Bio-electrochemical enhancement of anaerobic reduction of nitrobenzene and its effects on microbial community

The ambivalent role of graphene oxide in anaerobic digestion: A review

The capability of graphene oxide (GO) to enhance direct interspecies electron transfer (DIET) and improve anaerobic digestion (AD) performance is gaining attention in AD literature. The present review discusses the implications of GO and its ambivalent role in AD. Under anaerobic conditions, GO is rapidly converted to biologically reduced graphene oxide (bioRGO) through microbial respiration. GO addition could promote the release of extracellular polymeric substances and lead to toxic effects on anaerobic microorganisms. However, further research is needed to determine the GO toxic concentration thresholds. GO application can impact biogas production and organic micropollutants removal of anaerobic digesters. Nevertheless, most of the studies have been conducted at batch scale and further work in continuously operated anaerobic digesters is still needed. Finally, the review evaluates the economic potential of GO application in AD systems. Overall, this review lays the foundations to improve the applicability of GO in future full-scale digesters.

Recent advances in biological removal of nitroaromatics from wastewater

Various nitroaromatic compounds (NACs) released into the environment cause potential threats to humans and animals. Biological treatment is valued for cost-effectiveness, environmental friendliness, and availability when treating wastewater containing NACs. Considering the significance and wide use of NACs, this review focuses on recent advances in biological treatment systems for NACs removal from wastewater. Meanwhile, factors affecting biodegradation and methods to enhance removal efficiency of NACs are discussed. The selection of biological treatment system needs to consider NACs loading and cost, and its performance is affected by configuration and operation strategy. Generally, sequential anaerobic-aerobic biological treatment systems perform better in mineralizing NACs and removing co-pollutants. Future research on mechanism exploration of NACs biotransformation and performance optimization will facilitate the large-scale application of biological treatment systems.

Aerobic biocathodes with potential regulation for ammonia oxidation with concomitant cathodic oxygen reduction and their microbial communities

Aerobic biocathodes are effective construct for the simultaneous nitrification and denitrification, but the disturbance of cathodic oxygen reduction on ammonia oxidation and denitrification remains unclear. In this study, we revealed the oxygen reduction peak at -0.4 V (versus silver/silver chloride) by cyclic voltammetry analysis at a cathode without a biofilm. The reduction peak, however, showed a right shift from -0.4 to -0.3 V for the biocathode, indicating that the aerobic biocathode could simultaneously perform traditional nitrification and cathode oxygen reduction. Therefore, different electrode potentials ranging from -0.5 to -0.1 V were designed for regulating the ammonia oxidation rate, and the results showed that the highest oxidation rate reached 3.08 mg/h/L at a potential of -0.2 V under a low-aeration rate of 5 mL/min. High-throughput sequencing showed that Nitrosomonas and Rhodococcus were the dominant nitrogen removal genera in the biocathode, and the abundance of Devosia was related to the interactions between the aeration rate and the electrode potential. Furthermore, the amoC and hao genes responded to aeration and electrode potential regulation, and -0.2 V was more suitable for promoting the denitrification process under low-aeration conditions. Therefore, these findings provided new insights on cathodic potential control for ammonia oxidation and nitrogen removal as well as for the regulation of microbial communities.

Enhanced refractory organics removal by sponge iron-coupled microbe technology: performance and underlying mechanism analysis

Sponge iron (SFe) is a zero-valent iron (Fe⁰) composite with a high-purity and porous structure. In this study, SFe was coupled with microorganisms that were gradually domesticated to form a Fe⁰/iron-oxidizing bacteria system (Fe⁰-FeOB system). The enhancement effect of the Fe⁰-FeOB system on refractory organics was verified, the mechanism of its strengthening action was investigated, and the relationship and influencing factors between the Fe⁰ and microorganisms were revealed. The average removal rates of the Fe⁰-FeOB system were 8.98%, 5.69%, and 40.67% higher than those of the SBR system for AF, AN, and NB wastewater treatment, respectively. With the addition of SFe, the microbial community structure was gradually enhanced with a large number of FeOB were detected. Moreover, the bacteria with strong iron corrosion and Fe(II) oxidation abilities plays a critical role in improving the Fenton-like effect. Interestingly, the variation trend of ⋅OH was fairly consistent with that of Fe(II). Thus, the main drivers of the Fenton-like effect are biological corrosion and metabolism. Consequently, microbial degradation and Fenton-like effect contributed to the degradation performance of the Fe⁰-FeOB system. Among them, the microbial degradation accounted for 96.09%, of which the biogenic Fenton effect accounted for 8.9%, and the microbial metabolic activity accounted for 87.19%. However, the augmentation of the Fe⁰-FeOB system was strongly dependent on SFe for the strengthening effect of microorganisms disappeared after leaving the SFe 35 days.

Electrocatalytic reduction of trace nitrobenzene using a graphene-oxide@polymerized-manganese-porphyrin composite

A more stable composite of graphene oxide encapsulated by the polymerized porphyrins realize the electrocatalytic reduction of nitrobenzene to nitrosobenzene.

Sludge degradation and microbial community structures analysis in a microbial electrolysis cell-coupled up flow anaerobic blanket reactor with an ultrasound treatment system

This study proposed a sludge degradation system comprised of: (i) an ultrasound treatment (UT) system to disintegrate sludge; (ii) an up flow anaerobic sludge blanket (UASB) reactor to degrade the disintegrated sludge; and (iii) a microbial electrolysis cell (MEC) in replacement of a three-phase UASB separator to deeply degrade the disintegrated sludge. The influence of the ultrasound power, the temperature, and the voltage on the sludge degradation process was discussed. The experimental results showed that the UT unit effectively promoted sludge disintegration, thereby leading to deterioration of the quality of the reactor effluent. The temperature and the voltage parameters were found to be key for the anaerobic degradation (AD) process within this system. The volatile suspended solid concentration in the effluent was maintained at 320–380 mg L⁻¹ (ca. 0.08 times the raw sludge concentration), thereby validating the utilization of MEC as a three-phase separation unit. The total chemical oxygen demand removal was maintained at 61.3% during 5 days of AD upon intermittent exposure of the sludge to the UT unit, thereby showing that the system can effectively degrade solid organic matter. The bacterial community structure of the raw sludge significantly changed, with the high biodiversity of this system increasing the ecological stability. This system can degrade sludge with high efficiency and could be used in further engineering applications.

This study proposes an ultrasound treatment–up flow anaerobic sludge blanket–microbial electrolysis cell (UT-UASB-MEC) degradation system.

A review on the applications of microbial electrolysis cells in anaerobic digestion

Anaerobic digestion (AD) has been widely used for biogas or biofuel generation from waste treatment. Because a low production rate and instability of AD occur frequently, various technologies have been applied to improvement of AD. Microbial electrolysis cells (MECs), an emerging technology, can convert organic matter into hydrogen, methane, and other value-added products. Recent studies showed that application of MEC to AD (MEC-AD) can accelerate degradation of a substrate (including recalcitrant compounds) and alter AD microbial community by enriching exoelectrogens and methanogens thus increasing biogas production. With stable microbial communities established, improvement of MEC-AD for methane production was achieved. MEC-AD process can be monitored in real-time by detecting electric signals, which linearly correlate with substrate concentrations. This review attempts to evaluate interactions among the decomposition of substrates, MEC-AD system, and the microbial community. This analysis should provide useful insights into the improvement of methane production and the performance of MEC-AD.

Evaluating the effects of activated carbon on methane generation and the fate of antibiotic resistant genes and class I integrons during anaerobic digestion of solid organic wastes

The effects of activated carbon (AC) on methane production and the fate of antibiotic resistance genes (ARGs) were evaluated through comparing the anaerobic digestion performance and transformation of ARGs among anaerobic mono-digestion of food waste, co-digestion of food waste and chicken manure, and co-digestion of food waste and waste activated sludge. Results showed that adding AC in anaerobic digesters improved methane yield by at least double through the enrichment of bacteria and archaea. Conventional digestion process showed ability in removing certain types of ARGs, such as tetA, tetX, sul1, sul2, cmlA, floR, and intl1. Supplementing AC in anaerobic digester enhanced the removal of most of the ARGs in mono-digestion of food waste. The effects tended to be minimal in co-digestion of co-substrates such as chicken manure and waste activated sludge, both of which contain a certain amount of antibiotics.

Metagenomic insight into the microbial networks and metabolic mechanism in anaerobic digesters for food waste by incorporating activated carbon

Powdered activated carbon (AC) is commonly used as an effective additive to enhance anaerobic digestion (AD), but little is known about how the metabolic pathways resulting from adding AC change the microbial association network and enhance food waste treatment. In this work, the use of AC in an anaerobic digestion system for food waste was explored. Using bioinformatics analysis, taxonomic trees and the KEGG pathway analysis, changes in microbial network and biometabolic pathways were tracked. The overall effect of these changes were used to explain and validate improved digestion performance. The results showed that AC accelerated the decomposition of edible oil in food waste, enhancing the conversion of food waste to methane with the optimized dosage of 12 g AC per reactor. Specifically, when AC was added, the proponoate metabolic pathway that converts propanoic acid to acetic acid became more prominent, as measured by 16S rRNA in the microbial community. The other two metabolic pathways, Lipid Metabolism and Methane Metabolism, were also enhanced. Bioinformatics analysis revealed that AC promoted the proliferation of syntrophic microorganisms such as Methanosaeta and Geobacter, forming a highly intensive syntrophic microbial network.

Microbiome involved in microbial electrochemical systems (MESs): A review

Microbial electrochemical systems (MESs) are an attracting technology for the disposal of wastewater treatment and simultaneous energy production. In MESs, at the anode microorganisms through the catalytic activity generates electrons that can be converted into electricity or other valuable chemical compounds. Microorganisms those having ability to donate and accept electrons to and from anode and cathode electrodes, respectively are recognized as 'exoelectrogens'. In the MESs, it renders an important function for its performance. In the present mini-review, we have discussed the role of microbiome including pure culture, enriched culture and mixed culture in different BESs application. The effects of operational and biological factors on microbiome development have been discussed. Further discussion about the molecular techniques for the evaluation of microbial community analysis is addressed. In addition different electrochemical techniques for extracellular electron transfer (EET) mechanism of electroactive biofilms have been discussed. This review highlights the importance of microbiome in the development of MESs, effective operational factors for exo-electrogens activities as well their key challenges and future technological aspects are also briefly discussed.

Three-stage anaerobic co-digestion of food waste and horse manure

A novel compact three-stage anaerobic digester (HM3) was developed to combine the advantages of high solids anaerobic digestion (AD) and wet AD for co-digestion of food waste and horse manure. By having three separate chambers in the three-stage anaerobic digester, three different functional zones were created for high-solids hydrolysis, acidogenesis and wet methanogenesis. The results showed that the functionalized partitioning in HM3 significantly accelerated the solubilization of solid organic matters and the formation of volatile fatty acids, resulting in an increase of 11~23% in methane yield. VS reduction in the HM3 presents the highest rate of 71% compared to the controls. Pyrosequencing analysis indicated that different microbial communities in terms of hydrolyzing bacteria, acidogenic bacteria and methanogenic archaea were selectively enriched in the three separate chambers of the HM3. Moreover, the abundance of the methanogenic archaea was increased by 0.8~1.28 times compared to controls.

Efficient removal of nitrobenzene and concomitant electricity production by single-chamber microbial fuel cells with activated carbon air-cathode

Single-chamber microbial fuel cells (S-MFCs) with bio-anodes and activated carbon (AC) air-cathodes showed high nitrobenzene (NB) tolerance and NB removal with concomitant electricity production. The maximum power over 25Wm^-3 could be obtained when S-MFCs were operated in the NB loading range of 1.2-6.2molm^-3d^-1, and stable electricity production over 13.7Wm^-3 could be produced in a NB loading range of 1.2-14.7molm^-3d^-1. The present S-MFCs exhibited high NB removal performance with NB removal efficiency over 97% even when the NB loading rate was increased to 17.2molm^-3d^-1. The potential NB reduced product (i.e. aniline) could also be effectively removed from influents. The findings in this study means that single-chamber MFCs assembled with pre-enriched bio-anodes and AC air-cathodes could be developed as effective bio-electrochemical systems to remove NB from wastewaters and to harvest energy instead of consuming energy.

Facilitated biological reduction of nitroaromatic compounds by reduced graphene oxide and the role of its surface characteristics

How reduced graphene oxide (RGO) mediates the reductive transformation of nitroaromatic pollutants by mixed cultures and the role of its surface characteristics were evaluated in this study. Different electron donors were applied to investigate the interaction between RGO and anaerobic microbes. Moreover, the influence of the surface properties of RGO on biological nitroaromatic removal was further elucidated. The results show that RGO could achieve an approximate one-fold rate increase of nitrobenzene reduction by mixed culture with glucose as an electron donor. Selective elimination of oxygen moieties on the RGO surface, such as quinone groups, decreased the nitrobenzene transformation rate, whereas doping nitrogen into the RGO framework exhibited a positive effect. The study indicates that graphene-based carbon nanomaterials have the potential to accelerate the biological transformation of nitroaromatic compounds and that the functionalization of these carbon nanomaterials, especially through surface modification, would further enhance the conversion efficiency of contaminants.