Bioelectrochemical anoxic ammonium nitrogen removal by an MFC driven single chamber microbial electrolysis cell

Emerging biotechnological applications of anaerobic ammonium oxidation

Anaerobic ammonium oxidation (anammox) is an energy-efficient method for nitrogen removal that opens the possibility for energy-neutral wastewater treatment. Research on anammox over the past decade has primarily focused on its implementation in domestic wastewater treatment. However, emerging studies are now expanding its use to novel biotechnological applications and wastewater treatment processes. This review highlights recent advances in the anammox field that aim to overcome conventional bottlenecks, and explores novel and niche-specific applications of the anammox process. Despite the promising results and potential of these advances, challenges persist for their real-world implementation. This underscores the need for a transition from laboratory achievements to practical, scalable solutions for wastewater treatment which mark the next crucial phase in the evolution of anammox research.

High-efficient nutrient removal in a single-stage electrolysis-integrated sequencing batch biofilm reactor (E-SBBR) for low C/N sanitary sewage treatment

To efficiently remove nutrients from low C/N sanitary sewage by conventional biological process is challenging due to the lack of sufficient electron donors. A novel electrolysis-integrated sequencing batch biofilm reactor (E-SBBR) was established to promote nitrogen and phosphorus removal for sanitary sewage with low C/N ratios (3.5-1.5). Highly efficient removal of nitrogen (>79%) and phosphorus (>97%) was achieved in the E-SBBR operating under alternating anoxic/electrolysis-anoxic/aerobic conditions. The coexistence of autotrophic nitrifiers, electron transfer-related bacteria, and heterotrophic and autohydrogenotrophic denitrifiers indicated synergistic nitrogen removal via multiple nitrogen-removing pathways. Electrolysis application induced microbial anoxic ammonia oxidation, autohydrogenotrophic denitrification and electrocoagulation processes. Deinococcus enriched on the electrodes were likely to mediate the electricity-driven ammonia oxidation which promoted ammonia removal. PICRUSt2 indicated that the relative abundances of key genes (hyaA and hyaB) associated with hydrogen oxidation significantly increased with the decreasing C/N ratios. The high autohydrogenotrophic denitrification rates during the electrolysis-anoxic period could compensate for the decreased heterotrophic rates resulting from insufficient carbon sources and nitrate removal was dramatically enhanced. Electrocoagulation with iron anode was responsible for phosphorus removal. This study provides insights into mechanisms by which electrochemically assisted biological systems enhance nutrient removal for low C/N sanitary sewage.

Anammox with alternative electron acceptors: perspectives for nitrogen removal from wastewaters

In the context of the anaerobic ammonium oxidation process (anammox), great scientific advances have been made over the past two decades, making anammox a consolidated technology widely used worldwide for nitrogen removal from wastewaters. This review provides a detailed and comprehensive description of the anammox process, the microorganisms involved and their metabolism. In addition, recent research on the application of the anammox process with alternative electron acceptors is described, highlighting the biochemical reactions involved, its advantages and potential applications for specific wastewaters. An updated description is also given of studies reporting the ability of microorganisms to couple the anammox process to extracellular electron transfer to insoluble electron acceptors; particularly iron, carbon-based materials and electrodes in bioelectrochemical systems (BES). The latter, also referred to as anodic anammox, is a promising strategy to combine the ammonium removal from wastewater with bioelectricity production, which is discussed here in terms of its efficiency, economic feasibility, and energetic aspects. Therefore, the information provided in this review is relevant for future applications.

Implemented impediment of extracellular electron transfer-dependent anammox process :Unstable nitrogen removal efficiency and decreased abundance of anammox bacteria

The present study investigates the extracellular electron transfer (EET)-dependent anammox process as a promising approach for sustainable wastewater treatment. The study examines the performance and metabolic pathway of the EET-dependent anammox process in comparison to the nitrite-dependent anammox process. The EET-dependent reactor successfully achieved nitrogen removal with a maximum removal efficiency of 93.2%, although it exhibited a lower ability to sustain high nitrogen removal load when compared to the nitrite-dependent anammox process, which poses opportunity and challenge for ammonia-wastewater treatment under applied voltage conditions. Nitrite was identified as a critical factor responsible for the changes in microbial community structure, resulting in a significant reduction in nitrogen removal load in the absence of nitrite. The study further suggests that the Candidatus Kuenenia species could dominate the EET-dependent anammox process, while nitrifying and denitrifying bacteria also contribute to the nitrogen removal in this system.

Recent Advances in Metal-Organic Framework-Based Nanomaterials for Electrocatalytic Nitrogen Reduction

The production of ammonia under moderate conditions is of environmental and sustainable importance. The electrochemical nitrogen reduction reaction (E-NRR) method has been intensively investigated in the recent decades. Nowadays, the further development of E-NRR is largely hindered by the lack of competent electrocatalysts. Metal-organic frameworks (MOFs) are considered as the next-generation catalysts for E-NRR, featuring their tailorable structures, abundant active sites and favorable porosity. To present a comprehensive review on both the fundamental and advanced development in MOFs catalyst-based E-NRR field, this paper first introduces the basic principles of E-NRR, including the reaction mechanism, major apparatus components, performance criteria, and ammonia detection protocols. Next, the synthesis and characterization methods for MOFs and their derivatives are discussed. In addition, a reaction mechanism study via density functional theory calculations is also presented. After that, the recent advancement of MOF-based catalysts in the E-NRR field as well as the modification approaches on MOFs for E-NRR optimization is elaborated. Finally, the current challenges and outlook of MOF catalyst-based E-NRR field are emphasized.

Anodic ammonium oxidation in microbial electrolysis cell: Towards nitrogen removal in low C/N environment

Biological nitrogen removal in low C/N environment is challenging in wastewater treatment for a long time. Autotrophic ammonium oxidation is promising due to the no need of carbon source addition, but alternative electron acceptors other than oxygen has to be widely investigated. Recently, microbial electrolysis cell (MEC), which applies a polarized inert electrode as the electron harvester, has been proved effective to oxidize ammonium with electroactive biofilm. That is, anodic microbes stimulated by exogenous low power can extract electron from ammonium and transfer electron to electrodes. This review aims to consolidate the recent advances in anodic ammonium oxidation in MEC. Various technologies based on different functional microbes and mechanisms of these processes are reviewed. Thereafter, the crucial factors influencing the ammonium oxidation technology are discussed. Challenges and prospects of anodic ammonium oxidation in ammonium-containing wastewater treatment are also proposed to provide valuable insights on the technologic reference and potential value of MEC in ammonium-containing wastewater treatment.

Direct ammonium oxidation to nitrogen gas (Dirammox) in Alcaligenes strain HO-1: The electrode role

It has been recently suggested that Alcaligenes use a previously unknown pathway to convert ammonium into dinitrogen gas (Dirammox) via hydroxylamine (NH₂OH). This fact alone already implies a significant decrease in the aeration requirements for the process, but the process would still be dependent on external aeration. This work studied the potential use of a polarised electrode as an electron acceptor for ammonium oxidation using the recently described Alcaligenes strain HO-1 as a model heterotrophic nitrifier. Results indicated that Alcaligenes strain HO-1 requires aeration for metabolism, a requirement that cannot be replaced for a polarised electrode alone. However, concomitant elimination of succinate and ammonium was observed when operating a previously grown Alcaligenes strain HO-1 culture in the presence of a polarised electrode and without aeration. The usage of a polarised electrode together with aeration did not increase the succinate nor the nitrogen removal rates observed with aeration alone. However, current density generation was observed along a feeding batch test representing an electron share of 3% of the ammonium removed in the presence of aeration and 16% without aeration. Additional tests suggested that hydroxylamine oxidation to dinitrogen gas could have a relevant role in the electron discharge onto the anode. Therefore, the presence of a polarised electrode supported the metabolic functions of Alcaligenes strain HO-1 on the simultaneous oxidation of succinate and ammonium.

Comparative assessment of energy generation from ammonia oxidation by different functional bacterial communities

Bioelectrochemical ammonia oxidation (BEAO) in a microbial fuel cell (MFC) is a recently discovered process that has the potential to reduce energy consumption in wastewater treatment. However, level of energy and limiting factors of this process in different microbial groups are not fully understood. This study comparatively investigated the BEAO in wastewater treatment by MFCs enriched with different functional groups of bacteria (confirmed by 16S rRNA gene sequencing): electroactive bacteria (EAB), ammonia oxidizing bacteria (AOB), and anammox bacteria (AnAOB). Ammonia oxidation rates of 0.066, 0.083 and 0.082 g NH₄⁺-N L^-1 d^-1 were achieved by biofilms enriched with EAB, AOB, and AnAOB, respectively. With influent 444 ± 65 mg NH₄⁺-N d^-1, nitrite accumulation between 84 and 105 mg N d^-1 was observed independently of the biofilm type. The AnAOB-enriched biofilm released electrons at higher potential energy levels (anode potential of 0.253 V vs. SHE) but had high internal resistance (R_int) of 299 Ω, which limits its power density (0.2 W m^-3). For AnAOB enriched biofilm, accumulation of nitrite was a limiting factor for power output by allowing conventional anammox activity without current generation. AOB enriched biofilm had R_int of 18 ± 1 Ω and yielded power density of up to 1.4 W m^-3. The activity of the AOB-enriched biofilm was not dependent on the accumulation of dissolved oxygen and achieved 1.5 fold higher coulombic efficiency when sulfate was not available. The EAB-enriched biofilm adapted to oxidize ammonia without organic carbon, with R_int of 19 ± 1 Ω and achieved the highest power density of 11 W m^-3. Based on lab-scale experiments (scaling-up factors not considered) energy savings of up to 7 % (AnAOB), 44 % (AOB) and 475 % (EAB) (positive energy balance), compared to conventional nitrification, are projected from the applications of BEAO in wastewater treatment plants.

Efficient elimination of nonylphenol and 4-tert-octylphenol by weak electrical stimulated anaerobic microbial processes

The investigation into the degradation of alkylphenol pollutants (APs) has become a hotspot due to their harmful effects on the environment and human health. In this study, microbial electrolysis cells (MECs) were used to degrade nonylphenol (NP) and 4-tert-octylphenol (4-tert-OP). The study found that the degradation rates of NP and 4-tert-OP for a 6-day period were 83.6% and 96.3%, respectively, which were 30.53% and 26.7% higher than those of the group without applied voltage. The double layer area in the degradation of 4-tert-OP was larger than that of NP, and the resistance exhibited by 4-tert-OP (87.47 Ω) in MEC was lower than that of NP (99.42 Ω). Meanwhile, NP had a greater effect on the bioenzyme activity than 4-tert-OP. GC-MS analysis showed that the degradation pathways of both pollutants mainly included oxidation and hydroxylation reactions. Furthermore, the microbial community analysis indicated that the main functional bacteria in NP degradation were Citrobacter, Desulfovibrio and Advenella, and those in 4-tert-OP degradation were Stenotrophomonas, Chryseobacterium, Dokdonella, and the key microbiomes underlying the cooperative relationship. The biotoxicity test indicated that the toxicity of residual substances was significantly reduced. Therefore, the MEC system is efficient and environmentally friendly and has broad application prospects in phenol refractory organics.

Bioelectrochemical systems for enhanced nitrogen removal with minimal greenhouse gas emission from carbon-deficient wastewater: A review

Nitrogen pollution and the rising amount of wastewater generation are calling for advanced wastewater treatments, which is particularly necessary for carbon-deficient wastewater that contains multi-species inorganic nitrogen, since conventional heterotrophic denitrification processes cannot remove nitrogen completely when carbon sources are insufficient. For that, bioelectrochemical systems (BES) have been recently developed because they can simultaneously produce electricity and remove resistant nitrogen from the carbon-deficient wastewater. However, the simultaneous removal of multi-species inorganic nitrogen cannot be achieved by electroautotrophic denitrification using BES alone. Moreover, the efficiency of nitrogen removal and power generation has been thwarted by the low energy output, high internal resistance of the device, and electron competition in non-denitrification pathways. This review article discusses the latest developments for nitrogen removal through BES-enhanced denitrification and elucidates multiple coupled BES-based denitrification pathways to remove multi-species inorganic nitrogen simultaneously. Focus points of the research area include coupling BES technologies with emerged methods, electron transfer enhancement, and avoiding electron competition that improves performance with less cost. The prospect of reducing emissions of greenhouse gases is also critically reviewed, in the hope of reducing potential intermediate products of denitrification, such as nitrous oxide (a potent greenhouse gas), through multi-factor regulation. We imply that BES is a good choice for future scale-up applications of MFC coupled with MEC to treat carbon-deficient wastewater. Overall, this review will provide useful information for the development of advanced technologies to treat carbon-deficient wastewater with less emission of greenhouse gases.

Ammonia/ammonium removal/recovery from wastewaters using bioelectrochemical systems (BES): A review

This review updates the current research efforts on using BES to recover NH₃/NH₄⁺, highlighting the novel configurations and introducing the working principles and the applications of microbial fuel cell (MFC), microbial electrolysis cell (MEC), microbial desalination cell (MDC), and microbial electrosynthesis cell (MESC) for NH₃/NH₄⁺ removal/recovery. However, commonly studied BES processes for NH₃/NH₄⁺ removal/recovery are energy intensive with external aeration needed for NH₃ stripping being the largest energy input. In such a process bipolar membranes used for yielding a local alkaline pool recovering NH₃ is not cost-effective. This gives a chance to microbial electrosynthesis which turned out to be a potential alternative option to approach circular bioeconomy. Furtherly, the reactor volume and NH₃/NH₄⁺ removal/recovery efficiency has a weakly positive correlation, indicating that there might be other factors controlling the reactor performance that are yet to be investigated.

High electrical energy harvesting performance of an integrated microbial fuel cell and low voltage booster-rectifier system treating domestic wastewater

To harvest directly usable electrical energy from real domestic wastewater, a new power management system (PMS), transistor-based low voltage boosters followed by a voltage rectifier (LVBR), was developed and tested for its energy harvesting performance. Three air-cathode MFCs were individually linked with LVBs, which were electrically stacked in parallel and then connected with a single voltage rectifier (MFC-LVBR). The MFC-LVBR system could increase V_MFCto 11.9 ± 0.6 V without voltage reversal, which was capable of charging a lithium-ion batteryand supercapacitor-based power banks. When the integrated MFC-LVBR system was linked with a lithium-ion battery, the highest normalized energy recovery (NER_COD) of 0.76 kWh/kg-COD (NER_volumeof 0.22 kWh/m³) was achieved with a minimal energy loss of 14.4%, whichwas much higher than those previously reported values.Furthermore, the electrical energy charged in the lithium-ion battery successfully powered a DC peristaltic pump requiring a minimum operating power of 0.46 W.

Simultaneous removal of organic matter and nitrogen compounds by partitioned aeration in a 226 L-scale microbial fuel cell

Although microbial fuel cells (MFCs) have been widely studied as wastewater treatment technologies that convert organic matter to electricity, there are few reports of large-scale MFCs that treat both organic matter and nitrogen compounds. In this study, a 226 L reactor equipped with 27 MFC units was partially aerated at 10% of its total volume. The MFC unit consists of a cylindrical air core covered with a carbon-based air cathode, an anion exchange membrane, and a graphite non-woven fabric anode. The air-cathode MFC with 13 L min^-1 aeration rate produced a current density of 0.0012-0.15 A m^-2 with 40 to >93% biological oxygen demand (BOD) removal to have an effluent BOD of <5-36 mg L^-1 at a hydraulic retention time (HRT) of 12-47 h. Meanwhile, 55 ± 17% of the total nitrogen (TN) was removed, resulting in 9.7 ± 3.8 mg L^-1 TN in the effluent, although the TN removal was limited at ≥20 °C. The mono-exponential regression for BOD and TN (≥20 °C) estimated that an HRT of 21 h could meet the Japanese effluent quality standards of BOD and TN. Calculation of the total energy recovered via current generation and energy consumed by aeration suggested an energy consumption of 0.22 kW h m^-3. Decreasing the aeration rate and HRT in the reactor would further reduce energy consumption and increase energy production.

Exerting applied voltage promotes microbial activity of marine anammox bacteria for nitrogen removal in saline wastewater treatment

To date, the application of marine anammox bacteria (MAB) is still a challenge in saline wastewater treatment due to the low growth rate and high sensitivity. Herein, bioelectrochemical system with applied voltage was exerted for the first time to promote the activity of MAB for removing nitrogen from saline wastewater. At the optimal voltage of 1.5 V, the mean total nitrogen removal rate (TNRR) reached the maximum of 0.65 kg/m³•d, which was 27.45% higher than that without applied voltage. Besides, applied voltage reduced the microbial diversity of MAB-based consortia, but the relative abundance of Candidatus Scalindua increased by 4.63% at 1.5 V compared with that without applied voltage. Also, proper applied voltage promoted the secretion of EPS and heme c, which resulted in the enhancement of MAB activity. Based on the remodified Logistic model analysis, the lag time of the nitrogen removal process was shortened by 0.72 h at the voltage of 1.5 V. Furthermore, it was found that higher voltage (> 2.0 V) had a negative effect on the MAB activity for low TNRR of 0.33 kg/m³•d (2.5 V). However, TNRR increased back to 0.61 kg/m³•d after removing the high applied voltage, which implied that the bioactivity was recoverable after being inhibited. These findings demonstrated that external electrical stimulation is an effective strategy to promote nitrogen removal and MAB activity for treating saline wastewater.

Microbial Electrolysis Cell as a Diverse Technology: Overview of Prospective Applications, Advancements, and Challenges

Microbial electrolysis cells (MECs) have been explored for various applications, including the removal of industrial pollutants, wastewater treatment chemical synthesis, and biosensing. On the other hand, MEC technology is still in its early stages and faces significant obstacles regarding practical large-scale implementations. MECs are used for energy generation and hydrogen peroxide, methane, hydrogen/biohydrogen production, and pollutant removal. This review aimed to investigate the aforementioned uses in order to better understand the different applications of MECs in the following scenarios: MECs for energy generation and recycling, such as hydrogen, methane, and hydrogen peroxide; contaminant removal, particularly complex organic and inorganic contaminants; and resource recovery. MEC technology was examined in terms of new concepts, configuration optimization, electron transfer pathways in biocathodes, and coupling with other technologies for value-added applications, such as MEC anaerobic digestion, combined MEC–MFC, and others. The goal of the review was to help researchers and engineers understand the most recent developments in MEC technologies and applications.

Long-term evaluation of an air-cathode microbial fuel cell with an anion exchange membrane in a 226L wastewater treatment reactor

Although the treatment of municipal wastewater using microbial fuel cells (MFCs) has been extensively studied, scaling the systems up for practical use remains challenging. In this study, a 226 L sewage treatment reactor was equipped with 27 MFC units, and its chemical oxygen demand (COD) removal and electricity production were evaluated. The MFC units were tubular air cores with a diameter of 5 cm and length of 100 cm, which were wrapped with a carbon-based cathode, anion exchange membrane (AEM), and nonwoven graphite fabric. The air-cathode-AEM MFC generated 0.12-0.30 A/m², 0.072-0.51 W/m³, and 1.7-4.6 Wh/m³ in a chemostat reactor with a COD of 140-36 mg/L and hydraulic retention time (HRT) of 9-42 h throughout a year. The decrease in the COD was represented as the first-order rate constant of 0.038. The rate constant was comparable to that of other air-cathode MFCs with cation exchange membranes, indicating the necessity of a posttreatment to meet the discharge standard. It has been estimated that the MFC operation for 24 h before post-aeration can reduce the energy required to meet the discharge standard by 70%, suggesting the potential applicability of MFC in long HRT-treatments such as oxidation ditch. The resistances of the anode, cathode, and AEM were 15, 7.0, and 0.51 mΩ m², respectively, and surface dirt rather than deterioration primarily increased the AEM resistance. A current exceeding 0.2 A/m² significantly increases the anode potential, indicating that the current was limited by low COD. Increasing the anode-specific surface area can improve air-AEM MFCs used for practical applications.

Effect of poised cathodic potential on anodic ammonium nitrogen removal from domestic wastewater by air-cathode microbial fuel cells

Performances of anodic ammonia oxidation have been investigated for various bioelectrochemical systems at a wide range of poised anodic potentials in the literature. The effect of poised cathodic potential on ammonium nitrogen (NH₄⁺-N) and total nitrogen (TN, sum of NH₄⁺-N, NO₂^--N, and NO₃^--N) removal from domestic wastewater by single chamber air-cathode microbial fuel cells (MFCs) was investigated. Poising the air-cathode potential at +0.7 V vs. SHE significantly increased current generation (from 11 ± 1 mA to 22.8 ± 5 mA) and oxygen permeation into the MFC through the air-cathode (from 75.4 ± 1.2 g-O₂/m³/d to 151 ± 3.7 g-O₂/m³/d), which consequently resulted in a high NH₄⁺-N removal rate of 150 ± 13 g-NH₄⁺-N/m³/d and TN removal rate of 63 ± 16 g-TN/m³/d. These high NH₄⁺-N and TN removal rates could be attributed to the enhancement of dual respiratory pathways: the electrode-assisted anodic and aerobic NH₄⁺ oxidation.

From Waste to Watts: Updates on Key Applications of Microbial Fuel Cells in Wastewater Treatment and Energy Production

Due to fossil fuel depletion and the rapid growth of industry, it is critical to develop environmentally friendly and long-term alternative energy technologies. Microbial fuel cells (MFCs) are a powerful platform for extracting energy from various sources and converting it to electricity. As no intermediate steps are required to harness the electricity from the organic substrate’s stored chemical energy, MFC technology offers a sustainable alternative source of energy production. The generation of electricity from the organic substances contained in waste using MFC technology could provide a cost-effective solution to the issue of environmental pollution and energy shortages in the near future. Thus, technical advancements in bioelectricity production from wastewater are becoming commercially viable. Due to practical limitations, and although promising prospects have been reported in recent investigations, MFCs are incapable of upscaling and of high-energy production. In this review paper, intensive research has been conducted on MFCs’ applications in the treatment of wastewater. Several types of waste have been extensively studied, including municipal or domestic waste, industrial waste, brewery wastewater, and urine waste. Furthermore, the applications of MFCs in the removal of nutrients (nitrogen and sulphates) and precious metals from wastewater were also intensively reviewed. As a result, the efficacy of various MFCs in achieving sustainable power generation from wastewater has been critically addressed in this study.

Novel biotechnologies for nitrogen removal and their coupling with gas emissions abatement in wastewater treatment facilities

Wastewaters contaminated with nitrogenous pollutants, derived from anthropogenic activities, have exacerbated our ecosystems sparking environmental problems, such as eutrophication and acidification of water reservoirs, emission of greenhouse gases, death of aquatic organisms, among others. Wastewater treatment facilities (WWTF) combining nitrification and denitrification, and lately partial nitrification coupled to anaerobic ammonium oxidation (anammox), have traditionally been applied for the removal of nitrogen from wastewaters. The present work provides a comprehensive review of the recent biotechnologies developed in which nitrogen-removing processes are relevant for the treatment of both wastewaters and gas emissions. These novel processes include the anammox process with alternative electron acceptors, such as sulfate (sulfammox), ferric iron (feammox), and anodes in microbial electrolysis cells (anodic anammox). New technologies that couple nitrate/nitrite reduction with the oxidation of methane, H₂S, volatile methyl siloxanes, and other volatile organic compounds are also described. The potential of these processes for (i) minimizing greenhouse gas emissions from WWTF, (ii) biogas purification, and (iii) air pollution control is critically discussed considering the factors that might trigger N₂O release during nitrate/nitrite reduction. Moreover, this review provides a discussion on the main challenges to tackle towards the consolidation of these novel biotechnologies.

Nitrate removal from groundwater using a batch and continuous flow hybrid Fe-electrocoagulation and electrooxidation system

During the last two decades nitrate contaminated groundwater has become an extensive worldwide problem with wide-reaching negative effects on human health and the environment. In this study, a combination of electrocoagulation (EC) and electrooxidation (EO) was studied as a denitrification process to efficiently remove nitrates and ammonium (a by-product produced during EC) from real polluted groundwater. Initially, EC experiments under batch operating mode were performed using iron electrodes at different applied current density values (20-40 mA cm^-2). Nitrate percentage removal of 100 % was recorded, however high ammonium concentrations were performed (4.5-6.5 mg NH₄⁺-Ν L^-1). Therefore, a continuous flow system was examined for the complete removal of both nitrates and EC-generated ammonium cations. The system comprised an EC reactor, a settling tank and an EO reactor. The applied current densities to the EC process were the same as those in the batch experiments, while the volumetric flow rates were 4, 6 and 8 mL min^-1. Regarding the current density of the EO process was kept constant at the value of 75 mA cm^-2. The percentage nitrate removal recorded during the EC process ranged between 52.0 and 100 %, while the NH₄⁺-N concentration at the outlet of the EO reduced significantly (53-100 %) depending on the applied current density and the volumetric flow rate. Also, the dissolved iron concentration in the treated water was always below the legislated limit of 0.2 mg L^-1 (up to 0.027 mg L^-1). These results indicate that the proposed hybrid system is capable of denitrifying real nitrate contaminated groundwater without generating toxic by-products, therefore making the water suitable for human consumption.

Role of bioelectrochemical systems for the remediation of emerging contaminants from wastewater: A review

Bioelectrochemical systems (BESs) are a unique group of wastewater remediating technology that possesses the added advantage of valuable recovery with concomitant wastewater treatment. Moreover, due to the application of robust microbial biocatalysts in BESs, effective removal of emerging contaminants (ECs) can be accomplished in these BESs. Thus, this review emphasizes the recent demonstrations pertaining to the removal of complex organic pollutants of emerging concern present in wastewater through BES. Owing to the recalcitrant nature of these pollutants, they are not effectively removed through conventional wastewater treatment systems and thereby are discharged into the environment without proper treatment. Application of BES in terms of ECs removal and degradation mechanism along with valuables that can be recovered are discussed. Moreover, the factors affecting the performance of BES, like biocatalyst, substrate, salinity, and applied potential are also summarized. In addition, the present review also elucidates the occurrence and toxic nature of ECs as well as future recommendations pertaining to the commercialization of this BES technology for the removal of ECs from wastewater. Therefore, the present review intends to aid the researchers in developing more efficient BESs for the removal of ECs from wastewater.

Estimation of total energy requirement for sewage treatment by a microbial fuel cell with a one-meter air-cathode assuming Michaelis–Menten COD degradation

Calculations of chemical oxygen demand (COD) degradation in sewage by a microbial fuel cell (MFC) were used to estimate the total energy required for treatment of the sewage. Mono-exponential regression (MER) and the Michaelis–Menten equation (MME) were used to describe the MFC's COD removal rate (CRR). The tubular MFC used in this study (ϕ 5.0 × 100 cm) consisted of an air core surrounding a carbon-based cathode, an anion exchange membrane, and graphite non-woven fabric immersed in sewage. The MFC generated 0.26 A m⁻² of the electrode area and 0.32 W m⁻³ of the sewage water, and 3.9 W h m⁻³ in a chemostat reactor supplemented continuously with sewage containing 180 mg L⁻¹ of COD with a hydraulic retention time (HRT) of 12 h. The COD removal and coulombic efficiency (CE) were 46% and 19%, respectively, and the energy generation efficiency (EGE) was 0.054 kW h kg⁻¹-COD. The CRR and current in the MFC were strongly dependent on the COD, which could be controlled by varying the HRT. The MER model predicted first-order rate constants of 0.054 and 0.034 for reactors with and without MFC, respectively. The difference in these values indicated that using MFC significantly increased the COD removal. The results of fitting the experimental data to the MME suggested that the total COD can be separated into nondegradable CODs (C_n) and degradable CODs (C_d) via MFC. The values of CRR for C_d and CE suggest that MFC pretreatment for 12 hours prior to aeration results in a 75% decrease in net energy consumption while reducing sewage COD from 180 to 20 mg L⁻¹.

Calculations of chemical oxygen demand (COD) degradation in sewage by a microbial fuel cell (MFC) were used to estimate the total energy required for treatment of the sewage.