Integrated Kinetic Modelling and Microbial Profiling Provide Insights Into Biological Sulfate-Reducing Reactor Design and Operation

Abstract

Biological sulfate reduction (BSR) is an attractive approach for the bioremediation of sulfate-rich wastewater streams. Many sulfate-reducing microorganisms (SRM), which facilitate this process, have been well-studied in pure culture. However, the role of individual members of microbial communities within BSR bioreactors remains understudied. In this study we investigated the performance of two up-flow anaerobic packed bed reactors (UAPBRs) supplemented primarily with acetate and with lactate, respectively, during a hydraulic retention time (HRT) study set up to remediate sulfate-rich synthetic wastewater over the course of 1,000 + days. Plug-flow hydrodynamics led to a continuum of changing volumetric sulfate reduction rates (VSRRs), available electron donors, degrees of biomass retention and compositions of microbial communities throughout these reactors. Microbial communities throughout the successive zones of the reactors were resolved using 16S rRNA gene amplicon sequencing which allowed the association of features of performance with discrete microorganisms. The acetate UAPBR achieved a maximum VSRR of 23.2 mg.L−1. h−1 at a one-day HRT and a maximum sulfate conversion of the 1 g/L sulfate of 96% at a four-day HRT. The sulfate reduction reactions in this reactor could be described with a reaction order of 2.9, an important observation for optimisation and future scale-up. The lactate UAPBR achieved a 96% sulfate conversion at one-day HRT, corresponding with a VSRR of 40.1 mg.L−1. h−1. Lactate was supplied in this reactor at relatively low concentrations necessitating the subsequent use of propionate and acetate, by-products of lactate fermentation with acetate also a by-product of incomplete lactate oxidation, to achieve competitive performance. The consumption of these electron donors could be associated with specific SRM localised within biofilms of discrete zones. The sulfate reduction rates in the lactate UAPBR could be modelled as first-order reactions, indicating effective rates were conferred by these propionate- and acetate-oxidising SRM. Our results demonstrate how acetate, a low-cost substrate, can be used effectively despite low associated SRM growth rates, and that lactate, a more expensive substrate, can be used sparingly to achieve high VSRR and sulfate conversions. We further identified the preferred environment of additional microorganisms to inform how these microorganisms could be enriched or diminished in BSR reactors.

Harnessing Fermentation May Enhance the Performance of Biological Sulfate-Reducing Bioreactors

“…We previously modeled the rate of sulfate reduction observed in the packed bed reactors as irreversible n th -order reactions ( eq 7 ) along an ideal plug-flow reactor ( eq 8 ) according to the derived eq 9 and eq 10 (Methodology). 39 This analysis aimed to determine the reaction order and rate constant across the different packed bed reactors in a generalized sulfate reduction model assuming a constant rate constant. We anticipated the rate of sulfate reduction in the lactate-supplemented packed bed to substantially decrease once lactate was depleted, leading to a higher modeled reaction order.…”

“… Observed and modeled sulfate reduction rates observed in the (a) lactate- and (b) acetate-supplemented UAPBRs at a range of flow rate-to-volume ratios. Adapted from Hessler et al 39 The determined reaction order ( n ) and rate constant ( k ) are shown. (a, b) The modeled reaction rates are shown over a range of applied dilution rates where the starting substrate concentration ( C 0 ) is 1,000 mg/L with observed reaction rate data from the inlet zones (0.33 L), composite inlet and middle zones (0.66 L), and entire reactors (1.0 L).…”

“…Nonlinear regression using the iterative, generalized reduced gradient method, employed by SOLVER (Microsoft Excel, Microsoft Office 365 ProPlus), was used to solve for the rate constant and reaction order. Further details are discussed in Hessler et al r A = − k × C A n r A = V F ( ( C 0 ( − n + 1 ) + true( n − 1 true) × k × V F ) false 1 ( − n + 1 ) − C 0 ) r A = − F V ( C 0 e V × K F − C 0 ) …”

“…This required the SRM in the remainder of the column to consume the less energy-rich propionate and acetate. Surprisingly, the volumetric sulfate reduction rates in the lactate-supplemented packed bed reactor were best described as a first-order reaction, indicating that the modeled sulfate reduction rate was consistently proportional to the sulfate concentration but unaffected by changing electron donors 39 (Figure 3a). This is noteworthy because the oxidation of propionate and acetate coupled to sulfate reduction is less energetically favorable than that linked to lactate oxidation (Table 1).…”

“…By employing multiple bioreactors, we aimed to study many stable microbial communities, arising from a single inoculum, that facilitate effective sulfate-reducing reactor performance. Some of the bioreactor performance data were previously published, − including the performance of the packed bed reactors and the acetate channel reactor, but in the current study, we integrate performance data from all six bioreactors to contextualize the metagenomic results. Prior to this study, there was essentially no genome-resolved metagenome information available regarding the biological underpinnings of the biological sulfate reduction bioreactors.…”