• Energy Research

The present work is the first detailed study, which is about the performance of CW after MFC integration with it. The experiments were run in open and closed circuit mode for assessment purpose. The findings of this study indicate towards a more practical application of MFC in wastewater treatment along with electricity production. The closed circuit operations of CW-MFCs have performed 12-20% better than open circuit operation and 27-49% better than Normal-CW for chemical oxygen demand (COD) removal. The maximum power density of 320.8 mW/m(3) and current density of 422.2 mA/m(3) have been achieved in granular graphite anode and Pt coated carbon cloth cathode based CW-MFC.

The present study explores low-cost cathode development possibility using radial oxygen loss (ROL) of Canna indica plants and intermittent aeration (IA) for wastewater treatment and electricity generation in constructed wetland-microbial fuel cell (CW-MFC) system. Two CW-MFC microcosms were developed. Amongst them, one microcosm was planted with Canna indica plants for evaluating the ROL dependent cathode reaction (CW-MFC dependent on ROL) and another microcosm was equipped with intermittent aeration for evaluating the intermittent aeration dependent cathode reaction (CW-MFC with additional IA). The CW-MFC with additional IA has achieved 78.71% and 53.23%, and CW-MFC dependent on ROL has achieved 72.17% and 46.77% COD removal from synthetic wastewater containing glucose loads of 0.7gL-1and 2.0gL-1, respectively. The maximum power density of 31.04mWm-3 and 19.60mWm-3 was achieved in CW-MFC with additional IA and CW-MFC dependent on ROL, respectively.

This is the first study that demonstrates developing a Power Management System (PMS) for initiating a self-automated Constructed wetlands (CW)-microbial fuel cell (MFC). The developed PMS helps by harvesting and storing low power generated from CW-MFC and using it to operate an air-pump used as a part of treatment processes. The great potential for self generating energy harvestors to power electrical instruments makes the CW-MFC technology a sustainable candidate for many applications. In this paper, two laboratory-scale CW-MFC were used for this purpose, the cathode of one CW-MFC (R1) intermittently aerated (IA) with the self-automated air-pump, and the performance compared with the second CW-MFC (R2). An energy harvesting system was configured specifically optimised to suit the low energy output from the CW-MFC. The maximum power generated by R1 and R2 CW-MFC was 54.6 µW and 41.2 µW, respectively, which increased up to 90 µW after IA. The pollutant treatment performance of R1 with IA increased significantly from R1 without aeration and R2 microcosms. The ammonium (NH4+) removal in R1_IA was 10% higher than R1 without aeration and 12% higher than in R2. With the IA, dissolved oxygen at the cathode of R1 increased, which influenced redox potential for better electron recovery and reduced the internal resistance. The use of generated energy from the system makes the technology self-sustainable and high performing.

Constructed wetlands (CWs) integrated with bioelectrochemical systems (BESs) are being intensively researched with the names like constructed wetland-microbial fuel cell (CW-MFC), electro-wetlands, electroactive wetlands, and microbial electrochemical technologies-based constructed wetland since the last decade. The implantation of BES in CW facilitates the tuning of redox activities and electron flow balance in aerobic and anaerobic zones in the CW bed matrix, thereby alleviating the limitation associated with electron acceptor availability and increasing its operational controllability. The benefits of CW-MFC include high treatment efficiency, electricity generation, and recalcitrant pollutant abatement. This article presents CW-MFC technology's journey since its emergence to date, encompassing the research done so far, including the basic principle and functioning, bio-electrocatalysts as its machinery, influential factors for microbial interactions, and operational parameters controlling different processes. A few key challenges and potential applications are also discussed for the CW-MFC systems.

This investigation is the first of its kind to evaluate the interrelation of sulphate (SO42-) with conductive materials as well as their individual and synergetic effects on the removal of ammonium and organic pollutants in electroactive wetlands, also known as constructed wetland (CW) - microbial fuel cell (MFC). The role of MFC components in CW was investigated to treat the sulphate containing wastewater under a long-term operation without any toxicity build-up in the system. A comparative study was also performed between CW-MFC and CW, where sulphate containing wastewater (S-replete) and without sulphate wastewater (S-deplete) was assessed. The S-replete showed high NH4+ removal than the S-deplete, and the requesnce of removal was: CW-MFC-replete>CW-MFC-deplete>CW-replete>CW-deplete. The chemical oxygen demand (COD) removal was high in the case of CW-MFC-replete, and the sequence of removal was CW-MFC-replete>CW-MFC-deplete>CW-deplete>CW-replete. X-ray photon spectroscopic study indicates 0.84% sulphur accumulation in CW-MFC-replete and 2.49% in CW-replete, indicating high sulphur precipitation in CW without the MFC component. The high relative abundance of class Deltaproteobacteria (7.3%) in CW-MFC-replete along with increased microbial diversity (Shannon index=3.5) rationalise the symbiosis of sulphate reducing/oxidising microbes and its impact on the treatment performance and electrochemical activity.

This study provides a new approach of electrode dependent anaerobic ammonium oxidation (electroanammox) in microbial fuel cell (MFC) integrated hybrid constructed wetlands (CWs). The study was carried out in three CWs, each with a horizontal flow (HF) followed by a vertical upflow (VUF). Two of the CWs were integrated with MFC, one was operated in closed circuit (CL) mode and the other in open circuit (OP) mode to determine the influence of electron transfer through an external electrical circuit. The initial nitrogen and carbon concentration were 40 mg/l and 880 mg/l respectively. The total nitrogen (TN), NH4+-N, TOC and COD removal achieved in CW-MFC-CL were 90.0 ± 1.15%, 94.4 ± 0.75%, 64.8 ± 3.0% and up to 99.5 ± 3.4%, respectively. The TN and NH4+-N removal in CW-MFC-CL was 20.0% and 13.6% higher than normal CW. Maximum current density achieved in CW-MFC-HF was of 75 mA/m3 and in CW-MFC-VUF was 156 mA/m3. Furthermore, the study revealed that even at low microbiological biomass, an MFC integrated CW operating in closed circuit gave higher removal of NH4+-N and COD than the normal CW and open circuit CW-MFC. Microbiological analysis shows the presence of already known nitrifier and denitrifer which indicates their role in electrode dependent nitrogen removal.

A two-stage hybrid Constructed Wetland (CW) integrated with a microbial fuel cell (MFC), and microbial electrolysis cell (MEC) has been assessed for treatment performance and clogging assessment and further compared with CW. The CW-MEC was operated with applied potential to the working electrode and compared with the performance of naturally adapted redox potential of the CW-MFC system. A complex synthetic municipal wastewater was used during the study, which was composed of trace metals, organics, inorganics, and dye. The study demonstrated that providing a constant potential to the working electrode in CW-MEC has resulted in high treatment performance and reduced sludge generation. The maximum chemical oxygen demand (COD), ammonium (NH4+), and phosphate (PO43-) removal achieved during treatment by CW-MEC at 24 h hydraulic retention time was 89 ± 6%, 72 ± 6% and 93 ± 2%, respectively. ICP-MS results indicated that trace metal removals were also higher in CW-MEC than in CW alone (p < 0.05). At the end of the experiment, significant volumetric change (total volume of the microcosm) occurred in CW (1.3 L), which indicates high sludge generation, whereas it was lesser in CW-MEC (0.3 L) and in CW-MFC (0.5 L). Further, Energy Dispersive X-ray (EDX) spectroscopy results indicated low levels of metal precipitation in the CW-MEC system. Based on the Shannon diversity index, the CW-MEC was assessed to be characterised by high species richness and diversity. The observations from this study indicate that the applied potential at the working electrode has a significant impact on treatment performance and clogging behaviour of the system.

In less than a decade, bioelectrochemical systems/microbial fuel cell integrated constructed wetlands (electroactive wetlands) have gained a considerable amount of attention due to enhanced wastewater treatment and electricity generation. The enhancement in treatment has majorly emanated from the electron transfer or flow, particularly in anaerobic regions. However, the chemistry associated with electron transfer is complex to understand in electroactive wetlands. The electroactive wetlands accommodate diverse microbial community in which each microbe set their own potential to further participate in electron transfer. The conductive materials/electrodes in electroactive wetlands also contain some potential, due to which, several conflicts occur between microbes and electrode, and results in inadequate electron transfer or involvement of some other reaction mechanisms. Still, there is a considerable research gap in understanding of electron transfer between electrode-anode and cathode in electroactive wetlands. Additionally, the interaction of microbes with the electrodes and understanding of mass transfer is also essential to further understand the electron recovery. This review mainly deals with the electron transfer mechanism and its role in pollutant removal and electricity generation in electroactive wetlands.