Abstract
Anaerobic digestion (AD) is a well-established technology used for the treatment of wastes and wastewaters with high organic content. During AD organic matter is converted stepwise to methane-containing biogas—a renewable energy carrier. Methane production occurs in the last AD step and relies on methanogens, which are rather sensitive to some contaminants commonly found in wastewaters (e.g. heavy metals), or easily outcompeted by other groups of microorganisms (e.g. sulphate reducing bacteria, SRB). This review gives an overview of previous research and pilot-scale studies that shed some light on the effects of sulphate and heavy metals on methanogenesis. Despite the numerous studies on this subject, comparison is not always possible due to differences in the experimental conditions used and parameters explained. An overview of the possible benefits of methanogens and SRB co-habitation is also covered. Small amounts of sulphide produced by SRB can precipitate with metals, neutralising the negative effects of sulphide accumulation and free heavy metals on methanogenesis. Knowledge on how to untangle and balance sulphate reduction and methanogenesis is crucial to take advantage of the potential for the utilisation of biogenic sulphide as a metal detoxification agent with minimal loss in methane production in anaerobic digesters.
Highlights
Anaerobic digestion (AD) is a well-established and efficient process for waste and wastewater treatment
Methanogens are a key group in AD, because when methanogenic activity is inhibited digestion is blocked at the acidogenesis step leading to an incomplete degradation of the organic matter
Granular sludge shows higher resistance to toxicity than flocculent sludge (Lin and Chen 1997). Such higher resistance of the granules is explained by their layered microstructure where the most sensitive microorganisms, such as methanogens, are found mainly in the interior while the exterior of the granule is mainly composed of fermentative bacteria which are more resistant to metal toxicity (Fang and Hui 1994)
Summary
Anaerobic digestion (AD) is a well-established and efficient process for waste and wastewater treatment. The process is based on the degradation of organic matter by a network of diverse microorganisms, with ultimate formation of methane-containing biogas (a renewable energy carrier) (Fig. 1a). The different groups of microorganisms involved in AD (fermenters, volatile fatty acids (VFA) oxidizers, and methanogens) have diverse nutritional demands and growth properties. Methanogens are a key group in AD, because when methanogenic activity is inhibited digestion is blocked at the acidogenesis step leading to an incomplete degradation of the organic matter. Optimisation of methanogenesis is still a challenge, and that is mainly due to the low growth rates of methanogens and their high susceptibility to changes in environmental conditions and sensibility to toxic compounds (Chen et al 2008).
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