Abstract
Anaerobic digestion (AD) is an effective biological treatment for stabilizing organic compounds in waste/wastewater and in simultaneously producing biogas. However, it is often limited by the slow reaction rates of different microorganisms’ syntrophic biological metabolisms. Stable and fast interspecies electron transfer (IET) between volatile fatty acid-oxidizing bacteria and hydrogenotrophic methanogens is crucial for efficient methanogenesis. In this syntrophic interaction, electrons are exchanged via redox mediators such as hydrogen and formate. Recently, direct IET (DIET) has been revealed as an important IET route for AD. Microorganisms undergoing DIET form interspecies electrical connections via membrane-associated cytochromes and conductive pili; thus, redox mediators are not required for electron exchange. This indicates that DIET is more thermodynamically favorable than indirect IET. Recent studies have shown that conductive materials (e.g., iron oxides, activated carbon, biochar, and carbon fibers) can mediate direct electrical connections for DIET. Microorganisms attach to conductive materials’ surfaces or vice versa according to particle size, and form conductive biofilms or aggregates. Different conductive materials promote DIET and improve AD performance in digesters treating different feedstocks, potentially suggesting a new approach to enhancing AD performance. This review discusses the role and potential of DIET in methanogenic systems, especially with conductive materials for promoting DIET.
Highlights
Organic compounds in waste and wastewater are generated from various municipal and industrial sources and can be treated by biological processes
We demonstrated that its use enhanced organic degradation and methanogenesis rates
The main limitation of the interspecies electron transfer (IIET) mechanism is that any stagnation in this process causes the loading conditions, fast growth of acidogenic bacteria can lead to excessive accumulation of volatile fatty acids (VFAs) accumulation of VFAs, and high VFA concentrations are toxic to methanogens
Summary
Organic compounds in waste and wastewater are generated from various municipal and industrial sources and can be treated by biological processes. In AD, organic matter is converted into CO2 and CH4 through a series of metabolic reactions of microorganisms This process affords the advantages of both energy production and wastewater treatment. Interspecies electron transfer (IET) between syntrophic partners plays an important role in oxidizing higher organic matters and reducing CO2 to CH4 in AD environments [5] The balance of this syntrophic relationship provides thermodynamically favorable conditions for the degradation of carboxylic acids, and for realizing stable anaerobic digesters. The specific characteristics of different types of conductive materials vary, all show similar behavior with regards to facilitating DIET in AD This suggests the potential for establishing a relatively simple method to improve the efficiency and rate of AD by using conductive materials as promoters. This review discusses the role and significance of DIET in the AD process and summarizes recent findings on its mechanisms and potential applications
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