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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]

• Biohythane production from MEC led to positive energy recovery • Dual-chamber MEC could stabilize biohythane generation and composition • Methanogen in single-chamber MEC consumed hydrogen to unbalance composition • CEM was key part to maintain hydrogen production for stable biohythane composition • Geobacter and methanogens were key species to affect biohythane composition Biohythane is alternative fuel to replace fossil fuel for car combustion, and biohythane generation could be potential pathway for energy recovery from wastewater treatment. Microbial electrolysis cell (MEC) is electrochemical technique to convert waste to methane and hydrogen gas for biohythane generation, but the feasibility and stability of MEC needs further investigation to assure sustainable energy recovery. System configuration is paramount factor for electrochemical reaction and mass transfer, and this study was to investigate the configuration impact (single vs dual chamber) of MEC for biohythane generation rate and stability. This study showed that dual-chamber MEC could separate methane and hydrogen gas production in the anode and cathode, and combined both together to produce biohythane. To reduce ohmic resistance for higher current, cation exchange membrane (CEM) was removed from dual-chamber to single-chamber MEC. However, free hydrogen diffusion was allowed in the single chamber since CEM was removed. The diffused hydrogen and substrate towards the cathode would favor the methanogen growth, and thus the hydrogen was consumed to reduce the biohythane generation and energy recovery efficiency (i.e., 7.5 × 10−3 reduced to 5.7 × 10−3 kWh kg−1 degraded COD day−1 after converting dual-chamber to single-chamber MEC). Absolute abundance of methanogen in single-chamber MEC was greatly boosted, as Methanosarcina and Methanobacteriale on the anode surface, increased by 132% and 243%, respectively, while the original dual-chamber MEC could maintain Geobacter growth for high current generation. This is the keystone study to demonstrate the importance of dual-chamber MEC for the feasibility and stability for the biohythane generation, building up the foundation to use electrochemical device to convert the organic waste to the alternative biohythane. [Display omitted] [ABSTRACT FROM AUTHOR]

• Artificial EAB shortens startup time of biosensor. • High repeatability of biosensor can be obtained by using artificial EAB. • Low concentration of electroactive bacteria improves biosensor's sensitivity. • 1% of sodium alginate was optimum for sensitivity of artificial EAB. • Artificial EAB realizes real-time online water quality early-warning. A major challenge for devising an electrochemically active biofilm (EAB)-based biosensor for real-time water quality early-warning is the formation of EAB that requires several days to weeks. Besides the onerous and time-consuming preparation process, the naturally formed EABs are intensively concerned as they can hardly deliver repeatable electrical signals even at identical experimental conditions. To address these concerns, this study employed sodium alginate as immobilization agent to encapsulate Shewanella oneidensis MR-1 and prepared EAB for devising a biosensor in a short period of less than 1 h. The artificial EAB were found capable of delivering highly consistent electrical signals with each other when fed with the same samples. Morphology and bioelectrochemical properties of the artificial EAB were investigated to provide interpretations for these findings. Different concentrations of bacteria and alginate in forming the EAB were investigated for their effects on the biosensor's sensitivity. Results suggested that lower concentration of bacteria would be beneficial until it increased to 0.06 (OD 660). Concentration of sodium alginate affected the sensitivity as well and 1% was found an optimum amount to serve in the formation of EAB. A long-term operation of the biosensor with artificial EAB for 110 h was performed. Clear warning signals for incoming toxicants were observed over random signal fluctuations. All results suggested that the artificial EAB electrode would support a rapid devised and highly sensitivity biosensor. [Display omitted] [ABSTRACT FROM AUTHOR]