Biological performance and fouling mitigation in the biochar-amended anaerobic membrane bioreactor (AnMBR) treating pharmaceutical wastewater

Study on membrane fouling mechanisms and mitigation strategies in a pilot-scale anaerobic membrane bioreactor (P-AnMBR) treating digestate

Anaerobic Membrane Bioreactor (AnMBR) are employed for solid-liquid separation in wastewater treatment, enhancing process efficiency of digestion systems treating digestate. However, membrane fouling remains a primary challenge. This study operated a pilot-scale AnMBR (P-AnMBR) to treat high-concentration organic digestate, investigating system performance and fouling mechanisms. P-AnMBR operation reduced acid-producing bacteria and increased methane-producing bacteria on the membrane, preventing acid accumulation and ensuring stable operation. The P-AnMBR effectively removed COD and VFA, achieving removal rates of 82.3 % and 92.0 %, respectively. Higher retention of organic nitrogen and lower retention of ammonia nitrogen were observed. The membrane fouling consisted of organic substances (20.3 %), predominantly polysaccharides, and inorganic substances (79.7 %), primarily Mg ions (10.1 %) and Ca ions (4.5 %). To reduce the increased transmembrane pressure (TMP) caused by fouling (a 10.6-fold increase in filtration resistance), backwash frequency experiment was conducted. It revealed a 30-min backwash frequency minimized membrane flux decline, facilitating recovery to higher flux levels. The water produced amounted to 70.3 m³ over 52 days. The research provided theoretical guidance and practical support for engineering applications, offering practical insights for scaling up P-AnMBR.

Performance Investigation of PSF-nAC Composite Ultrafiltration Membrane for Protein Separation

As a promising wastewater treatment technology, ultrafiltration membranes face challenges related to fouling and flux reduction. To enhance these membranes, various strategies have been explored. Among them, the incorporation of nano-activated carbon (nAC) powder has emerged as an effective method. In this study, composite polysulfone (PSF) ultrafiltration membranes were fabricated using nAC powder at concentrations ranging from 0 to 8 wt.%. These membranes underwent comprehensive investigation, including assessments of membrane morphology, hydrophilicity, pure water flux, equilibrium water content, porosity, average pore size, and protein separation. The addition of activated carbon improved several desirable properties. Specifically, the hydrophilicity of the PSF membranes was enhanced, with the contact angle reduced from 69° to 58° for 8 wt.% of nAC composite membranes compared to the pristine PSF membrane. Furthermore, the water flux test revealed that 6 wt.% activated carbon-based membranes exhibited the highest flux, with a nearly 3 times improvement at 2 bar. Importantly, this enhancement did not compromise the protein rejection. Additionally, the introduction of nAC had a significant effect on the membrane's pore size by improving lysozyme rejection up to 40%. Overall, these findings will guide the selection of the optimal concentration of nAC for PSF ultrafiltration membranes.

The evolution of calcified anaerobic granular sludge bed informs the deep insight into its agglomeration process

Calcium-induced agglomeration of anaerobic granular sludge bed (AGSB) has become a critical factor in performance decline of calcified anaerobic reactors. However, the agglomeration process of AGSB and the underlying mechanisms remain unclear and elusive. This study delved into the evolution of calcified AGSB, and four typical states of normal AGSB (Nor-AGSB), calcified dispersed AGSB (Dis-AGSB), calcified dimeric AGSB (Dim-AGSB), and calcified polymeric AGSB (Pol-AGSB) were characterized. It was found that the minimum transport velocity of Dis-AGSB was 3.14-3.79 times higher than that of Nor-AGSB, and surpassed both the superficial velocity and the bubble-induced wake velocity. This led to the sedimentation of AGS at the bottom of reactor, resulting in stable contacts with each other. Solid fillers between AGS, namely cement, were observed within Dim-AGSB and Pol-AGSB, and could be classified as tightly- and loosely- bonded cement (T- and L-cement). Further analysis revealed that T-cement was rich in extracellular polymeric substances and intertwining pili/flagella, serving as the primary driving force for robust inter-AGS adhesion. While the L-cement was primarily in the form of calcite precipitation, and blocked the convective mass transfer pathways in Pol-AGSB, leading to the decreased convective mass transfer capacity. The critical distance between calcite and AGS was further revealed as 5.33 nm to form stable initial adhesion. Consequently, the agglomeration mechanism involving the evolution of AGSB was proposed as calcium-induced sedimentation, calcium-induced adhesion, and calcium-induced stasis in order. This study is expected to offer deep insight into the calcium-induced agglomeration especially from the overlooked perspective of AGSB, and provides feasible control strategies to manage the pressing calcification issues in engineering applications.

A comparison study on membrane fouling in A/O-MBR and A/A-MBR at different mixed liquor-suspended solids concentrations

Membrane fouling leads to decreased membrane flux, increases the frequency of membrane tissue replacement and membrane cleaning, and increases the operating cost of membrane bioreactor. In this study, the pollutant removal effects, membrane fouling differences and microbial characteristics of anaerobic/aerobic MBR (A/O-MBR) and anaerobic/anoxic MBR (A/A-MBR) were investigated at different mixed liquor suspended solids (MLSS) concentrations. The results showed that the chemical cleaning cycle of membrane contamination was 12, 28, 44 h and 24, 40, 104 h, respectively, and the cycle was prolonged with the increase of MLSS concentration (from 6000 to 9000 mg L-1). A/O-MBR was 1.4-2.4 times the rate of membrane fouling of A/A-MBR. In irreversible resistance, extracellular polymer substances (EPS) were the most significant contributors to membrane fouling. EPS concentration in A/A-MBR (118.33, 73.75, 54.26 mg/gMLSS) was lower than that in A/O-MBR (171.68, 91.92, 62.33 mg/gMLSS). Therefore, increasing MLSS concentration could mitigate membrane fouling. 16S rRNA high-throughput sequencing demonstrated that filamentous bacteria was the primary reason for the membrane fouling difference. Filamentous bacteria were more likely to be attached to the surface of the membrane, causing membrane fouling. The abundance percentage of filamentous bacteria in A/A-MBR was smaller than that in A/O-MBR. In summary, The excellent performance of A/A-MBR in membrane fouling behaviour, resistance analysis, EPS and microorganisms proved that A/A-MBR is more promising than A/O-MBR in wastewater nitrogen and phosphorus removal. This study can provide a theoretical basis for the application of MBR in the field of sewage treatment.

Effectiveness of cloth media filters on mitigating membrane fouling in anaerobic filter membrane bioreactors

Membrane fouling is a persistent challenge that has impeded the broader application of anaerobic membrane bioreactors (AnMBRs). To mitigate membrane fouling, between the outlet of the UASB anaerobic bioreactor and the PVDF membrane to form the anaerobic filter membrane bioreactor (AnFMBR) system. Through comprehensive experiments, the optimal pore size for cloth filters was determined to be 50 μm. A comprehensive assessment over 140 days of operation shows that the novel AnFMBR had significantly greater resistance to membrane pollution than the traditional AnMBR. The AnFMBR system membrane tank exhibited lower mixed liquor suspended solid and mixed liquor volatile suspended solid concentrations, smaller sludge particle sizes, increased hydrophilicity of sludge flocs, and optimized microbial community distribution compared to those of conventional AnMBRs. The total solids foulant accumulation rate in the AnMBR was 5.1 g/m2/day, while in the AnFMBR, the rate was 2.4 g/m2/day, marking a 53.7 % decrease in fouling rate for the AnFMBR compared with the AnMBR. This decrease indicates that integrating the filtration assembly significantly lowered the rate of solid foulant accumulation on the membrane surface, primarily by controlling the buildup of solid foulants in the cake layer, thereby alleviating membrane fouling. AnFMBR compared to AnMBR, the membrane fouling rate halved, effectively doubled the interval between membrane cleaning from seven days, as observed in the AnMBR system, to fourteen days. These findings underscore the potential of integrating cloth media filters into AnMBRs to improve operational efficiency, economic viability, and sustainability.

Effect of magnetic field coupled magnetic biochar on membrane bioreactor efficiency, membrane fouling mitigation and microbial communities

The excitation by magnetic field was established to mitigate the membrane fouling of magnetic biochar (MB)-supplemented membrane bioreactor (MBR) in this study. The results showed that the transmembrane pressure (TMP) increase rates decreased by about 8 % after introducing the magnetic field compared with the magnetic biochar-MBR (MB-MBR). Membrane characterization suggested that the flocs in the magnetic field-magnetic biochar-MBR (MF-MB-MBR) formed a highly permeable developed cake layer, and a fluffier and more porous deposited layer on membrane surface, which minimized fouling clogging of the membrane pores. Further mechanistic investigation revealed that the decrease in contact angle of fouled membrane surface in MF-MB-MBR, i.e. an enhanced membrane hydrophilicity, is considered important for forming highly permeable layers. Additionally, the magnetic field was demonstrated to have a positive effect on the improvement of the magneto-biological effect, the enhancement of charge neutralization and adsorption bridging between sludge and magnetic biochar, and the reduction of formation of extracellular polymeric substances (EPSs), which all yielded sludge flocs with a large pore structure conducive to form a fluffy and porous deposited layer in the membrane surface. Furthermore, high-throughput sequencing analysis revealed that the magnetic field also led to a reduction in microbial diversity, and that it promoted the enrichment of specific functional microbial communities (e.g. Bacteroidetes and Firmicutes) playing an important role in mitigating membrane fouling. Taken together, this study of magnetic field-enhanced magnetic biochar for MBR membrane fouling mitigation provides insights important new ideas for more effective and sustainable operation strategies.

Deciphering the behavior and potential mechanism of biochar at different pyrolysis temperatures to alleviate membrane biofouling

Biofouling limits applications of membrane technology in wastewater treatment, but dosing additives to membrane tanks is an effective method to alleviate biofouling. In this study, biochar derived from corncob and pyrolyzed at 300, 500, and 700°C was dosed to determine the underlying anti-biofouling mechanism. The effects of the biochar on the membrane properties and foulant behavior were systematically investigated. The results showed that biochar delayed the occurrence of the fouling transition (0.5-3.0 h), and decreased the flux decline rate, thus achieving a higher water flux (3.1-3.7 times of the control group). Biochar altered membrane surface properties, and increased the membrane surface charge, roughness, and hydrophilicity, which all contributed to higher membrane permeability. Moreover, adding biochar reduced the number of foulants in the fouling layer, particularly protein substances. The flux model fit and the XDLVO theory further revealed the mitigating effect of biochar on membrane biofouling. At the initial intermediate-blocking stage, the effect of biochar on membrane fouling was determined by its properties, and adsorption capacity to the foulants, BC500 presented the best mitigation performance. At the later cake-filtration stage, the role of biochar in membrane fouling was strongly associated with protein content in the fouling layer, and the minimum rate of flux decline occurred in BC300. This study promotes the understanding and development of biochar to alleviate membrane biofouling.

Volatile fatty acids production from kitchen waste slurry using anaerobic membrane bioreactor via alkaline fermentation with high salinity: Evaluation on process performance and microbial succession

In this study, a pilot-scale anaerobic membrane bioreactor (AnMBR) was developed to continuously produce volatile fatty acids (VFAs) from kitchen waste slurry under an alkaline condition. The alkaline fermentation effectively suppressed methanogenesis, thus achieving high VFAs production of 60.3 g/L. Acetic acid, propionic acid, and butyric acid accounted for over 95.0 % of the total VFAs. The VFAs yield, productivity, and chemical oxygen demand (COD) recovery efficiency reached 0.5 g/g-CODinfluent, 6.0 kg/m3/d, and 62.8 %, respectively. Moreover, the CODVFAs/CODeffluent ratio exceeded 96.0 %, and the CODVFAs/NH3-N ratio through ammonia distillation reached up to 192.5. The microbial community was reshaped during the alkaline fermentation with increasing salinity. The membrane fouling of the AnMBR was alleviated by chemical cleaning and sludge discharge, and membrane modules displayed a sustained filtration performance. In conclusion, this study demonstrated that high-quality VFAs could be efficiently produced from kitchen waste slurry using an AnMBR process via alkaline fermentation.

Insight into key interactions between diverse factors and membrane fouling mitigation in anaerobic membrane bioreactor

Anaerobic membrane bioreactors (AnMBRs) have garnered considerable attention as a low-energy and low-carbon footprint treatment technology. With an increasing number of scholars focusing on AnMBR research, its outstanding performance in the field of water treatment has gradually become evident. However, the primary obstacle to the widespread application of AnMBR technology lies in membrane fouling, which leads to reduced membrane flux and increased energy demand. To ensure the efficient and long-term operation of AnMBRs, effective control of membrane fouling is imperative. Nevertheless, the interactions between various fouling factors are complex, making it challenging to predict the changes in membrane fouling. Therefore, a comprehensive analysis of the fouling factors in AnMBRs is necessary to establish a theoretical basis for subsequent membrane fouling control in AnMBR applications. This review aims to provide a thorough analysis of membrane fouling issues in AnMBR applications, particularly focusing on fouling factors and fouling control. By delving into the mechanisms behind membrane fouling in AnMBRs, this review offers valuable insights into mitigating membrane fouling, thus enhancing the lifespan of membrane components in AnMBRs and identifying potential directions for future AnMBR research.

Biochar mediated high-rate anaerobic bioreactors: A critical review on high-strength wastewater treatment and management

Treatment of high-strength wastewater is critical for the aquatic environment and receiving water bodies around the globe. Untreated or partially treated high-strength wastewater may cause severe damage to the existing water bodies. Various high-rate anaerobic bioreactors have been developed in the last decades for treating high-strength wastewater. High-rate anaerobic bioreactors are effective in treating industrial wastewater and provide energy in the form of methane as well. However, the physical or chemical properties of high-strength industrial wastewater, sometimes, disrupt the functioning of a high-rate anaerobic bioreactor. For example, the disintegration of granular sludge in up flow anaerobic sludge blanket reactor or membrane blocking in an anaerobic membrane bioreactor are the results of a high-strength wastewater treatment which hamper the proper functioning and may harm the wastewater treatment plant economically. Biochar, if added to these bioreactors, may help to alleviate the ill-functioning of high-rate anaerobic bioreactors. The primary mechanisms by biochar work in these bioreactors are direct interspecies electron transfer, microbial immobilization, or gene level alternations in microbial structure. The present article explores and reviews the recent application of biochar in a high-rate anaerobic bioreactor treating high-strength industrial wastewater.

Advanced oxidation and biological integrated processes for pharmaceutical wastewater treatment: A review

The stress of pharmaceutical and personal care products (PPCPs) discharging to water bodies and the environment due to increased industrialization has reduced the availability of clean water. This poses a potential health hazard to animals and human life because water contamination is a great issue to the climate, plants, humans, and aquatic habitats. Pharmaceutical compounds are quantified in concentrations ranging from ng/Lto μg/L in aquatic environments worldwide. According to (Alsubih et al., 2022), the concentrations of carbamazepine, sulfamethoxazole, Lutvastatin, ciprofloxacin, and lorazepam were 616-906 ng/L, 16,532-21635 ng/L, 694-2068 ng/L, 734-1178 ng/L, and 2742-3775 ng/L respectively. Protecting and preserving our environment must be well-driven by all sectors to sustain development. Various methods have been utilized to eliminate the emerging pollutants, such as adsorption and biological and advanced oxidation processes. These methods have their benefits and drawbacks in the removal of pharmaceuticals. Successful wastewater treatment can save the water bodies; integrating green initiatives into the main purposes of actor firms, combined with continually periodic awareness of the current and potential implications of environmental/water pollution, will play a major role in water conservation. This article reviews key publications on the adsorption, biological, and advanced oxidation processes used to remove pharmaceutical products from the aquatic environment. It also sheds light on the pharmaceutical adsorption capability of adsorption, biological and advanced oxidation methods, and their efficacy in pharmaceutical concentration removal. A research gap has been identified for researchers to explore in order to eliminate the problem associated with pharmaceutical wastes. Therefore, future study should focus on combining advanced oxidation and adsorption processes for an excellent way to eliminate pharmaceutical products, even at low concentrations. Biological processes should focus on ideal circumstances and microbial processes that enable the simultaneous removal of pharmaceutical compounds and the effects of diverse environments on removal efficiency.

Insights into current bio-processes and future perspectives of carbon-neutral treatment of industrial organic wastewater: A critical review

With the rise of the concept of carbon neutrality, the current wastewater treatment process of industrial organic wastewater is moving towards the goal of energy conservation and carbon emission reduction. The advantages of anaerobic digestion (AD) processes in industrial organic wastewater treatment for bio-energy recovery, which is in line with the concept of carbon neutrality. This study summarized the significance and advantages of the state-of-the-art AD processes were reviewed in detail. The application of expanded granular sludge bed (EGSB) reactors and anaerobic membrane bioreactor (AnMBR) were particularly introduced for the effective treatment of industrial organic wastewater treatment due to its remarkable prospect of engineering application for the high-strength wastewater. This study also looks forward to the optimization of the AD processes through the enhancement strategies of micro-aeration pretreatment, acidic-alkaline pretreatment, co-digestion, and biochar addition to improve the stability of the AD system and energy recovery from of industrial organic wastewater. The integration of anaerobic ammonia oxidation (Anammox) with the AD processes for the post-treatment of nitrogenous pollutants for the industrial organic wastewater is also introduced as a feasible carbon-neutral process. The combination of AnMBR and Anammox is highly recommended as a promising carbon-neutral process for the removal of both organic and inorganic pollutants from the industrial organic wastewater for future perspective. It is also suggested that the AD processes combined with biological hydrogen production, microalgae culture, bioelectrochemical technology and other bio-processes are suitable for the low-carbon treatment of industrial organic wastewater with the concept of carbon neutrality in future.

Effects of biochar addition towards the treatment of blackwater in anaerobic dynamic membrane bioreactor (AnDMBR): Comparison among room temperature, mesophilic and thermophilic conditions

Effects of biochar addition on the performance of anaerobic dynamic membrane bioreactor (AnDMBR) under different temperatures for blackwater treatment was investigated. When the organic load ratios (OLR) was 1.0 g COD/L·d, the specific methane yield for the three biochar-amended reactors were 125.7, 148.0 and 182.3 mLCH₄/g COD_removed, respectively. Compare to those digesters without biochar participation, the methane production in the thermophilic reactor with biochar increased by 12% while the other two digesters increased by 6-10%. An analysis of membrane filtration resistance showed a reduction in total resistance (R_t) of 6.2 × 10¹¹-7.3 × 10¹¹ m^-1 when biochar was added to the three reactors. The thermophilic reactors with biochar increased the relative abundance of Methanothermobacter and promoted gene expression of metabolic pathways related to hydrolysis, acid production and methane production. Overall, biochar showed great potential as an inexpensive conductive material to increase methane production with reduced membrane fouling in AnDMBR systems.

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.

Recent advances in the biological treatment of wastewater rich in emerging pollutants produced by pharmaceutical industrial discharges

Pharmaceuticals and personal care products present potential risks to human health and the environment. In particular, wastewater treatment plants often detect emerging pollutants that disrupt biological treatment. The activated sludge process is a traditional biological method with a lower capital cost and limited operating requirements than more advanced treatment methods. In addition, the membrane bioreactor combines a membrane module and a bioreactor, widely used as an advanced method for treating pharmaceutical wastewater with good pollution performance. Indeed, the fouling of the membrane remains a major problem in this process. In addition, anaerobic membrane bioreactors can treat complex pharmaceutical waste while recovering energy and producing nutrient-rich wastewater for irrigation. Wastewater characterizations have shown that wastewater's high organic matter content facilitates the selection of low-cost, low-nutrient, low-surface-area, and effective anaerobic methods for drug degradation and reduces pollution. However, to improve the biological treatment, researchers have turned to hybrid processes in which all physical, chemical, and biological treatment methods are integrated to remove various emerging contaminants effectively. Hybrid systems can generate bioenergy, which helps reduce the operating costs of the pharmaceutical waste treatment system. To find the most effective treatment technique for our research, this work lists the different biological treatment techniques cited in the literature, such as activated sludge, membrane bioreactor, anaerobic treatment, and hybrid treatment, combining physicochemical and biological techniques.

Performance evaluation and energy potential analysis of anaerobic membrane bioreactor (AnMBR) in the treatment of simulated milk wastewater

An anaerobic membrane bioreactor (AnMBR) was employed as primary treatment unit for anaerobic treatment of simulated wastewater to produce high effluent quality. A lab scale hollow fiber membrane was used to scrutinize the performance of AnMBR as a potential treatment system for simulated milk wastewater and analyze its energy recovery potential. The 15 L bioreactor was operated continuously at mesophilic conditions (35 °C) with a pH constant of 7.0. The membrane flux was in the range of 9.6-12.6 L/m². h. The different organic loading rates (OLRs) of 1.61, 3.28, 5.01, and 8.38 g-COD/L/d, of simulated milk wastewater, were fed to the reactor and the biogas production rate was analyzed, respectively. The results revealed that the COD removal efficiencies of 99.54 ± 0.001% were achieved at the OLR of 5.01 gCOD/L/d. The highest methane yield was found to be at OLR of 1.61 gCOD/L/d at HRT of 30 d with the value of 0.33 ± 0.01 L-CH₄/gCOD. Moreover, based on the analysis of energy balance in the AnMBR system, it was found that energy is positive at all the given HRTs. The net energy production (NEP) ranged from 2.594 to 3.268 kJ/gCOD, with a maximum NEP value of 3.268 kJ/gCOD at HRT 10 d HRT. Bioenergy recovery with the maximum energy ratio, of 4.237, was achieved with an HRT of 5 d. The study suggests a sizable energy saving with the anaerobic membrane process.

Simultaneous sludge minimization and membrane fouling mitigation in membrane bioreactors by using a microaerobic - Settling pretreatment module

Membrane fouling is the major obstacle for membrane bioreactors operated at a long sludge retention time to reduce sludge production. In this study, a sludge process reduction (SPR) module, consisting of a microaerobic tank and a settler, was inserted before an anoxic/oxic MBR (AO-MBR) to achieve dual objectives of fouling alleviation and sludge reduction. Three SPR-MBRs were operated to investigate influences of sludge recirculation ratios from the SPR settler to the microaerobic tank on process performance. Compared to AO-MBR, the SPR-MBRs reduced sludge production by 43.1-56.4% by maintaining sludge retention times above 175 d, and decreased foulant layer resistance and pore clogging resistance. Inserting SPR reduced the accumulation of dissolved organic matters and extracellular polymeric substances, enlarged sludge flocs, and decreased sludge viscoelasticity. However, increasing R_SPR stimulated outward diffusion of extracellular polymeric substances and increased sludge viscosity. SPR-MBRs achieved effective sludge reduction by enriching hydrolytic (Trichococcus and Aeromonas) and fermentative genera (Lactococcus, Paludibacter, Macellibacteroides, and Acinetobacter) in the SPR, and alleviated membrane fouling by prohibiting the growth of extracellular polymeric substance-secreting bacteria and enriching filamentous bacteria to enlarge particle size. The results revealed that the SPR-MBR maximized sludge reduction with a very long sludge retention time, and alleviated membrane fouling synchronously.

Biochar-assisted anaerobic membrane bioreactor towards high-efficient energy recovery from swine wastewater: Performances and the potential mechanisms

A high-efficient energy recovery system of biochar-assisted anaerobic membrane bioreactor (BC-AnMBR) was established for swine wastewater treatment. Comparing with a conventional AnMBR, biochar addition accelerated volatile fatty acids (VFA) degradation during start-up stage, thereby shortened start-up duration by 44.0 %. Under a high organic loading rate (OLR) of 21.1 gCOD/L/d, BC-AnMBR promoted COD removal efficiency from 90.1 % to 95.2 %, and maintained a high methane production rate of 4.8L CH₄/L/d. The relative abundance of Methanosaeta declined from 53.9 % in conventional AnMBR to 21.0 % in BC-AnMBR, whereas that of Methanobrevibacter dramatically increased from 10.3 % to 70.9 %, respectively. Metabolic pathway analysis revealed that biochar not only strengthened hydrogenotrophic methanogenesis pathway, but also upregulated the genes encoding electron transfer carriers and riboflavin metabolism, suggesting the role of biochar facilitating direct interspecies electron transfer for syntrophic methanogenesis. The excellent energy yield performances under high OLR confirmed BC-AnMBR as an advanced system for high-strength swine wastewater treatment.

Material mass balance and elemental flow analysis in a submerged anaerobic membrane bioreactor for municipal wastewater treatment towards low-carbon operation and resource recovery

The anaerobic membrane bioreactor (AnMBR) has gained huge attention as a municipal wastewater (MWW) treatment process that combined high organics removal, a low sludge yield and bioenergy recovery. In this study, a 20 L AnMBR was set up and operated steadily for 70 days in temperate conditions with an HRT of 6 h and a flux of 12 LMH for the treatment of real MWW, focusing on the behavior of the major elements (C, N, P and S) from an elemental balance perspective. The results showed that the AnMBR achieved more than 85 % COD removal, a low sludge yield (0.081 gVSS/gCOD_removed) and high methane production (0.31 L-CH₄/gCOD_removed) close to the theoretical value. The elemental flow analysis revealed that the AnMBR converted 77 % of the influent COD to methane (57 % gaseous and 20 % dissolved) and 6 % of the COD for sludge production. In addition, the AnMBR converted 34 % of the total carbon to energy-generated carbon, and only 3 % was in the form of CO₂ in the biogas for further upgradation, which was in line with the concept of carbon neutrality. Since little nitrogen or phosphorus were removed, the permeate was nutrient-rich and further treatment to recover the nutrients would be required. This study illustrates the superior performance of the AnMBR for MWW treatment with a microscopic view of elemental behavior and provides a reference for implementing the mainstream AnMBR process in carbon-neutral wastewater treatment plants.

Landfill leachate treatment by graphite engineered anaerobic membrane bioreactor: Performance enhancement and membrane fouling mitigation

Low efficiency of anaerobic digestion and membrane fouling, treating landfill leachate, are big barriers in the application of anaerobic membrane bioreactor (AnMBR). Anaerobic digestion enhancement and membrane fouling mitigation of AnMBR with graphite addition, treating landfill leachate, were investigated in this study. The effect of graphite on organics removal, biogas production, methane content in biogas, membrane fouling, microbial responses and foulant compositions were analyzed. With the graphite addition, chemical oxygen demand (COD) removal of 78% was achieved for influent COD concentration of 3000 mg/l, which was significantly higher than the stage without graphite addition (65%) for influent COD concentration of 2000 mg/l. Similarly, methane content in biogas with graphite addition was 56%, while without graphite addition it was 46%. These digestion improvements were due to the promotion of organics degradation, facilitated by direct interspecies electron transfer (DIET) mechanism via graphite addition in AnMBR. The graphite addition prolonged membrane cleaning cycle from 13 days to 30 days. Protein content in loosely bound extracellular polymeric substance (LB-EPS) was the main fouling agent, which decreased with the graphite addition. The main mechanism behind membrane fouling mitigation was the protein content reduction in LB-EPS, which was biodegraded by Trichococcus being increased in relative abundance with the graphite addition. Furthermore, abundance of Denitratisoma decreased in anaerobic sludge and its accumulation reduced on membrane surface, subsequently membrane fouling was mitigated. Overall, graphite addition in AnMBR is a potential eco-innovative approach that efficiently removes pollutants from landfill leachate, enhances biogas quality and mitigates membrane fouling.

Prospecting cellulose fibre-reinforced composite membranes for sustainable remediation and mitigation of emerging contaminants

Many environmental pollutants caused by uncontrolled urbanization and rapid industrial growth have provoked serious concerns worldwide. These pollutants, including toxic metals, dyes, pharmaceuticals, pesticides, volatile organic compounds, and petroleum hydrocarbons, unenviably compromise the water quality and manifest a severe menace to aquatic entities and human beings. Therefore, it is of utmost importance to acquaint bio-nanocomposites with the capability to remove and decontaminate this extensive range of emerging pollutants. Recently, considerable emphasis has been devoted to developing low-cost novel materials obtained from natural resources accompanied by minimal toxicity to the environment. One such component is cellulose, naturally the most abundant organic polymer found in nature. Given bio-renewable sources, natural abundance, and impressive nanofibril arrangement, cellulose-reinforced composites are widely engineered and utilized for multiple applications, such as wastewater decontamination, energy storage devices, drug delivery systems, paper and pulp industries, construction industries, and adhesives, etc. Environmental remediation prospective is among the fascinating application of these cellulose-reinforced composites. This review discusses the structural attributes of cellulose, types of cellulose fibrils-based nano-biocomposites, preparatory techniques, and the potential of cellulose-based composites to remediate a diverse array of organic and inorganic pollutants in wastewater.

Anaerobic membrane bioreactors for pharmaceutical-laden wastewater treatment: A critical review

Pharmaceuticalsare a diverse group of chemical compounds widely used for prevention and treatment of infectious diseases in both humans and animals. Pharmaceuticals, either in their original or metabolite form, find way into the wastewater treatment plants (WWTPs) from different sources. Recently, anaerobic membrane bioreactors (AnMBR) has received significant research attention for the treatment of pharmaceuticals in various wastewater streams. This review critically examines the behaviour and removal of a wide array of pharmaceuticals in AnMBR with primary focus on their removal efficiencies and mechanisms, critical influencing factors, and the microbial community structures. Subsequently, the inhibitory effects of pharmaceuticals on the performance of AnMBR and membrane fouling are critically discussed. Furthermore, the imperative role of membrane biofouling layer and its components in pharmaceuticals removal is highlighted. Finally, recent advancements in AnMBR configurations for membrane fouling control and enhanced pharmaceuticals removal are systemically discussed.

Evaluation of a novel integrated membrane biological aerated filter for water reclamation: A practical experience

The use of treated wastewater in addition to solving the problem of water shortage, can increase soil fertility and reduce the use of chemical fertilizers. We aim to provide a high-quality effluent to feed membrane system, reduce treatment costs and enhance the efficiency of wastewater recycling. All experiments were conducted on a novel integrated membrane biological aerated filter (IMBAF) consisting of a down flow cylindrical biological aerated filter (BAF) filled by silica and a novel sand-coated polystyrene granules (SCP), followed by ultrafiltration (UF) and reverse osmosis (RO) membranes. IMBAF reactor, with 73.6 L volume, was operated for 270 days (in three 90-day stages) with different conditions of returning backwash water. Accordingly, BAF generated high quality water for feeding UF membrane with 94.2%, 68%, 54.4%, 91.2%, and 99.95% of turbidity, 5-day biochemical oxygen demand (BOD₅), chemical oxygen demand (COD), oil and grease (O&G), fecal coliform (FC) removal, respectively. At the end of stage 3, 99.88% of influent was recycled by UF and only 0.12% was disposed of as sludge. The BAF and UF module efficiently promote the quality of water entering RO system. After 75 days of continuous operation, the increase in trans-membrane pressure (TMP) and also decrease in RO membrane permeability were about 14% and 9.4%, respectively, indicating low clogging of the membrane. The use of BAF structure designed in this study increases the wastewater recycling rate, decreases membrane clogging and thereby reduces the costs of concentrate disposal and chemical cleaning.

Application of biochar immobilized microorganisms for pollutants removal from wastewater: A review

Microbial immobilization technology (MIT) has been rapidly developed and used to remove pollutants from water/wastewater in recent years, owing to its high stability, rapid reaction rate, and high activity. Microbial immobilization carrier with low cost and high removal efficiency is the key of MIT. Biochar is considered to be an efficient carrier for microbial immobilization because of its high porosity and good adsorption effect, which can provide a habitat for microorganisms. The use of biochar immobilized microorganisms to treat different pollutants in wastewater is a promising treatment method. Compared with the other biological treatment technology, biochar immobilized microorganisms can improve microbial abundance, repeated utilization ratio, microbial metabolic capacity, etc. However, current research on this method is still in its infancy. Little attention has been paid to the interaction mechanisms between biochar and microorganisms, and many studies are only carried out in the laboratory. There are still problems such as difficult recovery after use and secondary pollution caused by residual pollutants after biochar adsorption, which need further clarification. To have comprehensive digestion and an in-depth understanding of biochar immobilized microorganisms technology in wastewater treatment, the wastewater treatment methods based on biochar are firstly summarized in this review. Then the mechanisms of immobilized microorganisms were explored, and the applications of biochar immobilized microorganisms in wastewater were systematically reviewed. Finally, suggestions and perspectives for future research and practical application are put forward.

Antibiotic bioremediation by new generation biochar: Recent updates

The evolving multidrug resistance in microbes with increasing antibiotic pollution is becoming a severe global crisis. Recent developments on antibiotic remediations by biochar are promising. Advancements in biochar engineering enhanced biochar remediation efficiency to another level through developing new interactions and bonding abilities with antibiotic pollutants. Especially chemical/metal-composite modification significantly increased catalysis of biochar. The review's main focus is to underline biochar efficiency for the abatement of emerging antibiotic pollutants. Moreover, to relate feedstock, production conditions, and engineering techniques with biochar properties. Also, modification strategies are reviewed to obtain biochar or their composites before examining improved remediation potential ranging from 20 to 552 mg g^-1 for various antibiotics. Biochar offers different interactions depending on the surface functionalities e.g., π-π stacking, electrostatic, H-bonding, etc. Biochar and related composites have also been reviewed for remarkable properties e.g., photocatalysis, adsorption, and oxidation processes. Furthermore, future research directions and opportunities for biochar research are discussed.

Bacteriophage cocktail as a promising bio-enhancer for methanogenic activities in anaerobic membrane bioreactors

This study aimed to explore the effect of a bacteriophage cocktail, pyophage, on the treatment of wastewater containing antibiotics in an anaerobic membrane bioreactor (AnMBR). During the operational period, performance of the AnMBR was monitored through the changes in chemical oxygen demand (COD), antibiotic removal, transmembrane pressure, and biogas production. Microbial community structure and composition, as well as the occurrence of antibiotic resistance genes were analyzed through shotgun metagenomics analysis. When exposed to pyophage, COD removal efficiency was enhanced up to 96%, whereas membrane fouling was delayed by 25%. Average biogas production was doubled from 224.2 mL/d in control with antibiotics to 447.3 mL/d when exposed to pyophage cocktail with considerable alterations to the archaeal and bacterial community structures. Most notably, the methanogenic community shifted from dominance of Methanothermobacter to Methanoculleus, along with syntrophic bacteria. The results provide insight into the synergistic effects of phage-bacteria and methanogenic communities and illustrate the potential of bacteriophages as bio-enhancers.

How to select substrate for alleviating clogging in the subsurface flow constructed wetland?

Constructed wetland (CW) is a cost-effective and environmentally friendly ecological technology for contaminated water remediation, especially in dispersed communities and rural areas. Plants grow, biofilms form, and pollutants attach to the substrate, which is the main supporting structure of a subsurface flow CW (SSFCW) system. After long-term operation, the accumulation of clogs from physical, chemical, and biological processes in SSFCW substrates can easily cause clogging, thus reducing treatment efficiency reduction and service life and causing no discharge of sewage by intermittent until last indicates in the CW surface. Subsequently, stench and mosquito breeding occur, thus influencing environmental sanitation. Substrate clogging is the most serious, challenging, and inevitable problem in the long-term operation of SSFCWs. The present study reviews the effects of substrates on clogging categorized into physical, chemical, and biological clogging and analyzes the substrates that can alleviate/aggravate clogging in CWs. The recommended substrates that can relieve clogging include plastic, rubber, soil mixture, walnut shell, biochar, organic waste, alum sludge, and lightweight aggregate, while shell, steel slag, blast furnace slag, zeolite, and soil may easily generate phosphorus-clogging substances. CW substrate clogging is a mixture of three clogs with synergistic effects, and the corresponding clogging mitigation substrates mentioned above can be used to alleviate the most severe among the three types of clogs to reduce the synergy, and thus to promote stable operation and technology level of CWs. This review aims to promote the scientific selection of substrates for the stable operation and technical level of CW through targeted recommendations for substrates that relieve clogging. Future studies should focus the effects of influent water quality and substrate type on clogging, and waste as substrate to alleviate clogging, while mitigating the negative environmental impact of waste treatment.

Efficient remediation of antibiotic pollutants from the environment by innovative biochar: current updates and prospects

The increased antibiotic consumption and their improper management led to serious antibiotic pollution and its exposure to the environment develops multidrug resistance in microbes against antibiotics. The entry rate of antibiotics to the environment is much higher than its exclusion; therefore, efficient removal is a high priority to reduce the harmful impact of antibiotics on human health and the environment. Recent developments in cost-effective and efficient biochar preparation are noticeable for their effective removal. Moreover, biochar engineering advancements enhanced biochar remediation performance several folds more than in its pristine forms. Biochar engineering provides several new interactions and bonding abilities with antibiotic pollutants to increase remediation efficiency. Especially heteroatoms-doping significantly increased catalysis of biochar. The main focus of this review is to underline the crucial role of biochar in the abatement of emerging antibiotic pollutants. A detailed analysis of both native and engineered biochar is provided in this article for antibiotic remediation. There has also been discussion of how biochar properties relate to feedstock, production conditions and manufacturing technologies, and engineering techniques. It is possible to produce biochar with different surface functionalities by varying the feedstock or by modifying the pristine biochar with different chemicals and preparing composites. Subsequently, the interaction of biochar with antibiotic pollutants was compared and reviewed. Depending on the surface functionalities of biochar, they offer different types of interactions e.g., π-π stacking, electrostatic, and H-bonding to adsorb on the biochar surface. This review demonstrates how biochar and related composites have optimized for maximum removal performance by regulating key parameters. Furthermore, future research directions and opportunities for biochar research are discussed.

Mitigating membrane fouling in a high solid food waste thermophilic anaerobic membrane bioreactor by incorporating fixed bed bio-carriers

Hybrid anaerobic membrane bioreactor (Hy-AnMBR) was developed by incorporating polyurethane sponge carriers to mitigate membrane fouling. The results showed that the membrane fouling was well controlled in Hy-AnMBR from the aspects of sludge property and membrane filtration performance. The solid concentration, including TS and MLSS in the Hy-AnMBR was reduced after introducing the fixed bed carrier, which was 13% and 20% lower than the control AnMBR (Con-AnMBR), and this resulted in improved filtration performance. The scanning electron microscope (SEM) photograph showed that membrane pores could still be observed on the Hy-AnMBR surface, indicating that the cake layer fouling of the Hy-AnMBR was less than the Con-AnMBR. The increase of the EPS and SMP promoted the acceleration of the membrane fouling rate. Analysis through confocal laser scanning microscopy (CLSM) and membrane cleaning revealed that adding sponge carriers mitigated 3.3%-9% pore-blocking, and the total membrane resistance in the Hy-AnMBR was reduced by 52% compared to the Con-AnMBR. Chemical cleaning was essential for pollutant removal, and membrane permeability recovery was more than 97%.

Recent advances in attached growth membrane bioreactor systems for wastewater treatment

To tackle membrane fouling and limited removals of pollutants (nutrients and emerging pollutants) that hinder the wide applications of membrane bioreactor (MBR), attached growth MBR (AGMBR) combining MBR and attached growth process has been developed. This review comprehensively presents the up-to-date developments of media used in both aerobic and anaerobic AGMBRs for treating wastewaters containing conventional and emerging pollutants. It also elaborates the properties of different media, characteristics of attached biomass, and their contributions to AGMBR performance. Conventional media, such as biological activated carbon and polymeric carriers, induce formation of aerobic, anoxic and/or anaerobic microenvironment, increase specific surface area or porous space for biomass retention, improve microbial activities, and enrich diverse microorganisms, thereby enhancing pollutants removal. Meanwhile, new media (i.e. biochar, bioaugmented carriers with selected strain/mixed cultures) do not only eliminate conventional pollutants (i.e. high concentration of nitrogen, etc.), but also effectively remove emerging pollutants (i.e. micropollutants, nonylphenol, adsorbable organic halogens, etc.) by forming thick and dense biofilm, creating anoxic/anaerobic microenvironments inside the media, enriching special functional microorganisms and increasing activity of microorganisms. Additionally, media can improve sludge characteristics (i.e. less extracellular polymeric substances and soluble microbial products, larger floc size, better sludge settleability, etc.), alleviating membrane fouling. Future studies need to focus on the development and applications of more new functional media in removing wider spectrum of emerging pollutants and enhancing biogas generation, as well as scale-up of lab-scale AGMBRs to pilot or full-scale AGMBRs.

Biofilm formation as a method of improved treatment during anaerobic digestion of organic matter for biogas recovery

The efficiency of anaerobic digestion could be increased by promoting microbial retention through biofilm development. The inclusion of certain types of biofilm carriers has differentiated existing AD biofilm reactors through their respective mode of biofilm growth. Bacteria and archaea engaged in methanogenesis during anaerobic processes potentially build biofilms by adhering or attaching to biofilm carriers. Meta-analyzed results depicted varying degrees of biogas enhancement within AD biofilm reactors. Furthermore, different carrier materials highly induced the dynamicity of the dominant microbial population in each system. It is suggested that the promotion of surface contact and improvement of interspecies electron transport have greatly impacted the treatment results. Modern spectroscopy techniques have been and will continue to give essential information regarding biofilm's composition and structural organization which can be useful in elucidating the added function of this special layer of microbial cells.

Anaerobic Membrane Bioreactors for Municipal Wastewater Treatment: A Literature Review

Currently, there is growing scientific interest in the development of more economic, efficient and environmentally friendly municipal wastewater treatment technologies. Laboratory and pilot-scale surveys have revealed that the anaerobic membrane bioreactor (AnMBR) is a promising alternative for municipal wastewater treatment. Anaerobic membrane bioreactor technology combines the advantages of anaerobic processes and membrane technology. Membranes retain colloidal and suspended solids and provide complete solid–liquid separation. The slow-growing anaerobic microorganisms in the bioreactor degrade the soluble organic matter, producing biogas. The low amount of produced sludge and the production of biogas makes AnMBRs favorable over conventional biological treatment technologies. However, the AnMBR is not yet fully mature and challenging issues remain. This work focuses on fundamental aspects of AnMBRs in the treatment of municipal wastewater. The important parameters for AnMBR operation, such as pH, temperature, alkalinity, volatile fatty acids, organic loading rate, hydraulic retention time and solids retention time, are discussed. Moreover, through a comprehensive literature survey of recent applications from 2009 to 2021, the current state of AnMBR technology is assessed and its limitations are highlighted. Finally, the need for further laboratory, pilot- and full-scale research is addressed.

Production and beneficial impact of biochar for environmental application: A comprehensive review

This review focuses on a holistic view of biochar, production from feedstock's, engineering production strategies, its applications and future prospects. This article reveals a systematic emphasis on the continuation and development of biochar and its production methods such as Physical engineering, chemical and bio-engineering techniques. In addition, biochar alternatives such as nutrient formations and surface area made it a promising cheap source of carbon-based products such as anaerobic digestion, gasification, and pyrolysis, commercially available wastewater treatment, carbons, energy storage, microbial fuel cell electrodes, and super-capacitors repair have been reviewed. This paper also covers the knowledge blanks of strategies and ideas for the future in the field of engineering biochar production techniques and application as well as expand the technology used in the circular bio-economy.

Fouling Behavior in a High-Rate Anaerobic Submerged Membrane Bioreactor (AnMBR) for Palm Oil Mill Effluent (POME) Treatment

The characteristics of foulant in the cake layer and bulk suspended solids of a 10 L submerged anaerobic membrane bioreactor (AnMBR) used for treatment of palm oil mill effluent (POME) were investigated in this study. Three different organic loading rates (OLRs) were applied with prolonged sludge retention time throughout a long operation time (270 days). The organic foulant was characterized by biomass concentration and concentration of extracellular polymeric substances (EPS). The thicknesses of the cake layer and foulant were analyzed by confocal laser scanning microscopy and Fourier transform infrared spectroscopy. The membrane morphology and inorganic elements were analyzed by field emission scanning electron microscope coupled with energy dispersive X-ray spectrometer. Roughness of membrane was analyzed by atomic force microscopy. The results showed that the formation and accumulation of protein EPS in the cake layer was the key contributor to most of the fouling. The transmembrane pressure evolution showed that attachment, adsorption, and entrapment of protein EPS occurred in the membrane pores. In addition, the hydrophilic charge of proteins and polysaccharides influenced the adsorption mechanism. The composition of the feed (including hydroxyl group and fatty acid compounds) and microbial metabolic products (protein) significantly affected membrane fouling in the high-rate operation.

Addition of Trichocladium canadense to an anaerobic membrane bioreactor: evaluation of the microbial composition and reactor performance

Membrane bioreactors are powerful systems for wastewater treatment and the removal of toxic compounds. However, membrane biofouling stands in the way of their widespread usage. In this study, the saprophytic fungus Trichocladium canadense was used as the bioaugmentor in an anaerobic membrane bioreactor (AnMBR) and its impact on membrane biofouling, biogas production, the microbial communities of the reactor and removal of the common antibiotics erythromycin (ERY), sulfamethoxazole (SMX) and tetracycline (TET) from synthetic wastewater was investigated. The results indicated that through bioaugmentation with 20% T. canadense, membrane biofouling was slowed by 25%, the chemical oxygen demand removal increased by 16% and a higher efficiency removal of ERY and SMX was achieved. The presence of T. canadense significantly increased the abundance and diversity of the biofilm archaeal community and the bacterial phylum Firmicutes, a known bio-foulant.

A review on facilitating bio-wastes degradation and energy recovery efficiencies in anaerobic digestion systems with biochar amendment

In this review, progress in the potential mechanisms of biochar amendment for AD performance promotion was summarized. As adsorbents, biochar was beneficial for alleviating microbial toxicity, accelerating refractory substances degradation, and upgrading biogas quality. The buffering capacity of biochar balanced pH decreasing caused by volatile fatty acids accumulation. Moreover, biochar regulated microbial metabolism by boosting activities, mediating electron transfer between syntrophic partners, and enriching functional microbes. Recent studies also suggested biochar as potential useful additives for membrane fouling alleviation in anaerobic membrane bioreactors (AnMBR). By analyzing the reported performances based on different operation models or substrate types, debatable issues and associated research gaps of understanding the real role of biochar in AD were critically discussed. Accordingly, Future perspectives of developing biochar-amended AD technology for real-world applications were elucidated. Lastly, with biochar-amended AD as a core process, a novel integrated scheme was proposed towards high-efficient energy-resource recovery from various bio-wastes.

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.