Distinction between polymeric and ceramic membrane in AnMBR treating municipal wastewater: In terms of irremovable fouling

A novel, low-cost clay ceramic membrane for the separation of oil-water emulsions

The development of a facile and efficient method for the fabrication of ceramic membranes fills a vital gap in the ceramic membrane manufacture research field. Ceramic membranes are relatively high in cost due to the cost of raw materials (metal oxides) as well as the energy required during the sintering stage of the fabrication process. In this study, a ceramic membrane made of low-cost halloysite nanotubular (HNT) clay is fabricated through a die press process and sintered at temperatures notably lower than those required of raw materials in commercial membranes. The features of the membrane were evaluated in terms of chemical properties, surface characteristics, hydrophilicity, durability, oil rejection performance, and antifouling properties. The pore size, porosity, and water permeability corresponding to the optimum membrane composition: halloysite: Al2O3: starch as 60:25:15 wt% were found to be 230 nm, 62.4%, and 1040 LMH/bar, respectively. The membrane demonstrated to be superhydrophilic in air and superoleophobic underwater. The performance tests were conducted with oil emulsions. Oil-water rejection tests were conducted at different concentrations of oil-in-water emulsions (724 and 1014 mg/L) and oil rejection was observed to reach more than 99%. The flux recovery ratio (FRR) of the membrane in the first filtration cycle when treating the lower oil concentration emulsion was 54% greater than the FRR corresponding to the higher oil concentration emulsion. However, in the next filtration cycle, the FRR of the ceramic membrane was 30% greater for the higher oil concentration emulsion which could be attributed to the formation of an oil film, preventing further oil particle penetration in the membrane matrix. The testing with real produced water from gas extraction indicated that the novel HNTs-based ceramic membrane performed well in feed solutions with high total dissolved solids content and can be used for produced water pretreatment before reverse osmosis membranes if the produced water to be desalted. The results from this work show that the developed ceramic membrane could be a promising, low-cost alternative to the ones existing in the current market for oily wastewater treatment.

A deeper investigation of membrane fouling in anaerobic membrane bioreactors for wastewater treatment: Influencing factors and fouling layer characteristics

Identifying the core parameters affecting membrane fouling and analyzing fouling layer characteristics are crucial for membrane fouling mitigation of anaerobic membrane bioreactors (AnMBRs). This study investigated the influence of various operating parameters on membrane fouling and the characteristics of different fouling layers. The ratio of flux to specific gas demand per unit of membrane area (SGD) was proposed as a key parameter for membrane fouling control and was applicable under various flux, SGD, and sludge concentration conditions. The membrane resistance and specific filtration resistance of foulants at high flux and sludge concentration reached 1.56 × 1012 m-1 and 3.56 × 1015 m-1/kg, respectively. Solid foulants accumulated on the membrane surface during rapid fouling stage. Protein-like pollutants accounted for a higher proportion (85%) of soluble foulants on the membrane surface. Humic acids were enriched on the cake/gel layer and the longitudinal enrichment process from the cake layer to the gel layer was uneven. Proteocatella (>45%) at the phylum level, Desulfovibrio (>3.1%), Syntrophobacter (>2.8%), and Treponema (>0.25%) at the genus level colonized in the gel layer and were the pioneers of membrane biofouling. Their enrichment on the membrane surface was primarily based on their own characteristics and was less sensitive to the operating conditions of AnMBRs. Therefore, this study provides a deeper understanding of membrane fouling formation process, which contributes to the long-term stable operation of AnMBR and scales up its engineering application.

Role of bacterial quorum sensing and quenching mechanism in the efficient operation of microbial electrochemical technologies: A state-of-the-art review

Microbial electrochemical technologies (METs) are a group of innovative technologies that produce valuables like bioelectricity and biofuels with the simultaneous treatment of wastewater from microorganisms known as electroactive microorganisms. The electroactive microorganisms are capable of transferring electrons to the anode of a MET through various metabolic pathways such as direct (via cytochrome or pili) or indirect (through transporters) transfer. Though this technology is promising, the inferior yield of valuables and the high cost of reactor fabrication are presently impeding the large-scale application of this technology. Therefore, to overcome these major bottlenecks, a lot of research has been dedicated to the application of bacterial signalling, for instance, quorum sensing (QS) and quorum quenching (QQ) mechanisms in METs to improve its efficacy in order to achieve a higher power density and to make it more cost-effective. The QS circuit in bacteria produces auto-inducer signal molecules, which enhances the biofilm-forming ability and regulates the bacterial attachment on the electrode of METs. On the other hand, the QQ circuit can effectively function as an antifouling agent for the membranes used in METs and microbial membrane bioreactors, which is imperative for their stable long-term operation. This state-of-the-art review thus distinctly describes in detail the interaction between the QQ and QS systems in bacteria employed in METs to generate value-added by-products, antifouling strategies, and the recent applications of the signalling mechanisms in METs to improve their yield. Further, the article also throws some light on the recent advancements and the challenges faced while incorporating QS and QQ mechanisms in various types of METs. Thus, this review article will help budding researchers in upscaling METs with the integration of the QS signalling mechanism in METs.

Evaluating performance of nano-Fe3O4 modified granular activated carbon assisted wastewater treatment in anaerobic fluidized membrane bioreactor

Magnetic granular activated carbon (MGAC), a nano-Fe₃O₄ modified granular activated carbon, was used as the carrier in an anaerobic fluidized-bed membrane bioreactor (AFMBR) to promote domestic wastewater treatment efficiency and alleviate membrane biofouling. Chemical oxygen demand (COD) removal reached 89 ± 2.6% with the effluent concentration of 20 ± 3.9 mg/L in the MGAC-AFMBR, while it was 28 ± 5.2 mg/L in AFMBR at hydraulic retention time (HRT) of 4 h. Total nitrogen (TN) removal was also enhanced by 4.0% with MGAC. An increased proportion of Chloroflexi and Bacteroidetes in the sludge may be responsible for improved treatment performance. MGAC reduced the protein and polysaccharide content in extracellular polymeric substances (EPS) by 9.8% and 8.1%, respectively. Besides, Bacteroidetes and Proteobacteria abundance decreased by 4.0% and 16.6% in the membrane cake layer with MGAC addition. Therefore, the high-quality effluent and low membrane biofouling of AFMBR was sustained by MGAC.

In-situ membrane fouling control and performance improvement by adding materials in anaerobic membrane bioreactor: A review

Anaerobic membrane bioreactor (AnMBR) is a promising treatment technique for various types of wastewaters, and is preferred over other conventional aerobic and anaerobic methods. However, membrane fouling is considered a bottleneck in AnMBR system, which technically blocks membrane pores by numerous inorganics, organics, and other microbial substances. Various materials can be added in AnMBR to control membrane fouling and improve anaerobic digestion, and studies reporting the materials addition for this purpose are hereby systematically reviewed. The mechanism of membrane fouling control including compositional changes in extracellular polymeric substances (EPSs) and soluble microbial products (SMPs), materials properties, stimulation of antifouling microbes and alteration in substrate properties by material addition are thoroughly discussed. Nonetheless, this study opens up new research prospects to control membrane fouling of AnMBR, engineered by material, including compositional changes of microbial products (EPS and SMP), replacement of quorum quenching (QQ) by materials, and overall improvement of reactor performance. Regardless of the great research progress achieved previously in membrane fouling control, there is still a long way to go for material-mediated AnMBR applications to be undertaken, particularly for materials coupling, real scale application and molecular based studies on EPSs and SMPs, which were proposed for future researches.

Long term operation performance and membrane fouling mechanisms of anaerobic membrane bioreactor treating waste activated sludge at high solid concentration and high flux

High solid anaerobic membrane bioreactor (HSAnMBR) is widely applied in biomass treatment and energy regeneration, while membrane operation performance and membrane fouling control remain critical issues. In this study, a HSAnMBR was utilized for waste activated sludge (WAS) treatment at organic loading rates of 3.69-3.72 gCOD/L·d and biogas yield was ranged in 0.38-0.39 L/gVS_fed with the COD conversion efficiency of 40 %. The membrane operated stably when the average flux was 9.6, 4.5 and 1.2 L/m²/h at mixed liquor total solid of 25, 30 and 40 g/L with a filtration: relaxation of 4:1, 1:1 and 1:2, respectively. The distinctive characteristics of membrane fouling at high solid condition were that the polysaccharides and proteins had high fouling propensity and were the main composition of the foulant layer. Furthermore, phosphorus and magnesium were the predominant causes of inorganic fouling. The minerals precipitated on the membrane and were embedded into membrane pores, contributing to cake layer formation and pore blocking. This research provided a comprehensive analysis of the membrane operation characterization and fouling mechanisms of HSAnMBR, which was expected to push forward HSAnMBR applications to WAS treatment.

3D spray-coated gradient profile ceramic membranes enables improved filtration performance in aerobic submerged membrane bioreactor

Rational design of cross-sectional microstructure in ceramic membranes has shown to improve membrane filtration efficacy without affecting rejection performance. In this work, we adopted 3D spray-coating technique to generate multi-layered membrane layers on macro-porous flat-sheet ceramic supports. The thickness of each layer was controlled by spray-coating cycles, and a gradient membrane layer was rationalized by successively coating three ceramic slurries containing alumina powders of gradually refined particle sizes, followed by co-sintering. Gradient membrane layers on both sides of the various sized flat-sheet ceramic supports were fabricated. Compared to the non-gradient counterpart, the gradient membranes showed both higher pure water flux (at the same TMP) and lower membrane resistance, which clearly evidenced the benefits of gradient profile in the membrane layer. Further, their performance in aerobic membrane bioreactors (AeMBR) was comparably studied for the first time. The treatment performance was not significantly affected by the types of membranes used, while the gradient membrane showed better filtration performance (i.e., a slower rise in TMP). Although the fouling mechanisms were revealed to be similar, the fouling layer in the gradient membrane was composed of a higher percentage of smaller foulants compared to that of the non-gradient counterpart. The observed differences were closely correlated to the larger internal pore structure in the gradient membrane. The present work provides a feasible 3D spray-coating technique for the fabrication of gradient flat-sheet ceramic membranes, and clarifies the benefits in AeMBR for domestic wastewater treatment.

Energy-Efficient AnMBRs Technology for Treatment of Wastewaters: A Review

In recent years, anaerobic membrane bioreactor (AnMBRs) technology, a combination of a biological reactor and a selective membrane process, has received increasing attention from both industrialists and researchers. Undoubtedly, this is due to the fact that AnMBRs demonstrate several unique advantages. Firstly, this paper addresses fundamentals of the AnMBRs technology and subsequently provides an overview of the current state-of-the art in the municipal and domestic wastewaters treatment by AnMBRs. Since the operating conditions play a key role in further AnMBRs development, the impact of temperature and hydraulic retention time (HRT) on the AnMBRs performance in terms of organic matters removal is presented in detail. Although membrane technologies for wastewaters treatment are known as costly in operation, it was clearly demonstrated that the energy demand of AnMBRs may be lower than that of typical wastewater treatment plants (WWTPs). Moreover, it was indicated that AnMBRs have the potential to be a net energy producer. Consequently, this work builds on a growing body of evidence linking wastewaters treatment with the energy-efficient AnMBRs technology. Finally, the challenges and perspectives related to the full-scale implementation of AnMBRs are highlighted.

Polyaniline derived carbon membrane and its in-situ membrane fouling mitigation performance in MBR based on metal-free electro-Fenton

An electro-enhanced membrane bioreactor (EMBR) was constructed with polyaniline-based carbon (PAC) separation membrane as the membrane-electrode, which could realize the in-situ electro-generation and activation of H₂O₂ to ·OH depending on the graphitic and pyridinic N as active sites without metal catalyst. After the continuous operation of the bioreactor for 74 days, approximately 77.41% irreversible membrane fouling occurred on the electrochemically enhanced membrane, which was less than that on the control membrane (85.96%). The ·OH oxidation combined with electrostatic barrier formed by -1.0 V enhanced PAC membrane suppressed the extracellular polymeric substances deposition on membrane. After operation, the strength of total cell, proteins, β-polysaccharides and α-polysaccharides on the membrane without bias were 5.17, 4.32, 9.65 and 16.31, respectively. In EMBR, the corresponding strength were 2.03, 3.35, 2.15 and 6.73. After calculation, the unblocked pores accounted for 35.3% and 78.5% of the total membrane surface in MBR and EMBR, respectively, indicating the fouling was alleviated obviously. Meanwhile, the EMBR owned a satisfactory wastewater treatment effect with average effluent chemical oxygen demand and NH₄⁺-N around 18.98 mg/L and 0.68 mg/L. The successful implementation of this strategy achieved a green and metal-free method for ·OH production with electrochemical effect for membrane fouling control in MBR.

Evaluation of bio-energy recovery from the anaerobic treatment of municipal wastewater by a pilot-scale submerged anaerobic membrane bioreactor (AnMBR) at ambient temperature

The potential of bio-energy recovery from real municipal wastewater was investigated using a one-stage pilot-scale submerged anaerobic membrane bioreactor (AnMBR) for a range of HRTs from 24 h to 6 h at ambient temperature around 25 °C. This pilot-scale AnMBR demonstrated a high COD removal efficiency of over 90% during an operation of 217 days for municipal wastewater treatment. The energy balance of the AnMBR was calculated from both theoretical and practical aspects. The theoretical net energy potential was calculated as 0.174 kWh/m³ by applying operational data to empirical equations, obtaining a bio-energy recovery efficiency of 69.4%. The practical net energy potential was estimated as -0.014 kWh/m³ using the powers of engines applied in a full-scale wastewater treatment plant. This is considerably lower than that of the conventional activated sludge process. These results are evidence of the potential of the AnMBR and feasibility in the treatment of municipal wastewater treatment.

Application of two anaerobic membrane bioreactors with different pore size membranes for municipal wastewater treatment

Pore size is one of the most important properties in the successful operation of membrane-based bioprocesses for the treatment of municipal wastewater. The characteristics of two anaerobic membrane bioreactors (AnMBRs), one with a hollow fiber membrane of 0.4 μm pore size (AnMBR1), and the other with a membrane of 0.05 μm pore size (AnMBR2) were investigated for the treatment of real municipal wastewater at room temperature (25 °C) under varied hydraulic retention times (HRTs). Process performance was evaluated in terms of organic removal efficiency, biogas production and membrane filtration behaviours during a long-term continuous operation. Both AnMBRs showed good organic removal performance with COD and BOD removal efficiencies of around 89% and 93%, respectively. High energy recovery potential was achieved, with the biogas yield ranging between 0.20 and 0.26 L-gas/g-COD_rem and a methane content of approximately 75%. The differences in the membrane filtration behaviours in the two AnMBRs included different permeate flux and total filtration resistance (R_t). In the AnMBR with a 0.4 μm pore size membrane, an average R_t of 1.08 × 10^12 m^-1 was obtained even when the permeate flux was a high 0.274 m/day, while a higher average R_t of 1.51 × 10^12 m^-1 was observed in the AnMBR with 0.05 μm pore size membrane even when the flux was a low 0.148 m/day. The off-line membrane cleaning strategy used for AnMBR1 indicated that the membrane restoration efficiency was 90.2%.

A brief review of anaerobic membrane bioreactors emphasizing recent advancements, fouling issues and future perspectives

This review summarizes the recent development and studies of anaerobic membrane bioreactor (AnMBR) to control fouling issues. AnMBR is an emerging waste water treatment technology mainly because of its low sludge residual, high volumetric organic removal rate, complete liquid-solid separation, better effluent quality, efficient resource recovery and the small footprint. This paper surveys the fundamental aspects of AnMBRs, including its applications, membrane configurations, and recent progress for enhanced reactor performance. Furthermore, the membrane fouling, a major restriction in the practical application of AnMBR, its mechanism and antifouling strategies like membrane cleaning, quorum quenching, ultrasonic treatment, membrane modifications, and antifouling agents are briefly discussed. Based on the review, the key issues that require urgent attention to facilitate large scale and integrated application of AnMBR technology are identified and future research perspectives relating to the prevalent issues are proposed.

Optimization of In Situ Backwashing Frequency for Stable Operation of Anaerobic Ceramic Membrane Bioreactor

The cost-effective and stable operation of an anaerobic ceramic membrane bioreactor (AnCMBR) depends on operational strategies to minimize membrane fouling. A novel strategy for backwashing, filtration and relaxation was optimized for stable operation of a side stream tubular AnCMBR treating domestic wastewater at the ambient temperature. Two in situ backwashing schemes (once a day at 60 s/day, and twice a day at 60 s × 2/day) maintaining 55 min filtration and 5 min relaxation as a constant were compared. A flux level over 70% of the initial membrane flux was stabilized by in situ permeate backwashing irrespective of its frequency. The in situ backwashing by permeate once a day was better for energy saving, stable membrane filtration and less permeate consumption. Ex situ chemical cleaning after 60 days’ operation was carried out using pure water, sodium hypochlorite (NaOCl), and citric acid as the order. The dominant cake layer was effectively reduced by in situ backwashing, and the major organic foulants were fulvic acid-like substances and humic acid-like substances. Proteobacteria, Firmucutes, Epsilonbacteria and Bacteroides were the major microbes attached to the ceramic membrane fouling layer which were effectively removed by NaOCl.