Direct Contact Membrane Distillation Operation Research Articles

Process intensification in the direct contact membrane distillation (DCMD) desalination by patterning membrane surface: A CFD study

In this research, the impact of pattern geometry on the intensification of the performance of surface patterned membranes for saline water desalination by direct contact membrane distillation (DCMD) is investigated by computational fluid dynamics (CFD) simulation. The Comsol Multiphysics software was applied to solve the governing transport equations for heat, momentum, and mass transfer. The result of the model was validated by the published experimental data, and the maximum deviation was less than 10%. The target was to maximize the permeate flux by altering surface pattern geometry and dimensions. Based on the previous studies, a prism pattern was chosen in this work, and the influences of pattern type (3 types), pattern dimension (25-150 µm valley depth), and the distance between the valleys (0-400 µm) were studied on the temperature polarization coefficient (TPC) and DCMD permeate flux. The results showed that the pattern with a valley depth of 25 µm and a distance between the valleys of 300 µm had the best performance in DCMD operation. In this situation and feed temperature of 80°C, a TPC of 0.78 and a water flux of 49.3 kg/m2.h were attained. The characteristics of flow close to the patterned membrane surface were also investigated, and it was observed that there are weak shear stresses in the lower zone of the valleys, while stronger shear stresses are created in the upper regions that are responsible for improving the TPC and water flux in the patterned membranes.

Surface functionalization of polytetrafluoroethylene membrane by polydopamine and quaternary ammonium compound for fouling mitigation in membrane distillation of secondary effluent

Considering the worldwide water scarcity issue, membrane distillation (MD) offers a promising solution for producing reclaimed water from secondary effluent generated by municipal wastewater treatment plants. However, prolonged operation of MD systems can result in membrane fouling, which impairs separation efficiency and shortens the membrane's lifespan. To overcome these challenges, novel surface-modified polytetrafluoroethylene (PTFE) membranes have been developed through a combination of polydopamine deposition and quaternary ammonium compound (QAC) grafting (named as PDQ membranes). The objectives of this preparation are to reduce the membrane hydrophobicity, impart anti-adhesive and bactericidal properties, and improve water permeability and antifouling performance in the MD system. In particular, the membrane coated by a QAC solution with the concentration of 40 mM (named as M3 membrane), achieved a flux of 47.0 LMH, surpassing the original PTFE membrane by 11.4%. During the long-term direct contact membrane distillation operation with the secondary effluent, the M3 membrane exhibited significantly superior antifouling capability than the PTFE membrane. Undergoing three runs of the MD process, the flux reduction of M3 was only 38.1%, whereas the flux of original PTFE membrane experienced a 71.4% decline. The PDQ membranes were also capable of providing stable distillate quality (over 40 h). Notably, the M3 demonstrated a high rejection capability, with an impressive removal efficiency of 96.9% for low molecular weight organics and 92.5% for humus-like substances. Additionally, the PDQ membrane exhibited good anti-bioadhesion and bactericidal properties. This study highlights the promising application potential of the PDQ membrane for fouling mitigation in water and wastewater treatment.

Engineered eco-friendly composite membranes with superhydrophobic/hydrophilic dual-layer for DCMD system

Considerable advancements have been made in the development of hydrophobic membranes for membrane distillation (MD). Nonetheless, the environmentally responsible disposal of these membranes poses a critical concern due to their synthetic composition. Herein, an eco-friendly dual-layered biopolymer-based membrane was fabricated for water desalination. The membrane was electrospun from two bio-polymeric layers. The top hydrophobic layer comprises polycaprolactone (PCL) and the bottom hydrophilic layer from cellulose acetate (CA). Additionally, silica nanoparticles (SiO2 NPs) were electrosprayed onto the top layer of the dual-layered PCL/CA membrane to enhance the hydrophobicity. The desalination performance of the modified PCL-SiO2/CA membrane was compared with the unmodified PCL/CA membrane using a direct contact membrane distillation (DCMD) unit. Results revealed that silica remarkably improves membrane hydrophobicity. The modified PCL-SiO2/CA membrane demonstrated a significant increase in water contact angle of 152.4° compared to 119° for the unmodified membrane. In addition, PCL-SiO2/CA membrane has a smaller average pore size of 0.23 ± 0.16 μm and an exceptional liquid entry pressure of water (LEPw), which is 3.8 times higher than that of PCL/CA membrane. Moreover, PCL-SiO2/CA membrane achieved a durable permeate flux of 15.6 kg/m2.h, while PCL/CA membrane showed unstable permeate flux decreasing approximately from 25 to 12 kg/m2.h over the DCMD test time. Furthermore, the modified PCL-SiO2/CA membrane achieved a high salt rejection value of 99.97% compared to a low value of 86.2% for the PCL/CA membrane after 24 h continuous DCMD operation. In conclusion, the proposed modified PCL-SiO2/CA dual-layer biopolymeric-based membrane has considerable potential to be used as an environmentally friendly membrane for the MD process.

Clean-In-Place (CIP) wastewater management using nanofiltration (NF)-forward osmosis (FO)-direct contact membrane distillation (DCMD): Effects of draw salt

A substantial amount of water is being used during Clean-in-Place (CIP) operation, and is transformed into wastewater that can cause eutrophication to the nearby ecosystem. The present study proposed the Nanofiltration (NF) – Forward Osmosis (FO) – Direct Contact Membrane Distillation (DCMD) to recover the cleaning agents and reclaim freshwater from the model CIP wastewater. NF steps were suggested as prefiltration steps to remove organic compounds from the CIP wastewater. NF steps reduced the lactose and protein contents by 100 % and 95.6 %, respectively. The permeates from NF steps were further managed by the integrated FO-DCMD system. Several draw salts such as NaCl, KCl, MgCl2, and CaCl2 were compared to investigate the influence on FO and DCMD performance. It was found that monovalent salts (NaCl and KCl) outperformed the divalent salts (MgCl2 and CaCl2) in terms of water flux for both FO and DCMD. This can be attributed to the lower viscosity and higher mass transfer coefficient. In addition, the replenishment costs of each salt were evaluated since salts loss occurred during FO and DCMD operation. The cost evaluation revealed that NaCl is most the cheapest salts per reclaimed water. All of this observation indicates that NaCl is preferred in terms of water flux and replenishment cost. The NF permeate kept concentrated using the integrated FO-DCMD or single FO with 2 M of NaCl. Compared to a single FO that showed a consistent decline in draw solution concentration, FO-DCMD could maintain the concentration of the draw solution. Despite the constant concentration, flux decline of FO was observed due to fouling formation caused by the high-temperature operation. However, the FO-DCMD could accomplish the recovery of pure water. Finally, the cleaning agents recovered by the NF-FO-DCMD showed the cleaning efficacy comparable to the fresh NaOH. These results suggest the potential of the proposed system to manage the CIP wastewater.

Fabrication and characterization of hydrophobic/hydrophilic dual-layer polyphenyl sulfone Janus membrane for application in direct contact membrane distillation

In this study, single-layer (SL) and dual-layer (DL) polyphenyl sulfone (PPSU) flat sheet nanocomposite Janus membranes were fabricated via a combination of VIPS and NIPS techniques for direct contact membrane distillation (DCMD) for the first time. Hydrophobic silica nanoparticles (SiNP-B) were incorporated into the SL membranes and the top layer of the DL membranes, while hydrophilic silica nanoparticles (SiNP-L) were added into the down layer of the DL membranes. The effect of the SiNP-B concentration (1–5 wt%) and the top layer thickness (50 and 100 μm) on the morphology, mean pore size (MPS), thickness, porosity, surface roughness, water contact angle (WCA), liquid entry pressure, mechanical strength, and thermal conductivity were investigated. All of the membranes showed a finger-like structure surrounded by a sponge-like structure. The membranes were examined in the DCMD of a 35,000 ppm saline solution at different feed temperatures (50–70 °C) and flow rates (250–1000 mL/min). The DL membranes possessed higher fluxes than the SL membranes through increasing MPS and WCA. The DL membrane with a SiNP-B concentration of 2 wt% and a hydrophobic thickness of 50 μm (DL- 2,50) achieved the highest flux (42.68 ± 0.35 kg/m2h) and salt rejection (94.90 %) during 5 h. This membrane has an MPS of 0.22 ± 0.03 μm, a thickness of 201 ± 4 μm, and a porosity of 76 ± 2 %. It could be concluded the proper pore size distribution is the most important factor in achieving a successful DCMD operation.

Recovery of cleaning agents from Clean-In-Place (CIP) wastewater using nanofiltration (NF) and direct contact membrane distillation (DCMD)

Increasing concerns about freshwater sources necessitate the management of wastewater, such as the wastewater generated from Clean-in-Place (CIP) operations. In this investigation, a membrane system composed of nanofiltration (NF) and direct contact membrane distillation (DCMD) was proposed to manage model dairy CIP wastewater that contained NaOH as an alkaline cleaning agent. During the NF step, prefiltration by a 4 kDa membrane or a 4 kDa membrane followed by a 200 Da membrane (4 kDa/200 Da) was used to remove the whey protein and lactose. With these two membranes in series of NF, the protein concentration was reduced by 92.4% and the lactose content was reduced to a non-detectable level when compared to the model CIP wastewater. Before concentrating the permeates from NF steps, three DCMD membranes (FR, Solupor, and ST) with different characteristics were evaluated to manage the NF permeates from 4 kDa or 200 Da NF. An increase in the feed temperature from 40 °C to 60 °C resulted in an increase in the water flux during DCMD operation, except for FR. In addition, it was found that ST generated the highest water flux when compared to the other membranes. Using ST and a feed temperature of 60 °C, the permeates from 4 kDa or 4 kDa/200 Da were continuously concentrated for 7 h with DCMD. During this concentration, there was no significant decline in flux. The cleaning effectiveness of the cleaning agent (NaOH) recovered by NF and DCMD was compared with a fresh cleaning solution using quartz crystal microbalance with dissipation (QCM-D). It was found that the cleaning agents recovered by 4 kDa/200 Da NF presented a statistically identical cleaning rate compared to fresh NaOH. This research highlights the potential of NF and DCMD to regenerate alkaline cleaning agents, while reclaiming water from dairy CIP wastewater.

Development and optimization of low surface free energy of rGO-PVDF mixed matrix membrane for membrane distillation

In this research work, mixed matrix membranes were fabricated by synthesising and blending reduced graphene oxide (rGO) in polyvinylidene fluoride (PVDF) membrane via a facile phase inversion process. Low and high loadings of rGO were incorporated into the PVDF membrane matrix to study its physicochemical properties which could be analysed using field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX), surface analyser and atomic force microscopes (AFM). All fabricated mixed matrix membranes exhibit low surface free energy (SFE) properties compared with the pristine membrane which is suitable for the application in membrane distillation (MD) system. Besides, high rGO loadings in PVDF membrane resulted in a smaller membrane pore size and rougher membrane surface. The successful blend-in of rGO in PVDF membrane could be further confirmed with FESEM, EDX, SFE and AFM results. The performance of fabricated membranes was evaluated with a prolonged desalination process via direct contact membrane distillation (DCMD). It was found that low rGO-PVDF mixed matrix membranes exhibit a better permeate flux of 31.92 ± 2.85 L/m2.hr which has 31.79 % increase in flux compared with pristine membrane. All membranes have 99.99 ± 0.05 % of salt rejection for 40 h of DCMD operation. Membranes after the desalination process were analysed and it was found that the rGO is stable in the membrane matrix and low loadings of rGO in PVDF membrane could improve MD desalination performance. Besides, optimization of rGO loadings using response surface methods (RSM) was employed and the result showed that 1.36 wt% of rGO-loading in PVDF membrane matrix could result in 34.69 ± 2.74 L/m2.hr of permeate flux.

Influences of Permeate Solution and Feed pH on Enhancement of Ammonia Recovery from Wastewater by Negatively Charged PTFE Membranes in Direct Contact Membrane Distillation Operation.

This research investigated the feasibility of enhancing ammonia recovery from wastewater using a negatively charged poly(tetrafluoroethylene) (PTFE) membrane in a direct contact membrane distillation (DCMD) system. The influences of phosphate solution types (as the permeate solutions) and feed pH on ammonia recovery were analyzed. Three types of permeate solutions—DI water and two types of phosphate solutions (H3PO4 and KH2PO4)—were investigated for recovery of ammonia gas on the permeate side. From the obtained results, the H3PO4 solution was found to be the most suitable permeate solution to recover ammonia gas in the DCMD operation with the highest overall ammonia mass transfer coefficient of 7.4 × 10–5 m/s, compared to values of 1.2 × 10–5 and 2.4 × 10–5 m/s for DI water and KH2PO4 solution, respectively. Moreover, an increase in the H3PO4 concentration from 0.3 to 0.5 M in the permeate solution also could significantly enhance ammonia recovery. With an increase in the feed pH from 10.0 to 11.8, the ammonia recovery could be enhanced to 92.98% at a pH of 11.8. Liquid ammonium phosphate fertilizer could be produced by the DCMD system with the use of 0.5 M H3PO4 solution. Therefore, the DCMD process using a negatively charged PTFE membrane with an appropriate permeate solution is one of the challenging processes for ammonia recovery from wastewater to promote the circular economy concept.

Omniphobic surface modification of silica sand ceramic hollow fiber membrane for desalination via direct contact membrane distillation

Omniphobic silica sand ceramic hollow fiber membrane (SCHFM) was successfully fabricated for the reduction of the wetting and fouling in direct contact membrane distillation (DCMD). Surface functionalization of the laboratory spun SCHFM was performed following a series of steps 1) deposition of spherical silica nanoparticles (SiNPs), 2) fluorination by 1H,1H,2H,2H perfluorodecyltriethoxysilane (FAS17) and 3) poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) coating to glue SiNPs to the SCHFM surface. Initially, the pristine SCHFM was hydrophilic, but when the surface modification was fully applied the hollow fiber became omniphobic, exhibiting high contact angles for all tested liquids of different surface tensions, such as distilled water, red palm oil and ethanol. For example, one of the surface modified hollow fibers, called SiNPs-FAS-PVDF/FAS, was superomniphobic toward water (167°) and red palm oil (157°), and near superomniphobic toward ethanol (146.1°). As for the DCMD experiments with the feed solution containing 35 g/L NaCl and 10 mg/L humic acid, two of the surface modified hollow fibers, called SiNPs-PVDF/FAS and SiNPs-FAS-PVDF/FAS, showed high anti-fouling and anti-wetting resistance during the entire DCMD operation period of 400 min with little flux reduction and no observable increase in the conductivity of the permeate stream. The anti-fouling resistance of these hollow fibers was further evidenced by the cleanness of the surface in the micrographic picture taken after the DCMD run. The omniphobicity of these hollow fibers is ascribed to the combined effects of surface tension decrease by the FAS17 fluorination and the formation of the re-entrant structure by the densely packed SiNPs. With reasonably high fluxes of 35.1 kg/m2∙h and 43.6 kg/m2∙h, for SiNPs-PVDF/FAS and SiNPs-FAS-PVDF/FAS, respectively, and 100% of salt rejection, these hollow fibers have potential for seawater desalination by DCMD.

Study of a poly(vinylidene fluoride)/hydrophobic silica sol hybrid hollow fiber membrane for treatment of produced water via direct contact membrane distillation

Direct contact membrane distillation (DCMD) has shown great promises in remediating high-salinity oilfield produced water (PW). In our previous work, a hydrophobic silica sol (Si-R) was successfully synthesized to engineer anti-wetting ability of poly(vinylidene fluoride) (PVDF) membrane. Here our research interest lies in customizing PVDF hollow fiber membrane (HFM) with desirable stability for treating challenging oilfield PW via a DCMD process. The PVDF/Si-R hybrid HFM was fabricated under an optimized spinning condition. It is found that the fabricated PVDF/Si-R HFM is sandwich-structured, consisting a highly porous and thick middle layer and small-size macrovoids of outer layer and inner layer with different pore geometry, and the unique sandwich structure endows the membrane with superior wetting resistance. The membrane characterization showed that the liquid entry pressure (LEP) and water contact angle (WCA) of the fabricated PVDF/Si-R HFM impressively reached up to 5.25 bar and 126°, respectively. As a result, the PVDF/Si-R HFM possessed a desirable performance in treating high-salinity oilfield PW in terms of wetting resistance and salt rejection. Specifically, with a 60 h continuous DCMD operation, the PVDF/Si-R membrane exhibited over 99.9% salt rejection with only 7% reduction in permeate water flux. Clearly, the synthesized hydrophobic Si-R is a favorable additive for enhancing the antiwetting ability of PVDF HFM to treat high-salinity oilfield PW via DCMD process.

Crosslinked PVDF based hydrophilic-hydrophobic dual-layer hollow fiber membranes for direct contact membrane distillation desalination: from the seawater to oilfield produced water

Crosslinked polyvinylidene fluoride (PVDF) based hydrophilic-hydrophobic dual-layer hollow fiber membranes were fabricated for direct contact membrane distillation (DCMD). PVDF/ammonia/water dope solution was formulated with a spinning process delay of 9 days for the formation of a mechanically robust and hydrophobic crosslinked PVDF outer layer, and polyethylene glycol 6000 (PEG-6000) was introduced to create a thick hydrophilic PVDF/PEG-6000 inner layer. The influences of ammonia/water contents and the spinning process delay on the membrane morphology were studied. The DCMD performances were investigated using both simulated seawater and actual oilfield produced water as the feed solutions. The incorporation of ammonia/water mixture allowed a slow PVDF crystallization in the dope solution during the 9 days of spinning process delay. The obtained membrane DN2H2-9 showed contact angles of 133.6° and 47.1°on the surfaces of the crosslinked PVDF outer layer and PVDF/PEG-6000 inner layer, respectively. The membrane DN2H2-9 showed a superior permeate water flux of 97.6 kg m−2 h−1 and an energy efficiency of 92.8% to desalinate a 3.5% NaCl solution. The result from a 200 h continuous DCMD operation revealed a stable permeate water flux and more than 99.9% of salt rejection, which was attributed to a relatively high liquid entry pressure (LEP) of 1.95 bar and the simultaneously enhanced mechanical strength of the membrane. The membrane DN2H2-9 also demonstrated promising DCMD performance in the desalination of the real oilfield produced water with a high total dissolved solids (TDS), including almost 100% of permeate water recovery and more than 99.9% of salt rejection in 72 h of operation.

Engineering hierarchically structured superhydrophobic PTFE/POSS nanofibrous membranes for membrane distillation

Membrane distillation (MD) is an attractive thermally-driven membrane process for desalination, brine and wastewater treatment. One of the main obstacles impeding MD application is the absence of effective MD membranes with sufficient water permeability and long-term stability. Attributed to the outstanding hydrophobicity, excellent thermal and chemical resistance, the most ideal polymer to develop MD membrane is polytetrafluoroethylene (PTFE). However, the fabrication of PTFE membranes is challenging due to its poor processability. This work describes a novel strategy to develop superhydrophobic and highly porous PTFE membranes via electrospinning. The hydrophobic nanoparticles 8 vinyl-grafted polyhedral oligosilicone (vinyl-POSS) were firstly incorporated into PTFE nanofibers to promote the crystallization of PTFE nanofiber, enhance nanofiber roughness, and improve membrane mechanical robustness and porosity. The effects of vinyl-POSS concentration, temperature and salt concentration of feed solutions on MD performance of PTFE/POSS nanofibrous membranes were investigated systemically in this study. The optimized PTFE/POSS nanofibrous membrane #POSS-2 has a three-dimensional (3D) superhydrophobic property with a water contact angle of 151 ± 4°. It exhibited a competitive water flux of 40 ± 2 Lm−2 h−1 in the direct contact membrane distillation (DCMD) process when the feed and permeate temperatures were 60 and 20 °C, respectively. Moreover, #POSS-2 possessed an excellent long-term stability in a 200-h continuous DCMD operation.

Biofouling phenomenon of direct contact membrane distillation (DCMD) under two typical operating modes: Open-loop mode and closed-loop mode

The biofouling mechanism of Direct Contact Membrane Distillation (DCMD) was comprehensively investigated in the present study under two typical operating modes of open-loop mode (mode O) and closed-loop mode (mode C). The experimental results revealed that organic substances tended to attach onto the membrane surface under the mode C, significantly promoting the initial colonization of live bacteria on the membrane. The 16S rRNA analysis revealed that the abundance of live bacteria in the mode C was higher at the early period, which sharply reduced at the later stage. It might be attributed to the increased deposition of salt crystals on the membrane at the later period of DCMD operation. Under the mode C, increasing depositions of organic and inorganic substances can be obtained on the membrane, contributing to the continuous changes of membrane surface conditions and then the remarkable succession of biofilm bacterial community. Comparatively, the biofilms formed under the mode O contained less nutrients and a stable microbial community, resulting in a milder flux decline compared to mode C.

Effect and mechanism of an anionic surfactant on membrane performance during direct contact membrane distillation

This study aimed to investigate the effect of a typical anion surfactant sodium dodecyl sulfate (SDS) on direct contact membrane distillation (DCMD) process using three different membranes, including polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) hydrophobic membranes and an omniphobic membrane. The PVDF membrane was the most likely to be wetted, and the pores wetting got more serious for both the PVDF and PTFE membranes with SDS concentration increasing. The omniphobic membrane presented robust anti-surfactant-wetting ability, even facing the saline feed with 0.5 mM SDS. The calcium ions can promote the surfactant SDS micellization, and the formed SDS colloids could cause membrane fouling. After 50 h continuous DCMD operation, the permeate flux of the hydrophobic membranes declined more than 50% due to membrane fouling, while the loss of permeate flux for the omniphobic membrane was only about 20%. The mechanism of membrane fouling was quantitatively investigated by the extended Derjaguin-Landau-Verwey-Overbeek theory. The calculated results manifested that the omniphobic membrane had the lowest attractive force to the foulant, indicating a less risk of being fouled. The energy barrier of the omniphobic membrane was the highest, resulting in the strongest resistance against the SDS colloids deposition on membrane surface.

Membrane distillation as post-treatment for anaerobic fluidized bed membrane bioreactor for organic and nitrogen removal

To observe feasibility of membrane distillation (MD) as post-treatment for anaerobic fluidized bed membrane bioreactor (AFMBR), removals of organic and total nitrogen were investigated by using the commercial polyvinylidene difluoride (PVDF) membrane for direct contact membrane distillation (DCMD) at various operational conditions. Test solutions for MD experiments were permeate produced by staged AFMBR (SAF-MBR), permeate from single AFMBR and synthetic wastewater fed to both reactors. Increasing in feed temperature improved permeate flux through PVDF membrane, but it decreased total nitrogen (TN) removal efficiency. Effect of chemical oxygen demand (COD) concentrations in feed solutions for DCMD on TN removal efficiency was almost negligible. However, the COD removal efficiency was lower at lower feed concentration in DCMD operation. At constant feed temperature, TN removal efficiency was improved by increasing a recirculation flow rate on PVDF membrane across DCMD system. Both organic and inorganic fouling were observed on PVDF membrane surface and pore matrix after conducting DCMD operation. The organic fouling on PVDF membrane consisted mainly of protein and fatty acids, supporting that the permeate produced by AFMBR should have potentials to foul the membrane applied in DCMD system as post-treatment.

Robust surface modified polyetherimide hollow fiber membrane for long-term desalination by membrane distillation

Surface modified polyetherimide (PEI) hollow fibers were prepared by surface segregation of a fluorinated polyurethane additive (FPA). This undergoes spontaneous segregation principally to their outer surfaces resulting in PEI hollow fibers with an improved hydrophobic property of a contact angle up to 132.7°. This high contact angle is attributed mainly to the protrusion of fluorine of FPA at the hollow fiber surface. The addition of FPA also enhanced the mechanical strength and liquid entry pressure of water (LEP) reaching high values in the range 329–541 kPa. The effects of the PEI concentration in the blend on the morphological and structural characteristics of the hollow fibers were studied maintaining the FPA at 2 wt% in the PEI spinning solution. The hollow fibers were tested for desalination by direct contact membrane distillation (DCMD). A very stable DCMD performance was obtained for the FPA hollow fiber prepared with 14 wt% PEI with permeate fluxes around 23.8 kg/m2 h and low permeate electrical conductivity (4.9–5.8 μS/cm) over a period of almost two months (54 days DCMD operation with 29.8 g/L NaCl aqueous solution, 80 °C and 20 °C feed and permeate inlet temperatures, respectively).

3D printed spacers for organic fouling mitigation in membrane distillation

3D printing offers the flexibility to achieve favorable spacer geometrical modification. The role of 3D printed spacers for organic fouling mitigation in direct contact membrane distillation (DCMD) is evaluated. Compared to a commercial spacer, the design of 3D printed triply periodic minimal surfaces spacers (Gyroid and tCLP) - varying filament thickness and smaller hydraulic diameter enhanced DCMD fluxes by 50–65%. The highest DCMD flux was obtained with the 3D tCLP spacer due to its specific geometrical design feature. However, its design characteristics resulted in higher channel pressure drop compared to 3D Gyroid spacer. Moreover, 3D Gyroid spacer exhibited superior fouling mitigation (lower membrane organic mass deposition and reversible membrane hydrophobicity with humic acid solution), attributed to its tortuous design that repelled foulants. 3D Gyroid spacer was effective in achieving high water recovery (85%) while maintaining good quality distillate (10–15 μS/cm, 99% ion rejection) in DCMD with wastewater concentrate that contained high organics, mixed with inorganics. In MD, high organic contents minimally affected MD fluxes but reduced membrane hydrophobicity. Repeated DCMD cycles showed that organic pre-treatment as well as cleaning-in-place of membrane and spacer are essential for achieving high recovery rate while maintaining a stable long-term DCMD operation with wastewater concentrate.

Direct Contact Membrane Distillation for Desalination Brine Solution

Desalination is a process where fresh water produces from high salinity solutions, many ways used for this purpose and one of the most important processes is membrane distillation (MD). Direct contact membrane distillation (DCMD) can be considered as the most prominent type from MD types according to ease of design and modus operandi. This work studies the efficiency of using DCMD operation for desalination brine with different concentration (1.75, 3.5, 5 wt. % NaCl). Frame and plate cell was used with flat sheet PTFE hydrophobic type membrane. The study proves that MD is an effective process for desalination brines with feed temperature less than 60˚C especially for feed with low TDS. 37˚C, 47˚C, and 57˚C was feed temperature and 17, 22, 27˚C as permeate temperatures used in study, temperature in both sides of membrane are recorded and TDS for permeate collected to assure that there is no penetration of brine to permeate side, the results took every 30 min for time experiment of 180 min. From results, the flux increases with increasing feed temperature and flow rate, and decreasing with increasing feed concentration, experiment time, and permeate temperature. 

Removal of selected pesticides from groundwater by membrane distillation.

The removal of five selected pesticide compounds in a brackish model groundwater solution was examined using a bench scale direct contact membrane distillation (DCMD) system. It was found that the rejection rate of the pesticides in DCMD is mainly influenced by its properties. Compounds with low hydrophobic characteristics and low vapour pressure showed a high rejection rate (70-99%), whereas compounds with a high vapour pressure or high hydrophobicity (LogD) showed a reduced rejection (30-50%) at a water recovery of 75%. The influence of groundwater feed solution contents such as the presence of organics (humic acid) and inorganic ions (Na+, Ca2+, Mg2+, Cl- and SO42-) as well as feed temperature (40, 55 and 70°C) on the rejection of the pesticides in DCMD operation was also evaluated. The results showed that the presence of inorganic ions and organics in the feed solution influences the pesticides rejection in DCMD operation to a minor degree. In contrast, reduced rejection of pesticides with high vapour pressure was observed. A rapid small-scale column test (RSSCT) was carried out to study the removal of any remaining substances in the permeate by adsorption onto granular activated carbon (GAC). RSSCT showed promising performance of GAC as a post-treatment option.

Fluoride removal from groundwater using direct contact membrane distillation (DCMD) and vacuum enhanced DCMD (VEDCMD)

Abstract Groundwater resources are under growing pressures in water scarce countries. Membrane distillation (MD) is an interesting option for drinking water production from groundwater with elevated salinity and fluoride (F) contamination. A direct contact MD (DCMD) at a moderate feed temperature of 55 °C was able to concentrate a synthetic solution representing F contaminated groundwater. An average initial flux of 13.3 L/m 2 /h was achieved at 75% water recovery, resulting in only 15–17% flux decline, while producing high quality permeate (96–99% F rejection). Membrane autopsy showed presence of Ca, Na, S and Mg on the used membrane. Particularly, 51–53% F precipitation was estimated in line with the saturation index (SI) model. The used MD membrane with groundwater showed only 10–12% reduction in membrane hydrophobicity, which was largely recovered with simple cleaning. Meanwhile, synthetic groundwater solution spiked with humic substances resulted in brownish deposition on MD membrane, reducing the membrane hydrophobicity significantly by 37–40%. Additionally, DCMD operation with vacuum at the permeate side (vacuum enhanced DCMD; VEDCMD) was beneficial in increasing the permeate flux by 42%. Continuous VEDCMD operation with intermediate membrane cleaning showed positive results in treating F contained groundwater while producing good quality permeate at 67% water recovery.