Volumetric Sulphate Reduction Rate Research Articles

Effect of temperature change on the performance of the hybrid linear flow channel reactor and its implications on sulphate-reducing and sulphide-oxidising microbial community dynamics

Semi-passive bioremediation is a promising strategy to mitigate persistent low volume mine-impacted wastewater containing high sulphate concentrations. Building on the proof of concept demonstration of the hybrid linear flow channel reactor (LFCR), capable of simultaneous biological sulphate reduction and partial sulphide oxidation with elemental sulphur recovery, the impact of key operating parameters, such as temperature, on process performance is critical to real-world application. Temperature fluctuates seasonally and across the diurnal cycle, impacting biological sulphate reduction (BSR) and partial sulphide oxidation. The process is reliant on the metabolic activity and synergistic interactions between sulphate-reducing (SRB) and sulphide-oxidising (SOB) microbial communities that develop within discrete oxic and anoxic microenvironments within the hybrid LFCR. In this study, the impact of operating temperature on process performance was evaluated by decreasing temperature with time from 30 to 10°C in each of three laboratory-scaled hybrid LFCR units operating in pseudo-steady state at 1 g/L sulphate. Using lactate as a carbon source, two reactor sizes (2 and 8 L) were considered, while the impact of lactate vs. acetate as carbon source was evaluated in the 2 L reactors. On incremental decrease in temperature from 30 to 10°C, a decrease in volumetric sulphate reduction rate was observed: from 0.144 to 0.059 mmol/L.h in the 2 L lactate-fed reactor; from 0.128 to 0.042 mmol/L.h in the 8 L lactate-fed reactor; and from 0.127 to 0.010 mmol/L.h in the 2 L acetate-fed reactor. Similarly, sulphate conversion efficiency decreased (2 L lactate-fed: 66% to 27%; 8 L lactate-fed: 61% to 20%; 2 L acetate-fed: 61% to 5%). A decrease in temperature below the critical value (15°C) led to considerable loss in metabolic activity and overall BSR performance. Sessile and planktonic microbial communities were represented by bacterial phyla including Proteobacteria, Synergistetes, Bacteroidetes, and Firmicutes. A diverse group of putative SRB (Deltaproteobacteria) and SOB, including Alpha, Beta, Gamma, and Epsilonproteobacteria phylotypes, were prevalent and shifted in relative abundance and community composition in response to decreasing temperature. Specifically, the decrease in the relative abundance of Deltaproteobacteria with decreasing temperature below 15°C corresponded with a loss of BSR performance across all three reactors. This study demonstrated the impact of low temperature on the physiological selection and ecological differentiation of SRB and SOB communities within the hybrid LFCR and its implications for real-world process performance.

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

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.

Linking performance and microbial ecology in a biological sulphate reducing reactor system with biomass retention developed for the treatment of acid rock drainage

Biological sulphate reduction (BSR) represents a sustainable process option for the remediation of low-flow acid rock drainage (ARD) effluents. The linear flow channel reactor (LFCR) was developed as a semi-passive bioreactor facilitating sulphate reduction and sulphide removal, through the action of sulphate-reducing bacterial (SRB) and sulphur oxidising bacterial consortia, within a single reactor unit. A consensus on key microorganism(s) required within BSR bioreactor consortia for successful operation is still to be reached. In this study next-generation gene amplicon sequencing was used to assess the microbial communities within an anaerobic LFCR supplemented with acetate as the primary electron donor. The performance of this reactor was monitored for over 1000 days throughout a hydraulic retention time (HRT) study and the composition of the microbial communities within this system assessed at several time points. The reactor exhibited a maximum volumetric sulphate reduction rate of 17.8 mg/L.h at a dilution rate of 0.042 h−1. Microbial biofilm communities formed on the incorporated carbon microfibres, as well as planktonic communities showed marked changes in community composition over the course of the study. The presence and shifting abundance of SRBs in the biofilm and planktonic communities that had been identified in previous studies to dominate under defined operating conditions allowed the functional role of individual microorganisms to be elucidated. A Desulfatitalea, whose presence had been previously linked to sulphate scavenging at low sulphate concentrations, was found at high abundance at long residence times. Conversely, a Desulfobacter which was previously shown to dominate under conditions of high sulphate loadings and associated high reaction rates and high substrate concentrations was found at a high dilution rate and low residence time in both the planktonic and biofilm communities. Assessment of the microbial communities and overlaying of performance within this system has enabled us to provide a preliminary link between the structure of the microbial community and the performance of this BSR reactor system. These findings provide considerable insights into the improved design and operation of acetate-supplemented BSR reactors to ensure successful performance.

Effect of hydraulic residence time on biological sulphate reduction and elemental sulphur recovery in a single-stage hybrid linear flow channel reactor

Biological sulphate reduction and partial sulphide oxidation, occurring simultaneously within the hybrid linear flow channel reactor (LFCR) were evaluated, under controlled conditions at laboratory scale, as a function of hydraulic residence time (HRT) using a synthetic media containing 1 g/L sulphate. The hybrid LFCR comprises a rectangular channel containing carbon microfibers as a support matrix for attachment of sulphate-reducing bacteria and an exposed air-liquid interface to facilitate the formation of a floating sulphur biofilm. Exposure to decreasing HRT, from 3 days to 12 h, resulted in an increase in the volumetric sulphate reduction rate (0.14 to 0.63 mmol/L.h), achieving levels typically associated with active reactors. Sulphate conversion was highest (97 %) at a 3 day HRT, decreasing to 73 % at 12 h. The highest sulphide removal efficiency (82 %) and accompanying sulphur recovery through harvesting of the floating sulphur biofilm (FSB) was observed at a 2 day HRT. The sulphur fraction not recovered through the biofilm was predominantly released within the effluent as colloidal elemental sulphur and fragments of the sulphur-rich biofilm, with minimal re-oxidation to sulphate occurring in the reactor. The hybrid LFCR technology was able to achieve high rates of sulphate reduction and effective sulphide removal within a single, semi-passive reactor unit.

Demonstration of simultaneous biological sulphate reduction and partial sulphide oxidation in a hybrid linear flow channel reactor

Semi-passive remediation systems have the potential to treat low-volume, sulphate-rich, mining impacted waters in a sustainable way. This paper describes the “proof of concept” evaluation of a hybrid linear flow channel reactor, capable of efficient biological sulphate reduction and partial oxidation of the sulphide to elemental sulphur. Key elements include a sulphate-reducing microbial community, immobilised onto carbon fibres and the rapid development of a floating sulphur biofilm at the air-liquid interface. The biofilm impedes oxygen mass transfer, creating a suitable pH-redox microenvironment for partial sulphide oxidation. Demonstration of the concept was successful, with near complete sulphate reduction (1 g/L feed), effective sulphide management (95–100 % removal) and partial recovery of sulphur by harvesting the biofilm. The biofilm re-formed within 24 h, with no decrease in volumetric sulphate reduction rate during this period. Colonisation of the carbon microfibers ensured biomass retention, suggesting the reactor could remain effective at high volumetric flow rates.

Stratification of microbial communities throughout a biological sulphate reducing up-flow anaerobic packed bed reactor, revealed through 16S metagenomics

Biological sulphate reduction (BSR) is a promising low-cost treatment of acid rock drainage effluents. In this paper, the system performance and microbial ecology of a lactate supplemented BSR up-flow anaerobic packed bed reactor (UAPBR) are evaluated across reactor height and compared to a continuous stirred tank reactor (CSTR). The biomass concentrations of planktonic and biofilm communities were quantified and subsequently characterised by 16S rRNA gene amplicon sequencing. The defined microbial communities were shown to correlate with differing availability of lactate, volatile fatty acids produced from lactate degradation and sulphate concentration. The UAPBR was able to achieve near complete sulphate conversion at a 4-day hydraulic residence time (HRT) at a sulphate feed concentration of 10.41 mM (1 g/L). The high volumetric sulphate reduction rate of 0.184 mM/L.h achieved in the first third of the reactor was attributed to OTUs present in the planktonic and biofilm communities. While the scavenging of sulphate within the final third of the UAPBR was attributed to an acetate oxidising genus of SRB which was not detected in the lactate-fed CSTR. The detailed analyses of the microbial communities throughout the UAPBR and CSTR contribute to the growing understanding of the impact of the microbial communities of BSR reactors on system performance.

Lignocellulosic biowastes as carrier material and slow release electron donor for sulphidogenesis of wastewater in an inverse fluidized bed bioreactor

Industrial wastewaters containing high concentrations of sulphate, such as those generated by mining, metallurgical and mineral processing industries, require electron donor for biological sulfidogenesis. In this study, five types of lignocellulosic biowastes were characterized as potential low-cost slow release electron donors for application in a continuously operated sulphidogenic inverse fluidized bed bioreactor (IFBB). Among them, natural scourer and cork were selected due to their high composition of volatile solids (VS), viz. 89.1 and 96.3%, respectively. Experiments were performed in batch (47days) and in an IFBB (49days) using synthetic sulphate-rich wastewater. In batch, the scourer gave higher sulphate reduction rates (67.7mgSO42-L-1day-1) in comparison to cork (12.1mgSO42-L-1day-1), achieving >82% sulphate reduction efficiencies. In the IFBB packed with the natural scourer, the average sulphate reduction efficiency was 24 (±17)%, while the volumetric sulphate reduction rate was 167 (±117)mgSO42- L-1day-1. The long incubation time in the batch experiments (47days) allowed higher sulphate reduction efficiencies in comparison to the short hydraulic retention time (24h) in the IFBB. This suggests the hydrolysis-fermentation was the rate-limiting step and the electron donor supply (through hydrolysis of the lignocellulosic biowaste) was limiting the sulphate reduction. Lignocellulose as carrier material and slow release electron donor for sulphidogenesis.

Kinetic analysis of biological sulphate reduction using lactate as carbon source and electron donor: Effect of sulphate concentration

This study investigated the effect of feed sulphate concentration on the kinetics of anaerobic sulphate reduction by a mixed SRB culture, using lactate as the sole carbon source and electron donor. Chemostat cultures were operated across a range of residence times (0.5–5 d) and feed sulphate concentrations (1.0–10.0 g l −1). Similar phenomena were observed at feed sulphate concentrations of 1.0 and 10.0 g l −1 with the volumetric sulphate reduction rate increasing linearly with increasing volumetric sulphate loading rate. These reactors were characterised by higher specific volumetric sulphate reduction rates with maximum values of 0.24 and 0.20 g h −1 g −1. Contrastingly, the reactors fed with sulphate concentrations of 2.5 and 5.0 g l −1 showed distinctly different trends in which the volumetric sulphate reduction rate passed through a maximum at the dilution rates of 0.014 and 0.021 h −1, respectively, followed by a decline with further increase in sulphate loading rate. The maximum specific volumetric sulphate reduction rates observed were 2–6-fold lower than those observed at 1.0 and 10.0 g l −1 feed sulphate concentrations. Profiles of specific volumetric sulphate reduction rate and biomass concentration suggested a shift in lactate utilisation from oxidation to fermentation at high dilution rates, implying a change in the dominant components of the microbial consortium. The data suggest that population structure was influenced by lactate affinity and dissolved sulphide concentration. The trends observed were attributed to the greater ability of lactate oxidisers to scavenge lactate under limiting concentrations of the substrate and their greater resilience to dissolved sulphide species in comparison to lactate fermenters .

Sulfate reduction during the acidification of sucrose at pH 5 under thermophilic (55 °C) conditions. II: Effect of sulfide and COD/[formula omitted] ratio

This work studied the effect of the sulfide concentration and COD/SO42- ratios (4 and 1) on sulfate reduction and acidification in a thermophilic (55°C) UASB reactor fed with sucrose (4gCOD(lreactord)−1) operated at a reactor mixed liquor pH controlled at 5 for a period of 301days. When implementing N2 stripping, sulfate reduction efficiencies up to 95%, corresponding to volumetric sulfate reduction rates of 0.87 and 4.2g (lreactord)−1 at the COD/SO42- ratios of 4 and 1, respectively, were achieved. Sulfide was toxic to sulfate reduction at a total dissolved sulfide concentration of 100mgl−1. Acidification was always complete and acetate was the only form of substrate in the effluent at a COD/SO42- ratio of 1. The sludge was well retained in the reactor and kept its granular shape throughout the reactor run.

Seasonal dynamics of microbial sulfate reduction in temperate intertidal surface sediments: controls by temperature and organic matter

The role of microbial sulfate reduction on organic matter oxidation was studied quantitatively in temperate intertidal surface sediments of the German Wadden Sea (southern North Sea) on a seasonal base in the years 1998–2007. The sampling sites represent the range of sediments found in the back-barrier tidal area of Spiekeroog Island: sands, mixed and muddy flats. The correspondingly different contents in organic matter, metals, and porosities lead to significant differences in the activity of sulfate-reducing bacteria with volumetric sulfate reduction rates (SRR) in the top 15 cm of up to 1.4 μmol cm−3 day−1. Depth-integrated areal SRR ranged between 0.9 and 106 mmol m−2 day−1, with the highest values found in the mudflat sediments and lower rates measured in sands at the same time, demonstrating the impact of both temperature and organic matter load. According to a modeling approach for a 154-km2 large tidal area, about 39, 122, and 285 tons of sulfate are reduced per day, during winter, spring/autumn, and summer, respectively. Hence, the importance of areal benthic organic matter mineralization by microbial sulfate reduction increases during spring/autumn and summer by factors of about 2 and 7, respectively, when compared to winter time. The combined results correspond to an estimated benthic organic carbon mineralization rate via sulfate reduction of 78 g C m−2 year−1.

Effect of Sulphate Concentration on the Community Structure and Activity of Sulphate Reducing Bacteria

This study investigated the effect of sulphate concentration and residence time on the performance of anaerobic sulphate reduction by a mixed sulphate reducing bacteria (SRB) culture using lactate as the sole carbon source and electron donor. The process performance is related to the population structure of the microbial consortia and dominant metabolic reactions. Laboratory scale chemostat cultures at different residence times (1-4 d) and sulphate concentrations (1.0-10.0 g/L) were employed. Lactate oxidation was prevalent at feed sulphate concentrations of 1.0 to 5.0 g/L. A corresponding increase in the volumetric sulphate reduction rate with increasing volumetric loading rate was also observed at this range. However, at the higher feed sulphate concentration range (10.0-15.0 g/L), sulphate inhibition, lactate fermentation and an increased microbial diversity were evident. At each feed concentration of sulphate in the range 5.0 to 15.0 g/L, varying dilution rates resulted in significant shifts in dominant metabolic reactions. Sulphate concentration and residence time have significant effects on both the structure of the microbial population and kinetics of biological sulphate reduction.

Oxygen isotope biogeochemistry of pore water sulfate in the deep biosphere: Dominance of isotope exchange reactions with ambient water during microbial sulfate reduction (ODP Site 1130)

Microbially mediated sulfate reduction affects the isotopic composition of dissolved and solid sulfur species in marine sediments. Experiments and field data show that the δ 18 O SO 4 2 - composition is also modified in the presence of sulfate-reducing microorganisms. This has been attributed either to a kinetic isotope effect during the reduction of sulfate to sulfite, cell-internal exchange reactions between enzymatically-activated sulfate (APS), and/or sulfite with cytoplasmic water. The isotopic fingerprint of these processes may be further modified by the cell-external reoxidation of sulfide to elemental sulfur, and the subsequent disproportionation to sulfide and sulfate or by the oxidation of sulfite to sulfate. Here we report δ 18 O SO 4 2 - values from interstitial water samples of ODP Leg 182 (Site 1130) and provide the mathematical framework to describe the oxygen isotope fractionation of sulfate during microbial sulfate reduction. We show that a purely kinetic model is unable to explain our δ 18 O SO 4 2 - data, and that the data are well explained by a model using oxygen isotope exchange reactions. We propose that the oxygen isotope exchange occurs between APS and cytoplasmic water, and/or between sulfite and adenosine monophosphate (AMP) during APS formation. Model calculations show that cell external reoxidation of reduced sulfur species would require up to 3000 mol/m 3 of an oxidant at ODP Site 1130, which is incompatible with the sediment geochemical data. In addition, we show that the volumetric fluxes required to explain the observed δ 18 O SO 4 2 - data are on average 14 times higher than the volumetric sulfate reduction rates (SRR) obtained from inverse modeling of the porewater data. The ratio between the gross sulfate flux into the microbes and the net sulfate flux through the microbes is depth invariant, and independent of sulfide concentrations. This suggests that both fluxes are controlled by cell density and that cell-specific sulfate reduction rates remain constant with depth.

A 300 m long depth profile of metabolic activity of sulfate‐reducing bacteria in the continental margin sediments of South Australia (ODP Site 1130) derived from inverse reaction‐transport modeling

Reaction‐transport modeling of dissolved species in interstitial water allows for the inversion of transport processes and thus facilitates the detailed investigation of signals which are usually blurred by diffusion and advection. Here I present a case study from the South Australian continental margin (ODP Leg 182, Site 1130) where I use reaction‐transport modeling to derive a depth transect of volumetric sulfate reduction rates. Site 1130, located on the shelf slope in 500 m deep water, is of special interest as an upwelling sulfate‐rich brine allows for an extended sulfate reduction zone which reaches to a depth of at least 300 mbsf. The obtained reduction rates vary from 600 pmol/cm−3 yr−1 at 30 mbsf to 63 pmol/cm−3 yr−1 at 300 mbsf. The depth‐integrated sulfate consumption equals 65 × 10−6 mol/yr cm−2, which is similar to other shelf slope settings without advecting sulfate. This suggests that the primary control on sulfate reduction rates is organic matter reactivity, rather than sulfate availability. However, similar to other ODP Leg 182 sites, the interstitial water chemistry in the upper 30 mbsf is inconsistent with a diffusive/advective transport system. While the actual process causing this remains elusive, pyrite burial rates from this zone suggest that sulfate reduction rates in this zone are at least 60 times higher than those derived from reaction‐transport modeling assuming diffusion and advection alone.

Treatment of acid and sulphate-rich effluents in an integrated biological/chemical process

A novel chemical/biological process is described in which sulphate and sulphide are removed simultaneously during biological treatment. Partial sulphate removal is achieved during chemical pre-treatment. In the biological stage sulphate is reduced to sulphide in a complete-mixed reactor through addition of sucrose or ethanol as a carbon and energy source. Sulphide is oxidised by allowing oxygen to enter the system in a controlled way. The experimental investigation of the process showed that sulphate and sulphide could be removed simultaneously due to co-existence of sulphate-reducing bacteria and sulphur oxidising bacteria. The volumetric sulphate reduction rate in a complete-mixed reactor, with sucrose as an organic carbon and energy source, amounts to 12.4 g SO4/(ℓ.d). The rate of biological sulphate removal was found to be directly related to the square root of sulphate, COD and VSS concentrations respectively, and inversely proportional to sulphide concentration. The practical value of simultaneous sulphate and sulphide removal is that only one stage is required for removal of both sulphate and sulphide; a conventional completemixed reactor can be used; and sulphate can be removed in a consistent way to below 200 mg/ℓ (as SO4) due to the stability of the process. By combining the biological stage with CaCO3- neutralisation and/or lime pre-treatment, the chemical cost can be reduced. Sulphate, associated with the over-saturated fraction after treatment with CaCO3 or lime, can be removed through gypsum crystallisation. In the integrated sulphate removal process (CaCO3- neutralisation, lime treatment and biological stages), sulphate can be removed from 9 200 mg/ℓ (typical sulphate concentration of coal discard leachate) to 2410 mg/ℓ, 1 230 mg/ℓ and 205 mg/ℓ (as SO4) in the various stages respectively. The chemical cost with the integrated process amounts to R2.94/m3, versus R12.44/m3 when all the sulphate is removed using the biological stage only. Similarly, the cost for treating magnesium sulphaterich mine water amounts to R1.92/m3 for the integrated process, versus R3.11/m3 for biological treatment only. WaterSA Vol.30 (2) 2004: 183-189