Browsing by Author "Fernandes, Sarah"

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    Investigation of alternative carbon sources for the biological treatment of synthetic sulphate-laden water and mine impacted water in a linear flow channel reactor
    (2025) Tawodzera, Nyasha; Harrison, Susan; Fernandes, Sarah
    South Africa grapples with the generation of acid mine drainage (AMD), which adversely affects surface and ground water quality. Existing treatment methods typically treat the acid and heavy metal components of AMD but often fail to meet sulphate removal standards, necessitating additional polishing steps that add to expense. They also carry drawbacks such as high operational costs and metal sludge generation for active treatment and reduced process control and the need for large land areas for installation for passive treatment. These treatment methods are not cost-efficient when treating low-volume, circum-neutral wastewater. Biological sulphate reduction (BSR) offers a sustainable alternative for sulphate removal and is applicable to partially treated AMD as well as circum- neutral, mine-impacted water. Coupled with partial sulphide oxidation, it also has the potential to convert sulphate to elemental sulphur, touching on waste valorisation as sulphur is a value-added product. However, BSR systems require supplementation with organic carbon and are characterised as slow, while partial oxidation of sulphide is difficult to control in many reactor systems; these are key drawbacks in the economic feasibility of these processes. A semi-passive linear flow channel reactor (LFCR) which simultaneously reduces sulphate to sulphide using sulphate reducing bacteria (SRB) and partially oxidises the sulphide formed to elemental sulphur using sulphur oxidising bacteria (SOB) within a floating sulphur biofilm, was developed at the Centre for Bioprocess Engineering Research at the University of Cape Town, South Africa. It may be operated as a one- or two-stage reactor system, with the second stage providing additional surface area for partial sulphide oxidation. During its development, carbon fibres were added to enable biomass retention and thereby enhance reaction rates. For this study, the primary reactor was further modified to include baffles for improved directional flow and enhanced contacting and the carbon microfibre biomass support was replaced with polyurethane foam (PUF) previously shown to effect efficient biomass retention. A secondary reactor was included to provide 30% of operating volumes with no baffles; it was connected in series to increase surface area for sulphur recovery. The baffled hybrid linear flow channel reactor (BaH-LFCR) was supplied with an organic substrate, lactate, and was tested for its treatment of a synthetic sulphate laden feed. Lactate has been shown to be highly effective carbon source and electron donor for sulphate reduction but is expensive and not available at sufficient scale or low enough cost to be a cost-effective option at an industrial scale. The synthetic feed, unlike AMD from the field, was nutrient-rich with a stable, neutral pH that promoted SRB function. This study investigated the selection and use of an alternative, cheap and readily available carbon source and electron donor for BSR as well as the treatment of AMD from the field in the BaH-LFCR. Alternative carbon sources investigated included molasses, acetate, honey and algal biomass; each have a high chemical oxygen demand making them potentially suitable organic carbon sources for BSR. Honey and algal biomass can be produced on-site enhancing availability and negating transportation costs. As a byproduct of the sugar industry, the equivalent COD as molasses costs less than 0.1% of that of lactate. Acetate is a byproduct of most fermentation processes making it readily available. Field AMD presents several challenges for BSR due to its acidic nature, lack of nutrients, and potential toxins. To address these challenges, the AMD was characterised and pre-treated to increase the pH before introduction into the BSR system. Use of an alternative substrate and real AMD from the field demonstrates the ability of the LFCR to achieve real-world application and implementation. Three small-scale reactor configurations were tested with lactate and sulphate-laden feed: a 1 L fed-batch Schott bottle, a 93 mL continuous mini column, and a 1 L continuous Schott bottle. Continuous reactors performed poorly due to oxygen ingress, achieving only 43.4% sulphate conversion in the Schott bottle and no conversion after 23 days in the mini column. The fed-batch reactor demonstrated better stability with 78.1% conversion. Oxygen ingress impact was found to be inversely proportional to reactor size. Based on superior stability and conversion efficiency, the fed-batch reactor was selected for carbon source testing. The four alternative substrates were tested against lactate, as base case, in the fed-batch reactor. Molasses showed the highest performance among the alternative carbon sources, achieving 82.7% sulphate conversion and producing the highest sulphide concentration. In contrast, honey and algal lysate performed poorly, with average sulphate conversions of 7.4% and 14.1% respectively. The poor performance of honey was attributed to its antimicrobial properties and acidic nature. Poor performance of algal lysate likely resulted from its composition of predominantly unusable COD which would require fermentation to produce more accessible compounds. Acetate had an average conversion of 41.4% which was approximately half the conversion achieved using molasses as a carbon source. VFA analysis revealed that molasses was the only substrate where all measurable sugars and VFAs were consumed by the end of each cycle, as indicated by HPLC analysis. Additionally, molasses contained fermenting microorganisms that converted sugars into small concentrations of lactate, enhancing sulphate reduction. These fermenters were introduced into the BSR system along with the molasses substrate. Before introducing AMD from the field to the BaH-LFCR, various AMD pretreatment methods were evaluated using lactate-fed batch reactors. Four AMD conditions were tested: untreated AMD, lime-treated AMD, lime and sulphate-treated AMD, and lime-treated sterilised AMD. The untreated AMD batch showed the lowest sulphate conversion (9%) due to its acidic pH, which inhibited SRB activity. The highest conversion of 89% was achieved with lime-treated, sterilised AMD. Sterilisation eliminated competition for the carbon source between native microorganisms present in AMD and the SRB, resulting in enhanced sulphate conversion. Three experiments were conducted in the BaH-LFCR system to evaluate its performance using different combinations of carbon sources (lactate vs. molasses) and feed solutions (synthetic Postgate media vs. pre-treated AMD from the field). The first experiment established a base case using lactate and sulphate-rich synthetic feed to determine the optimal hydraulic residence time (HRT). At the optimal 3-day HRT, this base case achieved 64.8% sulphate conversion and the highest volumetric sulphate reduction rate (VSRR) of 0.187 mmol/L.h in the primary reactor, nearly two-fold that achieved previously in the LFCR. The second experiment, using lactate with partially treated AMD, achieved the highest sulphate conversion of 87.4% and the second-highest VSRR (0.216 mmol/L.h) in the primary reactor. It had 41.6% of the sulphur entering the system through the feed converted to sulphur. Sulphur formation was observed to decline, likely due to the development of a thin, impervious surface film hypothesised to consist of calcium crystal complexes. This film may have impaired oxygen di`usion at the air-liquid interface more severely than the typical floating sulphur biofilm (FSB), thereby reducing the efficiency of sulphide oxidation to elemental sulphur. The third experiment, combining molasses with partially treated AMD, achieved second highest sulphate conversion of 85.6% in the primary reactor. Overall sulphate conversion however dropped to 27.2% due to extensive re-oxidation in the secondary reactor. This re-oxidation linked to poor FSB formation and limited carbon availability in the secondary reactor. However, in the primary reactor the molasses configuration achieved the highest proportion of expected sulphide converted to sulphur of 30.7%, with a comparable expected sulphide amount of 518 mmol. The synthetic feed + lactate experiment achieved approximately only 9.4% sulphide conversion in the primary reactor, with an expected sulphide amount of 428 mmol while the AMD + lactate experiment had the highest expected sulphide amount of 543 mmol. In summary, the introduction of partially treated AMD into the LFCR system on a lactate carbon source not only maintained but enhanced system performance, achieving the highest sulphate conversion despite lacking the additional nutrients present in the SRB-specific feed. While using molasses as a complex waste stream carbon source achieved high sulphate conversion in the primary reactor, sulphur recovery was compromised due to re-oxidation at the primary reactor effluent port and because of limited carbon availability there was poor sulphur formation and high sulphate concentrations. The effective treatment of circum-neutral, sulphate-laden mine-impacted water using a readily available, cost-effective substrate demonstrates the system's suitability for industrial-scale deployment.
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    Optimisation of a linear flow channel reactor for semi-passive, simultaneous biological sulphate reduction and partial sulphide oxidation
    (2021) Fernandes, Sarah; Harrison, Susan; Huddy, Robert; van Hille, Rob P
    Acid rock drainage (ARD) is a growing concern, particularly in South Africa, as a country already classified as water scarce. ARD is defined as water that has been impacted by mining activities and typically has high levels of sulphate and heavy metals, at acidic pH. Similarly, high sulphate neutral rock drainage is of increasing concern. High sulphate content increases water salinity leading to adverse effects on human health as well as agriculture. Types of ARD and neutral rock drainage can be categorised into those that are produced in high volumes from groundwater rebound, and those that are generated from diffuse sources, as low-flow ARD. Low-flow ARD and neutral rock drainage are amenable to biological treatment of the sulphate component using sulphate reducing bacteria (SRB). Biological sulphate reduction (BSR) generates sulphide, which requires further treatment to remove it from the stream in gaseous form or as a solid sulphur-containing compound. Alternatively, it can also be used, in part, to precipitate metals present within ARD waters. Key challenges associated with SRBbased bioremediation include the cost of the supplemented electron donor needed for SRB to reduce sulphate, as well as the downstream management or treatment of the excess sulphide remaining. This investigation aimed to optimise a semi-passive treatment process which integrates BSR, and concomitant partial oxidation, by sulphur oxidising bacteria (SOB), of the sulphide produced to elemental sulphur. This is generated as a floating sulphur biofilm (FSB). These processes occur simultaneously within a linear flow channel reactor (LFCR), facilitating both treatment of the water stream to a fit-for-purpose water product, and recovery of the sulphur for use within fertilisers or fungicides. The work focused on the effective utilisation of electron donors and a sustainable option thereof, as well as the optimisation of partial sulphide oxidation and sulphur recovery. In addressing the cost of a supplemented electron donor, the use of a waste product is of interest. To explore this and build on earlier work within the Centre for Bioprocess Engineering Research (CeBER) labs, this study investigated the efficient use of the volatile fatty acids (VFAs) acetate, propionate and lactate. Firstly, the study investigated propionate, a common fermentation product of waste organic sources. It is found as a component of the effluent or digestate, of anaerobic digestion (AD) processes such as algal AD. Propionate was proposed as an attractive option for a sustainable source of electron donor. An LFCR fed with synthetic propionate showed sulphate reduction occurring via the utilisation of both propionate as well as acetate produced from propionate metabolism. Fermentative bacteria were seen to work syntrophically within the system availing a significant amount of acetate to the SRB community. This acetate was the preferred electron donor over propionate. Maximum volumetric sulphate reduction rates (VSRRs) of 190 mg/L/day were achieved in the reactor. However, detailed analysis showed few propionate-utilising SRB in the community. It was concluded that a more diverse inoculum was needed to investigate the potential of propionate more fully. Secondly, the study investigated lactate as a means to explore the efficiency of electron donors that are incompletely oxidised to acetate. Higher chain VFAs such as lactate are partially oxidised to acetate under biosulphidogenesis; however, acetate oxidation by SRB appears to be a rate-limiting step in most systems. Simultaneous incomplete oxidation of the more complex VFAs with complete oxidation of acetate are rarely reported. Acetate accumulates in the effluents of these processes and results in high chemical oxygen demand (COD) remaining which can lead to environmental impacts such as eutrophication if released into river systems. Further, utilisation of the electron donor is inefficient. In order to address this, the study presents a sequential LFCR system to increase utilisation efficiency of the incompletely oxidised VFA feed. The sequential system was developed by coupling a second reactor unit, specifically colonised with acetate-utilising SRB, to a primary reactor unit utilising lactate. The SRB community in the secondary reactor was able to oxidise acetate from the primary reactor resulting in further sulphate reduction and lower residual COD levels. Residual acetate decreased from 475 mg/L in the primary reactor to 275 mg/L in the secondary reactor. Similarly, residual sulphate decreased from 533 mg/L in the primary reactor to 150 mg/L in the secondary reactor, on a 1 g/L sulphate feed, achieving a much improved effluent water quality. A VSRR of 213 mg/L/day across the sequential system, an overall conversion of 85% and a two-fold increase in sulphate reduced / lactate consumed from 0.45 to 0.85 (g/g) were achieved. Lastly, this study investigated sulphide removal via the incorporation of elemental sulphur into a floating sulphur biofilm (FSB). The generation of an FSB within a sulphate reducing bioreactor is a passive and sustainable method of sulphide remediation, producing a value-added product of elemental sulphur. The sequential reactor system resulted in an increase of two to three-fold in the amount of elemental sulphur recovered, with an improved conversion of sulphide formed to elemental sulphur. Further, the effect of the feed inorganic ions, magnesium and phosphate, on sulphur yield and sulphide removal was studied using the sequential LFCR system. It was found that a decrease of magnesium in the media supplied resulted in an increase in sulphide conversion to sulphur from 21 to 39%, while a concomitant reduction in feed phosphate resulted in a further increase to 50% of the same. In both cases the sulphur concentration in the FSB was substantially increased. Overall, the thesis addresses the need to decrease the cost associated with the supply of a suitable electron donor, as well as improves the water quality of a BSR effluent both in terms of residual sulphate, sulphide and COD. The LFCR system studied was improved both by a sequential reactor system, allowing greater sulphate reduction, sulphide removal via elemental sulphur recovery, and substrate utilisation efficiency. Additionally, changes to the inorganic component of the feed led to further sulphide removal and elemental sulphur recovery. Propionate was concluded to have been only partially used as an electron donor for SRB, however fermentative bacteria present in the mixed community degraded propionate to acetate which was then used for BSR. The work presented here contributes towards the broader research into a semi-passive, environmentally sustainable and economically viable treatment solution for low-flow, circumneutral ARD.