Flux model for the membrane distillation process to treat wastewater: Effect of solids concentration

Breaking the Saturated Vapor Layer with a Thin Porous Membrane

The main idea of membrane distillation is to use a porous hydrophobic membrane as a barrier that isolates vapor from aqueous solutions. It is similar to the evaporation process from a free water surface but introduces solid-liquid interfaces and solid-vapor interfaces to a liquid-vapor interface. The transmembrane mass flux of a membrane-distillation process is affected by the membrane's intrinsic properties and the temperature gradient across the membrane. It is interesting and important to know whether the evaporation process of membrane distillation is faster or slower than that of a free-surface evaporation under the same conditions and know the capacity of the transmembrane mass flux of a membrane-distillation process. In this work, a set of proof-of-principle experiments with various water surface/membrane interfacial conditions is performed. The effect and mechanism of membrane-induced evaporation are investigated. Moreover, a practical engineering model is proposed based on mathematical fitting and audacious simplification, which reflects the capacity of transmembrane flux.

Optimal cleaning strategy to alleviate fouling in membrane distillation process to treat anaerobic digestate

This paper deals with the membrane fouling issue in the Direct Contact Membrane Distillation (DCMD) process treating a wasted sludge from an anaerobic digestion process. The main objective is to define an optimal cleaning strategy to alleviate fouling. Using a lab scale DCMD process, a cleaning strategy based on DI water flushing followed by 0.2% sodium hypochlorite (NaOCl) and 3% citric acid (C₆H₈O₇) cleaning was tested with different cleaning frequencies and various chemical cleaning durations at different cross-flow velocities. To avoid severe fouling, the optimal cross-flow velocity was found at 0.18 m/s (0.8 L/min). Moreover, even if higher cross-flow velocity allows higher flux, it could increase fouling risks. For a better membrane regeneration and process productivity, a cleaning of 60 min duration for each chemical cleaning applied every two days was defined as the optimal cleaning strategy. Such conditions allowed the preservation of 75.5% of the initial flux after 96 h of operation. Furthermore, the effect on membrane flux regeneration of DI water flushing, sodium hypochlorite, and citric acid cleaning registered were, 31.52%, 11.95% and 20.65%, respectively. This study revealed that in the MD process treating real wastewater both external and internal fouling are responsible of permeate flux decline due to the accumulation of organic and inorganic matter on the membrane surface as well as within the pores.

Organic Fouling Impact in a Direct Contact Membrane Distillation System Treating Wastewater: Experimental Observations and Modeling Approach

This study aims to investigate the effect of operational conditions on organic fouling occurring in a direct contact membrane distillation (DCMD) system used to treat wastewater. A mixed solution of sodium alginate (SA) and bovine serum albumin (BSA) was used as a feed solution to simulate polysaccharides and proteins, respectively, assumed as the main organic foulants. The permeate flux was observed at two feed temperatures 35 and 50 °C, as well as three feed solution pH 4, 6, and 8. Higher permeate flux was observed for higher feed temperature, which allows higher vapor pressure. At higher pH, a smaller particle size was detected with lower permeate flux. A mathematical model based on mass balance was developed to simulate permeate flux with time by assuming (i) the cake formation controlled by attachment and detachment of foulant materials and (ii) the increase in specific cake resistance, the function of the cake porosity, as the main mechanisms controlling membrane fouling to investigate the fouling mechanism responsible of permeate flux decline. The model fitted well with the experimental data with R² superior to 0.9. High specific cake resistance fostered by small particle size would be responsible for the low permeate flux observed at pH 8.

An Improved Modelling Approach for the Comprehensive Study of Direct Contact Membrane Distillation

Direct Contact Membrane Distillation (DCMD) is a promising and feasible technology for water desalination. Most of the models used to simulate DCMD are one-dimensional and/or use a linear function of vapour pressure which relies on experimentally determined parameters. In this study, the model of DCMD using Nusselt correlations was improved by coupling the continuity, momentum, and energy equations to better capture the downstream alteration of flow field properties. A logarithmic function of vapour pressure, which is independent from experiments, was used. This allowed us to analyse DCMD with different membrane properties. The results of our developed model were in good agreement with the DCMD experimental results, with less than 7% deviation. System performance metrics, including water flux, temperature, and concentration polarisation coefficient and thermal efficiency, were analysed by varying inlet feed and permeate temperature, inlet velocity, inlet feed concentration, channel length. In addition, twenty-two commercial membranes were analysed to obtain a real vision on the influence of membrane characteristics on system performance metrics. The results showed that the feed temperature had the most significant effect on water flux and thermal efficiency. The increased feed temperature enhanced the water flux and thermal efficiency; however, it caused more concentration and temperature polarisation. On the other hand, the increased inlet velocity was found to provide increased water flux and reduced temperature and concertation polarisation as well. It was also found that the membrane properties, especially thickness and porosity, can affect the DCMD performance significantly. A two-fold increase of feed temperature increased the water flux and thermal efficiency, 10-fold and 27%, respectively; however, it caused an increase in temperature and concertation polarisation, at 48% and 34%, respectively. By increasing Reynolds number from 80 to 1600, the water flux, CPC, and TPC enhanced by 2.3-fold, 2%, and 21%, respectively. The increased feed concentration from 0 to 250 [g/L] caused a 26% reduction in water flux. To capture the downstream alteration of flow properties, it was shown that the ratio of inlet value to outlet value of system performance metrics decreased significantly throughout the module. Therefore, improvement over the conventional model is undeniable, as the new model can assist in achieving optimal operation conditions.

Membrane processes

This literature review provides a review for publications in 2018 and 2019 and includes information membrane processes findings for municipal and industrial applications. This review is a subsection of the annual Water Environment Federation literature review for Treatment Systems section. The following topics are covered in this literature review: industrial wastewater and membrane. Bioreactor (MBR) configuration, membrane fouling, design, reuse, nutrient removal, operation, anaerobic membrane systems, microconstituents removal, membrane technology advances, and modeling. Other sub-sections of the Treatment Systems section that might relate to this literature review include the following: Biological Fixed-Film Systems, Activated Sludge, and Other Aerobic Suspended Culture Processes, Anaerobic Processes, and Water Reclamation and Reuse. This publication might also have related information on membrane processes: Industrial Wastes, Hazardous Wastes, and Fate and Effects of Pollutants.

Zero-liquid discharge (ZLD) technology for resource recovery from wastewater: A review

Water resources are becoming scarce meaning that reuse options are receiving more and more attention. In this perspective, zero-liquid discharge (ZLD) is considered as an emerging technique to minimize waste, recover resources, treat toxic industrial waste streams, and mitigate potential water quality impacts in receiving water streams. Although ZLD systems are capable of minimizing contamination of water sources and amplifying water supply, its industrial scale applications are restricted due to their high cost and intensive energy consumption. In ZLD systems, membrane-based technologies are an attractive future strategy for industrial wastewater reclamation. Therefore, this review examines why a greater focus on environmental protection and water security is leading to more widespread adoption of ZLD technology in various industries. We highlight existing ZLD processing schemes, including thermal and membrane-based processes, and discuss their limitations and potential solutions. We also investigated global application of ZLD systems for resource recovery from wastewater. Finally, we discuss the potential environmental impacts of ZLD technologies and provide some focus on future research needs.

Influence of dye class on the comparison of direct contact and vacuum membrane distillation applied to remediation of dyeing wastewater

This work investigated the influence of dye class on permeate flux and color rejection by comparing direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) applied to remediation of dyeing wastewater. The same operating system at the feed side was used and the driving force of each configuration was determined. Reactive and disperse dye solutions were considered, and a commercial membrane was employed. Final color rejection > 90.79% was obtained, and water was recovered at the permeate side (final normalized permeate flux up to 38.92 kg m^-2day^-1kPa^-1). VMD showed higher normalized permeate flux when compared to DCMD. However, the performance according to dye class depended on MD configuration. Reactive dye resulted in higher permeate flux than the disperse dye solution in DCMD. Contrarily, disperse dye solution showed higher permeate flux in VMD. The formation of a concentration boundary layer at the permeate membrane interface was suggested with disperse dye solution in DCMD, decreasing thus the driving force. In VMD, the boundary effect is negligible with disperse dye solution. This result implies that the VMD performance in the textile industry may depend more on driving force rather than the dye class of the dyeing bath.