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Addressing the Challenges Facing Biological Sulphate Reduction as a Strategy for AMD Treatment: Reactor stage - raw materials, products and process kinetics
Expanded Title:The contamination of surface and groundwater by acid mine drainage (AMD) and acid rock drainage (ARD) and the consequences for the environment, agriculture and human health are serious concerns. Biological treatment of ARD is centred on the activity of sulphate-reducing bacteria (SRB), which reduce sulphate to sulphide, coupled to the oxidation of an electron donor, typically an organic carbon molecule. Commercial processes based on biological sulphate reduction have been constrained by the cost of the electron donor, the relatively slow growth of sulphate reducers and the associated kinetic constraints, and the management of the sulphide product. This report addresses the first two constraints by assessing the potential of whole and partially digested microalgae as the electron donor and investigating novel reactor configurations aimed at biomass retention and recycling. The handling of the sulphide product was previously addressed in WRC report no. 2139/1/13. Together, these studies demonstrate potential for development of a feasible integrated process for operation in a passive, semi-passive or active configuration, allowing treatment of these ARD sources with concomitant sulphur recovery. The aims of this project were as follows: (1) Critically evaluate existing SRB-based technologies, (2) evaluate microalgae as a carbon source and electron donor for SRB systems, both in terms of digestibility and the ability to cultivate sufficient quantities, (3) evaluate the effect of decoupling the hydrolysis and acidogenesis steps from the sulphate reduction, (4) review reactor options for the retention of biomass and the creation of specific reaction zones, (5) investigate the use of cross-flow microfiltration as a model biomass retention strategy for the sulphate reduction reactor, and (6) investigate novel reactor configuration to achieve biomass retention that has the potential for application in active and passive systems. Baseline data were generated for suspended culture, using continuously stirred tank reactors (CSTRs) and a growth medium with lactate as the electron donor and carbon source. Three feed sulphate concentrations were tested, at a constant chemical oxygen demand (COD) to sulphate ratio (0.7). The reactors were operated at hydraulic retention times (HRTs) from 5 days to 12 hours and steady state data were used to determine volumetric sulphate reduction rates (VSRR). Three experimental reactor configurations were evaluated: a standard reactor without active agitation, a standard reactor coupled to a cross-flow microfiltration unit for biomass recycling, and a novel linear flow channel reactor (LFCR) with carbon fibres for biomass retention. The potential of whole cell and anaerobically digested microalgae as electron donors was evaluated in standard CSTRs. The potential for growing algae on treated ARD (raw and nutrient supplemented) was evaluated in small-scale growth studies. A number of known species as well as uncharacterised environmental isolates were tested. Selected isolates were grown in airlift photobioreactors to compare growth on treated ARD against defined growth media. The baseline data were consistent with previous studies conducted under similar conditions. At a feed sulphate concentration of 1 g/ℓ, >85% sulphate reduction was observed at HRTs from 5 days to 1 day, with a linear increase in the VSRR and complete utilisation of the lactate substrate. The results suggest washout of part of the sulphate reducing community and a shift toward a lactate fermenting community. The data generated at the feed sulphate concentrations of 2.5 and 5 g/ℓ showed lower sulphate reduction efficiencies (50-60%) at the longer HRTs (4-5 days), becoming even less efficient at the shorter HRTs. Lactate conversion was complete in the reactor receiving 2.5 g/ℓ sulphate across the HRTs and ranged from 90% to 60% in the 5 g/ℓ reactor, indicating that the majority of the lactate was consumed by fermenters, rather than sulphate reducers. Sulphate reduction in the non-agitated control reactor was similar to that in the CSTR, indicating that constant agitation was not necessary. The inclusion of the cross-flow microfiltration unit to ensure biomass recycle significantly improved the sulphate reduction efficiency of the system at low HRTs. The VSRR at a 12 hour HRT was approximately 50% higher than in the CSTR under similar conditions, demonstrating the benefit of biomass recycling and the associated increase in the biomass concentration. The carbon microfibres used as the site of biofilm formation for cell retention in the LFCR proved to be an excellent support material. They were rapidly colonised, and the benefit of biomass retention was apparent at HRTs below 24 hours. The lack of turbulent mixing in the LFCR increased the selective pressure on non-sulphate reducing species and prevented the proliferation of lactate fermenters at low HRTs. Biological sulphate reduction was achieved using whole Spirulina biomass as the electron donor and carbon source, although the performance was inconsistent. More predictable performance was achieved when the hydrolysis and acidogenesis reactions were decoupled from the sulphate reduction. Spirulina proved to be a good substrate for anaerobic digestion with relatively rapid liberation of volatile fatty acids, predominantly acetate and butyrate. Digestion of whole cell Scenedesmus by anaerobic digestion was less efficient than Spirulina, probably due to the recalcitrance of the cellulosic cell wall. The use of effluent from the anaerobic digestion of Spirulina as an electron donor and carbon for sulphate reduction was successfully demonstrated at feed sulphate concentrations of 2.5 and 5 g/ℓ. The digestate was blended with the sulphate feed to ensure the COD:sulphate ratio was maintained at 0.7. A high degree of sulphate reduction was obtained at hydraulic HRTs of 5, 4 and 3 days. In order to operate a sulphate reducing system cost effectively on raw or partially digested microalgae, the biomass should be cultivated on site, preferably using wastewater or treated ARD as the basis of the medium. Effluent from the LFCR showed a consistently low sulphide concentration, which was reduced to zero following a brief period of aeration. While Spirulina could not grow on the aerated channel reactor effluent (ACRE), a number of other known species and uncharacterised environmental isolates could, although at growth rates lower than in defined media.
Date Published:01/11/2014
Document Type:Research Report
Document Subjects:Mine water - Mine water treatment
Document Keywords:Technology
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:2110/1/14
ISBN No:978-1-4312-0605-6
Authors:Harrison STL; Van Hille RP; Mokone T; Motleleng L; Smart M; Legrand C; Marais T
Project No:K5/2110
Organizations:University of Cape Town
Document Size:8 813 KB
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