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This study assessed the impact of antimicrobial AgNPs (50 mg L -1 , 30-50 nm) on the electrocatalytic activity of a mixed-culture anode biofilm enriched with Geobacter species. The current densities and electrochemical kinetics were maintained after exposure to AgNPs in consecutive fed-batch cycles, despite significant changes in morphological structures and bacterial communities. Bacterial community analysis showed a substantial increase in the Geobacter population in response to AgNPs exposure, indicating their higher tolerance to AgNPs. In contrast, the population of other anode respiring bacteria (ARB) belongs to Acinetobacter, Dysgonomonas, and Cloacibacillus genera appeared to be very sensitive to AgNPs toxicity as their relative abundance significantly decreased. Microscopic imaging showed that AgNPs were accumulated within anode biofilm matrix without penetration inside the cells. Moreover, the anode biofilm became denser because of enhanced extracellular polymeric substances (EPSs) production by ARB after exposure of AgNPs, implying that EPS could protect ARB against AgNPs toxicity., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

The microbial electrolysis cell assisted anaerobic digestion holds great promises over conventional anaerobic digestion. This article reports an experimental investigation of extracellular polymeric substances (EPS), reactive oxygen species (ROS), and the expression of genes associated with extracellular electron transfer (EET) in methanogenic biocathodes. The MEC-AD systems were examined using two cathode materials: carbon fibers and stainless-steel mesh. A higher abundance of hydrogenotrophic Methanobacterium sp. and homoacetogenic Acetobacterium sp. appeared to play a major role in superior methanogenesis from stainless steel biocathode than carbon fibers. Moreover, the higher secretion of EPS accompanied by the lower ROS level in stainless steel biocathode indicated that higher EPS perhaps protected cells from harsh metabolic conditions (possibly unfavorable local pH) induced by faster catalysis of hydrogen evolution reaction. In contrast, EET-associated gene expression patterns were comparable in both biocathodes. Thus, these results indicated hydrogenotrophic methanogenesis is the key mechanism, while cathodic EET has a trivial role in distinguishing performances between two cathode electrodes. These results provide new insights into the efficient methanogenic biocathode development. [ABSTRACT FROM AUTHOR]

In this study, mesophilic acidogenic fermentation, thermophilic acidogenic fermentation, and thermal hydrolysis process (THP) were compared to generate sludge liquors for bioenergy recovery with microbial electrolysis cells (MECs). The results showed that THP at 170 °C was the most effective for hydrolysis of particulate organics in sewage sludge, while fermentation under thermophilic temperature led to the highest accumulation of volatile fatty acids (VFAs) in sludge liquor. However, THP yielded the highest percentage of acetate in VFAs, which resulted in superior MEC performance compared to fermented sludge liquors in terms of current density (2.7 vs. ~1.3 A/m2), coulombic efficiency (50% vs. 31–34%), bio-H 2 potential (1114 vs. 839–881 mL), and H 2 production rate (50.3 mL/d vs. 28–32 mL/d). The utilization sequence of the VFAs was found to be acetate > butyrate > propionate. Overall, our results show that generating sludge liquors through THP could provide a feasible solution to produce bio-H 2 from sewage sludge; however, coulombic efficiencies should be further improved before practical application. • MECs were operated with sludge liquors produced through three different processes. • Thermal hydrolysis process (170 °C) led to superior MEC performance. • Sludge liquors from fermentations (38 °C and 55 °C) showed comparable performance. • VFAs degradation profiles followed the order of acetate > butyrate > propionate. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • MEC followed by chemical precipitation was studied for N&P recovery from centrate. • Inadequate biodegradable COD in centrate led to ineffective N recovery in MEC. • Centrate was mixed with sludge fermentation liquor to enhance N recovery in MEC. • Recovered H 2 gas was adequate to offset the electricity cost of MEC operation. • P was efficiently recovered from MEC anodic effluent using Ca2+ or Mg2+ salts. Abstract A two-step sidestream process was investigated for nitrogen (N) and phosphorus (P) recovery from digested sludge centrate. In the first step, a dual-chamber microbial electrolysis cell (MEC) was used for N recovery on the cathode. In the second step, P was recovered as solid precipitates by the addition of Ca2+ or Mg2+ salts in the anodic effluent. The operation of MEC with centrate indicate that N transport from the anode to the cathode chamber is primarily driven by anodic electron transport rather than diffusional transport. Low concentration of readily biodegradable organics in centrate significantly hindered current density (<0.15 A/m2) and led to trivial N recovery on the cathode chamber. The addition of primary sludge fermentation liquor (25 vol%) with centrate as an exogenous source of readily biodegradable organics substantially increased current density up to 6.4 A/m2, along with high TAN removal efficiency of 53 ± 5%. The energy requirement was calculated at 5.8 ± 0.1 kWh/kg-TAN; however, the recovered H 2 gas from the cathode was adequate to offset this energy input completely. The addition of Ca2+ salt at a Ca:P molar ratio of 3:1 was optimum for P recovery from the anodic effluent; Mg:P molar ratio of 2:1 was found to be optimum for Mg2+ salt addition. However, optimum doses of both salts resulted in maximum P recovery efficiency of ∼85%, while Mg2+ addition provided an additional 38% TAN removal. These results demonstrate that microbial electrolysis followed by chemical precipitation can promote sustainable nutrients recovery from centrate at municipal wastewater treatment plants where sludge fermentation has already been adopted to provide readily biodegradable carbon source in the biological nutrient removal process. [ABSTRACT FROM AUTHOR]

• Impact of intermittent applied potential on electro-methanogenesis was assessed. • Turning off the applied potential for 6 h/day did not alter methane production. • Performance deteriorated when applied potential switched off for 12 h/day. • Microbial communities considerably altered by intermittent applied potential. • Applying external potential for 18 h/day led to the highest net energy income. From the perspective of energy saving in the operation of microbial electrolysis cell assisted anaerobic digester (MEC-AD), this study focused on developing an intermittent power supply scheme. The applied potential was switched off for 12 and 6 hours/day during the operation of a laboratory-scale MEC-AD system fed with glucose. The results from the operation under continuous applied potential served as the control. The overall biomethane generation and net energy income from the process were unaffected when the applied potential turned off for 6 hours/day. Both quantitative and qualitative analyses of microbial communities suggested that a balanced microbiome could be maintained under short-term switching-off the applied potential. However, performance substantially deteriorated when the applied potential turned off for 12 hours/day. Overall, the results of this study suggest that MEC-AD operation does not need a continuous power supply, and higher energy efficiency can be effectively achieved by intermittently powering the reactor. [ABSTRACT FROM AUTHOR]