Microbial electrochemical nutrient recovery in anaerobic osmotic membrane bioreactors

Membrane-based technology in water and resources recovery from the perspective of water social circulation: A review

In this review, the application of membrane-based technology in water social circulation was summarized. Water social circulation encompassed the entire process from the acquirement to discharge of water from natural environment for human living and development. The focus of this review was primarily on the membrane-based technology in recovery of water and other valuable resources such as mineral ions, nitrogen and phosphorus. The main text was divided into four main sections according to water flow in the social circulation: drinking water treatment, agricultural utilization, industrial waste recycling, and urban wastewater reuse. In drinking water treatment, the acquirement of water resources was of the most importance. Pressure-driven membranes, such as ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) were considered suitable in natural surface water treatment. Additionally, electrodialysis (ED) and membrane capacitive deionization (MCDI) were also effective in brackish water desalination. Agriculture required abundant water with relative low quality for irrigation. Therefore, the recovery of water from other stages of the social circulation has become a reasonable solution. Membrane bioreactor (MBR) was a typical technique attributed to low-toxicity effluent. In industrial waste reuse, the osmosis membranes (FO and PRO) were utilized due to the complex physical and chemical properties of industrial wastewater. Especially, membrane distillation (MD) might be promising when the wastewater was preheated. Resources recovery in urban wastewater was mainly divided into recovery of bioenergy (via anaerobic membrane bioreactors, AnMBR), nitrogen (utilizing MD and gas-permeable membrane), and phosphorus (through MBR with chemical precipitation). Furthermore, hybrid/integrated systems with membranes as the core component enhanced their performance and long-term working ability in utilization. Generally, concentrate management and energy consumption control might be the key areas for future advancements of membrane-based technology.

Recent Advances in Bioelectrochemical Systems for Nitrogen and Phosphorus Recovery Using Membranes

Bioelectrochemical systems (BESs) have emerged as a technology that is able to recover resources from different kinds of substrates, especially wastewater. Nutrient recovery, mostly based on membrane reactor configuration, is a clear niche for BES application. The recovery of nitrogen or phosphorus allows for treatment of wastewater while simultaneously collecting a concentrated stream with nutrients that can be reintroduced into the system, becoming a circular economy solution. The aim of this study is to review recent advances in membrane-based BESs for nitrogen and phosphorus recovery and compare the recovery efficiencies and energy requirements of each system. Finally, there is a discussion of the main issues that arise from using membrane-based BESs. The results presented in this review show that it would be beneficial to intensify research on BESs to improve recovery efficiencies at the lowest construction cost in order to take the final step towards scaling up and commercialising this technology.

Membrane Technologies for Nitrogen Recovery from Waste Streams: Scientometrics and Technical Analysis

The concerns regarding the reactive nitrogen levels exceeding the planetary limits are well documented in the literature. A large portion of anthropogenic nitrogen ends in wastewater. Nitrogen removal in typical wastewater treatment processes consumes a considerable amount of energy. Nitrogen recovery can help in saving energy and meeting the regulatory discharge limits. This has motivated researchers and industry professionals alike to devise effective nitrogen recovery systems. Membrane technologies form a fundamental part of these systems. This work presents a thorough overview of the subject using scientometric analysis and presents an evaluation of membrane technologies guided by literature findings. The focus of nitrogen recovery research has shifted over time from nutrient concentration to the production of marketable products using improved membrane materials and designs. A practical approach for selecting hybrid systems based on the recovery goals has been proposed. A comparison between membrane technologies in terms of energy requirements, recovery efficiency, and process scale showed that gas permeable membrane (GPM) and its combination with other technologies are the most promising recovery techniques and they merit further industry attention and investment. Recommendations for potential future search trends based on industry and end users' needs have also been proposed.

Recovery of phosphorus from wastewater: A review based on current phosphorous removal technologies

Phosphorus (P) as an essential nutrient for life sustains the productivity of food systems; yet misdirected P often accumulates in wastewater and triggers water eutrophication if not properly treated. Although technologies have been developed to remove P, little attention has been paid to the recovery of P from wastewater. This work provides a comprehensive review of the state-of-the-art P removal technologies in the science of wastewater treatment. Our analyses focus on the mechanisms, removal efficiencies, and recovery potential of four typical water and wastewater treatment processes including precipitation, biological treatment, membrane separation, and adsorption. The design principles, feasibility, operation parameters, and pros & cons of these technologies are analyzed and compared. Perspectives and future research of P removal and recovery are also proposed in the context of paradigm shift to sustainable water treatment technology.

Bio-membrane integrated systems for nitrogen recovery from wastewater in circular bioeconomy

Wastewater contains a significant amount of recoverable nitrogen. Hence, the recovery of nitrogen from wastewater can provide an option for generating some revenue by applying the captured nitrogen to producing bio-products, in order to minimize dangerous or environmental pollution consequences. The circular bio-economy can achieve greater environmental and economic sustainability through game-changing technological developments that will improve municipal wastewater management, where simultaneous nitrogen and energy recovery are required. Over the last decade, substantial efforts were undertaken concerning the recovery of nitrogen from wastewater. For example, bio-membrane integrated system (BMIS) which integrates biological process and membrane technology, has attracted considerable attention for recovering nitrogen from wastewater. In this review, current research on nitrogen recovery using the BMIS are compiled whilst the technologies are compared regarding their energy requirement, efficiencies, advantages and disadvantages. Moreover, the bio-products achieved in the nitrogen recovery system processes are summarized in this paper, and the directions for future research are suggested. Future research should consider the quality of recovered nitrogenous products, long-term performance of BMIS and economic feasibility of large-scale reactors. Nitrogen recovery should be addressed under the framework of a circular bio-economy.

Enhanced microalgae cultivation using wastewater nutrients extracted by a microbial electrochemical system

Cultivating algae using wastewater nutrients is a potential approach to realize resource recovery that can contribute to circular economy. However, growing algae directly in a wastewater has problems such as bacterial contamination and a low biomass density. To address those problems, we investigated microalgal cultivation in a photobioreactor (PBR) fed with the nutrients extracted from wastewater by a microbial nutrient recovery cell (MNRC). With an external voltage of 0.3 V, the MNRC-PBR system removed 96% of COD and recovered 44% of NH₄⁺-N and 39% of PO₄^3--P at a hydraulic retention time of 7.2 h. Microalgae cultivated in the nutrient recovery medium from the MNRC had 8.3-fold biomass density and 1.4-fold lipid contents, versus that cultivated in a food wastewater containing more nutrients. More significantly, 90% of biomass yielded from the MNRC-PBR system was microalgae, much higher than ∼30% in the food wastewater. A liquid exchange ratio of 30% achieved the highest microalgal density of 0.61 ± 0.06 g L^-1, comparable to that in a standard BG11 medium. There was a tradeoff between recycling PBR medium and microalgal growth. The accumulated salinity was observed in the extended operation of the MNRC-PBR system treating an actual food wastewater. The results of this study have demonstrated an effective approach to extract nutrients from wastewater for enhanced microalgal growth and improved biomass quality.

Progress in osmotic membrane bioreactors research: Contaminant removal, microbial community and bioenergy production in wastewater

Renewable energy, water conservation, and environmental protection are the most important challenges today. Osmotic membrane bioreactor (OMBR) is an innovative process showing superior performance in bioenergy production, eliminating contaminants, and low fouling tendency. However, salinity build-up is the main drawback of this process. Identifying the microbial community can improve the process in bioenergy production and contaminant treatment. This review aims to study the recent progress and challenges of OMBRs in contaminant removal, microbial communities and bioenergy production. OMBRs are widely reported to remove over 80% of total organic carbon, PO₄^3-, NH₄⁺ and emerging contaminants from wastewater. The most important microbial phyla for both hydrogen and methane production in OMBR are Firmicutes, Proteobacteria and Bacteroidetes. Firmicutes' dominance in anaerobic processes is considerably increased from usually 20% at the beginning to 80% under stable condition. Overall, OMBR process has great potential to be applied for simultaneous bioenergy production and wastewater treatment.

Donnan Dialysis-Osmotic Distillation (DD-OD) Hybrid Process for Selective Ammonium Recovery Driven by Waste Alkali

This work proposed an innovative and energy-efficient Donnan Dialysis (DD) and Osmotic Distillation (OD) hybrid process for alkali-driven ammonium recovery from wastewater. The efficiency and feasibility of ammonium removal and recovery from synthetic and real wastewater using NaOH and waste alkali were investigated. Ammonium in the feed first transported across the cation exchange membrane and accumulated in the receiver chamber. It is then deprotonated as ammonia, passing through the gas permeable membrane and finally is fixed as ammonium salt in the acid chamber. Our results indicated that employing waste alkali (red mud leachate) as driving solution led to excellent ammonium recovery performances (recovery efficiency of >80%), comparable to those of NaOH solution. When the initial ammonium concentration was 5 and 50 mM, the waste alkali driven DD-OD process achieved acceptable NH₄⁺-N flux density of 16.8 and 169 g N m^-2 d^-1, at energy cost as low as 8.38 and 2.06 kWh kg^-1 N, respectively. Since this alkali driven DD-OD hybrid process is based on solute concentration (or partial pressure) gradient, it could be an energy-effective technology capable of treating wastewaters containing ammonium using waste alkali to realize nutrients recovery in a sustainable manner.

Integrating microbial electrolysis cell based on electrochemical carbon dioxide reduction into anaerobic osmosis membrane reactor for biogas upgrading

It has been reported that anaerobic osmosis membrane bioreactors have the potential for energy recovery since dissolved methane was almost rejected by commercial forward osmosis membranes. Notwithstanding, upgraded biogas has to be achieved by removing as much carbon dioxide as possible. In this study, a novel anaerobic osmotic membrane bioreactor-microbial electrolysis cell (AnOMBR-MEC) system was developed for simultaneous biogas upgrading and wastewater treatment. The AnOMBR-MEC elicited an excellent and stable soluble chemical oxygen demand and phosphorus removal. As the experiment progressed, unwanted carbon dioxide produced from biogas was reduced to formate using a SnO₂ nanoparticles electrocatalytic cathode in an electrocatalytic-assisted MEC, with the highest faradic efficiency of formate being 85% at 1.2V. Compared to AnOMBR, the methane content increased from 55% to 90% at the end of operation and methane yield experienced a1.6-fold increment in the AnOMBR-MEC. Microbial community analysis revealed that hydrogenotrophic methanogens (e.g. Methanobacterium and Methanobrevibacter) converted the produced H₂ and formate to methane at saline conditions. These results have demonstrated an efficient strategy based on the integration of an electrocatalytic-assisted MEC into AnOMBR for upgrading biogas, enhancing methane yield and wastewater treatment.

Effects of operating parameters on salinity accumulation in a bioelectrochemically-assisted osmotic membrane bioreactor

Salinity accumulation in osmotic membrane bioreactors (OMBRs) is one of the key challenges, which can be mitigated in situ by reverse-fluxed solute transport through integration of bioelectrochemical systems (BES). The effects of several key operating parameters on salinity accumulation were investigated. Salinity accumulation depended on balance between reversal solute flux (RSF) and reverse-fluxed ammonium (RFA) transport, which was driven by electrical migration and concentration diffusion. DS concentration was the primary factor influencing RSF, and the lowest DS concentration exhibited the minimum solute leakage. Aeration played a vital role in RFA transport, and a higher aeration helped to enhance RFA transport. Increased current generation (i.e., influent flow rate of 0.5 mL min^-1 and external resistance of 5.0 Ω) contributed to RFA migration. The lack of electrolyte addition in catholyte contributed to RFA diffusion. These optimal parameters encourage the further development of an effective strategy for salinity mitigation in BES-based OMBR technology.

A comprehensive review of nutrient-energy-water-solute recovery by hybrid osmotic membrane bioreactors

Hybrid osmotic membrane bioreactor (OMBR) takes advantage of the cooperation of varying biological or desalination processes and can achieve NEWS (nutrient-energy-water-solute) recovery from wastewater. However, a lack of universal parameters hinders our understanding. Herein, system configurations and new parameters are systematically investigated to help better evaluate recovery performance. High-quality water can be produced in reverse osmosis/membrane distillation-based OMBRs, but high operation cost limits their application. Although bioelectrochemical system (BES)/electrodialysis-based OMBRs can effectively achieve solute recovery, operation parameters should be optimized. Nutrients can be recovered from various wastewater by porous membrane-based OMBRs, but additional processes increase operation cost. Electricity recovery can be achieved in BES-based OMBRs, but energy balances are negative. Although anaerobic OMBRs are energy-efficient, salinity accumulation limits methane productions. Additional efforts must be made to alleviate membrane fouling, control salinity accumulation, optimize recovery efficiency, and reduce operation cost. This review will accelerate hybrid OMBR development for real-world applications.

pH Dependence of Phosphorus Speciation and Transport in Flow-Electrode Capacitive Deionization

Electrochemical processes such as capacitive deionization have shown great promise for salt removal and nutrient recovery, but their effectiveness on phosphate removal was lower than other charged ions. This study hypothesized that the speciation and transport behaviors of phosphate ions are highly influenced by electrolyte pH, and it used experimental and modeling approaches to elucidate such impacts in flow-electrode capacitive deionization (FCDI) cells. Phosphate removal was investigated in either constant current (CC) or constant voltage (CV) charging mode with pH ranged from 5 to 9 in the feed solution. Results showed that the average P removal rate increased from 20.8 (CC mode) and 16.8 mg/min (CV mode) at pH 9 to 38.3 (CC mode) and 34.3 mg/min (CV mode) at pH 5 (84-104% in improvement), respectively. Correspondingly, the energy consumption reduced from 1.04 kWh/kg P at pH 9 to 0.59 kWh/kg P at pH 5 (42.9-56.1% in saving). Such benefits were attributed to the shift in dominant P-species from HPO₄^2- to H₂PO₄^-. Conversely, high-electrolyte pH (pH = 11) for flow-electrode led to ∼74.8% higher phosphate recovery during discharge compared with pH 5, which was associated with the higher distribution of phosphate ions in the electrolyte versus on the flow-electrodes due to surface charge change. These results improved our understanding in ion distribution and migration and indicate that solution pH is critical for operating FCDI reactors. It shed lights on the best practices on electrochemical phosphate removal and recovery.

Nutrient conversion and recovery from wastewater using electroactive bacteria

Wastewater is widely recognized as a sink of active nitrogen and phosphorus, and the recovery of both nutrients as fertilizers is widely studied in recent years. Electroactive bacteria increasingly attract attentions in this area because they are able to produce an electric field in microbial electrochemical systems to concentrate ammonium and phosphate for recovery. Importantly, these unique bacteria are able to convert nitrate and nitrite directly to ammonium, maximizing the active nitrogen species capable of recovery. Ferric ions produced by electroactive bacteria can be precipitated with phosphate to recover as vivianite in neutral wastewaters. All these processes employed electroactive bacteria as both nitrate and iron reducer and bioelectric field generator. The mechanism as well as technologies are summarized, and the challenges to further improve their performance are discussed.

Bioelectrochemically assisted osmotic membrane bioreactor with reusable polyelectrolyte draw solutes

The aim of this work was to study reverse solute flux (RSF) from osmotic membrane bioreactor (OMBR) and consequent solute buildup in the feed side. A polyelectrolyte (PAA-Na) served as a draw solute (DS) to minimize RSF in OMBRs. In addition, a bioelectrochemical system (BES) was employed to drive accumulated cations from the feed/anode side into the cathode compartment, subsequently achieving PAA-Na DS recovery with the aid of high catholyte pH. Compared to the 1 M NH₄HCO₃ DS, the 0.48 g mL^-1 PAA-Na DS produced consistently stable water flux, enhanced water recovery and increased ammonium removal efficiency. Due to a dynamic balance between PAA removal and continuing RSF, the residual PAA concentration was 72 mg L^-1 on the feed side (27.0% of TOC). These results demonstrate the advantages of integrating a PAA-Na DS with a BES to mitigate RSF and to support further development of OMBR technology.

Enhanced nutrients enrichment and removal from eutrophic water using a self-sustaining in situ photomicrobial nutrients recovery cell (PNRC)

Nutrients removal and recovery from surface water are attracting wide attention as nutrients contamination can cause eutrophication even threaten human health. In this study, a novel in-situ photomicrobial nutrient recovery cell (PNRC) was developed, which employed the self-generated electric field to drive nutrient ions to migrate and subsequent recovery as microalgae biomass. At an external resistance of 200 Ω, the current density of the PNRC reactor reached 2.0 A m^-2, more than 92% of ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), and total phosphorus (TP) were separated from eutrophic water, which represented <0.19 mg L^-1 of NH₄⁺-N, <0.23 mg L^-1 of NO₃^--N, <0.02 mg L^-1 of TP were left in the eutrophic water effluent. Meanwhile these separated NH₄⁺-N, NO₃^--N, and TP were highly enriched in the cathode and anode chambers, and further removed from the system with the removal efficiencies of 91.8%, 90.6%, and 94.4%. The analysis of microbial communities unraveled that high nitrate removal was attributed to the abundant denitrifying bacteria (Thauera, Paracoccus, Stappia, and Azoarcus). The removal of ammonia was attributed to the algae assimilation (69.3%) and nitrification process (22.5%), and the phosphorus removal was mainly attributed to C. vulgaris. The preliminary energy balance analysis indicated that the electricity generation and biodiesel production could achieve energy neutrality theoretically, further demonstrating the huge potential of the PNRC system in cost-effective nutrients recovery from eutrophic water.

Hydrophobic Gas Transfer Membranes for Wastewater Treatment and Resource Recovery

Gaseous compounds, such as CH₄, H₂, and O₂, are commonly produced or consumed during wastewater treatment. Traditionally, these gases need to be removed or delivered using gas sparging or liquid heating, which can be energy intensive with low efficiency. Hydrophobic membranes are being increasingly investigated in wastewater treatment and resource recovery. This is because these semipermeable barriers repel water and create a three-phase interface that enhances mass transfer and chemical conversions. This Critical Review provides a first comprehensive analysis of different hydrophobic membranes and processes, and identifies the challenges and potential for future system development. The discussions and analyses were grouped based on mechanisms and applications, including membrane gas extraction, membrane gas delivery, and hybrid processes. Major challenges, such as membrane fouling, wetting, and limited selectivity and functionality, are identified, and potential solutions articulated. New opportunities, such as electrochemical coating, integrated membrane electrodes, and membrane functionalization, are also discussed to provide insights for further development of more efficient and low-cost membranes and processes.

Innovative upflow anaerobic sludge osmotic membrane bioreactor for wastewater treatment

A novel upflow anaerobic sludge-forward osmotic membrane bioreactor was developed for simultaneous wastewater treatment, membrane fouling reduction, and nutrient recovery. An upflow anaerobic sludge blanket (UASB) reactor was incorporated into the system, suspending the anaerobic sludge at the bottom of the reactor. A forward osmosis membrane replaced the traditional three-phase separator of the UASB technology. The removals of chemical oxygen demand, PO₄^3-, and NH₄⁺ were all more than 95% with low membrane fouling in this system. Halotolerant Fusibacter, which can ferment organics to acetate, was increased rapidly from 0.1% to 5% in this saline environment. Acetoclastic Methanosaeta was the most dominant prokaryotes and responsible for majority of methane production. Reduction of membrane fouling in this system was verified by the fluorescence excitation-emission matrix spectrophotometry. Furthermore, phosphorus recovery and salinity build-up mitigation were achieved using periodic microfiltration to recover 57-105 mg/L phosphorus from pH 9 to 12.

Entrapped-cells-based anaerobic forward osmosis membrane bioreactor treating medium-strength domestic wastewater: Fouling characterization and performance evaluation

A novel entrapped cells-based-anaerobic forward osmosis membrane bioreactor (E-FOMBR) was developed. Its performance and fouling were investigated in comparison with suspended cells-based-anaerobic forward osmosis membrane bioreactor (S-FOMBR). E-FOMBR and S-FOMBR were operated under the same conditions with two widely used draw solutions (NaCl and (NH₄)₂SO₄). The membrane fouling especially irreversible fouling in S-FOMBR was more severe than that in E-FOMBR regardless of the type of draw solution. The permeate flux of E-FOMBR were 1.79 and 1.85 LMH while those of S-FOMBR were 1.49 and 1.14 LMH with NaCl and (NH₄)₂SO₄ as draw solutions, respectively. More deterioration of biological activity (suggested by lower organic removal) due to accumulation of salt was observed in S-FOMBR compared to E-FOMBR. Proteobacteria dominated in both FOMBRs but was more abundant in E-FOMBR than S-FOMBR. The superiority of E-FOMBR over S-FOMBR included higher and stable system performance, higher flux, and longer operation time.

Polyamidoamine dendrimer grafted forward osmosis membrane with superior ammonia selectivity and robust antifouling capacity for domestic wastewater concentration

Developing a forward osmosis (FO) membrane with superior ammonia selectivity and robust antifouling performance is important for treating domestic wastewater (DW) but challenging due to the similar polarities and hydraulic radii of NH₄⁺ and water molecules. Herein, we investigated the feasibility of using polyamidoamine (PAMAM) dendrimer to simultaneously enhance the ammonia rejection rate and antifouling capacity of the thin-film composite (TFC) FO membrane. PAMAM dendrimer with abundant, easily-protonated, terminal amine groups was grafted on TFC-FO membrane surface via covalent bonds, which inspired the TFC-FO membrane surface with appreciable Zeta potential (isoelectric point: pH = 5.5) and outstanding hydrophilicity (water contact angle: 39.83 ± 0.57°). Benefiting from the electrostatic repulsion between the protonated amine layer and NH₄⁺-N as well as the concentration-induced diffusion resistance, the introduction of PAMAM dendrimer endowed the grafted membrane with a superior NH₄⁺-N rejection rate of 98.23% and a significantly reduced the reverse solute flux when using NH₄Cl solutions as feed solution. Meanwhile, the perfect balance between the electrostatic repulsion to positively-charged micromoleculer ions (metal ions and NH₄⁺-N) and the electrostatic attraction to negatively-charged macromolecular organic foulants together with the hydrophilic nature of amine groups facilitated the enhancement of the grafted membranes in antifouling capacity and hence the NH₄⁺-N selectivity (rejection rate of 91.81%) during the concentration of raw DW. The overall approach of this work opens up a frontier for preparation of ammonia-selective and antifouling TFC-FO membrane.

Carbon dioxide and organic waste valorization by microbial electrosynthesis and electro-fermentation

Carbon-rich waste materials (solid, liquid, or gaseous) are largely considered to be a burden on society due to the large capital and energy costs for their treatment and disposal. However, solid and liquid organic wastes have inherent energy and value, and similar as waste CO₂ gas they can be reused to produce value-added chemicals and materials. There has been a paradigm shift towards developing a closed loop, biorefinery approach for the valorization of these wastes into value-added products, and such an approach enables a more carbon-efficient and circular economy. This review quantitatively analyzes the state-of-the-art of the emerging microbial electrochemical technology (MET) platform and provides critical perspectives on research advancement and technology development. The review offers side-by-side comparison between microbial electrosynthesis (MES) and electro-fermentation (EF) processes in terms of principles, key performance metrics, data analysis, and microorganisms. The study also summarizes all the processes and products that have been developed using MES and EF to date for organic waste and CO₂ valorization. It finally identifies the technological and economic potentials and challenges on future system development.

Modeling methane production in anaerobic forward osmosis bioreactor using a modified anaerobic digestion model No. 1

Anaerobic membrane bioreactor (AnMBR) using microfiltration (MF) or ultrafiltration (UF) membranes was introduced to enhance poor biomass retention of conventional anaerobic digestion (CAD). Recently, forward osmosis (FO) membrane have been applied to AnMBR, which is called AnFOMBR. FO membrane assures not only high biomass retention but also high removal efficiency for low molecular weight (LMW) matters. Methane production rates in CAD, AnMBR, and AnFOMBR were compared using a modified IWA anaerobic digestion model No. 1 (ADM1) in this work. Accumulation of biomass in AnMBR/AnFOMBR results in enhanced biochemical reaction and gains more methane production. AnFOMBR may experience a significant inhibition by accumulated free ammonia and cations, although concentrated soluble substrates rejected by FO membrane are favorable for more methane production. Rejection rate of inorganic nitrogen is a key parameter to determine the inhibition in methane production of AnFOMBR.

Nickel-Based Membrane Electrodes Enable High-Rate Electrochemical Ammonia Recovery

Wastewater contains significant amounts of nitrogen that can be recovered and valorized as fertilizers and chemicals. This study presents a new membrane electrode coupled with microbial electrolysis that demonstrates very efficient ammonia recovery from synthetic centrate. The process utilizes the electrical potential across electrodes to drive NH₄⁺ ions toward the hydrophilic nickel top layer on a gas-stripping membrane cathode, which takes advantage of surface pH increase to realize spontaneous NH₃ production and separation. Compared with a control configuration with conventionally separated electrode and hydrophobic membrane, the integrated membrane electrode showed 40% higher NH₃-N recovery rate (36.2 ± 1.2 gNH₃-N/m²/d) and 11% higher current density. The energy consumption was 1.61 ± 0.03 kWh/kgNH₃-N, which was 20% lower than the control and 70-90% more efficient than competing electrochemical nitrogen recovery processes (5-12 kWh/kgNH₃-N). Besides, the negative potential on membrane electrode repelled negatively charged organics and microbes thus reduced fouling. In addition to describing the system's performance, we explored the underlying mechanisms governing the reactions, which confirmed the viability of this process for efficient wastewater-ammonia recovery. Furthermore, the nickel-based membrane electrode showed excellent water entry pressure (∼41 kPa) without leakage, which was much higher than that of PTFE/PDMS-based cathodes (∼1.8 kPa). The membrane electrode also showed superb flexibility (180° bend) and can be easily fabricated at low cost (<20 $/m²).

Salinity build-up in osmotic membrane bioreactors: Causes, impacts, and potential cures

Osmotic membrane bioreactor (OMBR), which integrates forward osmosis (FO) with biological treatment, has been developed to advance wastewater treatment and reuse. OMBR is superior to conventional MBR, particularly in terms of higher effluent quality, lower membrane fouling propensity, and higher membrane fouling reversibility. Nevertheless, advancement and future deployment of OMBR are hindered by salinity build-up in the bioreactor (e.g., up to 50 mS/cm indicated by the mixed liquor conductivity), due to high salt rejection of the FO membrane and reverse diffusion of the draw solution. This review comprehensively elucidates the relative significance of these two mechanisms towards salinity build-up and its associated effects in OMBR operation. Recently proposed strategies to mitigate salinity build-up in OMBR are evaluated and compared to highlight their potential in practical applications. In addition, the complementarity of system optimization and modification to effectively manage salinity build-up are recommended for sustainable OMBR development.

Permeability recovery of fouled forward osmosis membranes by chemical cleaning during a long-term operation of anaerobic osmotic membrane bioreactors treating low-strength wastewater

Anaerobic osmotic membrane bioreactor (AnOMBR) has gained increasing interests in wastewater treatment owing to its simultaneous recovery of biogas and water. However, the forward osmosis (FO) membrane fouling was severe during a long-term operation of AnOMBRs. Here, we aim to recover the permeability of fouled FO membranes by chemical cleaning. Specifically speaking, an optimal chemical cleaning procedure was searched for fouled thin film composite polyamide FO (TFC-FO) membranes in a novel microfiltration (MF) assisted AnOMBR (AnMF-OMBR). The results indicated that citric acid, disodium ethylenediaminetetraacetate (EDTA-2Na), hydrochloric acid (HCl), sodium dodecyl sulfate (SDS) and sodium hydroxide (NaOH) had a low cleaning efficiency of less than 15%, while hydrogen peroxide (H₂O₂) could effectively remove foulants from the TFC-FO membrane surface (almost 100%) through oxidizing the functional group of the organic foulants and disintegrating the colloids and microbe flocs into fine particles. Nevertheless, the damage of H₂O₂ to the TFC-FO membrane was observed when a high cleaning concentration and a long duration were applied. In this case, the optimal cleaning conditions including cleaning concentration and time for fouled TFC-FO membranes were selected through confocal laser scanning microscope (CLSM) and scanning electron microscopy (SEM) images and the flux recovery rate. The results suggested that the optimal cleaning procedure for fouled TFC-FO membranes was use of 0.5% H₂O₂ at 25 °C for 6 h, and after that, the cleaned TFC-FO membrane had the same performance as a virgin one including water flux and rejection for organic matters and phosphorus during the operation of AnMF-OMBR.