Ex-situ electrochemical characterisation of fixed-bed denitrification biocathodes: A promising strategy to improve bioelectrochemical denitrification

Unveiling the mechanism of microbial nitrogen conversion by acclimating bioanode potential for enhancing ammonia removal and reducing N2O emission

The biodegradation mechanism of enhancing ammonia removal and reducing N2O emission by acclimating bioanode potential in bioelectrochemical systems (BES) was studied in this work. The BES was started up and optimized with different parameters (i.e., aeration volume, external resistance, and pH value). The cyclic voltammetry was then performed to determine the potential range for acclimating bioanode, suggesting that potentials of -0.3, -0.1, 0.1, 0.2, and 0.3 V were suitable for acclimation. The experimental results indicated that optimum total nitrogen removal was achieved at potential of 0.1 V with a removal efficiency of 83.7 %, which was 1.6 times higher than that of the control group, while the corresponding N2O emission of 1.3 mg-N/L was detected which reduced 2.8 times. The microbial community analysis indicated that the dominant genera for enhancing nitrogen conversion were Sphaerocheata, Petrimonas, and Lentimicrobium. The electrochemical tests suggested that potential acclimation could simultaneously enhance direct electron transfer and mediated electron transfer (MET), thereby increasing extracellular electron transfer, in which the proportion of MET process mediated by riboflavin played a decisive role. Meanwhile, the intracellular electron transfer process was also strengthened by the increase of the related enzyme activity and the corresponding electron transport system activity increased 3.4 times. Additionally, nitrogen conversion pathways were proposed, indicating that potential acclimation could regulate the ammonia degradation pathway, and reduce the intermediates accumulation. These findings provide new understanding of the nitrogen conversion mechanisms in BES for ammonia removal by acclimating bioanode potential.

Inorganic bioelectric system for nitrate removal with low N2O production at cold temperatures of 4 and 10 °C

Groundwater, essential for ecological stability and freshwater supply, faces escalating nitrate contamination. Traditional biological methods struggle with organic carbon scarcity and low temperatures, leading to an urgent need to explore efficient approaches for groundwater remediation. In this work, we proposed an inorganic bioelectric system designed to confront these challenges. At 10 and 4 °C, the system achieved total nitrogen (TN) removal efficiencies of 95.4 ± 2.7% and 90.9 ± 1.9% at 2 h hydraulic retention time (HRT), while maximum TN removal rates were recorded as 206.0 ± 6.3 and 178.3 ± 9.4 g N·m-3·d-1 at 1 h HRT. The microbial analysis uncovered shifts in dominant genera across temperatures, with Dechloromonas prevalent at 10 °C and Chryseobacterium at 4 °C, highlighting adaptability to cold-tolerant species. Gene analysis on narG, napA, nirS, nirK, norB, nosZI, nosZII, and nifA examined the nitrate reduction processes, and analysis on mtrC and omcA hinted at electrotrophic processes. Additionally, we demonstrated system resilience to disruptions of power outage and short periods without flow through. These findings establish a foundational understanding of electricity-driven nitrate bioreduction in cold environments, crucial in groundwater remediation strategies and paving the way for future optimization and upscaling efforts.

Electro-bioremediation of wastewater: Transitioning the focus on pure cultures to elucidate the missing mechanistic insights upon electro-assisted biodegradation of exemplary pollutants

Electro-bioremediation of exemplary water pollutants such as nitrogenous, phosphorous, and sulphurous compounds, hydrocarbons, metals and azo dyes has already been studied at a macro-scale level using mixed cultures. The technology has been generally established as a proof of concept at the technology readiness level (TRL) of 3, and there are already specific cases where the technology reached TRL 5. However, this technology is less utilized compared to traditional approaches. Although, mixed cultures result in high electro-biodegradation efficiency, their use hinders process' mechanistic insights which are better determined through pure cultures studies. This knowledge can lead to improved technologies. Therefore, this manuscript focuses on the specific pollutants' electro-biodegradation by pure cultures, assessing the availability of information regarding genes, enzymes, proteins and metabolites involved. Furthermore, studies characterizing the dominant genera or species are assessed, in which the available information at molecular level is evaluated. In total, less than 40 studies were found which were predominantly focused on the electro-biodegradation potential rather than the mechanistic insights. This highlights a gap in the field featuring a motivation to transitioning the focus on the study of pure cultures to unravel the mechanistic insights. Therefore, specific actions are suggested. Characterization of the mixed cultures followed by microorganisms' isolation is crucial. Thus, electroactive and biodegradation characteristics will be revealed using omics, genome annotation and transcriptional kinetics. This can lead to optimization at the microbiological level through genetic engineering, synthetic biology, mathematical modelling and strategic building of co-cultures. This research focus offers new avenues for sustainable wastewater treatment.

Nitrate electro-bioremediation and water disinfection for rural areas

Nitrate-contaminated groundwater is a pressing issue in rural areas, where up to 40 % of the population lacks access to safely managed drinking water services. The high costs and complexity of centralised treatment in these regions exacerbate this problem. To address this challenge, the present study proposes electro-bioremediation as a more accessible decentralised alternative. Specifically, the main focus of this study is developing and evaluating a compact reactor designed to accomplish simultaneous nitrate removal and groundwater disinfection. Significantly, this study has established a new benchmark for nitrate reduction rate within bioelectrochemical reactors, achieving the maximum reported rate of 5.0 ± 0.3 kg NO3- m-3NCC d-1 at an HRTcat of 0.7 h. Furthermore, thein-situ generation of free chlorine was effective for water disinfection, resulting in a residual concentration of up to 4.4 ± 1.1 mg Cl2 L-1 in the effluent at the same HRTcat of 0.7 h. These achievements enabled the treated water to meet the drinking water standards for nitrogen compounds (nitrate, nitrite, and nitrous oxide) as well as pathogens content (T. coliforms, E. coli, and Enterococcus). In conclusion, this study demonstrates the potential of the electro-bioremediation of nitrate-contaminated groundwater as a decentralised water treatment system in rural areas with a competitive operational cost of 1.05 ± 0.16 € m-3.