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 (A(0)). Three different model extensions (A(1), A(2), A(3)) are implemented, simulated and evaluated. The first extension (A(1)) considers P transformations by accounting for the kinetic decay of polyphosphates (X-PP) and potential uptake of volatile fatty acids (VFA) to produce polyhydroxyalkanoates (X-PHA) by phosphorus accumulating organisms (X-PAO). Two variant extensions (A(2,1)/A(2,2)) describe biological production of sulfides (S-IS) by means of sulfate reducing bacteria (X-SRB) utilising hydrogen only (autolithotrophically) or hydrogen plus organic acids (heterorganotrophically) as electron sources, respectively. These two approaches also consider a potential hydrogen sulfide (Z(H2S)) inhibition effect and stripping to the gas phase (G(H2S)). The third extension (A(3)) accounts for chemical iron (III) (SFe3+) reduction to iron (II) (SFe2+) using hydrogen (S-H2) and sulfides (S-IS) 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 (A(1) - A(3)) 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 A(1) and A(2,1)/A(2,2) lead to a reduction in the predicted methane/biogas production (and potential energy recovery) compared to reference ADM1 predictions (A(0)). This reduction is attributed to two factors: (1) loss of electron equivalents due to sulfate (S-SO4) reduction by X-SRB and storage of X-PHA by X-PAO; and, (2) decrease of acetoclastic and hydrogenotrophic methanogenesis due to Z(H2S) inhibition. Model A(3) shows the potential for iron to remove free S-IS (and consequently inhibition) and instead promote iron sulfide (X-FeS) precipitation. It also reduces the quantities of struvite (X-MgNH4PO4) and calcium phosphate (X-Ca3(PO4)2) 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. (C) 2016 Elsevier Ltd. All rights reserved.