Enhanced denitrification and power generation of municipal wastewater treatment plants (WWTPs) effluents with biomass in microbial fuel cell coupled with constructed wetland

Advancement in constructed wetland microbial fuel cell process for wastewater treatment and electricity generation: a review

The constructed wetland coupled with a microbial fuel cell (CW-MFC) is a wastewater treatment process that combines contaminant removal with electricity production, making it an environmentally friendly option. This hybrid system primarily relies on anaerobic bioprocesses for wastewater treatment, although other processes such as aerobic bioprocesses, plant uptake, and chemical oxidation also contribute to the removal of organic matter and nutrients. CW-MFCs have been successfully used to treat various types of wastewater, including urban, pharmaceutical, paper and pulp industry, metal-contaminated, and swine wastewater. In CW-MFC, macrophytes such as rice plants, Spartina angalica, Canna indica, and Phragmites australis are used. The treatment process can achieve a chemical oxygen demand removal rate of between 80 and 100%. Initially, research focused on enhancing power generation from CW-MFC, but recent studies have shifted towards resource recovery from wastewater. This review paper provides an overview of the development of constructed wetland microbial fuel cell technology, from its early stages to its current applications. The paper also highlights research gaps and potential directions for future research.

Reduction of Toxic Metal Ions and Production of Bioelectricity through Microbial Fuel Cells Using Bacillus marisflavi as a Biocatalyst

Industrialization has brought many environmental problems since its expansion, including heavy metal contamination in water used for agricultural irrigation. This research uses microbial fuel cell technology to generate bioelectricity and remove arsenic, copper, and iron, using contaminated agricultural water as a substrate and Bacillus marisflavi as a biocatalyst. The results obtained for electrical potential and current were 0.798 V and 3.519 mA, respectively, on the sixth day of operation and the pH value was 6.54 with an EC equal to 198.72 mS/cm, with a removal of 99.08, 56.08, and 91.39% of the concentrations of As, Cu, and Fe, respectively, obtained in 72 h. Likewise, total nitrogen concentrations, organic carbon, loss on ignition, dissolved organic carbon, and chemical oxygen demand were reduced by 69.047, 86.922, 85.378, 88.458, and 90.771%, respectively. At the same time, the PDMAX shown was 376.20 ± 15.478 mW/cm2, with a calculated internal resistance of 42.550 ± 12.353 Ω. This technique presents an essential advance in overcoming existing technical barriers because the engineered microbial fuel cells are accessible and scalable. It will generate important value by naturally reducing toxic metals and electrical energy, producing electric currents in a sustainable and affordable way.

Nitrogen and phosphorus removal performance of sequential batch operation for algal cultivation through suspended-solid phase photobioreactor

Nitrogen (N) and phosphorus (P) absorbed by algae in the suspended-solid phase photobioreactor (ssPBR) have emerged as an efficient pathway to purify the effluent of wastewater treatment plants (WWTPs). However, the key operational parameters of the ssPBR need to be optimized. In this study, the stability of the system after sequential batch operations and the efficiency under various influent P concentrations were evaluated. The results demonstrated that the ssPBR maintained a high N/P removal efficiency of 96 % and 98 %, respectively, after 5 cycles. When N was kept at 15 mg/L and P ranged from 1.5 to 3.0 mg/L, the system yielded plenty of algae products and guaranteed the effluent quality that met the discharge standards. Notably, the carriers were a key contributor to the high metabolism of algae and high performance. This work provided theoretical ideas and technical guidance for effluent quality improvement in WWTPs.

Application of alkali-heated corncobs enhanced nitrogen removal and microbial diversity in constructed wetlands for treating low C/N ratio wastewater

Lack of carbon source is the main limiting factor in the denitrification of low C/N ratio wastewater in the constructed wetlands (CWs). Agricultural waste has been considered as a supplementary carbon source but research is still limited. To solve this problem, ferric carbon (Fe-C) + zeolite, Fe-C + gravel, and gravel were used as substrates to build CWs in this experiment, aiming to investigate the effects of different carbon sources (rice straw, corncobs, alkali-heated corncobs) on nitrogen removal performance and microbial community structure in CWs for low C/N wastewater. The results demonstrated that the microbial community and effluent nitrogen concentration of CWs were mainly influenced by the carbon source rather than the substrate. Alkali-heated corncobs significantly enhanced the removal of NO2--N, NH4+-N, NO3-N, and TN. Carbon sources addition increased microbial diversity. Alkali-heated corncobs addition significantly increased the abundance of heterotrophic denitrifying bacteria (Proteobacteria and Bacteroidota). Furthermore, alkali-heated corncobs addition increased the copy number of nirS, nosZ, and nirK genes while greenhouse gas fluxes were lower than common corncobs. In summary, alkali-heated corncobs can be considered as an effective carbon source.

Biochar boosts nitrate removal in constructed wetlands for secondary effluent treatment: Linking nitrate removal to the metabolic pathway of denitrification and biochar properties

Constructed wetlands (CWs) amended with biochar have attracted much attention for nitrate removal treating secondary effluent. However, little is acknowledged about the linkage among the nitrate removal performance, microbial metabolic pathway of nitrate, and biochar properties. Herein, biochars pyrolyzed under 300 °C, 500 °C, and 700 °C (BC300, BC500, and BC700, respectively) were used in CWs to reveal the relationship. Results showed that CWs amended with BC300 (59.73%), BC500 (53.27%), and BC700 (49.07%) achieved higher nitrogen removal efficiency, compared with the control (39.51%). Metagenomic analysis showed that biochars could enrich the genes, which encoded key enzymes (adenosine triphosphate production, and electrons generation, transportation, and consumption) involved in carbon and nitrate metabolism. Further, biochar pyrolyzed under lower temperature, with higher oxygen content, molar O/C ratio, and the electron donating capacity, in CWs could obtain higher nitrate removal efficiency. Overall, this research offers new understandings for the promotion of denitrification in CWs amended with biochar.

Acorus calamus recycled as an additional carbon source in a microbial fuel cell-constructed wetland for enhanced nitrogen removal

Acorus calamus was recycled as an additional carbon source in microbial fuel cell-constructed wetlands (MFC-CWs), for efficient nitrogen removal of low carbon wastewater. The pretreatment methods, adding positions, and nitrogen transformations were investigated. Results indicated that alkali-pretreatment cleaved the benzene rings in dominant released organics, producing chemical oxygen demand of 164.5 mg from per gram of A. calamus. Pretreated biomass addition in the anode of MFC-CW attained the maximum total nitrogen removal of 97.6% and power generation of 12.5 mW/m², which were higher than those with biomass in the cathode (97.6% and 1.6 mW/m², respectively). However, the duration of a cycle with biomass in the cathode (20-25 days) was longer than that in the anode (10-15 days). Microbial metabolisms related to organics degradation, nitrification, denitrification, and anammox were intensified after biomass recycling. This study provides a promising method to improve nitrogen removal and energy recovery in MFC-CWs.

Investigation of electrochemical properties, leachate purification, organic matter characteristics, and microbial diversity in a sludge treatment wetland- microbial fuel cell

Sludge treatment wetland-microbial fuel cell (STW-MFC) is a unique sludge treatment process that produces bioelectricity, but its technology is still in its infancy. This study investigated the electrochemical properties, organic matter characteristics, leachate purification, and microbial community structure of STW-MFCs as affected by electrode location. When electrodes were placed in the filler layer, the STW-MFC system presented a higher power generation capacity (maximum output power density: 0.498 W/m³; peak cell voltage: 0.879 V) and organic matter degradation efficiency. The hydrophilic fraction was the main dissolved organic carbon fraction in sludge extracellular biological organic matter (EBOM) and leachate dissolved organic matter (DOM). Aromatics were mainly concentrated in the hydrophobic acid fraction. The UV-254 content of sludge EBOM decreased mainly in the hydrophilic and transphilic acid fractions. The excitation-emission matrix analysis showed that tryptophan-like protein was more easily eliminated than tyrosine-like protein. In addition, there was a strong correlation between voltage and NH₄⁺ removal efficiency; a negative correlation between total chemical oxygen demand (TCOD), total nitrogen (TN), and total phosphorus (TP) removal efficiency, and a negative correlation between pH and TN, TP, and NH₄⁺ removal efficiencies. High-throughput sequencing showed that the system was most abundant in Thermomonas, Geothrix and Geobacter when the electrodes were placed in the filled layer, while the levels of genes for membrane transport, carbohydrate metabolism and energy metabolism functions were higher than in other systems. This work will support STW- MFC widespread implementation by illuminating the underlying mechanics of different anode positions.

Accelerating denitrification and mitigating nitrite accumulation by multiple electron transfer pathways between Shewanella oneidensis MR-1 and denitrifying microbial community

The bio-denitrification was usually retarded by the unbalance of electron generation and consumption. In this study, mixing S. oneidensis MR-1 with denitrifying microbial community increased the nitrogen removal rate by 74.74 % via the interspecies electron transfer (IET), and reduced the accumulated nitrite from 9.90 ± 0.81 to 0.02 ± 0.03 mg/L. Enhanced denitrification still appeared but relatively decreased, when S. oneidensis MR-1 was separated by a dialysis bag (MW < 3000), indicating mediated interspecies electron transfer (MIET) counted in IET. The results of electron transfer activity and sludge conductivity suggested DIET and MIET jointly transfer electrons from MR-1 to electroactive denitrifying bacteria (EDB), improving denitrifying reductase activities. Electron distribution among denitrifying reductases was found to be associated with the IET rate. Microbial insights showed the total abundance of EDB was increased, and denitrifying genes were correspondingly enriched. Pseudomonas was found to cooperate with exoelectrogens in a complicated microbial community.

Comparison of constructed wetland performance coupled with aeration and tubesettler for pharmaceutical compound removal from hospital wastewater

Pharmaceutical compounds being able to alter, retard, and enhance metabolism has gained attention in recent time as emerging pollutant. However, hospitals which are part of every urban landscape have yet to gain attention in terms of its hospital wastewater treatment to inhibit pharmaceutical compounds from reaching environment. Hence this study evaluated performance of constructed wetland in combination with tubesettler and aeration based on removal efficiency and ecological risk assessment (HQ). The removal efficiency of constructed wetland with plantation was higher by 31% (paracetamol), 102% (ibuprofen), 46%, (carbamazepine), 57% (lorazepam), 54% (erythromycin), 31% (ciprofloxacin) and 20% (simvastatin) against constructed wetland without plantation. Constructed wetland with aeration efficiency increased for paracetamol, ibuprofen, carbamazepine, lorazepam, erythromycin, ciprofloxacin, and simvastatin removal efficiency were higher by 58%, 130%, 52%, 79%, 107%, 57%, and 29% respectively. In constructed wetland with plantation, removal efficiency was higher by 20% (paracetamol), 13% (ibuprofen), 4% (carbamazepine), 14% (lorazepam), 34% (erythromycin), 19% (ciprofloxacin) and 7% (simvastatin). High ecological risk was observed for algae, invertebrate and fish with hazard quotient values in range of 2.5-484, 10-631 and 1-78 respectively. This study concludes that if space is the limitation at hospitals aeration with constructed wetland can be adopted. If space is available, constructed wetland with tubesettler is suitable, economic and environmentally friendly option. Future research works can focus on evaluating other processes combination with constructed wetland.

Microbial electro deionization for waste water treatment - A critical review on methods, applications and mechanism

Electro deionization using microbial communities has been proven as a competent method for desalination and abatement of water pollution by removing ionic chemicals from the target waters. Microbial Desalination Cell (MDC) facilitates microbial deionization which can either support or be a substitute for the conventional desalination methods. Generation of electricity is accomplished by the bio electrochemical oxidation of organic compounds present as contaminants in wastewater which in turn attribute to the migration of ions in MDC system. The present review aims to elucidate the theory, principles and the application of microbial desalination cell and microbial fuel cell (MFC) in treatment of saline and wastewaters. Air cathode MDC and stacked MDC for purification of saline water are found to give promising results. Air pump assisted microbial desalination cell reported 150.39 ppm h^-1 of salt removal with an operational time period of 80 h and showed consistent results. Hence the air cathode assisted MDC showed dominant capacity of salt removal compared to stacked MDC. Also, three major types of microbial fuel cell, namely photosynthetic biofilm MFC, constructive wetland MFC and ceramic membrane supported MFC are reviewed for their potentials in wastewater treatment by deionization method and electricity generation. Complete (100%) removal of chemical oxygen demand was reported by photosynthetic microbial fuel cell operated for 16 days having 435.8 Ω of external resistance. When constructive wetland microbial fuel cell was operated for 10 days with 1000 ohms of external resistance, it exhibited complete (100%) removal of chemical oxygen demand from the wastewater. About 92% of chemical oxygen demand removal was demonstrated by ceramic membrane supported microbial fuel. Compared to ceramic membrane microbial fuel cell, photosynthetic and constructive wetland microbial fuel cell displayed better performance in terms of pollutant removal capacity and economical factor. Ability of the electrogenic species, namely Geobacter, Shewanella, Clostridium and Bacillus and the photosynthetic species, namely Chorella Vulgaris Rhodopsuedomonas, and Scenedesmus abundans in microbial deionization methods and their performance levels reported by several researchers are presented.

Enhanced performance and microbial interactions of shallow wetland bed coupling with functional biocathode microbial electrochemical system (MES)

It is essential to remediate the polluted urban river, which endangers the aquatic creatures and affected human body's senses. The treatment wetland combined with microbial electrochemical system (MES) used for the remediation is becoming a new research focus due to its ideal pollutants removal efficiency and small footprint. Here this paper provided a kind of novel shallow wetland bed coupling with close-circuit microbial electrochemical system (WB-CMES) to remove pollutants in surface water. In contrast to the shallow wetland bed coupling with open-circuit MES (WB-OMES) and the shallow wetland bed without MES (WB), the enhancing effects and pollutants removal pathway were evaluated. After 62-day operation, average TN removal efficiency in WB-CMES was 87.7%, which was 19.7% and 13.8% higher than that of WB-OMES and WB respectively. The rate coefficient k of NO₃^--N degradation in WB-CMES was 1.6 and 1.8 times higher than that in WB-OMES and WB. The results of chlorophyll, protein and superoxide dismutase (SOD) in WB-CMES were 27.3%, 44.3% and 12.9% higher than those in WB. The microbial community structure analysis indicated that electroactive bacteria on anode like Desulfobulbus could oxidize organics and generate electrons to compensate cathode, meanwhile, cathode could enrich more species of functional bacteria like Rhodobacter, Pirellula, Hyphomicrobium, Thauera, which had a synergistic effect on oxygen reduction, nitrogen removal and plant growth. The results indicated that oxygen produced by submerged plants could be utilized by the oxygen-reducing functional biocathode of MES and the proper aerobic and anoxic environment might enhance nitrate removal mainly through simultaneous nitrification and denitrification (SND), aerobic denitrification and anammox. This research provided a novel technology with advantages of simple operation, flexible configuration, easy scale-up and low cost for application in remediation of highly polluted surface water.

Recycled utilization of ryegrass litter in constructed wetland coupled microbial fuel cell for carbon-limited wastewater treatment

To solve wetland plant litter disposal and improve the nitrogen removal of carbon-limited wastewater, the integration of microbial fuel cell (MFC) and recycled utilization of ryegrass litter planted in constructed wetland (CW) may be effective. CW and MFC-CW with periodical ryegrass litter addition (10 days one cycle) were constructed to study the effects of ryegrass litter on nitrogen removal, electricity production and microorganism community. The results showed that total nitrogen removal of CW and MFC-CW after ryegrass litter addition reached 80.54 ± 10.99% and 81.94 ± 7.30%, increased by 22.19% and 17.50%, respectively. Three-dimensional excitation emission matrix fluorescence spectroscopy results revealed that the soluble organic matters produced by the hydrolyzed ryegrass litter were mainly tryptophan, tyrosine and fulvic acid, which promoted the growth of microorganisms and denitrification. The dosage of 200 g m^-2 did not cause the rise of refractory organic matter in the effluent. The ryegrass litter addition promoted the average voltage and power density slightly in MFC-CW, but the internal resistance also increased temporarily. Compared to the sole CW, current stimulation caused by MFC not only helped to increase the denitrification, but also accelerated the biomass hydrolysis. MFC could contribute to the enrichment and growth of functional microorganisms related to denitrification and organic degradation, such as Vogesella, Devosia, Thermomonas and Brevibacterium. The bacterial genera involved in the ryegrass litter degradation were mainly Thermomonas, Propionicimonas, TM7a, Clostridium_sensu_stricto_1 and so on. This study provided a promising way for practical applications of MFC-CW in the treatment of carbon-limited wastewater, especially in small ecological facilities.

A novel sulfur-driven autotrophic denitrification coupled with bio-cathode system for bioelectricity generation and groundwater remediation

This study explored the feasibility of treating wastewater using sulfur-driven autotrophic denitrification (SAD) coupled with the bio-cathode of microbial fuel cell (MFC), focusing on simultaneous bioelectricity generation, denitrification, and desulphurization. A maximum output voltage of 360 mV was obtained with a power generation cycle of 25 h when simulated wastewater with 100.0 mg/L of each NO₃^--N and S^2--S was employed as the influent in the SAD-BMFC. Compared with solo SAD or MFC, SAD-BMFC obtained a higher NO₃^--N removal rate (E_{12 h} = 87.7%, E_{24 h} = 100%), and less NO₂^--N accumulation. S^2--S of the influent was almost completely removed, oxidized to S⁰-S (88.6-90.2 mg/L) and SO₄^2--S (9.8-11.4 mg/L). The reaction system achieved self-balance of acidity-alkalinity (pH 7.05-7.35). The SAD process was the main pathway for NO₃^--N removal (80.2%) and a smaller proportion of electrons came from the bio-cathode. This study effectively combined SAD with a bio-cathode system for simultaneous energy harvest and bio-enhanced remediation of groundwater contaminated by both NO₃^--N and S^2--S.

Organic Waste Substrates for Bioenergy Production via Microbial Fuel Cells: A Key Point Review

High-energy consumption globally has raised questions about the low environmentally friendly and high-cost processes used until now for energy production. Microbial fuel cells (MFCs) may support alternative more economically and environmentally favorable ways of bioenergy production based on their advantage of using waste. MFCs work as bio-electrochemical devices that consume organic substrates in order for the electrogenic bacteria and/or enzyme cultures to produce electricity and simultaneously lower the environmental hazardous value of waste such as COD. The utilization of organic waste as fuels in MFCs has opened a new research path for testing a variety of by-products from several industry sectors. This review presents several organic waste substrates that can be employed as fuels in MFCs for bioenergy generation and the effect of their usage on power density, COD (chemical oxygen demand) removal, and Coulombic efficiency enhancement. Moreover, a demonstration and comparison of the different types of mixed waste regarding their efficiency for energy generation via MFCs are presented. Future perspectives for manufacturing and cost analysis plans can support scale-up processes fulfilling waste-treatment efficiency and energy-output densities.

Rice washing drainage (RWD) embedded in poly(vinyl alcohol)/sodium alginate as denitrification inoculum for high nitrate removal rate with low biodiversity

Immobilization technology with low maintenance is a promising alternative to enhance nitrate removal from water. In this study, washing rice drainage (RWD) was immobilized by poly(vinyl alcohol)/sodium alginate (PVA/SA) to obtain RWD-PVA/SA gel beads as inoculum for denitrification. When initial nitrate concentration was 50 mg N/L, nitrate was effectively removed at rates of 50-600 mg/(L∙d) using acetate as carbon source (C/N = 1.25). Arrhenius activation energy (Ea) of nitrate oxidoreductase was 28.64 kJ/mol for the RWD-PVA/SA gel beads. Temporal and spatial variation in microbial community structures were revealed along with RWD storage and in the reactors by Illumina high-throughput sequencing technology. RWD-PVA/SA gel beads has a simple (operational taxonomic units (OTUs) 〈100). Dechloromonas, Pseudomonas, Flavobacterium and Acidovorax were the most four dominant genera in the denitrification reactors inoculated with RWD-PVA/SA gel beads. This study provides an inoculum for denitrification with high nitrate removal performance and simple microbial community structures.

Enhanced treatment of low-temperature and low carbon/nitrogen ratio wastewater by corncob-based fixed bed bioreactor coupled sequencing batch reactor

In this study, a combined corncob-based fixed bed bioreactor and sequencing batch reactor system (CCF-SBR) was developed to treat low-temperature (3-12 °C) and low carbon/nitrogen ratio (C/N = 2) wastewater with a single SBR as the control. Results showed similarly low COD concentration of CCF-SBR (20.4 ± 3.7 mg·L^-1) and control SBR (24.9 ± 6.7 mg·L^-1) effluent. However, the total nitrogen (TN) removal rate of CCF-SBR was significantly higher than that of control SBR (29.6 ± 2.7% vs 8.6 ± 2.3%). According to the nitrification and denitrification activities and the analysis of microbial community, CCF mainly played the role of denitrification based on fermentation genera and denitrifying genera, and SBR mainly implemented nitrification with Nitrospira and Acinetobacter. This study explores a promising way for agricultural waste resource utilization and wastewater treatment under low-temperature and low C/N ratio.

Mitigation of tannery effluent with simultaneous generation of bioenergy using dual chambered microbial fuel cell

In this study, a dual chambered microbial fuel cell (MFC) was fabricated for the treatment of tannery wastewater with concurrent production of bio-energy. The tannery effluent acts as an anolyte and a synthetic electrolytic solution as the catholyte. Five electrochemically active bacteria from the biofilm were isolated that showed homology with Klebsiella quasipneumoniae, Klebsiella pneumoniae, Cloacibacterium normanese, Bacillus firmus and Pseudomonas reactans, using 16S rDNA analysis. The physiochemical studies of treated wastewater showcased the 88%, 74% and 94% reduction in COD, BOD and TDS level, respectively. The maximum voltage output and power density obtained using electroactive consortium in MFC was 940 mV and 7371 mW/cm³, respectively. The techno-economic feasibility of the bio-electrochemical system was studied for future bioprospecting. The present study reports a significant power generation with simultaneous effluent treatment up to a maximum of ∼85%, in a sustainable and eco-friendly manner.

Intensifying anoxic ammonium removal by manganese ores and granular active carbon fillings in constructed wetland-microbial fuel cells: Metagenomics reveals functional genes and microbial mechanisms

The conventional biological ammonium removal process is challenged for lack of electron acceptors. A lab-scale integrated constructed wetland coupled with microbial fuel cells (CW-MFC) filling manganese ores (MO) and granular active charcoal (GAC) has been developed, named CW-CM. It enhanced the nitrogen removal two times over the control. A metagenomic-based study illustrated the functional genes and taxonomic groups related to N transformations, explored metabolic mechanisms of nitrogen and carbon sources, and then revealed some characteristics of the extracellular electron transfer (EET). Many nitrifying bacteria and autotrophic and heterotrophic denitrifiers were enriched in CW-CM. Furthermore, most nitrification and denitrification reactions except for the conversion of ammonium to hydroxylamine were significantly enhanced in CW-CM. Glycolysis and the TCA cycle were also improved. Overall, a novel anoxic ammonia removal process was achieved in the experimental group with no need of anammox functional bacteria and anammox key genes.

Advanced bioelectrochemical system for nitrogen removal in wastewater

Nitrogen (N) pollution in water has become a serious issue that cannot be ignored due to the harm posed by excessive nitrogen to environmental safety and human health; as such, N concentrations in water are strictly limited. The bioelectrochemical system (BES) is a new method to remove excessive N from water, and has attracted considerable attention. Compared with other methods, it is highly efficient and has low energy consumption. However, the BES has not been applied for N removal in practice due to lack of in-depth research on the mechanism and construction of high-performance electrodes, separators, and reactor configurations; this highlights a need to review and examine the efforts in this field. This paper provides a comprehensive review on the current BES research for N removal focusing on the reaction principles, reactor configurations, electrodes and separators, and treatment of actual wastewater; the corresponding performances in these realms are also discussed. Finally, the prospects for N removal in water using the BES are presented.

A review of the bioelectrochemical system as an emerging versatile technology for reduction of antibiotic resistance genes

Antibiotic contamination and the resulting resistance genes have attracted worldwide attention because of the extensive overuse and abuse of antibiotics, which seriously affects the environment as well as human health. Bioelectrochemical system (BES), a potential avenue to be explored, can alleviate antibiotic pollution and reduce antibiotic resistance genes (ARGs). This review mainly focuses on analyzing the possible reasons for the good performance of ARG reduction by BESs and potential ways to improve its performance on the basis of revealing the generation and transmission of ARGs in BES. This system reduces ARGs through two pathways: (1) the contribution of BES to the low selection pressure of ARGs caused by the efficient removal of antibiotics, and (2) inhibition of ARG transmission caused by low sludge yield. To promote the reduction of ARGs, incorporating additives, improving the removal rate of antibiotics by adjusting the environmental conditions, and controlling the microbial community in BES are proposed. Furthermore, this review also provides an overview of bioelectrochemical coupling systems including the BES coupled with the Fenton system, BES coupled with constructed wetland, and BES coupled with photocatalysis, which demonstrates that this method is applicable in different situations and conditions and provides inspiration to improve these systems to control ARGs. Finally, the challenges and outlooks are addressed, which is constructive for the development of technologies for antibiotic and ARG contamination remediation and blocking risk migration.

Improving the outcomes from electroactive constructed wetlands by mixing wastewaters from different beverage-processing industries

Denitrification in electroactive constructed wetland (EW) systems is constrained by the carbon source and the carbon/nitrogen (C/N) ratio (the COD/TN ratio). In this study, wastewater with a high C/N from a brewery was added to wastewater with a low C/N (dairy wastewater) in an EW system, and the pollutant removal, bioelectricity generation, transformations of dissolved organic matter, and microbial community structures were evaluated. The results showed that the average removal rates of ammonium nitrogen, total nitrogen, and chemical oxygen demand from the wastewater mixture were 6.40%, 46.44%, and 23.85% higher than those from the wastewater with a low C/N, respectively. Dissimilatory nitrate reduction to ammonium was effectively inhibited, and the NH₄⁺-N removal was 25.52% higher, when the wastewater mixture was used instead of the high C/N wastewater. Similarly, the output voltage was significantly increased, and the internal resistance of the device was reduced, for the wastewater mixture. The structure of the microbial community improved, the relative abundance of electrochemically active bacteria was higher, and the protein-like and humic-like components were lower, in the mixture treatment than in the individual treatment. The results show that the nitrogen removal and biopower generation improved in an EW system when high C/N wastewater was used as the carbon source.

Effect of cathodic culture on wastewater treatment and power generation in a photosynthetic sediment microbial fuel cell (SMFC): Canna indica v/s Chlorella vulgaris

The aim of this work was to compare the performance of the two types of photosynthetic microbial fuel cells (MFCs) fed with real wastewater- one having plant Canna indica (PMFC) and the other having alga Chlorella vulgaris (AMFC) at the cathode. The chemical oxygen demand (COD), phosphate, and nitrate removal stood at 57.16% 88.81%, 59.82% for PMFC and 65.27%, 95.59%, 66.61% for the AMFC. While AMFC was slightly superior in water treatment, the power output was 6 times higher in PMFC (22.76 mW m^-2) than the AMFC (3.64 mW m^-2). The biomass growth was good in both systems, with biomass productivity of 0.031 Kg m^-3 day^-1 in AMFC and a leaf area index of 0.006 in PMFC. The study's findings suggest that PMFCs are equally good or even better than AMFCs when the goal is simultaneous water treatment and power generation.

Ecological impact of antibiotics on bioremediation performance of constructed wetlands: Microbial and plant dynamics, and potential antibiotic resistance genes hotspots

Constructed wetlands (CWs) are nature-based solutions for treating domestic and livestock wastewater which may contain residual antibiotics concentration. Antibiotics may exert selection pressure on wetland's microbes, thereby increasing the global antibiotics resistance problems. This review critically examined the chemodynamics of antibiotics and antibiotics resistance genes (ARGs) in CWs. Antibiotics affected the biogeochemical cycling function of microbial communities in CWs and directly disrupted the removal efficiency of total nitrogen, total phosphorus, and chemical oxygen demand by 22%, 9.3%, and 24%, respectively. Since changes in microbial function and structure are linked to the emergence and propagation of antibiotic resistance, antibiotics could adversely affect microbial diversity in CWs. The cyanobacteria community seemed to be particularly vulnerable, while Proteobacteria could resist and persist in antibiotics contaminated wetlands. Antibiotics triggered excitation responses in plants and increased the root activities and exudates. Microbes, plants, and substrates play crucial roles in antibiotic removal. High removal efficiency was exhibited for triclosan (100%) > enrofloxacin (99.8%) > metronidazole (99%) > tetracycline (98.8%) > chlortetracycline (98.4%) > levofloxacin (96.69%) > sulfamethoxazole (91.9%) by the CWs. This review showed that CWs exhibited high antibiotics removal capacity, but the absolute abundance of ARGs increased, suggesting CWs are potential hotspots for ARGs. Future research should focus on specific bacterial response and impact on microbial interactions.

Degradation of high concentration starch and biocathode autotrophic denitrification using photo microbial fuel cell

In the study, a dual-chamber photo MFC was constructed with a photosynthetic bacteria consortium PB-Z and a heterotrophic nitrifier C16 as anode and cathode inoculant, respectively. The electron released from starch degradation in the anode by photosynthetic bacteria was transferred to the cathode, which was utilized by the nitrifying bacteria C16 to realize autotrophic denitrification. Lower resistance was more conducive to the electron transfer and pollutants removal. Comparing with natural light, continuous light greatly promoted starch degradation by the photosynthetic bacteria in the anode and the denitrification by the nitrifying bacteria in the cathode. Under continuous light and external resistance of 500 Ω, high concentration starch was degraded by photosynthetic bacteria PB-Z and the COD removal efficiency reached up to 88.45% within 12 d, and nitrate of 95.8% was removed within 4 d by autotrophic denitrification by heterotrophic nitrifier C16. The study provides some enlightenment and reference for the application of MFC in the field of wastewater treatment.

Bioenergy generation and nitrogen removal in a novel ecological-microbial fuel cell

A novel ecological-microbial fuel cell (E-MFC) was constructed based on the mutualistic symbiosis relationship among wetland plants Ipomoea aquatic, benthic fauna Tubifex tubifex (T. tubifex) and microorganisms. The maximum power densities of sediment MFC (S-MFC), wetland plant MFC (WP-MFC) and E-MFC were 6.80 mW/m², 10.60 mW/m² and 15.59 mW/m², respectively. Ipomoea aquatic roots secreted organic matter as electricigens' fuel for electricity generation, while T. tubifex decomposed decaying leaves and roots into soluble organic matter and plant nutrients, forming a co-dependent and mutually beneficial system, which was conducive to bioelectricity production. The E-MFC obtained the highest nitrogen removal, and the removal efficiencies of NH₄⁺-N and NO₃^--N were 90.4% and 96.5%, respectively. Hydraulic retention time (HRT), cathodic aeration and T. tubifex abundance had significant effects on E-MFC power generation. The performeance boost of E-MFC was closely related to anodic microbial community change caused by the introduction of T. tubifex.

Improvement of Alcaligenes sp.TB performance by Fe-Pd/multi-walled carbon nanotubes: Enriched denitrification pathways and accelerated electron transport

Aiming at the accumulation of NO₂^--N and N₂O during denitrification process, this work focused to improve the denitrification performance by Pd-Fe embedded multi-walled carbon nanotubes (MWCNTs). The main conclusions were as follows: 30 mg/L Pd-Fe/MWCNTs have shown an excellent promotion on denitrification (completely TN removal at 36 h). Meanwhile, enzyme activity results indicated that the generation of NO₂^--N, NH₄⁺-N by Pd-Fe/MWCNTs led the occur of short-cut denitrification by increasing 203.9% the nitrite reductase activity. Furthermore, electrochemical results and index correlation analysis confirmed that the electron exchange capacity (1.401 mmol eg^-1) of Pd-Fe/MWCNTs was positively related to nitrite reductase activity, indicating its crucial role in electron transport activity (0.46 μg O₂/(protein·min) at 24 h) during denitrification process by Pd-Fe/MWCNTs played a role of chemical reductant and redox mediator.

Floating treatment wetlands integrated with microbial fuel cell for the treatment of urban wastewaters and bioenergy generation

The objective of the present study was to develop a combined system composed of anaerobic biofilter (AF) and floating treatment wetlands (FTW) coupled with microbial fuel cells (MFC) in the buoyant support for treating wastewater from a university campus and generate bioelectricity. The raw wastewater was pumped to a 1450 L tank, operated in batch flow and filled with plastic conduits. The second treatment stage was composed of a 1000 L FTW box with a 200 L plastic drum inside (acting as settler in the entrance) and vegetated with mixed ornamental plants species floating in a polyurethane support fed once a week with 700 L of wastewater. In the plant roots, graphite rods were placed to act as cathodes, while on the bottom of the box 40 graphite sticks inside a plastic hose with a stainless-steel cable acting as the anode chamber. Open circuit voltages were daily measured for 6 weeks, and later as closed circuit with the connection of 1000 Ω resistors. Plant harvestings were conducted, in which biomass production and plant uptake from each of the species were measured. On average, system was efficient in reducing BOD₅ (55.1%), COD (71.4%), turbidity (90.9%) and total coliforms (99.9%), but presented low efficiencies regarding total N (8.4%) and total P (11.4%). Concerning bioenergy generation, voltage peaks and maximum power density were observed on the feeding day, reaching 225 mV and 0.93 mW/m², respectively, and in general decaying over the 7 days. In addition, plant species with larger root development presented higher voltage values than plants with the smaller root systems, possible because of oxygen release. Therefore, the combined system presented potential of treating wastewater and generating energy by integrating FTW and MFC, but further studies should investigate the FTW-MFC combination in order to improve its treatment performance and maximize energy generation.

Biosynthetic FeS/BC hybrid particles enhanced the electroactive bacteria enrichment in microbial electrochemical systems

Modifying the surface of an anode can improve electroactive bacteria (EAB) enrichment, thereby enhancing the performance of the associated microbial electrochemical systems (MESs). In this study, biosynthetic FeS nanoparticles were used to modify the anode in MESs. The experimental results demonstrated that the stable maximum voltage of the FeS composited biochar (FeS/BC)-modified anode reached 0.72 V, which is 20% higher than that of the control. The maximum power density with the FeS/BC anode was 793 mW/m², which is 46.31% higher than that obtained with the control (542 mW/m²). According to cyclic voltammetry (CV) analysis, FeS/BC facilitates the direct electron transfer between bacteria and the electrode. The biomass protein concentration of the FeS/BC anode was 841.75 μg/cm², which is almost 1.5 times higher than that of the carbon cloth anode (344.25 μg/cm²); hence, FeS/BC modification can promote biofilm formation. The composition of Geobacter species on the FeS/BC anode (75.16%) was much higher than that on the carbon cloth anode (4.81%). All the results demonstrated that the use of the biosynthetic FeS/BC anode is an environmentally friendly and efficient strategy for enhancing the electroactive biofilm formation and EAB enrichment in MESs.

CO2 dynamic of Lake Donghu highlights the need for long-term monitoring

Inland freshwater lakes have been widely considered as significant sources of CO₂ to the atmosphere. However, long-term measurements of CO₂ dynamics in lakes are still lacking, but are necessary due to their large temporal variations. Herein, we present the long-term dynamics of water parameters in Lake Donghu from 2002 to 2016, and further calculate the partial pressure of CO₂ (pCO₂) based on the measurements of pH, water temperature, and alkalinity from 2008 to 2016. The results revealed that a significantly high pCO₂ occurred during the winter in Lake Donghu (p < 0.01), whereas no significant spatial difference was observed (p = 0.37). Statistical analysis indicated that the pCO₂ in the lake was only positively correlated with the total phosphorus (TP) concentration (p < 0.05). A multilinear regression model provided the best predictors for the pCO₂; however, it only explained 16% of the observed pCO₂ variability. This indicates the complex factors that influenced the pCO₂ in Lake Donghu between 2008 and 2016. Our estimated CO₂ flux revealed that Lake Donghu acted as a small CO₂ source to the atmosphere during this period, with a mean CO₂ flux of 10.8 ± 37.4 mg m^-2 day^-1 corresponding to a mean CO₂ emission of 0.13 ± 0.43 Gg year^-1. The CO₂ emission fluxes in Lake Donghu were much lower than the mean CO₂ fluxes reported for other lakes in China and globally. Furthermore, the long-term evolution of the CO₂ flux indicated that Lake Donghu has shifted between acting as a CO₂ source and sink, which highlights the need for long-term monitoring to accurately evaluate CO₂ emissions from lakes.

Mapping the field of constructed wetland-microbial fuel cell: A review and bibliometric analysis

The embedding microbial fuel cell (MFC) into constructed wetlands (CW) to form CW-MFC bears the potential to obtain bioelectricity and a clean environment. In this study, a bibliometric analysis using VOSviewer based on Web of Science data was conducted to provide an overview by tracing the development footprint of this technology. The countries, institutions, authors, key terms, and keywords were tracked and corresponding mapping was generated. From 2012 to September 2020, 442 authors from 129 organizations in 26 countries published 135 publications in 42 journals with total citation of 3139 times were found. The key terms analysis showed four clusters: bioelectricity generation performance, mechanism study, refractory pollutants removal, and enhanced conventional contaminants removal. Further research themes include exploring the biochemical properties of electrochemically active bacteria, emerging contaminants removal, effective bioelectricity harvest and the use, and biosensor development as well as scaling-up for real field application. The bibliometric results provide valuable references and information on potential research directions for future studies.

Dual role of macrophytes in constructed wetland-microbial fuel cells using pyrrhotite as cathode material: A comparative assessment

In the recent years many studies have shown that wetland plants play beneficial roles in bioelectricity enhancement in constructed wetland-microbial fuel cell (CW-MFC) because of the exudation of root oxygen and root exudates. In this study, the long-term roles of plants on the bioelectricity generation and contaminant removal were investigated in multi-anode (Anode1 and Anode2) and single cathode CW-MFCs. The electrode distances were 20 cm between Anode1-cathode and 10 cm between Anode2-cathode, respectively. Additionally, the employment of natural conductive pyrrhotite mineral as cathode material was firstly investigated in CW-MFC system. A cathode potential of -98 ± 52 mV to -175 ± 60 mV was achieved in the unplanted (CW-MFC 1), and planted CW-MFCs with Iris pseudacorus (CW-MFC 2), Lythrum salicaria (CW-MFC 3), and Phragmites australis (CW-MFC 4). The maximum power densities of Anode1-cathode and Anode2-cathode were 8.23 and 15.29 mW/m² in CW-MFC 1, 8.51 and 1.67 mW/m² in CW-MFC 2, 5.67 and 3.15 mW/m² in CW-MFC 3, and 7.59 and 14.71 mW/m² in CW-MFC 4, respectively. Interestingly, smaller power density was observed at Anode2-cathode, which has shorter electrode distance than Anode1-cathode in both CW-MFC 2 and CW-MFC 3, which indicates the negative role of oxygen released from the flourished plant roots at Anode2 micro-environment in power production. Therefore, recovering power from commercial CW-MFCs with flourished plants will be a challenge. The contradiction between keeping short electrode distance and avoiding the interference from plant roots to maintain anaerobic anode may be solved by the proposed modular CW-MFCs.

Responses of phytoremediation in urban wastewater with water hyacinths to extreme precipitation

Climate change not only intensifies eutrophication and enhances the rainfall, but also elevates the contents of greenhouse gases, which can further increase the intensity and frequency of extreme precipitation events. The effectivity of phytoremediation of urban wastewaters by water hyacinths under an extreme rainfall event (up to 380 mm d^-1) was investigated using self-designed fabrications with six flow rates (2-15 m³ d^-1) in situ on pilot scale for 30 days. The results suggest that water hyacinths had high N and P removal capacities even under adverse conditions such as low dissolved oxygen concentrations (DO, <1 mg L^-1) and high ammonium concentrations (NH₄⁺-N, >7 mg L^-1). Specifically, the highest removal yields of N and P were 13.14 ± 0.47 g N·m^-2·d^-1 and 2.12 ± 0.04 g P·m^-2·d^-1, respectively. The results indicate that water hyacinths can be used for water treatment to reduce the amounts of NH₄⁺-N, dissolved organic nitrogen (DON) and phosphate (PO₄^3-) even during extreme precipitation events. Moreover, DO increased due to wet deposition, runoff and surface flows during the extreme rainfall event, resulting in shifts between nitrification and denitrification processes which significantly altered nitrogen forms in urban wastewater. Results of this study suggest that water hyacinths could be recommended as a cost-effective and eco-friendly technology for urban wastewater phytoremediation in areas suffering from frequent extreme precipitation events.

Robust nitrate removal and bioenergy generation with elucidating functional microorganisms under carbon constraint in a novel multianode tidal constructed wetland coupled with microbial fuel cell

This study investigated synthetic wastewater treatment under low inflow C/N ratio and characterized NO₃^--N-transforming and electricity-producing bacteria in a multi-anode tidal constructed wetland-microbial fuel cell (TFCW-MFC). The optimal concurrent average removal rates of NH₄⁺-N and NO₃^--N were 73% and 78%, respectively, under a flood/rest/flood time of 4 h/2h/4h in "tide" mode accompanied by one recirculation. The lowest NO₃^--N concentration among all anodes was observed when the electrode gap was 45 cm. Similarly, the 45 cm anode exhibited selective enrichment of Variovorax and Azoarcus. Correction analysis showed that the high relative abundance of Azoarcus was crucial in enhancing NO₃^--N removal, and the internal resistance significantly decreased as the relative abundance of Acidovorax increased. These results suggest that NO₃^--N removal and bioelectricity generation can be promoted in a TFCW-MFC with limited carbon by improving the culture conditions for specific genera.

Effect of the C/N Ratio on Biodegradation of Ciprofloxacin and Denitrification from Low C/N Wastewater as Assessed by a Novel 3D-BER System

Emerging pollutants in the form of pharmaceuticals have drawn international attention during the past few decades. Ciprofloxacin (CIP) is a common drug widely found in effluents from hospitals, industrial and different wastewater treatment plants, as well as rivers. In this work, the lab-scale 3D-BER system was established, and more than 90% of the antibiotic CIP was removed from Low C/N wastewater. The best results were obtained with the current intensity being taken into account, and a different C/N ratio significantly improved the removal of CIP and nitrates when the ideal conditions were C/N = 1.5–3.5, pH = 7.0–7.5 and I = 60 mA. The highest removal efficiency occurred when CIP = 94.2%, NO3−-N = 95.5% and total nitrogen (TN) = 84.3%, respectively. In this novel system, the autotrophic-heterotrophic denitrifying bacteria played a vital role in the removal of CIP and an enhanced denitrification process. Thus, autotrophic denitrifying bacteria uses CO2 and H2 as carbon sources to reduce nitrates to N2. This system has the assortment and prosperous community revealed at the current intensity of 60 mA, and the analysis of bacterial community structure in effluent samples fluctuates under different conditions of C/N ratios. Based on the results of LC-MS/MS analysis, the intermediate products were proposed after efficient biodegradation of CIP. The microbial community on biodegrading was mostly found at phylum, and the class level was dominantly responsible for the NO3−-N and biodegradation of CIP. This work can provide some new insights towards the biodegradation of CIP and the efficient removal of nitrates from low C/N wastewater treatment through the novel 3D-BER system.

Horizontal sub surface flow Constructed Wetlands coupled with tubesettler for hospital wastewater treatment

Hospital wastewater are a lurking threat to environment and human health security for any given moment of time owing to its complexity and high vulnerability to cause disease outbreak. Though there are a number of treatment process for wastewater., there is a high need for employing cost-efficient and sustainable method of treatment. Hence a pilot scale horizontal surface flow Constructed Wetland (HSFCW) coupled with Tubesettler was installed at New Delhi, India (February to may 2019). This study reports comparative pollutants removal from hospital wastewater using Constructed Wetlands and associated tubesettler dosed with Hospital wastewater. A pilot scale CW system was used for treating 10m³/day of hospital wastewater. The system was tested for 3 Months to evaluate its performance for removing pollutants from the wastewater. The HSFCW coupled with tubesettler achieved over all removal efficiency of 94% (COD), MLSS (97%), TSS (98%), BOD₅ (96%), Phosphate (79%). However, process of nitrification was not observed and accumulation of Nitrate up to 197% was observed. The study concluded that it may be due to the presence of pharmaceuticals and other elements present in hospital wastewater. This conclusion was based on the fact that Alkalinity increased by 52% in effluent and pH value also exhibited an average increase of 12%. Further research studies are required to investigate effect of pharmaceutical originating from hospital on treatment efficiency, to incorporate anaerobic setup to complete denitrification-nitrification process and also to determine efficiency of thermophilic, mesophilic, and psychrophilic bacteria with respect to climate and temperature.

Electron transfer pathways and kinetic analysis of cathodic simultaneous nitrification and denitrification process in microbial fuel cell system

Microbial fuel cell (MFC) is an innovative bioconversion technology for wastewater treatment accompanied with electricity recovery. In this study, a kinetic model was developed base on Activated Sludge Model No.1 (ASM1) to describe electron transfer pathways during the simultaneous nitrification and denitrification (SND) process in the biocathode system of a dual-chamber MFC. The batch running of the dual-chamber MFC system showed that it produced a power density up to 2.96 W m^-3 within 48 h, the achieved SND efficiency and autotrophic denitrification ratio in the cathodic denitrification process were up to 87.3 ± 0.8% and 69.5 ± 6.6%, respectively. Meanwhile, by integrating nitrification, autotrophic denitrification, heterotrophic denitrification, organic carbon oxidation, and oxygen reduction in the cathode, the model was able to precisely fit the concentration variations of NH₃-N, dissolved oxygen (DO) and chemical oxygen demand (COD) during the cathodic SND process (R² ≥ 0.9876). The cathode electrons tended to be completely utilized with the increase of autotrophic denitrification ratio in the cathodic denitrification process. When the nitrification rate was enhanced, the autotrophic denitrification would prevail in the competition with the heterotrophic denitrification. In summary, the developed model was confirmed to be effective and reliable for describing the electron transfer pathways and predicting the performance of the nitrogen removal reactions during the cathodic SND process in a double-chamber MFC.

Leachate treatment and electricity generation using an algae-cathode microbial fuel cell with continuous flow through the chambers in series

Algae-cathode microbial fuel cells (MFCs) with various hydraulic retention times (HRTs) were investigated for electricity generation, and chemical oxygen demand (COD) and nutrient removal from diluted landfill leachate (15% v/v). The cell voltage and dissolved oxygen (DO) in the cathode were considerably affected by the HRT. The highest cell voltage was 303 mV at 20-h HRT, and DO concentration of 5.3 mg/L was only observed at 60-h HRT. Nutrient removal increased with increasing HRTs, and the maximum removal efficiency was 76.4% and 86.3% at 60-h HRT for ammonium and phosphorus, respectively. The highest COD removal of 26% was observed at 60-h HRT. The dominant phyla in the cathode were Proteobacteria, Cyanobacteria, Bacteroidetes, and Chlorophyta, which could have contributed to electricity generation and nutrient removal. This study suggests that an algae-cathode MFC with an appropriate HRT can continuously generate electricity and simultaneously remove nutrients from real leachate wastewater in field applications.

Response Characteristics of Nitrifying Bacteria and Archaea Community Involved in Nitrogen Removal and Bioelectricity Generation in Integrated Tidal Flow Constructed Wetland-Microbial Fuel Cell

This study explores nitrogen removal performance, bioelectricity generation, and the response of microbial community in two novel tidal flow constructed wetland-microbial fuel cells (TFCW-MFCs) when treating synthetic wastewater under two different chemical oxygen demand/total nitrogen (COD/TN, or simplified as C/N) ratios (10:1 and 5:1). The results showed that they achieved high and stable COD, NH₄ ⁺-N, and TN removal efficiencies. Besides, TN removal rate of TFCW-MFC was increased by 5-10% compared with that of traditional CW-MFC. Molecular biological analysis revealed that during the stabilization period, a low C/N ratio remarkably promoted diversities of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in the cathode layer, whereas a high one enhanced the richness of nitrite-oxidizing bacteria (NOB) in each medium; the dominant genera in AOA, AOB, and NOB were Candidatus Nitrosotenuis, Nitrosomonas, and Nitrobacter. Moreover, a high C/N ratio facilitated the growth of Nitrosomonas, while it inhibited the growth of Candidatus Nitrosotenuis. The distribution of microbial community structures in NOB was separated by space rather than time or C/N ratio, except for Nitrobacter. This is caused by the differences of pH, dissolved oxygen (DO), and nitrogen concentration. The response of microbial community characteristics to nitrogen transformations and bioelectricity generation demonstrated that TN concentration is significantly negatively correlated with AOA-shannon, AOA-chao, 16S rRNA V4-V5-shannon, and 16S rRNA V4-V5-chao, particularly due to the crucial functions of Nitrosopumilus, Planctomyces, and Aquicella. Additionally, voltage output was primarily influenced by microorganisms in the genera of Nitrosopumilus, Nitrosospira, Altererythrobacter, Gemmata, and Aquicella. This study not only presents an applicable tool to treat high nitrogen-containing wastewater, but also provides a theoretical basis for the use of TFCW-MFC and the regulation of microbial community in nitrogen removal and electricity production.