), is used as the basic platform (A0). Three different model extensions (A1, A2, A3) are implemented, simulated and evaluated. The first extension (A1) considers P transformations by accounting for the kinetic decay of polyphosphates (XPP) and potential uptake of volatile fatty acids (VFA) to produce polyhydroxyalkanoates (XPHA) by phosphorus accumulating organisms (XPAO). Two variant extensions (A2,1/A2,2) describe biological production of sulfides (SIS) by means of sulfate reducing bacteria (XSRB) utilising hydrogen only (autolithotrophically) or hydrogen plus organic acids (heterorganotrophically) as electron sources, respectively. These two approaches also consider a potential hydrogen sulfide (ZH2SÞ inhibition effect and stripping to the gas phase (GH2S). The third extension (A3) accounts for chemical iron (III) (SFe3þ ) reduction to iron (II) (SFe2þ ) using hydrogen (SH2 ) and sulfides (SIS) as electron donors. A set of pre/post interfaces between the Activated Sludge Model No. 2d (ASM2d) and ADM1 are furthermore proposed in order to allow for plant-wide (model-based) analysis and study of the interactions between the water and sludge lines. Simulation (A1 e A3) results show that the ratio between soluble/particulate P compounds strongly depends on the pH and cationic load, which determines the capacity to form (or not) precipitation products. Implementations A1 and A2,1/A2,2 lead to a reduction in the predicted methane/biogas production (and potential energy recovery) compared to reference ADM1 predictions (A0). This reduction is attributed to two factors: (1) loss of electron equivalents due to sulfate ðSSO4 Þ reduction by XSRB and storage of XPHA by XPAO; and, (2) decrease of acetoclastic and hydrogenotrophic methanogenesis due to ZH2S inhibition. Model A3 shows the potential for iron to remove free SIS (and consequently inhibition) and instead promote iron sulfide (XFeS) precipitation. It also educes the quantities of struvite (XMgNH4PO4) and calcium hosphate (XCa3ðPO4Þ2) that are formed due to its higher affinity for phosphate anions. This study provides a detailed analysis of the different model assumptions, the effect that operational/design conditions have on the model predictions and the practical implications of the proposed model extensions in view of plant-wide modelling/development of resource recovery strategies

Extending Wastewater Treatment Process Models for Phosphorus Removal and Recovery : A Framework for Plant-Wide Modelling of Phosphorus, Sulfur and Iron

As problems associated with shortage in resource supply arise, wastewater treatment plants turn to innovation to transform themselves into resource recovery facilities. Water groups worldwide recognize that wastewater treatment plants are no longer disposal facilities but rather sources of clean water, energy and nutrients.One of the most important resources that can be recovered from wastewater treatment plants is phosphorus. Mathematical modelling can be utilised to analyse various operational strategies to recover phosphorus from the wastewater. However, incorporating phosphorus transformation processes in plant-wide models is complex. Firstly, the tri-valence of phosphates suggests non-ideality, which requires the use of a physico-chemical model to account for this non-ideality. Secondly, phosphorus has strong interlinks with sulfur and iron, which necessitates inclusion of their transformations in biological and physico-chemical models. Lastly, consolidating these into a plant-wide model aimed at describing phosphorus removal and/or recovery requires interfacing, modifications to the plant layout, addition of recovery unit processes and development of new control and operational strategies. The research work presented in this thesis addresses the aforementioned challenges.A physico-chemical model is developed to take into account ion activity corrections, ion pairing effects, aqueous phase chemical equilibria, multiple mineral precipitation and gas stripping/adsorption. The model is then linked with standard approaches used in wastewater engineering, such as the Activated Sludge Model Nos. 1, 2d and 3 (ASM1, 2d, 3) and the Anaerobic Digestion Model No. 1 (ADM1). The extensions of the ASM2d and ADM1 with phosphorus, sulfur and iron-related conversions followed. And finally, the extended models and the physico-chemical model are consolidated into a plant-wide model provided by the Benchmark Simulation Model No. 2. The resulting model is used for simulation-based scenario analysis for finding ways to improve the operation of a wastewater treatment plant aimed at phosphorus removal and recovery

Process schemes for future energy-positive water resource recovery facilities

There are numerous successful studies on optimizing the performance of conventional activated sludge (CAS)-based wastewater treatment plants. However, recent studies have shown that a more significant improvement of the plant performance is achievable through integration of established technologies in novel process schemes. High-rate activated sludge system, chemically enhanced primary treatment, partial nitritation-anammox, partial nitrification-denitrification over nitrite and anaerobic digestion are integrated in two process schemes to determine to which extent energy savings and energy production can be achieved with these new process layouts compared to a CAS-based process scheme. The results presented in this paper show that there is potential for achieving future energy-positive water resource recovery facilities through novel integration of mature technologies for municipal wastewater treatment

Effects of influent fractionation, kinetics, stoichiometry and mass transfer on CH4, H-2 and CO2 production for (plant-wide) modeling of anaerobic digesters

This paper examines the importance of influent fractionation, kinetic, stoichiometric and mass transfer parameter uncertainties when modeling biogas production in wastewater treatment plants. The anaerobic digestion model no. 1 implemented in the plant-wide context provided by the benchmark simulation model no. 2 is used to quantify the generation of CH4, H-2 and CO2. A comprehensive global sensitivity analysis based on (i) standardized regression coefficients (SRC) and (ii) Morris' screening's (MS's) elementary effects reveals the set of parameters that influence the biogas production uncertainty the most. This analysis is repeated for (i) different temperature regimes and (ii) different solids retention times (SRTs) in the anaerobic digester. Results show that both SRC and MS are good measures of sensitivity unless the anaerobic digester is operating at low SRT and mesophilic conditions. In the latter situation, and due to the intrinsic nonlinearities of the system, SRC fails in decomposing the variance of the model predictions (R-2 < 0.7) making MS a more reliable method. At high SRT, influent fractionations are the most influential parameters for predictions of CH4 and CO2 emissions. Nevertheless, when the anaerobic digester volume is decreased (for the same load), the role of acetate degraders gains more importance under mesophilic conditions, while lipids and fatty acid metabolism is more influential under thermophilic conditions. The paper ends with a critical discussion of the results and their implications during model calibration and validation exercises