Membrane distillation treatment of landfill leachate: Characteristics and mechanism of membrane fouling

Improving membrane distillation performance by Fe(II) activated sodium percarbonate oxidation during the treatment of shale gas produced water

Membrane distillation (MD) offers promise for recycling shale gas produced water (SGPW), while membrane fouling is still a major obstacle in standalone MD. Herein, sodium percarbonate (SPC) oxidation was proposed as MD pretreatment, and the performance of the single MD, SPC-MD hybrid process and Fe(II)/SPC-MD hybrid process for SGPW treatment were systematically evaluated. Results showed that compared to raw SGPW, the application of SPC and Fe(II)/SPC led to the decrease of the fluorescent organics by 28.54 % and 54.52 %, respectively. The hydrophobic fraction decreased from 52.75 % in raw SGPW to 37.70 % and 27.20 % for SPC and Fe(II)/SPC, respectively, and the MD normalized flux increased from 0.19 in treating raw SGPW to 0.65 and 0.81, respectively. The superiority of SPC oxidation in reducing the deposited membrane foulants and restoring membrane properties was further confirmed through scanning electron microscopy observation, attenuated total reflection fourier transform infrared, water contact angle and surface tension analyses of fouled membranes. Correlation analysis revealed that hydrophobic/hydrophilic matters and fluorescent organics in SGPW took a crucial role in MD fouling. The mechanism of MD fouling mitigation by Fe(II)/SPC oxidation was attributed to the decrease in concentrations and hydrophobicity of organic by synergistic oxidation, coagulation and adsorption.

An in-depth analysis of membrane distillation research (1990-2023): Exploring trends and future directions through bibliometric approach

This bibliometric analysis offers a comprehensive investigation into membrane distillation (MD) research from 1990 to 2023. Covering 4389 publications, the analysis sheds light on the evolution, trends, and future directions of the field. It delves into authorship patterns, publication trends, prominent journals, and global contributions to reveal collaborative networks, research hotspots, and emerging themes within MD research. The findings demonstrate extensive global participation, with esteemed journals such as Desalination and the Journal of Membrane Science serving as key platforms for disseminating cutting-edge research. The analysis further identifies crucial themes and concepts driving MD research, ranging from membrane properties to strategies for mitigating membrane fouling. Co-occurrence analysis further highlights the interconnectedness of research themes, showcasing advancements in materials, sustainable heating strategies, contaminant treatment, and resource management. Overlay co-occurrence analysis provides temporal perspective on emerging research trends, delineating six key topics that will likely shape the future of MD. These include innovations in materials and surface engineering, sustainable heating strategies, emerging contaminants treatment, sustainable water management, data-driven approaches, and sustainability assessments. Finally, the study serves as a roadmap for researchers and engineers navigating the dynamic landscape of MD research, offering insights into current trends and future trajectories, ultimately aiming to propel MD technology towards enhanced performance, sustainability, and global relevance.

Mineral scaling induced membrane wetting in membrane distillation for water treatment: Fundamental mechanism and mitigation strategies

The scaling-induced wetting phenomenon seriously affects the application of membrane distillation (MD) technology in hypersaline wastewater treatment. Unlike the large amount of researches on membrane scaling and membrane wetting, scaling-induced wetting is not sufficiently studied. In this work, the current research evolvement of scaling-induced wetting in MD was systematically summarized. Firstly, the theories involving scaling-induced wetting were discussed, including evaluation of scaling potential of specific solutions, classical and non-classical crystal nucleation and growth theories, observation and evolution of scaling-induced processes. Secondly, the primary pretreatment methods for alleviating scaling-induced wetting were discussed in detail, focusing on adding agents composed of coagulation, precipitation, oxidation, adsorption and scale inhibitors, filtration including granular filtration, membrane filtration and mesh filtration and application of external fields including sound, light, heat, electromagnetism, magnetism and aeration. Then, the roles of operation conditions and cleaning conditions in alleviating scaling-induced wetting were evaluated. The main operation parameters included temperature, flow rate, pressure, ultrasound, vibration and aeration, while different types of cleaning reagents, cleaning frequency and a series of assisted cleaning measures were summarized. Finally, the challenges and future needs in the application of nucleation theory to scaling-induced wetting, the speculation, monitoring and mitigation of scaling-induced wetting were proposed.

Fenton pretreatment to mitigate membrane distillation fouling during treatment of landfill leachate membrane concentrate: Performance and mechanism

Membrane distillation (MD) is regarded as a promising technology for treatment of landfill leachate membrane concentrate (LLMC) due to its merits of low cost and high rejection of non-volatile components. However, the high concentration of pollutants in the wastewater will cause severe membrane fouling, resulting in costly cleaning and maintenance. In this study, Fenton pretreatment was applied to alleviate membrane fouling during MD treatment of LLMC. Compared to rapid flux decline of 88.2% at concentration factor (CF) of 3 for raw LLMC, MD flux only decreased by 17.4% at CF = 6 for treating acidic Fenton effluent without subsequent pH adjustment (Fe2+ and H2O2 concentration were 600 mg/L and 1457 mg/L, respectively). The pH neutralization of Fenton effluent or merely acidification of LLMC could not achieve such excellent fouling mitigation. It was concluded that both oxidation and acidification were critical and the collaboration mechanism was revealed to explain low membrane fouling. Firstly, Fenton oxidation removed organic contaminants, reduced the hydrophobicity of organic substances and increased the percentage of carboxylic group within LLMC. Thus, hydrophobic (HP) attraction was weakened but multivalent cation bridging became dominant fouling mechanism for neutral Fenton effluent. Then, acidification weakened multivalent cation bridging by inhibiting the deprotonation of carboxylic group, further mitigating membrane fouling. However, acidification of LLMC caused more severe organic fouling due to decrease in electrostatic (EL) repulsion. In addition to low membrane fouling, satisfactory total organic carbon (TOC) rejection rate of 96.23% was achieved during combined Fenton-MD process. This study demonstrated that Fenton pretreatment without pH neutralization could effectively alleviate MD fouling and elucidated the synergistic mechanism between oxidation and acidification for fouling mitigation.

Artificial neural network implementation for dissolved organic carbon quantification using fluorescence intensity as a predictor in wastewater treatment plants

Although spectroscopic methods provide a fast and cost-effective means of monitoring dissolved organic carbon (DOC) in natural and engineered water systems, the prediction accuracy of these methods is limited by the complex relationship between optical properties and DOC concentration. In this study, we developed DOC prediction models using multiple linear/log-linear regression and feedforward artificial neural network (ANN) and investigated the effectiveness of spectroscopic properties, such as fluorescence intensity and UV absorption at 254 nm (UV₂₅₄), as predictors. Optimum predictors were identified based on correlation analysis to construct models using single and multiple predictors. We compared the peak-picking and parallel factor analysis (PARAFAC) methods for selecting appropriate fluorescence wavelengths. Both methods had similar prediction capability (p-values >0.05), suggesting PARAFAC was not necessary for choosing fluorescence predictors. Fluorescence peak T was identified as a more accurate predictor than UV₂₅₄. Combining UV₂₅₄ and multiple fluorescence peak intensities as predictors further improved the prediction capability of the models. The ANN models outperformed the linear/log-linear regression models with multiple predictors, achieving higher prediction accuracy (peak-picking: R² = 0.8978, RMSE = 0.3105 mg/L; PARAFAC: R² = 0.9079, RMSE = 0.2989 mg/L). These findings suggest the potential to develop a real-time DOC concentration sensor based on optical properties using an ANN for signal processing.

Simultaneous ammonium and water recovery from landfill leachate using an integrated two-stage membrane distillation

Resources recovery from landfill leachate (LFL) has been attracting growing attention instead of merely purifying the wastewater. An integrated two-stage membrane distillation (ITMD) was proposed to simultaneously purify LFL and recover ammonia in this study. The results showed that organics could be always effectively rejected by the ITMD regardless of varying feed pH, with COD removal higher than 99%. With feed pH increased from 8.64 to 12, the ammonia migration (50-100%) and capture (36-75%) in LFL were considerably enhanced, boosting the separated ammonia enrichment to 1.3-1.7 times due to the improved ammonium diffusion. However, the corresponding membrane flux of the first MD stage decreased from 13.7 to 10.5 L/m²·h. Elevating feed pH caused the deprotonation of NOM and its binding with inorganic ions, constituting a complex fouling layer on the membrane surface in the first MD stage. In contrast, the membrane permeability and fouling of the second MD were not affected by feed pH adjustment because only volatiles passed through the first MD. More importantly, it was estimated that ITMD could obtain high-quality water and recover high-purity ammonium from LFL with relatively low ammonium concentration at an input cost of $ 2-3/m³, which was very competitive with existing techniques. These results demonstrated that the ITMD can be a valuable candidate strategy for simultaneous water purification and nutrient recovery from landfill leachate.

Challenges and engineering application of landfill leachate concentrate treatment

Landfill leachate concentrate (LLC) is a concentrated waste stream from landfill leachate treatment systems and has been recognized as a key challenge due to its high concentration of salts, heavy metals, organic matters, etc. Improper management of LLC (e.g. reinjection) would exacerbate the performance of upstream treatment processes and pose risks to the surrounding environments near landfill sites. Addressing the challenge and recovering resources from LLC have thus been attracting considerable attention. Although many LLC treatment technologies have been developed, a comprehensive discussion about the challenges still lacks. This review critically evaluates mainstream LLC treatment technologies, namely incineration, coagulation, advanced oxidation, evaporation and solidification/stabilization. We then introduce a geopolymer-based solidification (GS) process as a promising technology owning to its simple casting process and reusable final product and summarizes engineering applications in China. Finally, we suggest investigating hybrid systems to minimize LLC production and achieve the on-site reuse of LLC. Collectively, this review provides useful information to guide the selection of LLC treatment technologies and suggests a sustainable alternative for large-scale application, while also highlighting the need of joint efforts in the industry to achieve efficient, ecofriendly and economical on-site management of landfill waste streams.

Novel Approach to Landfill Wastewater Treatment Fouling Mitigation: Air Gap Membrane Distillation with Tin Sulfide-Coated PTFE Membrane

This study addressed the fouling issue in membrane distillation (M.D.) technology, a promising method for water purification and wastewater reclamation. To enhance the anti-fouling properties of the M.D. membrane, a tin sulfide (TS) coating onto polytetrafluoroethylene (PTFE) was proposed and evaluated with air gap membrane distillation (AGMD) using landfill leachate wastewater at high recovery rates (80% and 90%). The presence of TS on the membrane surface was confirmed using various techniques, such as Field Emission Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared Spectroscopy (FT-IR), Energy Dispersive Spectroscopy (EDS), contact angle measurement, and porosity analysis. The results indicated the TS-PTFE membrane exhibited better anti-fouling properties than the pristine PTFE membrane, and its fouling factors (FFs) were 10.4-13.1% compared to 14.4-16.5% for the PTFE membrane. The fouling was attributed to pore blockage and cake formation of carbonous and nitrogenous compounds. The study also found that physical cleaning with deionized (DI) water effectively restored the water flux, with more than 97% recovered for the TS-PTFE membrane. Additionally, the TS-PTFE membrane showed better water flux and product quality at 55 °C and excellent stability in maintaining the contact angle over time compared to the PTFE membrane.

Fouling characterisation in PVDF membrane contactors for dissolved methane recovery from anaerobic effluents: effect of surface organofluorosilanisation

Characterisation of the fouling attached to PVDF membranes treating an anaerobic effluent for dissolved CH₄ recovery was carried out. A commercial flat-sheet PVDF membrane and a PVDF functionalised by grafting of organofluorosilanes (mPVDF) that increased its hydrophobicity were subjected to a continuous flux of an anaerobic reactor effluent in long-term operation tests (> 800 h). The fouling cakes were studied by the membrane autopsy after these tests, combining a staining technique, FTIR, and FESEM-EDX, and the fouling extraction with water and NaOH solutions. Both organic and inorganic fouling were observed, and the main foulants were proteins, polysaccharides, and different calcium and phosphate salts. Also, a significant amount of live cells was detected on the fouling cake (especially on the non-modified PVDF). Although the fouling cake composition was quite heterogeneous, a stratification was observed, with the inorganic fouling mainly in the bulk centre of the cake and the organic fouling mainly located in the lower and upper surfaces of the cake. The mPVDF suffered a more severe fouling, likely owing to a stronger hydrophobic-hydrophobic interaction with the foulants. Irreversible fouling remained on both membranes after the extraction, although a higher irreversible fouling was detected in the mPVDF; however, a complete polysaccharide removal was observed. Regarding the operation performance, PVDF showed a lower stability and suffered a severe degradation, resulting in a lower thickness and perforations. Finally, the decrease in the methane recovery performance of both membranes was associated with the fouling depositions.

Integrated membrane electrochemical reactor-membrane distillation process for enhanced landfill leachate treatment

Treatment of recalcitrant landfill leachate (LFL) induces huge energy consumption and carbon emissions due to its complex composition. Although membrane distillation (MD) exhibits good potential in LFL treatment with waste heat utilization, membrane fouling and ammonia rejection are still the major problems encountered that hinder its application. Herein, membrane electrochemical reactor (MER) was coupled with MD for simultaneous membrane fouling control and resource recovery. LFL pretreatment with membrane-less electrochemical reactor (EO) and without pretreatment were also purified by MD for comparison. Results showed that the MER-MD system rejected almost all COD_Cr, total phosphorus, metal salts, and ammonia nitrogen (increased by 33.5%-43.5% without chemical addition), and recovered 31% of ammonia nitrogen and 48% of humic acid in the raw LFL. Owing to the effective removal of hardness (61%) and organics (77%) using MER, the MER-MD system showed higher resistance to the membrane wetting and fouling, with about 61% and 14% higher final vapor flux than those of the MD and EO-MD systems, respectively, and the pure water flux could be fully recovered by alkaline solution cleaning. Moreover, SEM-EDS, ATR-FTIR and XRD characterization further demonstrated the superiority of the MD membrane fouling reversibility of the MER-MD system. Energy consumption and carbon emissions analysis showed that the MER-MD system reduced the total energy consumption/carbon emissions by ∼20% and ∼8% compared to the MD and EO-MD systems, respectively, and the ammonia nitrogen recovered by MER could offset 8.25 kg carbon dioxide equivalent. Therefore, the introduction of MER pretreatment in MD process would be an option to decrease energy consumption and reduce carbon emissions for MD treatment of LFL.

Fouling-free membrane stripping for ammonia recovery from real biogas slurry

Hydrophobic gas permeable membranes (GPMs) exhibit great potential in stripping or recovering ammonia from wastewater, but they also suffer from severe fouling issues due to the complex water matrix, since the related process is often operated under highly alkaline conditions (pH > 11). In this study, we proposed a novel membrane stripping process by integrating a cation exchange membrane (CEM) in alkali-driven Donnan dialysis prior to GPM for efficient and robust ammonia recovery from real biogas slurry. During the conventional stripping for diluted biogas slurry, the ammonia removal across GPM finally decreased by 15% over 6 consecutive batches, likely due to the obvious deposition of inorganic species and penetration of organic compounds (rejection of 90% only). In contrast, a constant ammonia removal of 80% and organic matter rejection of more than 99%, as well as negligible fouling of both membranes, were found for the proposed novel stripping process operated over 120 h. Our results demonstrated that additional divalent cations clearly aggravated the fouling of GPM in conventional stripping, where only weak competition across CEM was found in the CEM-GPM hybrid mode. Then, for raw biogas slurry, the new stripping achieved a stable ammonia removal up to 65%, and no fouling occurrence was found, superior to that in the control (declined removal from 87% to 55%). The antifouling mechanism by integrating CEM prior to GPM involves size exclusion and charge repulsion towards varying foulants. This work highlighted that the novel membrane stripping process of hybrid CEM-GPM significantly mitigated membrane fouling and can be regarded as a potential alternative for ammonia recovery from high-strength complex streams.

An assessment of the UV/nFe0 /H2 O2 system for the removal of refractory organics in the effluent produced by the biological treatment of landfill leachate

The removal efficiency and mechanism of the ultraviolet/nanoscale Fe⁰ /H₂ O₂ (UV/nFe⁰ /H₂ O₂ ) system for refractory organics in membrane bioreactor effluent were investigated. The most effective removal of organics was achieved at initial pH = 3.0, H₂ O₂ dosage = 50 mM, nFe⁰ dosage = 1.0 g/L, and UV power = 15 W, with a reaction time of 60 min. Under these conditions, the absorbance at 254 nm, chromaticity, and total organic carbon removal efficiencies were 65.13%, 79.67%, and 61.51%, respectively, and the aromaticity, humification, molecular weight, and polymerization of organics were all significantly reduced. The surface morphology and elemental valence analysis of nano zero-valent iron (nFe⁰ ) before and after the reaction revealed the formation of iron-based (hydrated) oxides, such as Fe₂ O₃ , Fe₃ O₄ , FeOOH, and Fe (OH)₃ , on the surface of the nFe⁰ . Refractory organics were removed by Fenton-like reactions in the homogeneous and heterogeneous adsorption-precipitation of iron-based colloids. At the same time, UV radiation accelerated the formation of Fe²⁺ on the nFe⁰ surface and promoted the Fe³⁺ /Fe²⁺ redox cycle to a certain extent, enhancing the removal of refractory organics. The results provide a theoretical basis for the application of the UV/nFe⁰ /H₂ O₂ system to remove refractory organics in the effluent produced by the biological treatment of landfill leachate. PRACTITIONER POINTS: The UV/nFe⁰ /H₂ O₂ process is effective in refractory organics removal in leachate treatment. Humus in leachate was largely destroyed and mineralized by the UV/nFe⁰ /H₂ O₂ process. Active nFe⁰ material participated in the Fenton-like process and was promoted by UV. The effects of nFe⁰ material and UV introduction were investigated.

Fouling and Performance Investigation of Membrane Distillation at Elevated Recoveries for Seawater Desalination and Wastewater Reclamation

This study reports on the impact of elevated recovery (i.e., 80%, 85%, and 90%) on the fouling and performance of air gap membrane distillation (AGMD) with real seawater and landfill leachate wastewater samples using polytetrafluoroethylene (PTFE) polymer membranes. Increasing the feed temperature from 55 °C to 65 °C improved the water flux of seawater and wastewater and shortened the operating time by 42.8% for all recoveries. The average water flux in the 80%, 85%, and 90% recovery experiments at the 65 °C feed temperature was 32%, 37.32%, and 36.7% higher than the case of 55 °C for the same recoveries. The water flux decline was more severe at a higher temperature and recovery. The highest flux decline was observed with a 90% recovery at 65 °C feed temperature, followed by an 85% recovery at 65 °C. Close examination of the foulants layer revealed that seawater formed a cake fouling layer made predominantly of metal oxides. In contrast, the landfill leachate fouling was a combination of pore blocking and cake formation, consisting mainly of carbonous and nitrogenous compounds. Physical cleaning with deionized (DI) water at 55 °C and 65 °C and chemical cleaning with hydrogen peroxide (H2O2) were investigated for their efficiency in removing membrane foulants. Analytical results revealed that seawater fouling caused membrane pore blockage while wastewater fouling formed a porous layer on the membrane surface. The results showed that membrane cleaning with hydrogen peroxide restored >97% of the water flux. Interestingly, the fouling factor in seawater tests was 10%, while it was 16% for the wastewater tests.

Chloride-Enhanced Removal of Ammonia Nitrogen and Organic Matter from Landfill Leachate by a Microwave/Peroxymonosulfate System

Landfill leachate contains not only high concentrations of refractory organic matter and ammonia nitrogen, but also high concentrations of chloride ions (Cl−). The modification of reactive species of the peroxymonosulfate (PMS) oxidation system by Cl− and its priority sequence for the removal of NH4+-N and organic matter from landfill leachate remain unclear. This study investigated the removal characteristics of NH4+-N and organic matter in the microwave (MW)/PMS system with high Cl− content. The results show that increasing Cl− concentration significantly improves the production of hypochlorous acid (HOCl) in the MW/PMS system under acidic conditions, and that the thermal and non-thermal effects of MW irradiation have an important influence on the HOCl produced by PMS activation. The maximum cumulative concentration of HOCl was 748.24 μM after a reaction time of 2 min. The formation paths of HOCl are (i) SO4•− formed by the MW/PMS system interacting with Cl− and HO•, and (ii) the nucleophilic addition reaction of PMS and Cl−. Moreover, the high concentration of HOCl produced by the system can not only remove NH4+-N in situ, but also interact with PMS to continuously generate Cl• as an oxidant to participate in the reaction with pollutants (e.g., NH4+-N and organic matter). Common aqueous substances (e.g., CO32−, HCO3−, NO3−, and humic acid) in landfill leachate will compete with NH4+-N for reactive species in the system, and will thereby inhibit its removal to a certain extent. It was found that when NH4+-N and leachate DOM co-exist in landfill leachates, they would compete for reactive species, and that humic acid-like matter was preferentially removed, leading to the retention of fulvic acid-like matter. It is hoped that this study will provide theoretical support for the design and optimization of methods for removing NH4+-N and organic matter from landfill leachate with high chloride ion content.

Chemical Cleaning and Membrane Aging in MBR for Textile Wastewater Treatment

Membrane bioreactors have been widely used in textile wastewater treatment. Intensive chemical cleaning is indispensable in the MBR for textile wastewater treatment due to the severe membrane fouling implied. This work investigated the aging of three different membranes, polyvinylidene fluoride (PVDF), polyether sulfone (PES), and polytetrafluoroethylene (PTFE), in the MBRs for textile wastewater treatment. Pilot-scale MBRs were operated and the used membrane was characterized. Batch chemical soaking tests were conducted to elucidate the aging properties of the membranes. The results indicated that the PVDF membrane was most liable to the chemical cleaning, and the PES and PTFE membranes were rather stable. The surface hydrophobicity of the PVDF increased in the acid aging test, and the pore size and pure water flux decreased due to the elevated hydrophobic effect; alkaline oxide aging destructed the structure of the PVDF membrane, enlarged pore size, and increased pure water flux. Chemical cleaning only altered the interfacial properties (hydrophobicity and surface zeta potential) of the PES and PTFE membranes. The fluoro-substitution and the dehydrofluorination of the PVDF, chain scission of the PES molecules, and dehydrofluorination of the PTFE were observed in aging. A chemically stable and anti-aging membrane would be of great importance in the MBR for textile wastewater treatment due to the intensive chemical cleaning applied.