Enhancement of wastewater treatment under low temperature using novel electrochemical active biofilms constructed wetland

Voltage recovery from frozen microbial fuel cells in the laboratory and outdoor field reactors

Extreme temperature variations are a problem that must be faced in the practical application of microbial fuel cells (MFCs), but MFCs are not extensively described for low and even freezing temperatures. This study assessed the effect of low-temperature shock on the power generation performance and microbial community structure of MFCs. Two scales of MFCs, the small (mL-MFC) and the large (L-MFC), were constructed in the laboratory and their performance was evaluated before and after freezing at -18 °C. The experimental results demonstrate that both MFCs were capable of rapidly restoring their voltage to the previous level after thawing. For the mL-MFC (rGO/Ag), the power density recovered from 194.30 ± 10.84 mW/m2 to 195.57 ± 4.02 mW/m2 after thawing. For L-MFC (carbon felt electrodes), the power density increased significantly from the initial 1.79 mW/m2 to 173.90 mW/m2 after thawing, but the performance degradation problem after reactor amplification still needs to be solved. The sediment microbial fuel cell (SMFC) was successfully constructed and operated in a natural outdoor environment to maintain high voltage output after the period of frost. Microbial analysis indicated after the frost period, psychrotolerant microorganisms enriched on the anode, such as Flavobacterium and Psychrobacter, while the relative abundance of anaerobic methanogenic bacterium decreased. Overall, freeze-thaw operations had a non-negative impact on the performance of MFCs and provided some references for their practical applications.

Elucidating the impacts of cobalt (II) ions on extracellular electron transfer and pollutant degradation by anodic biofilms in bioelectrochemical systems during industrial wastewater treatment

Electrogenic biofilms in bioelectrochemical systems (BES) are critical in wastewater treatment. Industrial effluents often contain cobalt (Co2+); however, its impact on biofilms is unknown. This study investigated how increasing Co2+ concentrations (0-30 mg/L) affect BES biofilm community dynamics, extracellular polymeric substances, microbial metabolism, electron transfer gene expression, and electrochemical performance. The research revealed that as Co2+ concentrations increased, power generation progressively declined, from 345.43 ± 4.07 mW/m2 at 0 mg/L to 160.51 ± 0.86 mW/m2 at 30 mg/L Co2+. However, 5 mg/L Co2+ had less effect. The Co2+ removal efficiency in the reactors fed with 5 and 10 mg/L concentrations exceeded 99% and 94%, respectively. However, at 20 and 30 mg/L, the removal efficiency decreased substantially, likely because of reduced biofilm viability. FTIR indicated the participation of biofilm functional groups in Co2+ uptake. XPS revealed Co2+ presence in biofilms as CoO and Co(OH)2, indicating precipitation also aided removal. Cyclic voltammetry and electrochemical impedance spectroscopy tests revealed that 5 mg/L Co2+ had little impact on the electrocatalytic activity, while higher concentrations impaired it. Furthermore, at a concentration of 5 mg/L Co2+, there was an increase in the proportion of the genus Anaeromusa-Anaeroarcus, while the genus Geobacter declined at all tested Co2+ concentrations. Additionally, higher concentrations of Co2+ suppressed the expression of extracellular electron transfer genes but increased the expression of Co2+-resistance genes. Overall, this study establishes how Co2+ impacts electrogenic biofilm composition, function, and treatment efficacy, laying the groundwork for the optimized application of BES in remediating Co2+-contaminated wastewater.

The amelioration and improvement effects of modified biochar derived from Spartina alterniflora on coastal wetland soil and Suaeda salsa growth

Exotic species Spartina alterniflora (S. alterniflora) are widely invaded in the coastal zones of China and threaten the native ecosystem functions. In this study, phosphorus-magnesium modified BC (P-Mg modified BC) included PA-Mg-BC and DAP-Mg-BC derived from S. alterniflora were successfully prepared by co-pyrolysis of biomass and diammonium phosphate (DAP) or phosphoric acid (PA) and magnesium oxide (MgO). The preparation process markedly improved the surface morphologies, P loading amount, and P-containing functional groups of modified BC. The characterization results indicated that stable and low-solubility Mg-P complex formed on the surface of PA-Mg-BC and DAP-Mg-BC, which delayed the rapid release of P. Moreover, the MgO improved the buffering capacity of PA-Mg-BC and DAP-Mg-BC to competitive anions (SO42- and CO32-) during P release. Meanwhile, pot experiment showed that the suitable applications of PA-Mg-BC and DAP-Mg-BC could improve soil quality and fertility by enhancing SOC, DOC, TN, TP and AP contents, as well as β-glucosidase activities. The amended soil pH and salinity compared to the original soil also declined through precipitation and acid-base neutralization. In addition, P-Mg modified BC could improve bacterial community structure and promote the growth and biomass of Suaeda salsa (S. salsa). This study could provide a feasible method for realizing ecological restoration of coastal wetland and resource utilization of S. alterniflora.

Nutrient treatment of greywater in green wall systems: A critical review of removal mechanisms, performance efficiencies and system design parameters

Greywater has lower pathogen and nutrient levels than other mixed wastewaters, making it easier to treat and to reuse in nature-based wastewater treatment systems. Green walls (GWs) are one type of nature-based solutions (NBS) that are evolving in design to support on-site and low-cost greywater treatment. Greywater treatment in GWs involves interacting and complex physical, chemical, and biological processes. Design and operational considerations of such green technologies must facilitate these pivotal processes to achieve effective greywater treatment. This critical review comprehensively analyses the scientific literature on nutrient removal from greywater in GWs. It discusses nutrient removal efficiency in different GW types. Total nitrogen removal ranges from 7 to 91% in indirect green facades (IGF), 48-93% for modular living walls (MLW), and 8-26% for continuous living walls (CLW). Total phosphorus removal ranges from 7 to 67% for IGF and 2-53% for MLW. The review also discusses the specific nutrient removal mechanisms orchestrated by vegetation, substrates, and biofilms to understand their role in nitrogen and phosphorus removal within GWs. The effects of key GW design parameters on nutrient removal, including substrate characteristics, vegetation species, biodegradation, temperature, and operating parameters such as irrigation cycle and hydraulic loading rate, are assessed. Results show that greater substrate depth enhances nutrient removal efficiency in GWs by facilitating efficient filtration, straining, adsorption, and various biological processes at varying depths. Particle size and pore size are critical substrate characteristics in GWs. They can significantly impact the effectiveness of physicochemical and biological removal processes by providing sufficient pollutant contact time, active surface area, and by influencing saturation and redox conditions. Hydraulic loading rate (HLR) also impacts the contact time and redox conditions. An HLR between 50 and 60 mm/d during the vegetation growing season provides optimal nutrient removal. Furthermore, nutrient removal was higher when watering cycles were customized to specific vegetation types and their drought tolerances.

Bio-clogging mitigation in constructed wetland using microbial fuel cells with novel hybrid air-photocathode

Excessive accumulation of extracellular polymeric substances (EPS) in constructed wetland (CW) substrate can lead to bio-clogging and affect the long-term stable operation of CW. In this study, a microbial fuel cell (MFC) was coupled with air-photocathode to mitigate CW bio-clogging by enhancing the micro-electric field environment. Because TiO₂/biochar could catalyze and accelerate oxygen reduction reaction, further promoting the gain of electric energy, the electricity generation of the tandem CW-photocatalytic fuel cell (CW-PFC) reached 90.78 mW m^-3. After bio-clogging was mitigated in situ in tandem CW-PFC, the porosity of CW could be restored to about 62.5 % of the initial porosity, and the zeta potential of EPS showed an obvious increase (-14.98 mV). The removal efficiencies of NH₄⁺-N and chemical oxygen demand (COD) in tandem CW-PFC were respectively 31.8 ± 7.2 % and 86.1 ± 6.8 %, higher than those in control system (21.1 ± 11.0 % and 73.3 ± 5.6 %). Tandem CW-PFC could accelerate the degradation of EPS into small molecules (such as aromatic protein) by enhancing the electron transfer. Furthermore, microbiome structure analysis indicated that the enrichment of characteristic microorganisms (Anaerovorax) for degradation of protein-related pollutants, and electroactive bacteria (Geobacter and Trichococcus) promoted EPS degradation and electron transfer. The degradation of EPS might be attributed to the up-regulation of the abundances of carbohydrate and amino acid metabolism. This study provided a promising new strategy for synergic mitigation and prevention of bio-clogging in CW by coupling with MFC and photocatalysis.

Performance and mechanism of sacrificed iron anode coupled with constructed wetlands (E-Fe) for simultaneous nitrogen and phosphorus removal

Three anodic biofilm electrode coupled CWs (BECWs) with graphite (E-C), aluminum (E-Al), and iron (E-Fe), respectively, and a control system (CK) were constructed to evaluate the removal performance of N and P in the secondary effluent of wastewater treatment plants (WWTPs) under different hydraulic retention time (HRT), electrified time (ET), and current density (CD). Microbial communities, and different P speciation, were analyzed to reveal the potential removal pathways and mechanism of N and P in BECWs. Results showed that the optimal average TN and TP removal rates of CK (34.10% and 55.66%), E-C (66.77% and 71.33%), E-Al (63.46% and 84.93%), and E-Fe (74.93% and 91.22%) were obtained under the optimum conditions (HRT 10 h, ET 4 h, CD 0.13 mA/cm²), which demonstrated that the biofilm electrode could significantly improve N and P removal. Microbial community analysis showed that E-Fe owned the highest abundance of chemotrophic Fe(II) (Dechloromonas) and hydrogen autotrophic denitrifying bacteria (Hydrogenophaga). N was mainly removed by hydrogen and iron autotrophic denitrification in E-Fe. Moreover, the highest TP removal rate of E-Fe was attributed to the iron ion formed on the anode, causing co-precipitation of Fe(II) or Fe(III) with PO₄^3--P. The Fe released from the anode acted as carriers for electron transport and accelerated the efficiency of biological and chemical reactions to enhance the simultaneous removal of N and P. Thus, BECWs provide a new perspective for the treatment of the secondary effluent from WWTPs.

Towards advanced nutrient removal by microalgae-bacteria symbiosis system for wastewater treatment

In this study, the microalgae-bacteria symbiosis (ABS) system by co-culturing Chlorella sorokiniana with activated sludge was constructed for pollutants removal, and the according interaction mechanism was investigated. The results showed that the ABS system could almost completely remove ammonia nitrogen, and the removal efficiency of total nitrogen and total phosphorus could accordingly reach up to 65.3 % and 42.6 %. Brevundimonas greatly promoted microalgal biomass growth (maximum chlorophyll-a concentration of 9.4 mg/L), and microalgae contributed to the increase in the abundance of Dokdonella and Thermomonas in ABS system, thus facilitating nitrogen removal. The extended Derjaguin-Landau-Verwey-Overbeek theory indicated a repulsive potential barrier of 561.7 KT, while tryptophan-like proteins and tyrosine-like proteins were key extracellular polymeric substances for the formation of flocs by microalgae and activated sludge. These findings provide an in-depth understanding of interaction mechanism between microalgae and activated sludge for the removal of contaminants from wastewater.

Performance and mechanism of azo dyes degradation and greenhouse gases reduction in single-chamber electroactive constructed wetland system

A single-chamber microbial fuel cell-microbial electrolytic cell with a novel constructed wetland system was proposed for synergistic degradation of congo red and reduction in emissions of greenhouse gases. The closed-circuit system showed higher chemical oxygen demand and congo red removal efficiencies by 98 % and 96 % on average, respectively, than traditional constructed wetland. It could also significantly reduce the emissions of CH₄ and N₂O (about 52 % CO₂-equivalents) by increasing the electron transfer. Microbial community analysis demonstrated that the progressive enrichment of dye-degrading microorganisms (Comamonas), electroactive bacteria (Tolumonas, Trichococcus) and denitrifying microorganisms (Dechloromonas) promoted pollutant removal and electron transfer. Based on gene abundance of xenobiotics biodegradation, the congo red biodegradation pathway was described as congo red → naphthalene and alcohols → CO₂ and H₂O. In summary, the single-chamber closed-circuit system could significantly improve the degradation of congo red and reduce the emissions of greenhouse gases by influencing electron transfer and microbial activity.