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Bioremediation of Wastewater by Sulphate Reducing Bacteria

Introduction

Sulphate reducing bacteria (SRB) are a diverse group of microorganisms that have a vital role in the biogeochemical cycle of sulphur in anaerobic environments. Sulphate reducing bacteria are prokaryotic microorganisms that belong to bacteria and arcliaea domains. During the energy metabolism in SRB, sulphate acts as the terminal electron acceptor resulting in dissimilatory sulphate reduction. Due to then importance in the environmental remediation and ecosystem functioning, research interests in SRB have been ascending for the past few years. SRB can be considered as a problem due to the formation of hydrogen sulphide, as well as due to biocorrosion. However, there are various advantages of SRB in wastewater treatment, particularly the less sludge production and also the potential for removal of hazardous materials like heavy metals. Moreover, treatment of wastewater using SRB can be considered as a pretreatment for anaerobic digestion. Given the suitable environmental conditions such as presence of sulphate, temperature, and anoxic conditions, SRB has immense applications in wastewater treatment. Another highlight of SRB is their versatility. Most SRB are mesophilic, although there are some thermophilic and psychrophilic species available in the nature. Regarding pH, most SRB prefer acidic environment; however, there are SRB which thrive and prefer alkaline and hyper salute environments like marine sediments. Such vast adaptation makes SRB preferred candidates to treat various types of wastewater containing sulphate. This chapter explores the various applications of SRB in the treatment of different types of wastewater containing sulphate, heavy metals and chlorinated organic compounds.

Ecological significance of SRB

Sulphate reduction is one of the most commonly occurring and extensive microbiological processes on the earth. SRB are physiologically unique among living organisms in being able to reduce sulphate to sulphide. SRB was first discovered by Bejemeck (1895), and around 220 species of 60 genera have been identified until now. There are five divisions within the bacteria, and two divisions among the archea. These include: the spore-forming Desufotomaculiim, Desulfosporomusct and Desiilfosporosimis species within the Finnicutes division; Deltaproteobacteria and

Thermodesulfovibrio species within the Nitrospira division; and two phyla represented by Thermodesulfobium narugense and Thermodesulfobacteriuml Thermodesulfatator species, and two divisions within the archaea (euryarchaeotal genus Arcbaeoglobus, and the two crenarchaeotal genera Thermocladium and Caldivirga, affiliated with the Thermoproteales) (Castro et al. 2000, Itoh et al. 1998, 1999, Mori et al. 2003, Muyzer and Stains 2008, Ollivier et al. 2007, Rabus et al. 2006).

Representatives of sulphur reducing bacteria are found in environments ranging from brackish, super cooled Antarctic waters to hot artesian springs and deep Pacific sediments (Fauque 1995, Loubinoux et al. 2002, Muyzer and Stains 2008, Ollivier et al. 2007, Rabus et al. 2006). Generally, SRB prefer sulphate rich habitat (Cypionka 2000, Fareleria et al. 2003, Sass et al. 1992) like marine sediments, where sulphate reduction is a predominant terminal electron accepting process during the mineralisation of organic matter (Vincent et al. 2017). However, they are also prevalent in freshwater habitats, anaerobic digesters and gastrointestinal tract of humans and animals (Postage 1984). hi these environments, the SRB influences overall biogeochemistry by the oxidation of organic matter and concomitant sulphide production and/or metal reduction (Hao et al. 1996).

SRB are versatile in nature, and have the ability to use electron acceptors other than sulphates in facultative anerobic conditions, including elemental sulphur (Bottcher et al. 2005, Finster et al. 1998), fiunarate (Tomei et al. 1995), dimethyl sulfoxide (Jonkers et al. 1996), Fe (III) (Lowely et al. 1993, 2004), and nitrate (Krekeler and Cypionka 1995). Earlier, SRB was believed to utilize only a limited range of substrates as energy source such as lactate, molecular hydrogen, pyruvate, and ethanol; however, later studies revealed a wide range of electron donors used by SRB (Rabus et al. 2006) such as fructose, glucose, amino acids, monocarboxylic acids, oxalic acids, alcohols and aromatic compounds (Fauque et al. 1991, Rabus et al. 2006).

SRB in domestic wastew ater treatment

The application of SRB in domestic wastewater treatment is limited compared to industrial wastewater. This is because the concentration of sulphate in domestic wastewater is comparatively less (500 mg L_1) than industrial wastewater. The advantage of SRB to treat municipal wastewater is the low sludge yield that significantly reduces the amount of excess sludge produced. However, the challenge in application of SRB is the limitation of sulphate in wastewater. At higher sulphate levels, a maximum of 75% chemical oxygen demand (COD) is removed by SRB (van der Brand et al. 2018).

 
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