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• Conductivity is a comparable metric to characterize EET. • Nanowires and microbial aggregates have substantially different conductivity levels. • Several factors can cause a large variability in the measured conductivity. • Calculating the EET flux in microbial aggregates from conductivity measurement. Microbial extracellular electron transfer (EET) plays a crucial role in bioenergy production and resource recovery from wastewater. Interdisciplinary efforts have been made to unveil EET processes at various spatial scales, from nanowires to microbial aggregates. Electrical conductivity has been frequently measured as an indicator of EET efficiency. In this review, the conductivity of nanowires, biofilms, and granular sludge was summarized, and factors including subjects, measurement methods, and conducting conditions that affect the conductivity difference were discussed in detail. The high conductivity of nanowires does not necessarily result in efficient EET in microbial aggregates due to the existence of non-conductive substances and contact resistance. Improving the conductivity measurement of microbial aggregates is important because it enables the calculation of an EET flux from conductivity and a comparison of the flux with mass transfer coefficients. This review provides new insight into the significance, characterization, and optimization of EET in microbial aggregates during a wastewater treatment process. [ABSTRACT FROM AUTHOR]

• Biofilm disassembly agents helped to incorporate more Fe/Fe oxides into AGS. • Loading Fe/Fe oxides promoted flavins and c-Cyts secretion in EPS of AGS. • The resistance of EPS was reduced and interspecies electron transfer was increased. • AGS with Fe/Fe oxides showed an enhanced methane production capacity. Anaerobic granular sludge (AGS) is a promising technology for organic wastewater treatment and energy recovery. In this study, three different kinds of Fe and Fe oxides nanoparticles (Fe 3 O 4 , Fe 2 O 3 and ZVI) were tried to be incorporated into AGS through direct loading or aided with biofilm disassembly agents of norspermidine and D-tyrosine, which was aimed to enhance methane production capacity of AGS via increasing redox activity of extracellular polymer substance (EPS) and interspecies electron transfer. Despite the loading methods, incorporation of Fe and Fe oxides nanoparticles into AGS increased methane production capacity remarkably, with an enhancement of 36.49–85.17%, 20.37–204.95% and 189.71–243.32%, respectively, for the Fe 3 O 4 , Fe 2 O 3 and ZVI loaded AGS. Pretreatment of AGS using biofilm disassembly agents helped to incorporate more Fe and Fe oxides into the inner structure of AGS, which further enhanced methane production capacity by 48.68% and 184.58%, respectively, for the Fe 3 O 4 and Fe 2 O 3 loaded AGS. Loading Fe and Fe oxides into AGS not only introduced exogenous conductive substances and Fe(III)/Fe(II) redox couples into EPS matrix of AGS, but also stimulated the production of redox active components of flavins and c-Cyts. All these factors may contribute to the reduced resistance of EPS, enhanced interspecies electron transfer and methane production capacity of AGS. This study provides a novel strategy and facile method to accelerate interspecies electron transfer and enhance methane production for matured AGS. [Display omitted]. [ABSTRACT FROM AUTHOR]

Bioelectrochemical system (BES) is promising technology to simultaneously treat wastewater and recover energy, and electrode material is important for the system performance. Microbial fuel cell (MFC) is one of typical BES to be applied in wastewater treatment. How to improve the electrode material is significant to improve wastewater treatment, energy recovery and cost effectiveness. In this study, 3D-weaved carbon electrode entity, assembled by multiple pieces of carbon mesh (CM), was proposed to combine all electrode components as entity to facilitate electron conduction and ionic migration, compared with carbon brush (CB) and granular activated carbon (GAC). The result showed that current density and internal resistance of MFC using 3D-weaved CM as horizontally extended inside anode (CM(T)) were 30.9 A m-3 and 4.5 Ω, respectively, with higher output than traditional GAC (22.6 A m-3 and 6.2 Ω). Though GAC had greater electrode filling and surface area for biomass growth, the electron transfer efficiency per unit electrode biomass was only at 0.0019 ± 0.0002 mol g-1 d-1, much lower than CM(T) at 0.0077 ± 0.0009 mol g-1 day-1. Higher ionic migration rate of CM(T) suggested the assisting effect of composite electrode to enhance ionic transportation towards the cathode. Microbial analysis further indicated that 3D-CM electrode network could simultaneously enhance Geobacter abundance and methanogen activity, suggesting the importance of electrode network on electricigens. Furthermore, CM(T) could obtain 10 times higher energy output efficiency than traditional GAC when applied inside anode chamber. This study proved that network construction of anode electrode could promote the electrode performance and cost effectiveness, suggesting the future development of reactor design of bioelectrochemical system. Image 1 • Anode configuration impacted biomass growth and electron conduction. • 3D carbon mesh constructed as electron network entity for conducting facility. • Electrode entity construction greatly benefited electron generation efficiency. • 3D carbon mesh stimulated microbial activity and Geobacter growth. • Massive cost optimization achieved by 3D carbon mesh. [ABSTRACT FROM AUTHOR]

This study employed a titanium mesh-membrane assembly (MMA) as the current collector in flow-electrode capacitive deionization (FCDI) device (designated as M-FCDI), and obtained a much reduced charge transport distance as compared to traditional FCDI with plate-shaped current collectors located far from the exchange membrane. The average salt removal rate of M-FCDI was greatly improved by 76% under 10 wt% carbon content than the control experiment with graphite plate as current collector, and the charge efficiency remained over 75% even under low carbon loading. This improvement was attributed to the reduced resistance as revealed by electrochemical impedance spectroscopy tests. Further investigation on FCDI's performance with different specifications of titanium meshes showed that the implementation of MMA could provide a larger effective electron transfer area, which would lead to better desalting performance. Image 1 • Titanium mesh-membrane assembly (MMA) as current collector in FCDI. • Largely reduced charge transport distance with MMA than graphite plate. • Much increased salt removal rate up to 75% attributing to smaller cell resistance. • Stable operation over 24 h of FCDI with MMA and with lower energy consumption. [ABSTRACT FROM AUTHOR]