OpenUCT :: Browsing by Department "Centre for Bioprocess Engineering Research"

Browsing by Department "Centre for Bioprocess Engineering Research"

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Open Access
A Comparative Analysis of the Performance and the Microbial Ecology of Biological Sulphate Reducing Reactor Systems
(2020) Hessler, Tomas; Huddy, Robert; Harrison, Susan
Acid rock drainage (ARD) is defined as acidic waste-water contaminated with sulphate and heavy metals which is generated through the oxidation of sulphidic ores in the presence of water and oxygen. Mining activities accelerate this process by bringing these ores to the surface where they are further crushed and, eventually end up in waste rock dumps and tailing impoundments where they continue to generate ARD into perpetuity. Active mining operations are mandated to prevent the discharge of ARD into the environment. This ARD is commonly remediated by expensive yet highly effective active treatment strategies such as high-density sludge processes and reverse osmosis. South Africa has an extensive history of gold and coal mining which has left abandoned mine workings with associated waste rock dumps throughout northern and eastern parts of the country. As many of these mines have long been abandoned, the responsibility to mitigate the environmental impact of the generated ARD lies solely with government. Although these diffuse sites often generate smaller volumes of less aggressive ARD compared to that generated through mine water rebound, the sheer number and the continual ARD generation from these sites is a severe threat to South Africa's already poor water security. Biological sulphate reduction (BSR) has long been considered an attractive option for the longterm remediation of these low-volume sources of ARD – but its implementation has shown mixed success. BSR is a process catalysed through the innate metabolism of sulphate-reducing bacteria (SRB) which coexist within complex microbial communities. SRB themselves are a highly diverse group of anaerobic microorganisms which use sulphate as a terminal electron acceptor. The sulphide and bicarbonate produced during BSR can be used to precipitate heavy metals and aid in the neutralisation of the ARD, respectively. The implementation of BSR is, therefore, a comprehensive remediation strategy for diffuse sources of ARD. The study of BSR, using various reactor configurations and operating conditions shows much promise. However, the microbial ecology of the complex communities within BSR systems, and their links to the performance of BSR processes, has received far less attention in published literature. This is not a result of underappreciation of the role microbial communities but rather a historical lack of tools, specifically high-throughput techniques, available to assess complex microbial consortia. It is asserted that the success of a sustainable BSR process developed for the long-term remediation of ARD requires an in-depth understanding the microbial communities associated with this process. The identification of the microorganisms which are key to the process, thosewhich threaten the stability of the community and the optimal growth conditions of these microorganisms, can be used to inform how these bioreactors are designed and operated. This study investigated the performance and microbial ecology of several continuous BSR reactors using culture-independent metagenomic sequencing approaches. The performance and microbial ecology of these reactors were evaluated at a range of hydraulic residence times (HRT) over the course of approximately 1000 days of continuous operation, from five- through to one-day(s). The tested reactor configurations included a continuous stirred tank reactor (CSTR), an up-flow anaerobic packed bed reactor (UAPBR) and a linear flow channel reactor (LFCR) that were each operated in duplicate and supplemented with either lactate or acetate as an electron donor. The different reactor configurations and supplied electron donors, as well as the varied applied HRT, generated a range of microenvironments which were hypothesised to lead to the divergence of the initial microbial community of the inoculum and generate numerous distinct microbial communities throughout and across the reactor systems. 16S rRNA gene amplicon sequencing was used to assess the microbial community structure of the numerous populations across the reactor systems and monitor how these communities responded to the change in the applied HRT. Genome-resolved metagenomics was employed in parallel to recover the genomes of all predominant microorganisms identified through gene amplicon sequencing. This allowed the interrogation of the composition of the respective microbial communities as well as the genetic potential of each microorganism and encompassing the communities represented within specific reactor environments. The CSTRs were selected as these systems are characterised as well-mixed, support solely suspended biomass and kinetic equilibriums are achieved rapidly. This allows the performance of these reactors to be predictable and provides a benchmark to which the LFCRs and UAPBRs could be compared. The lactate-supplemented CSTR performed largely as anticipated based on available literature, demonstrating a maintained sulphate conversion of approximately 55% over the course of the study. The reactor achieved a maximum observed volumetric sulphate reduction rate (VSRR) of 17 mg/ℓ.h at a one-day HRT. The system supported a low SRB diversity, constituted almost entirely by a Desulfomicrobium and two Desulfovibrio operational taxonomic units (OTUs). The acetate-supplemented CSTR was able to maintain sulphate reducing performance at HRT where complete washout of SRB had been predicted based on literature. This reactor exhibited a maximum VSRR of 10.8 mg/ℓ.h at a 1.5-day HRT and was dominated by the same Desulfovibrio and Desulfomicrobium observed in the lactate-supplemented CSTR, along with several other SRB genera at lower abundance. The LFCRs demonstrated an approximately ten-fold greater biomass retention than the corresponding CSTRs. This was facilitated through the incorporation of carbon microfibres, whichfacilitated microbial colonisation and biofilm formation within the reactors. Surprisingly, the lactate-supplemented LFCR, underperformed compared to the lactate-supplemented CSTR, achieving a maximum VSRR of 14.8 mg/ℓ.h at a one-day HRT. This reduced performance, in spite of the enhanced biomass retention, was concluded to result from the out-competition of lactateoxidising SRB in the reactor by Veillonella and Enterobacter OTUs. The acetate-supplemented LFCR exhibited a period of underperformance before recovering and subsequently demonstrated a maximum VSRR of 17.1 mg/ℓ.h at a one-day HRT. Evaluations of the microbial communities of this system during the HRT study revealed a dramatic shift in the SRB communities from being dominated by Desulfatitalea and Desulfovibrio to being dominated predominantly by Desulfomicrobium and Desulfobacter. The UAPBRs are governed by plug-flow which resulted in the generation of gradients of decreasing substrates and increasing products throughout the height of the reactors. This, as hypothesised, resulted in the stratification of the microbial communities throughout the height of these reactors. This allowed many associations to be made between specific microorganisms and their ideal growth environments. Both UAPBRs demonstrated competitive sulphate reducing performance. The lactate-supplemented UAPBR proved especially successful as this system was able to maintain >95% sulphate conversion at one-day HRT, corresponding with a VSRR of 40.1 mg/ℓ.h. The performance of this reactor was attributed to the significant quantity of retained biomass and the successful harbouring of lactate-oxidising SRB towards the inlet zone of the reactor as well as propionate- and acetate-oxidising SRB towards the effluent zones of the reactor. The acetatesupplemented UAPBR exhibited a maximum VSRR of 23.2 mg/ℓ.h at a one-day HRT and a maximum sulphate conversion of 79% at a 2.3-day HRT. The stratification of the microbial communities within the acetate-supplemented UAPBR was less pronounced than the lactatesupplemented UAPBR, as a result of the fewer available volatile fatty acid species. However, the stratification which was observed in this system could be used to postulate the growth kinetics associated with the identified SRB – a Desulfobulbus was associated with rapid acetate oxidation in the inlet zone while a Desulfatitalea and a Desulfosarcina could be implicated in sulphate scavenging in the effluent zone of this reactor. This proved particularly valuable for elucidating the roles of these same SRB in the well-mixed reactor systems. Genome-resolved metagenomics was employed to recover the genomes of the microorganisms identified in these systems and determine the metabolic potential of these microorganisms. Hydrogen-evolving hydrogenase genes were found to be widespread in genomes not capable of sulphate reduction. In contrast, hydrogen-consuming hydrogenases as well as autotrophic gene pathways were common amongst SRB genomes. The ubiquity of hydrogenase genes in these environments indicated that inter-species hydrogen transfer was an important feature within thesemicrobial communities. The dual consumption of both acetate and hydrogen was concluded to have facilitated the maintained sulphate reducing performance of the acetate-supplemented reactor systems at short HRT where system failure had been predicted. Indices of replication (iRep) were used to estimate the instantaneous growth rates of the microorganisms from metagenomic shotgun sequencing datasets. This revealed that, at a four-day HRT, the microorganisms within the biofilms were comparably active to planktonic microorganisms. This, together with the dynamic changes in the composition of these biofilms during the HRT study, suggests these biofilms are even more active and competitive than previously thought. The combined use of next-generation gene amplicon sequencing and genome-resolved metagenomics has given unprecedented insights into the microbial communities of BSR reactor systems. Using this approach, it was possible to uncover a seldom discussed form of hydrogen cycling within BSR systems and has shown that there is no ‘one-size-fits-all' approach when inoculating BSR reactors. The SRB within these systems were often highly specialised to particular environments, specific electron donors and each showed differing growth kinetics. The success of long-term, semi-passive BSR reactor systems would benefit greatly from the tailoring of SRB inoculums informed by the chosen reactor configuration and operating conditions. The outcomes of the kinetic reactor experiments have led to several recommendations for the design and operation of these systems.
Open Access
A novel semi-passive process for sulphate removal and elemental sulphur recovery centred on a hybrid linear flow channel reactor
(2020) Marais, Tynan S; Harrison, Susan; van Hille, Rob; Huddy, Robert
South Africa (SA) currently faces a major pollution problem from mining impacted water, including acid rock drainage (ARD), as a consequence of the mining activities upon which the economy has been largely built. The environmental impact of ARD has been further exacerbated by the country's water scarce status. Increasingly scarce freshwater reserves require the preservation and strategic management of the country's existing water resources to ensure sustainable water security. In SA, the primary focus on remediation of ARDcontaminated water has been based on established active technologies. However, these approaches are costly, lead to secondary challenges and are not always appropriate for the remediation of lower volume discharges. Mostly overlooked, ARD discharges from diffuse sources, associated with the SA coal mining industry, have a marked impact on the environment, similar to those originating from underground mine basins. This is due to the large number of deposits and their broad geographic distribution across largely rural areas of SA. Semi-passive ARD treatment systems present an attractive alternative treatment approach for diffuse sources, with lower capital and operational costs than active systems as well as better process control and predictability than traditional passive systems. These semi-passive systems typically target sulphate salinity through biological sulphate reduction catalysed by sulphate reducing bacteria (SRB). These anaerobic bacteria reduce sulphate, in the presence of a suitable electron donor, to sulphide and bicarbonate. However, the hydrogen sulphide product generated is highly toxic, unstable, easily re-oxidised and poses a significant threat to the environment and human health, so requires appropriate management. An attractive strategy is the reduction of sulphate to sulphide, followed by its partial oxidation to elemental sulphur, which is stable and has potential as a value-added product. A promising approach to achieve partial oxidation is the use of sulphide oxidising bacteria (SOB) in a floating sulphur biofilm (FSB). These biofilms develop naturally on the surfaces of sulphide rich wastewater streams. Its application in wastewater treatment and the feasibility of obtaining high partial oxidation rates in a linear flow channel reactor (LFCR) has been described. The use of a floating sulphur biofilm overcomes many of the drawbacks associated with conventional sulphide oxidation technologies that are costly and require precise operational control to maintain oxygen limiting conditions for partial oxidation. In the current study a hybrid LFCR, incorporating a FSB with biological sulphate reduction in a single reactor unit, was developed. The integration of the two biological processes in a single LFCR unit was successfully demonstrated as a ‘proof of concept'. The success of this system relies greatly on the development of discrete anaerobic and microaerobic zones, in the bulk liquid and at the airliquid interface, that facilitate sulphate reduction and partial sulphide oxidation, respectively. In the LFCR these environments are established as a result of the hydrodynamic properties associated with its design. Key elements of the hybrid LFCR system include the presence of a sulphate-reducing microbial community immobilised onto carbon fibres and the rapid development of a floating sulphur biofilm at the air-liquid interface. The floating sulphur biofilm consists of a complex network of bacterial cells and deposits of elemental sulphur held together by an extracellular polysaccharide matrix. During the Initial stages of FSB development, a thin transparent biofilm layer is formed by heterotrophic microorganisms. This serves as ‘scaffolding' for the subsequent attachment and colonisation of SOB. As the biofilm forms at the air-liquid interface it impedes oxygen mass transfer into the bulk volume and creates a suitable pH-redox microenvironment for partial sulphide oxidation. Under these conditions the sulphide generated in the bulk volume is oxidised at the surface. The biofilm gradually thickens as sulphur is deposited. The produced sulphur, localised within the biofilm, serves as an effective mechanism for recovering elemental sulphur while the resulting water stream is safe for discharge into the environment. The results from the initial demonstration achieved near complete reduction of the sulphate (96%) at a sulphate feed concentration of 1 g/L with effective management of the generated sulphide (95-100% removal) and recovery of a portion of the sulphur through harvesting the elemental sulphur-rich biofilm. The colonisation of the carbon microfibres by SRB ensured high biomass retention within the LFCR. This facilitated high volumetric sulphate reduction rates under the experimental conditions. Despite the lack of active mixing, at a 4-day hydraulic residence time, the system achieved volumetric sulphate reduction rates similar to that previously shown in a continuous stirred-tank reactor. The outcome of the demonstration at laboratory scale generated interest to evaluate the technology at pilot scale. This interest necessitated further development of the process with a particular focus on evaluating key challenges that would be experienced at a larger scale. A comprehensive kinetic analysis on the performance of the hybrid LFCR was conducted as a function of operational parameters, including the effect of hydraulic residence time, temperature and sulphate loading on system performance. Concurrently, the study compared the utilisation of lactate and acetate as carbon source and electron donor as well as the effect of reactor configuration on system performance. Comparative assessment of the performance between the original 2 L LFCR and an 8 L LFCR variant that reflected the pilot scale design with respect to aspect ratio was conducted. Pseudo-steady state kinetics was assessed based on carbon source utilisation, volumetric sulphate reduction, sulphide removal efficiency and elemental sulphur recovery. Additionally, the hybrid LFCR provided a unique synergistic environment for studying the co-existence of the sulphate reducing (SRB) and sulphide oxidising (SOB) microbial communities. The investigation into the microbial ecology was performed using 16S rRNA amplicon sequencing. This enabled the community structure and the relative abundance of key microbial genera to be resolved. These results were used to examine the link between process kinetics and the community dynamics as a function of hydraulic residence time. Results from this study showed that both temperature and volumetric sulphate loading rate, the latter mediated through both sulphate concentration in the feed and dilution rate, significantly influenced the kinetics of biological sulphate reduction. Partial sulphide oxidation was highly dependent on the availability and rate of sulphide production. Volumetric sulphate reduction rates (VSRR) increased linearly as hydraulic residence time (HRT) decreased. The optimal residence time was determined to be 2 days, as this supported the highest volumetric sulphate reduction rate (0.21 mmol/L.h) and conversion (98%) with effective sulphide removal (82%) in the 2 L lactate-fed LFCR. Lactate as a sole carbon source proved effective for achieving high sulphate reduction rates. Its utilisation within the process was highly dependent on the dominant metabolic pathway. The operation at high dilution rates resulted in a decrease in sulphate conversion and subsequent increase in lactate metabolism toward fermentation. This was attributed to the competitive interaction between SRB and fermentative bacteria under varying availability of lactate and concentrations of sulphate and sulphide. Acetate as a sole carbon source supported a different microbial community to lactate. The lower growth rate associated with acetate utilising SRB required longer start-up period and was highly sensitive to operational perturbations, especially the introduction of oxygen. However, biomass accumulation over long continuous operation led to an increase in performance and system stability. Microbial ecology analysis revealed that a similar community structure developed between the 2 L and 8 L lactate-fed LFCR configurations. This, in conjunction with the kinetic data analysis, confirmed that the difference in aspect ratio and scale had minimal impact on process stability and that system performance can be reproduced. The choice of carbon source selected for distinctly different, highly diverse microbial communities. This was determined using principle co-ordinate analysis (PCoA) which highlighted the variation in microbial communities as a function of diversity and relative abundance. The SRB genera Desulfarculus, Desulfovibrio and Desulfomicrobium were detected across both carbon sources. However, Desulfocurvus was found in the lactate-fed system and Desulfobacter in acetate-fed system. Other genera that predominated within the system belonged to the classes Bacteroidetes, Firmicutes and Synergistetes. The presence of Veillonella, a lactate fermenter known for competing with SRB, was detected in the lactate-fed systems. Its relative abundance corresponded well with the lactate fermentation and oxidation performance, where an apparent shift in the dominant metabolic pathway was observed at high dilution rates. Furthermore, the data also revealed preferential attachment of selective SRB onto carbon microfibers, particularly among the Desulfarculus and Desulfocurvus genera. The microbial ecology of the floating sulphur biofilm was consistent across both carbon sources. Key sulphur oxidising genera detected were Paracoccus, Halothiobacillus and Arcobacter. The most dominant genera present in the FSB were Rhizobium, well-known nitrogen fixing bacteria, and Pannonibacter. Both genera are members of the class Alphaproteobacteria, a well-known phylogenetic grouping in which the complete sulphur-oxidising, sox, enzyme system is highly conserved. An aspect often not considered in the operation of these industrial bioprocess systems is the microbial community dynamics within the system. This is particularly evident within biomass accumulating systems where the proliferation of non-SRB over time can compromise the performance and efficiency of the process. Therefore, the selection and development of robust microbial inoculums is critical for overcoming the challenges associated with scaling up, particularly with regards to start-up period, and long-term viability of sulphate reducing bioreactor systems. In the current study, long-term operation demonstrated the robustness of the hybrid LFCR process to maintain relatively stable system performance. Additionally, this study showed that process performance can be recovered through re-establishing suitable operational conditions that favor biological sulphate reduction. The ability of the system to recover after being exposed to multiple perturbations, as explored in this study, confirms the resilience and long-term viability of the hybrid process. A key feature of the hybrid process was the ability to recover the FSB intermittently without compromising biological sulphate reduction. The current research successfully demonstrated the concept of the hybrid LFCR and characterised sulphate reduction and sulphide oxidation performance across a range of operating conditions. This, in conjunction with a clearer understanding of the complex microbial ecology, illustrated that the hybrid LFCR has potential as part of a semi-passive approach for the remediation of low volume sulphate-rich waste streams, critical for treatment of diffuse ARD sources.
Open Access
Ammonium hexachlororuthenate precipitation
(2011) De Klerk, Frederick Jacobus; Westra, Arjan; Hagemann, Justin; Petersen, Jochen
Ammonium hexachlororuthenate (ACR) is a salt precipitated during a substitution reaction between ammonium and ruthenate dissolved in a strong hydrochloric acid medium. This precipitation reaction is used within the platinum industry as a means of recovering and purifying ruthenium. Application of this process at the Precious Metals Refinery of Anglo Platinum has brought to light certain inefficiencies. In recent years, volatility in the Ru market price occurred, indicating a potential to benefit financially from an improved Ru recovery. Consequently, this study was conducted to understand the effect of certain parameters on the process of precipitating ammonium hexachlororuthenate.
Open Access
An investigation into the enzymatic activity of deepsea actinobacteria in decolourising crystal violet dye
(2019) Davids, Natasha; Petersen, Joachim; Huddy Robert
Crystal Violet (CV) decolourising deep-sea actinobacteria could provide a great source of novel redox biocatalysts that can be used in various applications such as removal of triphenylmethane dyes from contaminated wastewater and soil, degradation of aromatic environmental pollutants, biotransformation of antimicrobial agents and degradation of xenobiotics. CV is a triphenylmethane dye that has various applications, including use in medical, research and industrial applications, but its release into the environment poses a threat to aquatic life as it has characteristics of a biocide. Only a limited number of microorganisms are able to decolourise and degrade CV, and one of these proposed mechanisms by which they do so is the catalytic effect of oxidoreductase enzymes, including peroxidases, polyphenol oxidases and laccases. Triphenylmethane reductase has also been reported to be involved in decolourising CV, but the reaction involving this enzyme has not been studied systematically. Eleven deep-sea actinobacteria were investigated and found to decolourise CV by either biodegradation or biosorption. Gordonia sp. JC 51 was selected as a candidate for further study as it could decolourise CV efficiently and could tolerate high concentrations (1mM) of CV. A combination of spectral scan studies, dye decolourisation, biodegradation assays, enzymatic assays, SDS-PAGE, Native PAGE, TLC and LC/MS/MS methods revealed the mechanism involved in the decolourisation of CV. Gordonia sp. JC 51 decolourised CV via enzymatic and non-enzymatic mechanisms. However, true decolourisation of CV was performed via biodegrading enzymes. Triphenylmethane reductase and polyphenol oxidase was confirmed to be the enzymes involved. Leucocrystal Violet was identified as the metabolite produced. CV also was sequentially N-demethylated, oxidised and cleaved into smaller compounds such as Michler’s Ketone. In conclusion, Gordonia sp. JC 51 has potential as a whole cell biocatalyst and should be investigated further.
Open Access
An investigation into the fundamental understanding of an activated sludge bioremediation process and optimisation of thiocyanate and cyanide destruction
(2019) van Zyl, Andries Wynand; van Hille, Robert; Harrison, Susan
Cyanide (CN) is used in the gold mining industry to dissolve gold from free milling, complex and refractory gold containing ores. Processing sulphide containing refractory ores using biooxidation as a pre-treatment has become increasingly important due to the depletion of free milling ores. The reaction of CN with reduced sulphur species during the cyanidation process results in the formation of thiocyanate (SCN), often at relatively high concentrations (> 5 000 mg/L). The SCN and residual free CN are deported with the tailings as components of the liquid fraction. The concentration of SCN often exceeds the legislated discharge specification, necessitating on-site treatment, while water would also require treatment before on-site recycling and reuse. Biological degradation of CN and particularly SCN in these effluents provides an alternative to the more traditional processes such as SO2 treatment or UV destruction. The traditional destruction processes focus on breaking the chemical bonds, through physical or chemical means, thereby converting the toxic CN and SCN species to less toxic compounds. These processes generally suffer from high reagent cost, incomplete removal of CN and particularly SCN species and the generation of by-products which require further treatment. A number of microorganisms are capable of utilising CN and SCN as a source of sulphur, nitrogen and carbon, as well as generating energy from their oxidation. Additional removal of metal-CN complexes may be achieved by adsorption to the cell surface or extracellular polymeric substances secreted by the cells. The activated sludge tailings effluent remediation (ASTERTM) process was developed for the biological treatment of especially SCN, but also free CN and metal-cyanide complexes, such as CuCN and Zn(CN)2. The basic ASTERTM technology consists of an aerated reactor, in which SCN and CN species are oxidised and a settler to facilitate the recovery of water and potentially biomass. The desire to expand the commercial application of the technology necessitated a more complete, fundamental understanding of the ASTERTM process and required focused, in-depth research. This research aimed to define the viable operating window for SCN destruction, as well as optimising practical SCN and CN destruction process conditions. The ASTERTM process relies on a complex microbial community, so understanding the community structure and metabolic potential for SCN and CN destruction, further enhanced the fundamental and mechanistic understanding of this bioprocess. The research contributed to the fundamental understanding of this technology and enhanced the commercial application thereof. The first step in defining the operating window was to investigate the effect of feed SCN concentration on the SCN destruction ability of the mixed microbial community. Experiments were conducted at feed SCN concentrations ranging from 60- 1 800 mg/L. Complete SCN destruction was achieved across the range at ambient temperature. The maximum SCN destruction rate was 15.7 mg/L.h at an initial SCN concentration of 1 400 mg/L. Temperature was investigated in the range of 10-45°C with an initial SCN concentration range of 60-180 mg/L. A maximum SCN destruction rate of 17.4 mg/L.h was measured at 35°C, with an initial SCN concentration of 180 mg/L. A wide pH range (pH 5.0-10.0) was tolerated, with optimal performance recorded at pH 7.0. This evaluation identified not only the optimum operating pH, but also highlighted the negative impact of a sudden pH change on the efficiency of SCN destruction. Residual SCN concentrations below 1 mg/L were achieved in all cases, which would allow for discharge or recycling of treated water. Floc (sludge) formation was observed in experiments with high initial SCN concentrations and indicated a possible stress response during these batch experiments. Floc (sludge) formation were taken as microbial cells imbedded within extracellular polymeric substances and not only an aggregate of cells. Evaluating the maximum potential for SCN destruction and optimising the operating conditions and system configuration was investigated using continuous reactor experiments. A maximum SCN destruction rate of 87.4 mg/L.h (2 098 mg/L.d) was achieved at a feed SCN concentration of 1 000 mg/L and eight hour hydraulic retention time (HRT) during these experiments. The formation of substantial amounts of sludge was observed, with attachment to the reactor surfaces. The maximum feed SCN concentration, where substantial destruction was measured, was at 2 500 mg/L, achieving a practical SCN destruction rate of 972 mg/L.d. Significant inhibition of microbial inactivity was observed beyond this feed SCN concentration. The microbial community was able recover performance, within six days, after an extended period (54 days) of inactivity when the feed concentration was reduced from 3 500 mg/L SCN to 1 000 mg/L. The nature of the accumulated biofilm did not appear to change during the period of limited SCN destruction activity. Calculation of specific SCN destruction rates was not possible due to the nature of the sludge and heterogeneous dispersion of microbial members. Biomass (cells embedded in the EPS sludge) loading experiments showed SCN destruction rates increased with an increase in biomass loading, but this relationship was not proportional. A 25-fold increased biomass concentration resulted in only a 2-fold increase in destruction rate, suggesting a mass transfer limitation. The sludge most likely offers protection against unfavourable conditions, such as high residual SCN concentrations, by presenting a mass transfer barrier, resulting in an SCN concentration gradient across the sludge matrix. This enhances the robustness of the process and would facilitate rapid recovery in the case of a system upset at commercial scale. This research is the first to demonstrate the effective removal of SCN in the presence of suspended tailing solids, under conditions well suited for commercial application. The maximum SCN destruction rate achieved was 57 mg/L.h in the presence of 5.5% (m/v) solids. Sludge formation was not observed in the reactors containing solids, despite substantial sludge formation under similar operating conditions in the absence of solids, most likely due to shear-related effects. Fluctuations in pH, due to the nature of the solid material, were identified to negatively impact reactor performance and pH control was required. Moreover, the type of solid particle was found to influence the SCN destruction rate showing a need for adaptation not only to the presence of solids but also to various types of solids that are to be treated. Treatment of residual CN in solution is critical to ensure safe disposal or recycling of water. Treatment of SCN and CN was successfully demonstrated at feed concentrations up to 2 000 and 50 mg/L, respectively. The presence of residual CN (0.5 mg/L) prevented complete destruction of SCN, while complete SCN destruction was measured in the absence of CN under identical conditions. A range of reactor configurations were investigated and the optimum system required biomass retention, by means of attached biomass and complete destruction of any residual CN prior to SCN destruction. Conversion of SCN-S to SO4-S was stoichiometrically proportional in solution, while the majority of the liberated nitrogen appeared to be assimilated. Pre-colonisation of the reactor with attached biomass is beneficial and removed the need for a solid-liquid separation unit, reducing the potential footprint of the process. Additional treatment capacity could be created by operation of reactors in series. The diversity of the microbial community responsible for destruction of especially SCN were shown to be far more extensive than initially expected. Initial molecular characterisation of the microbial community selected for 185 representatives of bacterial 16S rRNA genes, of which 106 non-identical genotypes were sequenced. In contrast, for the reactor containing solids, only 48 representatives were selected and 30 genotypes were sequenced. Bacteria implicated in SCN destruction in the reactor containing suspended solids were members from the genera Bosea, Microbacterium and Thiobacillus. In the absence of solids, members capable of SCN destruction were identified from genera including Thiobacillus and Fusarium. High-throughput genome sequencing, followed by sequence assembly confirmed the dominance of Thiobacillus spp. Metabolic predictions indicated the autotrophs, gaining energy from the oxidation of reduced sulphur intermediates produced during SCN destruction were the dominant community members. The potential for ammonium oxidation and denitrification within the microbial community was identified during analysis of the metabolic potential, based on the metagenomic sequence data. These would be required for complete remediation of wastewater. The data generated during the research led to the development of a conceptual model to describe the evolution of system performance. Following inoculation with planktonic culture the SCN destruction is performed by the planktonic microbial community. An increased residual SCN concentration results in floc formation and the colonisation of reactor surfaces by attached biofilm. A concomitant decrease in planktonic cell concentration was observed, while SCN destruction rates increased. The extracellular material provided a matrix for biomass retention, resulting in high cell concentrations, and provided some protection against high SCN concentrations by providing a barrier to mass transfer. The attached biofilm developed to the point where overall SCN degradation rates may become limited by reduced oxygen penetration. The research presented in this thesis has been used to inform the design and operation of the ASTERTM process at commercial scale, specifically with respect to the benefits of attached biomass and the demonstration that the process can be used in the presence of suspended solids. The latter has been particularly important in applications where the available footprint is constrained.
Open Access
Anaerobic digestion of algal biomass for bioenergy production- a feasibility study
(2011) Inglesby, Alister Edward; Van Hille, Rob; Harrison, STL
AD technology is well developed, cost efficient and can be easily implemented in developing countries. Biogas production has become a very topical subject, with many European nations introducing initiatives to increase biogas production. AD of algal biomass was studied in detail during the 1980’s, however, with the current drive toward cleaner technology processes, there has been a renewed interest in the technology. This study investigated the feasibility of using algal biomass as a feedstock for AD.
Open Access
Anaerobic digestion of Spirulina sp. and Scenedesmus sp.: a comparison and investigation of the impact of mechanical pre-treatment
(Springer, 2015-05) Inglesby, A E; Griffiths, M J; Harrison, S T L; Van Hille, R P
AAnaerobic digestion (AD) is a unit process that integrates beneficially and sustainably into many bioprocesses. This study assesses and compares the production of methane from the biomass of the microalga Scenedesmus sp. and the cyanobacterium Spirulina sp. in batch anaerobic digesters. Anaerobic digestion of whole cell Spirulina resulted in a substantially higher methane productivity (0.18 L CH4 Lreactor −1 day−1) and methane yield (0.113 L CH4 g−1 volatile solids (VS)) compared to the digestion of whole cell Scenedesmus (0.12 L CH4 Lreactor −1 day−1 and 0.054 L CH4 g VS−1). Spirulina, possibly due to a combination of osmotic shock, the filamentous nature of the cells and lower mechanical strength of the non-cellulosic cell wall, was more readily degraded by hydrolytic and acidogenic microorganisms, resulting in the generation of a greater amount of acetic acid. This in turn provided greater substrate for methanogens and hence higher methane yields. In addition, Spirulina cells could be disrupted mechanically more quickly (1 h) than Scenedesmus cells (4 h) in a bead mill. Mechanical pre-treatment improved the final methane yields (L CH4 g VS−1) obtained from digestion of both substrates; however, the improvement was greater for Scenedesmus. Mechanical pre-treatment resulted in a 47 % increase in methane production for Spirulina compared to 76 % increase for Scenedesmus fed digesters. The more substantial increase observed for Scenedesmus was due to the relatively inefficient digestion of the whole, unruptured cells.
Open Access
Analysis of particle suspension and mixing in biological systems : an application of tomography
(2006) Stevenson, Ryan; Harrison, STL
Includes bibliographical references.
Open Access
Analysis of the microbial community associated with a bioprocess system for bioremediation of thiocyanate- and cyanide-laden mine water effluents
(Trans Tech Publications, 2015-11) Huddy, Rob; Kantor, Rose; Van Zyl, Wynand; Van Hille, Robert P; Banfield, Jill; Harrison, Susan T L
Gold extraction by cyanidation from refractory gold ores results in the formation of thiocyanate- and cyanide-contaminated wastewater effluents that must be treated before recycle or discard. Activated sludge processes, such as ASTER™, can be used for biodegradation of these effluent streams. The destruction of these compounds is catalyzed by a mixed microbial culture, however, very little is known about the community composition and metabolic potential of the thiocyanate- and cyanide-degrading microorganisms within the community. Here we describe our on-going attempts to better understand the key microorganisms, within the ASTER™ bioprocess, that contribute to the destruction of thiocyanate and cyanide, and how this knowledge relates to further process optimisation.
Open Access
The anodic dissolution of covellite in acidic, chloride solutions
(2010) Basson, Petrus; Petersen, Jochen
An electrochemical study was conducted on a stationary, synthetically produced, covellite electrode in acidic, chloride solutions at ambient conditions to investigate the dissolution behaviour of the mineral over a surface potential range from the open circuit potential (OCP) to about 0.62 V (vs. SHE). The electrode was mounted in an apparatus, which was designed to resemble leaching of the mineral under conditions applicable to heap leaching of whole ores, where the mineral occurs in cracks or pores in the gangue matrix or is covered (or partially covered) by reaction products.
Open Access
Application of electrical resistance tomography in evaluating the influence of nozzle design on the gas hold-up in boiling bubble column reactors
(2012) Sudhakaran, Harikrishna
Bubble column reactors are extensively used in the petro-chemicals industry due to the combined advantages of high rates of heat and mass transfer coupled with low operating and maintenance costs. The complex hydrodynamics brought about by the multiphase nature of such systems offer significant challenges in modelling, analysis and operation, thus making experimental measurement of system response of special importance. One such measurement technique is Electrical Resistance Tomography (ERT). In this study, the development of an experimental bubble column reactor fitted with 8 rings of 16 electrodes for ERT measurements using an instrument developed at the University of Cape Town (UCT) is described.
Open Access
Application of mineralogy in the interpretation of laboratory scale acid rock drainage (ARD) prediction tests : a gold case study
(2014) Dyantyi, Noluntu; Becker, Megan; Broadhurst, Jennifer Lee
The mining and beneficiation of gold generates large tonnages of waste, with up to 99% of mined gold ore discharged as waste. The waste generated contains unoxidized sulfides that when exposed to air and water react to form acid, which results in acid rock drainage (ARD). ARD is usually associated with low pH, high sulfate content and elevated concentrations of toxic elements. The mobility of ARD affects our scarce water resources, land and aquatic species. Methods applied to treat ARD do not provide a walk-away solution and they are either expensive or difficult to maintain. The best solution to completely eradicate ARD is to prevent it from the source. However, the effectiveness of ARD prevention depends on the accuracy of predicting future drainage quality. This can be done by using ARD prediction tests, which are generally classified as either static (acid base accounting, ABA, net acid generation, NAG) or kinetic (column leach, humidity cell, biokinetic test). There is no single test capable enough to accurately predict acid generating potential. It is therefore usual practise to conduct more than one test and cross-check results to ensure that the appropriate conclusions are made. In doing so, the reliability of the tests is improved but in some cases the different test results do not correlate. Mineralogy is an analytical technique that can be used to understand the nature of the errors and to better understand the leaching behaviour of minerals in the different tests. This study uses mineralogy to analyse both static and biokinetic test results of a Witwatersrand gold sample in order to improve the understanding of behaviour of mine wastes under different ARD prediction test conditions. A run-of-mine gold sample from the Witwatersrand region in South Africa was used as a case study to explore the mineral leaching behaviour for different ARD prediction tests.
Open Access
An approach of compartmentalisation in development of non-isothermal chemical reactor network models for the high speed simulation of iso-octane combustion
(2011) Khan, Zamier Ahmed; Rawatlal, Randhir
Every aspect of the modern day life relies on combustion, be it in motor vehicles, industrial equipment or power generation. The downside to the extensive use of combustion technology is the environmental pollution produced by the process. The lack of fast solving models to simulate combustion hampers the investigation into the optimisation of combustion processes. In this study, the compartment approach in developing a fast and accurate simulation is used to investigate combustion systems. A chemical reactor network (CRN) is proposed for the simulation of the combustion of iso-octane. The compartmentalisation of a combusting system involves proposing a reactor network based on the flow fields predicted by computational fluid dynamics (CFD). The first step in the development of such a model involves using of a reduced kinetic model representing thousands of combustion steps in a few elementary steps by lumping species. The reduced kinetic model used in this study consists of a five-step mechanism involving four pseudo species. The thermodynamic properties of the pseudo species in the system were regressed against experimental data and successfully validated using the plug flow and continuous stirred tank reactor sub-models. The reduced kinetic model was also further validated using Rapid Compression Machine data. The current study also modified the methodology for developing a CRN in order to make the CRN more predictive as compared to previous studies. This was achieved by incorporating non-isothermal sub-models into the network instead of isothermal sub-models that rely on the CFD temperature field. The network parameters were also correlated to the inlet Reynolds number in order to further increase the predictive nature of the network for industrial applications and to allow for the systems performance to be predicted over a wide range of input conditions. The investigation begins by conducting a CFD simulation of iso-octane combustion in a furnace and double inlet reactor assuming a one-step global reaction. On the basis of the CFD flow fields, a CRN was proposed and coupled to the reduced kinetics.
Open Access
Assessing environmental risks associated with ultrafine coal wastes using laboratory-scale tests
(Trans Tech Publications, 2015-11) Opitz, Alex; Broadhurst, Jenny; Harrison, Sue
Characterisation of the risk of acid rock drainage is typically achieved through the quantification of acid-generating and acid-consuming components present within a sample using initial laboratory-scale, chemical static tests. These static tests, however, consider ARD generation under chemical conditions and do not account for the role of micro-organisms. Their focus is exclusively on the net potential for acid generation, with no account of metal deportment or the relative rate of acid generation and consumption. The present study investigates the ARD potential of two ultrafine coal wastes samples using the standard static tests as well as the UCT biokinetic test to account for microbial ARD generation. The deportment of metal species under each test condition was also considered. The UCT biokinetic test results supported the static test classification, providing preliminary kinetic data on the ARD generation. Sequential chemical extraction tests allowed for differentiation of the host minerals according to their leaching potentials, providing supporting evidence for the deportment of metal species under the characterisation tests, thereby improving the knowledge base on which to classify coal wastes as benign or otherwise.
Open Access
Assessment of water pollution arising from copper mining in Zambia: a case study of Munkulungwe stream in Ndola, Copperbelt province
(2018) Mudenda, Lee; Harrison, Sue; Syampungani, Stephen
Water pollution is recognized as one of the major environmental problems in the mining industry. This has been compounded with an increase in agriculture activities. Water pollution is a major problem on copper and coal mines throughout the world and Zambia, the focus of this study, is no exception. Worldwide freshwater resources, which provide important ecosystem services to humans, are under threat from rapid population growth, urbanization, industrialization and abandonment of wastelands. There is an urgent need to monitor and assess these resources. In this context, the physical, chemical and ecological water quality of the Munkulungwe Stream located on the Copperbelt of Zambia, was assessed with possible contamination from Bwana Mkubwa TSF, agriculture activities and subsequent impact on the surrounding community. The chemical and physical parameters were assessed at four sampling locations. Sampling site S1 was located on the Munkulungwe stream upstream of Bwana Mkubwa TSF, S2, S3 and S4 were on the main stream downstream of Bwana Mkubwa TSF. In addition, a macroinvertebrate composition analysis was performed to estimate the quality of water using the biotic index score. Finally, the relationship between physiochemical parameters and biotic index score was analysed to interrogate their inter-relationship with respect to water quality. The results showed that the average values of dissolved oxygen (DO) of 4.52 mg/l, turbidity (40.96 NTU), Co (0.24 mg/l), Pb (0.25 mg/l), Fe (0.36 mg/l) and Mn (0.22 mg/l) downstream exceeded international standards for drinking water. Upstream, the values of Co, Pb, Fe and Mn were within acceptable standards for drinking water, DO and turbidity were above acceptable standards. The metal concentration and total dissolved solutes were impacted by closeness to the mine tailings deposit with the heavy metal concentration being highest at S2 and S3. Moreover, high turbidity levels revealed that land erosion induced by agriculture activities is a severe problem in the area. Physical parameters were high in the rainy season due erosion escalated by rains while chemical parameters were high post rainy season. During the rainy season, the chemical contaminants are diluted and thus they are not such a big impact, but they tend to concentrate up during the dry MDNLEE001 III season. The stream at sampling points S2 and S3 was dominated by species tolerant (leech, Isopod and Snail: Pouch) and semi tolerant (Blackfly larvae and Amphipod or Scud) to pollution. The change in season influenced the composition of macroinvertebrates, with the number of species increased post rainy season. The average biotic index score (2.5) showed that the stream condition is not good, it is slightly polluted. The results showed that water quality downstream was substantially affected by Bwana Mkubwa TSF, agriculture activities and is likely to affect human health and food security. It is recommended that groundwater surrounding tailings dams should be monitored in both active and abandoned mines. Curtain boreholes around a tailings dam can be drilled and the water extracted and treated so that it doesn't contaminate other water bodies. To improve the environmental management of mining related impacts in Zambia, mining areas should be completely rehabilitated. There is need for remediation strategies for abandoned mine sites. Constructed wetlands, roughing filtration and phytoremediation are highly promising techniques, as they are reliable, cheap, effective and sustainable.
Open Access
Atmospheric leaching of a saprolytic nickel laterite ore in chloride solutions
(2008) Field, Karen Louise; Gaylard, Peter
Open Access
Base metal heap and tank leaching of a platreef flotation concentrate using ammoniacal solutions
(2013) Muzawazi, Caroline; Petersen, Jochen
The technical feasibility of the ammonia leaching process of a Platreef flotation concentrate was investigated in different reactor settings i.e. shake flasks, columns and batch stirred tank reactors, respectively. The process investigated aims to use either a heap leaching environment or tank leaching of untreated low-grade concentrates under ambient conditions and mild temperatures. This process is proposed as an alternative primary treatment method for the recovery of base metal sulphides from a PGM containing concentrate that cannot be extracted economically by conventional milling, smelting and refining methods.
Open Access
Biocatalytic studies of phenol oxidases producing antioxidants
(2007) Ncanana, Sandile Welcome; Burton, Stephanie Gail
In recent years there has been increasing interest in the production of oligomers and polymers of economic importance using biocatalysts; the application of enzymes in dimerisation or polymerisation processes can lead to the synthesis of unique compounds, with novel properties, that could not be easily achieved by conventional methods. Further, the application of biocatalysts in polymerization processes can be exploited in development of bioremediation systems, and there is a demand for new technologies that can be utilized in the removal of organic pollutants such as phenolics from contaminated environment. This study reports on the potential application of laccase, obtained from the white rot fungi Trametes pubescens, in the synthesis of organic compounds which are dimers or polymers, and in the development of bioprocesses of potential economic importance. The focus of this study is, particularly, on the effect of organic solvents and the structure of the substrates on the nature of products formed. The thesis also gives some insight into the relationship between the structure of laccase products and their biological (antioxidant and antimalarial) activity. The compounds tyrosol, hydroxytyrosol, 8-hydroxyquinoline, and totarol were selected as the model compounds for laccase reactions. Tyrosol was oxidised by laccase, yielding dimeric and polymeric compounds which were identified by LC-MS and IH-NMR. As a comparative study, hydroxytyrosol was also oxidised by laccase yielding dimeric, trimeric and polymeric compounds which were identified by LC-MS. Manipulation of this biocatalytic system resulted in development of an efficient process that allows for selectivity with respect to the products. A system was then developed whereby oxidation of hydroxytyrosol by laccase would selectively yield either dimers or oligomeric products. Thus, use of 50 % acetone in the reaction medium favoured the synthesis of dimeric products, and 20 % methanol resulted in the formation of a polymeric product. These results showed that hydroxytyrosol-laccase reactions were more readily controlled than tyrosol-laccase reactions, and this difference was attributed to structural configuration of these substrates.
Open Access
Biolistic-mediated transformation of Eragrostis Curvula with the HSP 12 gene
(2003) Ncanana, Sandile Welcome; Farrant, Jill M; Lindsey, George G; Brandt, Wolf F
Eragrostis curvula is a desiccation sensitive monocotyledonous plant and an economically important forage grass in southern Africa. This species has a potential to be improved for drought and salt resistance among other important agronomic traits. In this study, E. curvula was used as a model system to explore the feasibility of producing pasture grasses with increased tolerance to water deficit. To date, no reports have been published on transformation of this species. This study reports the transformation of E. curvula with Saccharomyces cerevisiae Hsp 12 gene using biolistic-mediated transformation. Firstly, a tissue culture protocol was established for E. curvula that was suitable for transformation studies. Although this species has been previously regenerated in vitro using inflorescence tissues, this study established new protocols that utilize leaf and seed as source of material. The aim of which was to find the best regenerable tissue that could be used for transformation studies. Plant regeneration was achieved from shoot explants cultured on MS medium supplemented with either 0.5 mg/I thidiazuron (TDZ) or 0.5 mg/I 6-benzylaminopurine (BAP) through the process of direct organogenesis. It was found that TDZ was the most effective cytokinin. Plant regeneration was also achieved from callus induced from immature leaves on MS medium supplemented with 2 mg/I 2,4- 0,0.01 mg/I BAP and 6 % (w/v) sucrose. Histological experiments performed gave clear evidence that plant regeneration from callus was through a process of indirect organogenesis. The regeneration protocol was combined with an optimized biolistic mediated transformation protocol using the PDS-I00/He apparatus of which both shoot explants and callus were used as target tissues. In the molecular aspect of the study, the Hsp 12 gene was ligated in the Sac I restriction site of pCAMBIAUbeeQ vector. The successful cloning of the Hsp 12 gene was confirmed by PCR and restriction endonuclease digestion. The resultant vector pCAMBIAUbeeQ Hsp 12 was purified and subsequently used for biolistic transformation of E. curvula. The regenerable shoot explants and callus tissue were bombarded with DNA (pCAMBIAUBeeQHsp12) coated on gold particles. As a comparative study, Agrobacterium tumefaciens was transformed with pCAMBIAUBeeQHsp12 vector for subsequent transformation of Nicotiana tabacum. Transient expression of GUS gene in transformed E. curvula shoot explants was visualized 72 h after bombardment. Optimized conditions for expression of GUS gene were gold micro projectiles, 7 cm travel distance and helium pressure of 9100 kPa. Transformed tissues were cultured on the regeneration medium without antibiotic selection. Putative transformants were generated and the presence of Hsp 12 gene was verified by PCR and its expression at RNA level was confirmed by RT-PCR. The presence of Hsp 12 protein in the transformed plants was analyzed using SDS-PAGE and MALDI-TOF peptide mass spectrometry.
Open Access
Biological conversion of alkanes to dicarboxylic acids : an investigation into process challenges and optimisation in hydrocarbon-based bioprocesses
(2005) Williams, Peta Clair; Harrison, STL; Clarke, K G; Smit, MS
The focus of this project is bioconversion of alkanes to dicarboxylic acids. Dicarboxylic acids are versatile chemical intermediates that can be used in the manufacture of perfumes, polymers, adhesives and antibiotics. The use of a hydrocarbon in a biological process, however, introduces several process challenges related to the nature of the substrate. Many of these challenges are common to all hydrocarbon fermentations, regardless of the product formed, and include flammability, volatility and inhibition of cell growth (notably at low carbon chain lengths), insolubility (notably at high carbon chain lengths) and mass transfer limitations, with respect to both oxygen and alkane substrate. In particular, the provision of adequate oxygen transfer to the organism in hydrocarbon-based bioprocesses has been regarded as especially challenging because of the absence of oxygen in the hydrocarbon backbone. In contrast to carbohydrate-based bioprocesses in which the carbohydrate itself supplies about half of the oxygen, the metabolic requirement for oxygen in hydrocarbon-based bioprocesses has to be met entirely by the transfer of oxygen to the broth. This suggests a proportionately higher requirement for oxygen transfer under these conditions. Consequently, the oxygen transfer rate (OTR) has been mooted as a likely major process limitation, leading to a process which is transport, rather than kinetically controlled and correspondingly, a sub-optimal yield and productivity.