Theoretical and experimental study of organic fouling of loose nanofiltration membrane

A review of flow field characteristics in submerged hollow fiber membrane bioreactor: Micro-interface, module and reactor

As an important part of the membrane field, hollow fiber membranes (HFM) have been widely concerned by scholars. HFM fouling in the industrial application results in a reduction in its lifespan and an increase in cost. In recent years, various explorations on the HFM fouling control strategies have been carried out. In the current work, we critically review the influence of flow field characteristics in HFM-based bioreactor on membrane fouling control. The flow field characteristics mainly refer to the spatial and temporal variation of the related physical parameters. In the HFM field, the physical parameter mainly refers to the variation characteristics of the shear force, flow velocity and turbulence caused by hydraulics. The factors affecting the flow field characteristics will be discussed from three levels: the micro-flow field near the interface of membrane (micro-interface), the flow field around the membrane module and the reactor design related to flow field, which involves surface morphology, crossflow, aeration, fiber packing density, membrane vibration, structural design and other related parameters. The study of flow field characteristics and influencing factors in the HFM separation process will help to improve the performance of HFM in full-scale water treatment plants.

Towards effective recovery of humate as green fertilizer from landfill leachate concentrate by electro-neutral nanofiltration membrane

Under the environmental sustainability concept, landfill leachate concentrate can be up-cycled as a useful resource. Practical strategy for effective management of landfill leachate concentrate is to recover the existing humate as fertilizer purpose for plant growth. Herein, we designed an electro-neutral nanofiltration membrane to separate the humate and inorganic salts for achieving a sufficient humate recovery from leachate concentrate. The electro-neutral nanofiltration membrane yielded a high retention of humate (96.54 %) with an extremely low salt rejection (3.47 %), tremendously outperforming the state-of-the-art nanofiltration membranes and exhibiting superior promise in fractionation of humate and inorganic salts. With implementation of the pressure-driven concentration process, the electro-neutral nanofiltration membrane enriched the humate from 1756 to 51,466 mg∙L-1 at a fold of 32.6, enabling 90.0 % humate recovery and 96.4 % desalination efficiency from landfill leachate concentrate. Furthermore, the recovered humate not only exerted no phytotoxicity, but also significantly promoted the metabolism of red bean plants, serving as an effective green fertilizer. The study provides a conceptual and technical platform using high-performance electro-neutral nanofiltration membranes to extract the humate as a promising nutrient for fertilizer application, in view of sustainable landfill leachate concentrate treatment.

Differences in the Effect of Mn2+ on the Reverse Osmosis Membrane Fouling Caused by Different Types of Organic Matter: Experimental and Density Functional Theory Evidence

Landfill leachate from some sites contains a high concentration of Mn2+, which may cause reverse osmosis (RO) membrane fouling during RO treatment. In this study, the effect of Mn2+ on RO membrane fouling caused by typical organic pollutants (humic acid (HA), protein (BSA), and sodium alginate (SA)) was systematically investigated, and it was found that Mn2+ exacerbates RO membrane fouling caused by HA, SA, and HBS (mixture of HA + BSA + SA). When the Mn2+ concentration was 0.5 mM and 0.05 mM separately, the membrane fouling caused by HA and SA began to become significant. On the other hand, with for HBS fouling only, the water flux decreased significantly by about 21.7% and further decreased with an increasing Mn2+ concentration. However, Mn2+ has no direct effect on BSA. The effect degrees to which Mn2+ affected RO membrane fouling can be expressed as follows: HBS > SA > HA > BSA. The density functional theory (DFT) calculations also gave the same results. In modeling the reaction of the complexation of Mn2+ with the carboxyl group in these four types of organic matter, BSA has the highest energy (-55.7 kJ/mol), which predicts that BSA binding to Mn2+ is the most unstable compared to other organic matter. The BSA carboxylate group also has the largest bond length (2.538-2.574 Å) with Mn2+ and the weakest interaction force, which provides a theoretical basis for controlling RO membrane fouling exacerbated by Mn2+.

Confining Nano-Fe3O4 in the Superhydrophilic Membrane Skin Layer to Minimize Internal Fouling

Membrane surface fouling is often reversible as it can be mitigated by enhancing the crossflow shear force. However, membrane internal fouling is often irreversible and thus more challenging. In this study, we developed a new superhydrophilic poly(vinylidene fluoride) (P-PVDF) membrane confined with nano-Fe₃O₄ in the top skin layer via reverse filtration to reduce internal fouling. The surface of the P-PVDF membrane confined with nano-Fe₃O₄ had superwetting properties (water contact angle reaching 0° within 1 s), increased roughness (from 182 to 239 nm), and enhanced water affinity. The Fe₃O₄@P-PVDF membrane surface showed a thicker and enhanced hydration layer, which prevented foulants from approaching membrane surfaces and pores, thereby improving the rejection. For example, when 50 ppm humic acid (HA) solution was used as the feed, the removal efficiency of the Fe₃O₄@P-PVDF membrane was ∼67%, while the HA removal of the P-PVDF membrane was only ∼20%. The results from the resistance-in-series model showed that nanoconfinement of Fe₃O₄ in the top skin layer of the membrane allowed foulants to accumulate on the membrane surface (i.e., surface fouling) rather than within the internal pores (i.e., internal fouling). The filtration results under crossflow fouling and cleaning confirmed that the Fe₃O₄@P-PVDF membrane had higher surface fouling but it was much more reversible and much lower internal fouling compared with the control membrane. Our fouling analysis offers new insights into mass transfer mechanisms of the membrane with a nanoconfinement-enhanced hydration layer. This study provides an effective strategy to develop membranes with low internal fouling propensities.

Integrated loose nanofiltration-electrodialysis process for sustainable resource extraction from high-salinity textile wastewater

Effective extraction of useful resources from high-salinity textile wastewater is a critical pathway for sustainable wastewater management. In this study, an integrated loose nanofiltration-electrodialysis process was explored for simultaneous recovery of dyes, NaCl and pure water from high-salinity textile wastewater, thus closing the material loop and minimizing waste emission. Specifically, a loose nanofiltration membrane (molecular weight cutoff of ~800 Da) was proposed to fractionate the dye and NaCl in the high-salinity textile wastewater. Through a nanofiltration-diafiltration unit, including a pre-concentration stage and a constant-volume diafiltration stage, the dye could be recovered from the high-salinity textile wastewater, being enriched at a factor of ~9.0, i.e., from 2.01 to 17.9 g·L^-1 with 98.4% purity. Assisted with the subsequent implementation of electrodialysis, the NaCl concentrate and pure water were effectively reclaimed from the salt-containing permeate coming from the loose nanofiltration-diafiltration. Simultaneously, the produced pure water was further recycled to the nanofiltration-diafiltration unit. This study shows the potential of the integration of loose nanofiltation-diafiltration with electrodialysis for sufficient resource extraction from high-salinity textile wastewater.

Study of Turbulence Promoters in Prolonging Membrane Life

Nanofiltration membrane technology is an effective method for secondary treated sewage purification. However, membrane fouling, which is inevitable in the membrane-separation process, can reduce membrane performance and shorten membrane life. Installing a turbulence promoter is a promising means of improving the hydraulic conditions inside the membrane chamber. In this study, the effect of turbulence promoter on prolonging membrane life was studied for the first time. Flat-sheet polyethersulfone nanofiltration membrane was used to filter humic acid solution, used for simulating secondary treated sewage. By comparing photographs and SEM images of the membrane before and after the simulated secondary treated sewage filtration, it was found that humic acid tended to be deposited on the low-velocity region, which was reflected by COMSOL simulation. After incorporating a turbulence promoter, the reduction of the humic acid deposition area and membrane fouling resistance indicated that the turbulence promoter could reduce membrane fouling due to the improved hydraulic conditions. Additionally, the turbulence promoter also increased the flux and reduced the flux decay rate. The turbulence promoter was then place in the crossflow flat-sheet membrane filtration module, and the variation of flux with time was tested in simulated secondary treated sewage with different concentrations. The results showed that the membrane life for the filtration of simulated secondary treated sewage comprising 50, 250, and 500 ppm humic acid increased by 23.1%, 80.4%, and 85.7%, respectively. The results of this article can serve as a reference for the prediction of membrane life and the performance enhancement mechanism of membranes containing a turbulence promoter.

Electrically Polarized Graphene-Blended Spacers for Organic Fouling Reduction in Forward Osmosis

In membrane processes, a spacer is known to play a key role in the mitigation of membrane fouling. In this study, the effect of electric polarization on a graphene-blended polymer spacer (e.g., poly(lactic acid), PLA) for organic fouling on membrane surfaces was investigated. A pristine PLA spacer (P-S), a graphene-blended spacer (G-S), and an electrically polarized graphene-blended spacer (EG-S) were successfully fabricated by 3D printing. Organic fouling tests were conducted by the 5-h filtration of CaCl₂ and a sodium alginate solution through commercially available membranes, which were placed together with the fabricated spacers. Membranes utilizing P-S, G-S, and EG-S were characterized in terms of the fouling amount on the membrane surface and fouling roughness. Electrostatic forces of EG-S provided 70% less and 90% smoother fouling on the membrane surface, leading to an only 14% less water flux reduction after 5 h of fouling. The importance of nanomaterial blending and polarization was successfully demonstrated herein.

Biomimetic dynamic membrane (BDM): Fabrication method and roles of carriers and laccase

Biomimetic dynamic membrane (BDM) has been employed as a promising membrane separation technology regarding water/wastewater treatment (Model pollutant is methylene blue). Given its catalytic function on micro-pollutant removal and fouling control, detailed mechanism for impacts of fabrication method, carriers (CNT and GO) and laccase on the construction of biomimetic layer and enzyme immobilization have not been clear so far. In this work, the BDM performance with various fabrication methods, carriers and laccase were investigated and verified. The BDM fabrication tests demonstrated that BDM with mixed filtration method had better filtration performance (up to 120 L m^-2 h^-1 flux and 80% removal rate) than BDM with stepwise filtration method. Moreover, the laccases immobilized on GO exhibited a stronger laccase activity than those on CNT. Increasing CNT or GO dosage strengthened removal rate, but lowered flux, meanwhile flux and removal rate exhibited a significant fluctuation with certain laccase dosage. At 25 g m^-2 CNT or GO dosage and 50 g m^-2 laccase dosage, the optimized flux and removal rate values were obtained. Further study investigated the surface morphology and property of BDM, showing that BDM with mixed filtration method turned out to be the optimized enzyme immobilization mechanism and fabrication method. In addition, during multiple filtration cycles, with the optimized conditions, the removal rate, flux and laccase activity of BDM could maintain at high levels. On account of the finding of the present study, selecting a suitable fabrication method, appropriate CNT or GO dosage and laccase dosage can indeed optimize the structure of biomimetic layer and enzyme immobilization, expanding its possibility on sustainable operation.

Physico-chemical processes

The review scans research articles published in 2018 on physico-chemical processes for water and wastewater treatment. The paper includes eight sections, that is, membrane technology, granular filtration, flotation, adsorption, coagulation/flocculation, capacitive deionization, ion exchange, and oxidation. The membrane technology section further divides into six parts, including microfiltration, ultrafiltration, nanofiltration, reverse osmosis/forward osmosis, and membrane distillation. PRACTITIONER POINTS: Totally 266 articles on water and wastewater treatment have been scanned; The review is sectioned into 8 major parts;  Membrane technology has drawn the widest attention from the research community.