Understanding the role of in-situ ozonation in Fe(II)-dosed membrane bioreactor (MBR) for membrane fouling mitigation

Mechanisms and Strategies of Advanced Oxidation Processes for Membrane Fouling Control in MBRs: Membrane-Foulant Removal versus Mixed-Liquor Improvement

Membrane bioreactors (MBRs) are well-established and widely utilized technologies with substantial large-scale plants around the world for municipal and industrial wastewater treatment. Despite their widespread adoption, membrane fouling presents a significant impediment to the broader application of MBRs, necessitating ongoing research and development of effective antifouling strategies. As highly promising, efficient, and environmentally friendly chemical methods for water and wastewater treatment, advanced oxidation processes (AOPs) have demonstrated exceptional competence in the degradation of pollutants and inactivation of bacteria in aqueous environments, exhibiting considerable potential in controlling membrane fouling in MBRs through direct membrane foulant removal (MFR) and indirect mixed-liquor improvement (MLI). Recent proliferation of research on AOPs-based antifouling technologies has catalyzed revolutionary advancements in traditional antifouling methods in MBRs, shedding new light on antifouling mechanisms. To keep pace with the rapid evolution of MBRs, there is an urgent need for a comprehensive summary and discussion of the antifouling advances of AOPs in MBRs, particularly with a focus on understanding the realizing pathways of MFR and MLI. In this critical review, we emphasize the superiority and feasibility of implementing AOPs-based antifouling technologies in MBRs. Moreover, we systematically overview antifouling mechanisms and strategies, such as membrane modification and cleaning for MFR, as well as pretreatment and in-situ treatment for MLI, based on specific AOPs including electrochemical oxidation, photocatalysis, Fenton, and ozonation. Furthermore, we provide recommendations for selecting antifouling strategies (MFR or MLI) in MBRs, along with proposed regulatory measures for specific AOPs-based technologies according to the operational conditions and energy consumption of MBRs. Finally, we highlight future research prospects rooted in the existing application challenges of AOPs in MBRs, including low antifouling efficiency, elevated additional costs, production of metal sludge, and potential damage to polymeric membranes. The fundamental insights presented in this review aim to elevate research interest and ignite innovative thinking regarding the design, improvement, and deployment of AOPs-based antifouling approaches in MBRs, thereby advancing the extensive utilization of membrane-separation technology in the field of wastewater treatment.

Effects of High Pharmaceutical Concentrations in Domestic Wastewater on Membrane Bioreactor Treatment Systems: Performance and Microbial Community

Despite pharmaceuticals being widely detected in water-bodies worldwide, what remain unclear are the effects of high pharmaceutical concentrations on the treatment efficiency of biological wastewater treatment processes, such as membrane bioreactor (MBR) systems. This study investigated the efficiency of MBR technology in the treatment of synthetic wastewater containing a mixture of five typical pharmaceuticals (ofloxacin, sulfamethoxazole, sulfamethylthiadiazole, carbamazepine and naproxen) with a total concentration of 500 µg/L. Both the control MBR (MBRc) without pharmaceutical dosing and the MBR operated with high influent pharmaceutical concentrations (MBRe) were operated under room temperature with the same hydraulic retention time of 11 h and the same sludge retention time of 30 d. The removal efficiency rates of total nitrogen and total phosphorus were 83.2% vs. 90.1% and 72.6% vs. 57.8% in the MBRc vs. MBRe systems, and both MBRs achieved >98% removal of organics for a 180-day period. The floc size decreased, and membrane fouling became more severe in the MBRe system. Microbial diversity increased in the MBRe system and the relative abundances of functional microbe differed between the two MBRs. Furthermore, the total relative abundances of genes involved in glycolysis, assimilating nitrate reduction and nitrification processes increased in the MBRe system, which could account for the higher organics and nitrogen removal performance. This work provides insights for MBR operation in wastewater treatment with high pharmaceutical concentrations.

Polymer-flooding produced water treatment using an electro-hybrid ozonation-coagulation system with novel cathode membranes targeting alternating filtration and in situ self-cleaning

Polymer-flooding produced water is more difficult to treat for reinjection compared with normal produced water because of the presence of residual hydrolyzed polyacrylamide (HPAM). A novel cathode membrane integrated electro-hybrid ozonation-coagulation (CM-E-HOC) process was proposed for the treatment of polymer-flooding produced water. This process achieved in situ self-cleaning by generated microbubbles in the cathode membrane. The CM-E-HOC process achieved a higher suspended solid (SS), turbidity and PAM removal efficiency than the CM-EC process. The SS in the CM-E-HOC effluent was ≤ 20 mg/L SS, which met the reinjection requirements of Longdong, Changqing Oilfield, China (Q/SYCQ 08,011-2019) at different current densities (3, 5 and 10 mA/cm²). The CM-E-HOC process greatly mitigated both reversible and irreversible membrane fouling. Therefore, excellent flux recovery was obtained at different in situ self-cleaning intervals during the CM-E-HOC process. Furthermore, alternating filtration achieved continuous water production during the CM-E-HOC process. On one hand, the effective removal of aromatic protein-like substances and an increase in oxygen-containing functional groups were achieved due to the enhanced oxidation ability of the CM-E-HOC process, which decreased membrane fouling. On the other hand, the CM-E-HOC process showed improved coagulation performance because of the increased oxygen-containing functional groups and polymeric Fe species. Therefore, larger flocs with higher fractal dimensions were generated, and a looser and more porous cake layer was formed on the membrane surface during the CM-E-HOC process. Consequently, the CM-E-HOC process exhibited better in situ self-cleaning performance and lower filtration resistance than the CM-EC process.

Towards Effective, Sustainable Solution for Hospital Wastewater Treatment to Cope with the Post-Pandemic Era

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has spread across the globe since the end of 2019, posing significant challenges for global medical facilities and human health. Treatment of hospital wastewater is vitally important under this special circumstance. However, there is a shortage of studies on the sustainable wastewater treatment processes utilized by hospitals. Based on a review of the research trends regarding hospital wastewater treatment in the past three years of the COVID-19 outbreak, this review overviews the existing hospital wastewater treatment processes. It is clear that activated sludge processes (ASPs) and the use of membrane bioreactors (MBRs) are the major and effective treatment techniques applied to hospital wastewater. Advanced technology (such as Fenton oxidation, electrocoagulation, etc.) has also achieved good results, but the use of such technology remains small scale for the moment and poses some side effects, including increased cost. More interestingly, this review reveals the increased use of constructed wetlands (CWs) as an eco-solution for hospital wastewater treatment and then focuses in slightly more detail on examining the roles and mechanisms of CWs' components with respect to purifying hospital wastewater and compares their removal efficiency with other treatment processes. It is believed that a multi-stage CW system with various intensifications or CWs incorporated with other treatment processes constitute an effective, sustainable solution for hospital wastewater treatment in order to cope with the post-pandemic era.

Advanced nitrogen and phosphorus removal by the symbiosis of PAOs, DPAOs and DGAOs in a pilot-scale A2O/A+MBR process with a low C/N ratio of influent

Cooperating in harmony to avoid competition with dominant functional microbial symbiosis is an efficient way in advanced nitrogen and phosphorus removal in wastewater treatment processes. In this study, a niche-based coordinating strategy was implemented to cooperate in harmony with phosphorus-accumulating organisms (PAOs), denitrifying phosphorus-accumulating organisms (DPAOs) and denitrifying glycogen-accumulating organisms (DGAOs) to advance nitrogen and phosphorus removal based on an anaerobic-anoxic-oxic-anoxic-membrane bioreactor (A²O/A+MBR) under low C/N in municipal wastewater influent. The niche-based strategy was conducted based on the ORP change during the process as an indicator combined with the adjustment of recirculation and anoxic zone shifting. The results indicated that the strategy of the post-anoxic unit could enable significant enhancement of biological nitrogen and phosphorus removal (BNPR) by 9.9% and 16.3%, respectively, with low effluent concentrations of 7.0 ± 2.2 mg N/L and 0.36±0.32 mg P/L. The satisfactory performance was dominated along with the shift in the microbial community: the relative abundance of Tetrasphaera (PAO genus) increased from 0.14±0.08% to 0.32±0.12%, while the relative abundance of Decchloromonas (DGAO genus) and Candidatus Competibacter (DGAO genus) also increased. The advanced combination of anaerobic phosphorus release, anoxic denitrification, denitrifying phosphorus removal and endogenous denitrification was qualified by the modeling simulation of the biochemical kinetics mechanism of activated sludge in the A²O+MBR and A²O/A+MBR processes, which means that cooperation in the harmony of PAOs, DPAOs and DGAOs could be efficiently realized by a promising control strategy to enhance BNPR in an A²O+MBR with a post-anoxic unit. This study provides an efficient and simple novel control strategy to overcome the limitation of traditional nitrogen and phosphorus removal under an insufficient carbon source.

Study on the performance and mechanism of bio-electrochemical system to mitigate membrane fouling in bioreactors

To alleviate membrane fouling, a membrane of the membrane bioreactor was directly used as the anode of the bio-electrochemical system. On the 14th day, the control group had blocked, while the experimental group with a current of 0.44 mA, the increase in ΔTMP was only 2.2 kPa. The polysaccharide and protein concentrations in the open-circuit group were 4.2 and 2.9 times higher than those in the closed-circuit group, respectively. Three-dimensional fluorescence spectroscopy and gas chromatography mass spectrometry showed that most of the deposition in the control group contained high-molecular-weight compounds, especially long-chain ester derivatives, phenols, and complex hydrocarbons, whereas the experimental group was the opposite. Therefore, current (electrons) can change the composition of the cake layer. High-throughput sequencing indicated that a significantly higher abundance of electroactive microorganisms on the experimental than control group. Two-dimensional correlation spectroscopy showed that electrons promote the degradation of polysaccharides, thereby alleviating membrane fouling.

Novel strategy for membrane biofouling control in MBR with nano-MnO2 modified PVDF membrane by in-situ ozonation

In this study, ozonation catalyst nano-MnO₂ blended polyvinylidene fluoride (PVDF) membrane was fabricated via phase inversion method and applied to membrane bioreactors (MBR), and then coupled with in-situ ozonation to study the anti-biofouling performance and reveal its mechanism. Results showed that, compared with pristine PVDF membrane (MBR_M₀), 0.75 wt% and 1.00 wt% nano-MnO₂ modified PVDF membrane (MBR_M_0.75 and MBR_M_1.00) could mitigate the membrane biofouling rate. Meanwhile MBR_M_1.00 coupled with in-situ ozonation could increase the membrane cleaning cycle to 1.5 and 2.7 times, compared with MBR_M₀ and MBR_M_0.75 without in-situ ozonation. The possible mechanisms included that the nano-MnO₂ modification coupled with in-situ ozonation directly removed the biofouling on the membrane surface, improved the hydrophilicity of the membrane surface and enhanced the chemical oxidation and biodegradation of membrane biofouling contaminants in the sludge mixture. The results of this work provide a new strategy for the control of membrane biofouling in MBR to treat industrial wastewater.

Concentration Polarization Quantification and Minimization in Cork Process Wastewater Ultrafiltration by an Ozone Pretreatment

Concentration polarization and membrane fouling have been identified as the main problems during the ultrafiltration treatment of cork processing wastewaters. These problems drastically reduce the permeate fluxes and, therefore, their potential applications. In this work, a soft ozonation pretreatment was applied to minimize these undesirable effects. A new systematic study was carried out for membranes with different molecular weight cut-offs and at different operating conditions to monitor and quantify the concentration polarization caused by the wastewater’s remaining ozonated compounds. Film theory was used to correlate the mass transfer coefficient, k, and the intrinsic rejection coefficient, f′, with the resistance introduced by concentration polarization. The ultrafiltration treatment was carried out under varying hydrodynamic operating conditions (circulating flow rates of 100–200 L/h) and transmembrane pressures (1–3 bar) for a set of four cellulose acetate membranes covering a wide range of molecular weight cut-offs (5000–100,000 Da) and hydraulic permeabilities (25–110 kg/h/m2/bar). The ozone pretreatment (at wastewater pH) reduced the phenolic content selectively (direct oxidation) by more than 50%, reducing membrane fouling and concentration polarization and increasing permeate fluxes (by 22–45%) and mass transfer coefficients (up to six times).