Bioremediation is the use of living organisms, primarily microorganisms, to degrade environmental contaminants into less toxic forms. Research has demonstrated that there are very few environments where microbes have not been able to survive, adapt, and indeed, thrive. Microbes are able to utilise a near infinite combination of electron donors and electron acceptors to drive their metabolism. In addition to these redox (oxidation/reduction) reactions, they have also developed a myriad of other strategies enabling them to detoxify their environment. Bioremediation applies these principles to select a suitable combination of microbial community activity, electron donor/acceptor/contaminant concentrations and other physical and practical parameters to remediate/recover a targeted pollutant.

Bioremediation strategies are often more beneficial than traditional strategies, because it can be implemented in situ (directly at the site of the contaminant, with no need to transport the contaminated material). Innovative in situ technologies permit biological treatment of contaminated water by means of reactive molecules produced by microbes. This provides a simpler, less intrusive, and cheaper method than conventional ‘pump and treat’ systems which often employ hazardous chemicals that create an additional environmental risk.

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ACID MINE DRAINAGE

Acid mine drainage (AMD) is a worldwide problem commonly associated with coal mining, and is characterised by yellow tailings typical of low pH (<3) and containing high metal and sulphate concentrations (± 3 000 ppm). Approximately 80% of South African electricity demand is met by burning coal and understandably, coal mining is a massive industry with the resultant problem of AMD. The treatment approach relies on an active treatment (e.g. anaerobic filter bioreactors), passive treatment (e.g. anoxic ponds) or a combination of the two. These sulphate-reducing bioreactors increase the alkalinity, pH, and reduce the heavy-metal concentrations. Influent conditioning and effluent polishing treatment processes may be required to optimise the bioreactor performance and/or achieve the desired water and environmental quality objectives. The bioreactor employs natural biological processes to create anoxic conditions by controlling the oxidation reduction (redox) state.

In order to adapt this process to the removal of high levels of sulphate by a sulphate-reducing microbial consortium, the microbial portion of the process, during which hydrogen sulphide is produced, must be separated from processes in which significant mineral phase precipitation occurs. This will prevent clogging of the bioreactor and minimise operation and maintenance issues. Both biological and abiotic conditioning and polishing processes is investigated, with the most appropriate being incorporated into the final design.

PETROLEUM AND DIESEL BIOREMEDIATION

The basic requirements for bioremediation of petroleum to occur, include a food source (hydrocarbons), oxygen (and even this is now being re-evaluated), and nutrients (phosphorus and nitrogen), in a compatible environment (suitable pH, temperature and moisture). Other nutrients such as potassium, calcium, iron, manganese, cobalt, copper, and zinc, are generally present in adequate concentrations in most soil systems, and usually do not need amendment in the design of a bioremediation process. The challenge in designing a bioremediative strategy is how to provide these essential requirements to the microorganisms in a practical way. Populations of petroleum-degrading microbes in most environments are limited by the availability of hydrocarbons. Following an oil spill, this limitation is overcome and the microbial population expands.

In order for this to occur, the microbes need oxygen for respiration and other nutrients such as nitrogen or phosphorus that are not available in petroleum. Thus, the growth of oil-degrading microbes is generally limited by the availability of nitrogen and phosphorus. Since petroleum is a complex mixture of many different classes of hydrocarbons, of which any particular microorganism has the potential to degrade only part, it follows that the microbial population will consist of a changing consortium that will adapt according to the substrates available to them. Remediation focuses on stimulating aerobic microbial activity through aeration, moisture, and nutrient additions.

HEXAVALENT CHROMIUM BIOREMEDIATION

Heavy metal contamination of natural sources is a common and serious problem caused by various industrial activities. Chromium finds extensive use in many applications such as mining operations, metal-plating facilities, power-generation facilities, and tanneries. The industrial waste produced from these sources has a significant negative impact on the environment as a whole and remediation of ground- and wastewater and affected soil is a great concern worldwide.

Biological reduction of Cr(VI) with indigenous bacteria in semi-passive systems can be an environmentally sound alternative or complementary technology to active chemical treatment technologies, but at comparatively lower lifecycle costs with less specialised labour requirements. Bacterial reduction of Cr(VI) to its less toxic form is a complex phenomenon and can proceed under both aerobic and anaerobic conditions, utilising membrane or cytoplasmic proteins that may or may not require the presence of co-factors. Dissimilatory chromate reducing bacteria utilise Cr(VI) as terminal electron acceptor, ultimately precipitating it as insoluble chromium hydroxides, demonstrating the potential for remediation.

HEXAVALENT URANIUM BIOREMEDIATION

Since the 19th century, South Africa’s economy has been based on the production and export of minerals, which in turn, have contributed significantly to the country’s economic and industrial development. About 240 000 tons of uranium was exported in the early 1950s by uranium-producing gold mines in South Africa, but it is estimated that approximately 600 000 tons is still contained in gold-mining tailings. Groundwater plays a vital role in the migration of uranium and usually this is an uncontrolled process in the environment. As a result, it is not surprising that slime dams and areas around former gold-uranium mines are contaminated with uranium.

This project focuses on the biological reduction of soluble uranium by an indigenous bacterial community to interact and reduce U(VI). The geochemical modelling profile showed that uranium reduction was successful, by theoretically precipitating U(IV). These results showed that up-flow bioreactors can be used as a low-cost, low-maintenance and effective bioremediation strategy. These benchmark scale reactors can define all the parameters for up-scaled reactors on site bioremediation systems for uranium-impacted environments.