Improving anaerobic digestion via direct interspecies electron transfer requires development of suitable characterization methods

Abstract

Many recent AD studies mistakenly link performance improvements exclusively to DIET.• Unequivocally proving the occurrence of DIET requires a complex set of methods.• This paper groups DIET characterization methods in four categories and discusses their strengths and weaknesses. • This paper presents promising DIET detection approaches that merit further exploration.

A Comprehensive Understanding of Electro-Fermentation

“…In BESs, EF with an applied or in-situ potential can use acetate, propionate, and butyrate present majorly in waste effluents for chain elongation to produce MCFAs. Various experiments have been carried out for producing n-caprylate using ethanol as an electron donor [123]. Clostridium kluyveri observed the formation of caproate (C6) and caprylate (C8) from acetate in a BES at −0.9V against a normal hydrogen electrode (NHE) on the cathode with hydrogen provided in-situ as an electron donor [121].…”

Methane production by acetate dismutation stimulated by Shewanella oneidensis and carbon materials: An alternative to classical CO2 reduction

“…Our results reveal first that S. oneidensis MR-1 improved methane production. Indeed, although exoelectrogenic bacteria have been shown to favor DIET-mediated methane production [7,8,10], only Geobacter bacterial species have been found so far to participate in DIET with methanogens [19]. Shewanella species and methanogens have been previously detected simultaneously in the same niche [26,27], but no evidence was given for increasing methane production by exoelectrogenic activity.…”

“…Geobacter spp., with methanogenic archaea, but major electroactive microorganisms such as Geobacter spp. are not always detected in improved methane production systems [19]. Two recent studies suggested that conductive magnetite accelerates acetoclastic methanogenesis [20,21].…”

“…Until 2018, Geobacter spp. were the sole known bacterial species involved in DIET with methanogens [19]. Bioaugmentation of Geobacter has been shown to promote methanogenesis [22].…”

Enhanced Anaerobic Digestion by Stimulating DIET Reaction

“…Future researches should direct more effort for verifying DIET activation and/or other potential changes in anaerobic growth, which can be implicated in the observed enhancement in CH 4 production rate and/or yield. Recently, Van Steendam et al [132] evaluated some advanced DIET identification methods, including fluorescence in situ hybridization, cyclic voltammetry, scanning, transmission electron microscope, and various (meta-)omic approaches; the usage of combined methods for confirming DIET occurrence was finally recommended. Additionally, Neu et al [133] presented temperature-resolved THz spectroscopy that can easily measure the conductivity of wild-type pili.…”