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Diffusion behavior of humic acid during desalination with air gap and water gap membrane distillation
Abstract
Desalination and water reuse are important means to resolve local water scarcity and security issues worldwide where membrane distillation (MD) may be part of a solution. Natural organic matter and in particular, humic acids (HA), are widely present in water supplies to be treated but exhibit little understood behavior to diffuse through MD membranes into permeate. In this work, air gap (AGMD) and water gap (WGMD) were utilized to study HA behavior in MD using seawater and synthetic water over a range of typical MD temperatures, flow rates and membrane types. HA diffusion was first shown with seawater feed then on synthetic solutions at all process conditions. While electrical conductivity rejection was always above than 99%, HA rejection showed values of 33% and 90% for AGMD and 68% and 93% for WGMD with seawater and synthetic water, respectively. Analytical techniques were used to perform a preliminary organic matter characterization in permeate, obtaining clear differences between the feed and permeate HA property. Compared to hydrophobic membranes, uniquely oleophobic membranes inhibit HA diffusion suggesting hydrophobic surface diffusion of HA through the membrane. HA flux as well as potential undesirable effects of the organic matter in permeate should be considered for MD applications.
... Amaya-Vías et al. 31 and Naidu et al. 32 also reported an increased flux at higher operating temperatures. However, organic deposition on the membrane surface occurred due to the migration of organic substances in their experiments. ...
... The feed solution may change the feed vapor pressure and the diffusion of the vapor in the membrane pore. The diffusion model for organic filtration was suggested by Amaya-Vías et al. 31 Diffusion is the temperature function by the Fick's law that could affect the fate of humic substance within the membrane surface. The adsorption of humic substance on the membrane surface can be increased by the increased temperature and the diffusivity of humic substance. ...
... However, some research works detected humic acid concentration in the distillate side. Amaya-Vías et al. 31 detected an HA flux from 15.6 to 37.1 mg h-L m −2 for the AGMD system at higher feed temperatures. The deposition of humic acid on the membrane surface and the disaggregation of HA could occur by higher temperatures and migration of dissolved organic matters to the permeate side. ...
The main waste stream from the textile industry is its wastewater with high color, organic matters, and other contaminants. This study aims to investigate the effect of humic acid in mixed wastewater of humic acid and reactive dye on the treatment performance and permeate flux of a direct contact membrane distillation (DCMD) system. In this research, feed temperature and humic acid concentration were the main input parameters for the analysis of DCMD system operation. The fouling resistances significantly increased with higher humic acid concentrations in the mixed wastewater. As compared with the DI water test, 23% of flux decline occurred when the humic concentration in the wastewater was increased up to 20 mg/L. After the DCMD treatment, the 25 ADMI residual color was detected in the permeate when the mixed wastewater contained 20 mg/L humic acid. The mathematical model, based on the Antione equation, was proposed to predict the membrane flux decline of the DCMD system. The reduced pore size of the cake layer by a dimensionless constant β from the Kelvin equation was also considered for the fouling calculation to describe the transport mechanism.
... NOM (natural organic matter) is mainly composed of humic substances and is known to contribute color to the water [1,2]. These substances are originated from the degradation of plants and micro-organisms either by chemical or biological pathway [3,4]. In terms of chemical property, humic substances are amphipathic and show both hydrophobic and hydrophilic nature [5][6][7]. ...
... Fouling reduction in MD has been carried out by changing the flow regime across the membrane surface [31], use of antiscalants [20], and regular cleaning. We have also shown that immobilization of carbon nanotubes can reduce fouling in CaSO 4 and CaCO 3 contained water where the CNTs (carbon nanotubes) serve a screen that prevent the deposition of large particles [20,32,33]. ...
In this paper, we present the treatment of humic acid solution via carbon nanotube immobilized membrane (CNIM) distillation assisted by air sparging (AS). Carbon nanotubes offer excellent hydrophobicity to the modified membrane surface and actively transport water vapor molecules through the membrane to generate higher vapor flux and better rejection of humic acid. The introduction of air sparging in the membrane distillation (MD) system has changed the humic substance fouling by changing the colloidal behavior of the deposits. This modified MD system can sustain a higher run time of separation and has enhanced the evaporation efficiency by 20% more than the regular membrane distillation. The air sparging has reduced the deposition by 30% in weight and offered lesser fouling of membrane surface even after a longer operating cycle. The water vapor flux increased with temperature and decreased as the volumetric concentrating factor (VCF) increased. The mass transfer coefficient was found to be the highest for the air sparged—carbon nanotube immobilized membrane (AS-CNIM) integrated membrane distillation. While the highest change in mass transfer coefficient (MTC) was found for polytetrafluoroethylene (PTFE) membrane with air sparging at 70 °C.
... However, this process could result in high operational costs due to the implementation of a vacuum pump. Water-gap membrane distillation (WGMD) was introduced to mitigate both problems, that is, high heat transfer and low permeation flux [23][24][25]. WGMD employs water as the main fluid in the water gap, which helps reduces heat transfer from the feed solution to the coolant side. A cooling plate placed between the water gap and coolant side restricts the flow of the permeate to the coolant side. ...
Experimental and theoretical studies on material-gap membrane distillation (MGMD) and water-gap membrane distillation (WGMD) are presented in this paper to demonstrate the effect of gaps filled with different materials on permeation flux. A flat-sheet composite membrane composed of an active polytetrafluoroethylene layer and a support polypropylene layer was employed in the experiments. Graphite, water, silica gel, and zeolite were used to measure the effects of these materials on permeation flux and thermal resistance. In the experiments and simulations, the size of the material and water gaps was kept constant at 5 mm. The graphite-filled MGMD permeation flux was approximately 11–22% higher than that of the WGMD at inlet feed temperatures in the range of 40–70 °C. However, the permeation flux of MGMD filled with silica gel and zeolite was 17–24% and 18–27% lower than that of WGMD, respectively. At thermal conductivities below 5 W/mK, the permeation flux of the MGMD with a material packing density of 40% was higher than that with a material packing density of 60%. The MGMD permeation flux with a material packing density of 60% was higher than that with a material packing density of 40% above a thermal conductivity of 5 W/mK. Furthermore, the MGMD permeation flux and overall thermal resistance were primarily controlled by the material gap, with materials having thermal conductivities below 30 and 20 W/mK for the bead- and pellet-type materials (i.e., packing densities of 40 and 60%), respectively.
... Membrane distillation (MD) process is a combination of the thermal and membrane processes, where freshwater separation takes place using hydrophobic microporous membrane while heating the feed water to stimulate water vapor permeation through membrane pores [7]. Water vapor flows through membrane pores as a result of the difference in the vapor pressure across the membrane, which is induced by the temperature difference between the hot side and cold side of the membrane. ...
The performance of a solar-heated multistage direct contact membrane distillation system was experimentally evaluated for sustainable water production. Parallel and series connections of three DCMD stages for the feed and permeate streams were tested and compared. A mathematical model for a double glass evacuated tube solar collector was developed to predict the temperature variation of the stagnant feed water solar tank during heating. Results demonstrate the sustainability of water production using the integrated solar DCMD desalination system. Parallel connection of system stages is more productive compared to series connection. The system flux is decreasing over the day hours following the decreasing feed water temperature due to higher energy consumed by the DCMD desalination process compared to the solar energy received by the single solar collector. However, the system showed a very good level of water production sustainability over the day hours. The average fluxes from 9 AM to 6 PM are 51.6 kg/m².h for the parallel multistage system and 22.1 kg/m².h for the series multistage system, corresponding to an average solar tank (feed water) temperature of 66 °C. The parallel three-stage solar MD system produced 9 L of fresh water, with specific thermal energy consumption of about 1770 kWh/m³.
... Hence, the PFOA could attach to the PTFE and diffuse across the membrane. Similar phenomena were observed by Amaya-Vías et al. (Amaya-Vías et al., 2019) where humic acid diffused across the PTFE membrane and Dumee et al. in which PFAS diffused across the PVDF membrane. The diffusion could be suppressed by the PVA layer and incorporation of MOF into the PVA layer. ...
Landfill leachate is a highly polluted and complex wastewater as it contains large amounts of organic matters, ammonia‑nitrogen, heavy metals, and per−/poly-fluoroalkyl substances (PFAS), which makes its treatment very challenging. In this paper, hydrophilic/hydrophobic dual layer membranes combining advantages of pervaporation and membrane distillation was employed to treat leachate in a direct contact membrane distillation (DCMD) configuration. An aluminum fumarate (AlFu) metal organic framework (MOF) incorporated poly(vinyl alcohol) (PVA) hydrophilic layer was coated on hydrophobic PTFE membrane to overcome the low separation efficiency of PFAS and ammonia and wetting issues encountered by the conventional hydrophobic PTFE membrane used for DCMD. The rejections of dual layer membranes with different MOF loading to PFAS, ammonia, TOC and TDS were assessed based on the amount of AlFu MOF incorporated into the PVA layer. Based on the conducted adsorption tests, it was found that AlFu MOF increases the rejection of PVA layer to PFAS and ammonia. The coating of the hydrophilic layer could enhance the wetting resistance with/without MOF addition. In comparison with the pristine PTFE membrane using synthetic feed containing 3 wt% NaCl, 1 wt% addition of AlFu MOF into the PVA layer showed slightly increased flux. All the tested membranes showed more than 99% rejection to TOC. The rejection to ammonia was increased as more MOF was incorporated into the PVA layer. The maximum rejection of ammonia was 99.8% when the PVA layer containing 10% MOF. All the membranes showed more than 99% rejection to PFOS and PFHxS. However, PTFE membrane did not show any rejection to PFOA. As more MOF was added into the hydrophilic layer, the rejection to PFOA increased, but plateaued at 65.6% with 5% MOF incorporation into the hydrophilic layer.
... The commonly used designs of the MD process include direct contact, sweeping gas, air gap, and vacuum MD modules [5][6][7]. In air gap membrane distillation (AGMD) configuration, the feed stream flows directly over the membrane surface such that the hot water evaporates at the membrane surface interface [8]. On the other hand, an air gap is introduced to separate the condensation plate and the membrane's cold surface. ...
To improve productivity and reduce the energy consumption and production cost of the membrane distillation (MD) process, a new compact design of a multistage MD module is experimentally evaluated for water desalination. The compact multistage MD module uses two membranes in each feed channel and two cooling plates in each coolant channel. The module is utilized to compare the performances of the air gap and water gap MD configurations as a step forward to scaling up for possible commercialization. The performance metrics include the system productivity, gained output ratio (GOR), and production cost, which are calculated and compared at different operating parameters. Results revealed that the water gap produces more freshwater than the air gap under the same conditions with a lower cost of production. However, the air gap is better in terms of energy consumption with higher GOR. The multistage air gap system produced up to 0.98 L/h of freshwater with the best GOR of 0.6 and production cost of 12 /m³.
... In direct contact membrane distillation (DCMD) process; both feed and coolant are in direct contact with membrane surfaces. When water vapor passes through the membrane, it condenses directly in the coolant side [13]. In this regard, Jung-Gil Lee et al. [14] designed and investigated a multistage direct contact membrane distillation (MSDCMD) module for water desalination application. ...
A theoretical modeling of multistage water gap membrane distillation (MSWGMD) process is presented to predict its performance with economic evaluation. The theoretical model is based on coupled heat and mass transfer analysis inside the water gap MD module and considering the effect of gap natural convection. The model is validated and used to study the variation of feed temperature, coolant temperature, and system productivity with the number of stages for the MD system with and without insulation. It is also used to predict the maximum and optimal number of stages that can be employed under specific operating conditions. In addition, the cost analysis of MSWGMD system is carried out. Results show that under the conditions of 20 °C coolant temperature, 50–90 °C feed temperature, 2.3 L/min feed and coolant flow rates, 4 mm gap thickness; the maximum number of stages that can be used is found to be 35 stages. In this case, the system productivity is about 5.2 L/h. It is also found that, by insulating pipe connections and the MD module, the total productivity increases by 36%. The optimum number of stages was found to be 15 stages, and the corresponding cost of freshwater production of the insulated multistage water gap MD process is about 3 $/m³.
... WGMD emerged as an improvement of the two previous configurations. In WGMD, as it shown in Figure 1, the gap is filled with distilled or permeated water, therefore, the transmembrane flows are improved due to lower mass transfer resistance with respect to AGMD [28,35,36]. ...
Membrane distillation (MD) has a great deal of potential and this is currently being explored by the scientific community. However, this technology has not yet been implemented by industry, and an estimation of final product costs is key to its commercial success. In this study a techno-economic assessment of air gap MD (AGMD) and water gap MD (WGMD) for seawater desalination under different capacities and heat source scenarios was developed. The simplified cost of water (SCOW) method, which estimates investment costs, fixed and variable costs, as well as amortization factors and price influence over time was applied. In addition, experimental data from a laboratory-scale MD desalination plant were also used. The results showed water costs in the range of 1.56 to 7.53 €/m3 for WGMD and 2.38 to 9.60 €/m3 for AGMD. Specifically, the most feasible scenario was obtained for WGMD with a capacity of 1000 m3 daily using waste and solar heat. Finally, the costs obtained for MD were similar to those of conventional desalination technologies at the same scale factor. Therefore, although large-scale pilot studies and optimization of manufacturing processes are needed, MD shows very promising results that should be considered further.
This research explores the influence of electromagnetic forces (EMF) on Air Gap Membrane Distillation (AGMD) performance through the integration of Computational Fluid Dynamics (CFD) modeling and experimental validation. Introducing EMF within the feed channel of AGMD systems promotes turbulence, thereby augmenting mass transport and enhancing evaporation efficiency. Quantitative assessments reveal a 16 % improvement in mass flux and a 4 % enhancement in evaporation efficiency at an electromagnetic intensity of 0.5 Tesla.
The magnetic intensity effect is more significant within shorter module lengths that cover more feed channel area, enhancing 22.5 % mass flux. Moreover, the AGMD performance enhanced by EMF shows a reduction in specific thermal energy consumption to 69 kWh/m3 of produced water, a decrease from the 72 kWh/m3 noted in systems without EMF. This optimized AGMD framework not only exceeds traditional methodologies in energy efficiency but also lowers the cost of water production by approximately 14.1 %, thus improving its economic feasibility. These results support the viability of incorporating EMF into membrane distillation systems as a scalable strategy to boost desalination processes while reducing energy and cost requirements. This method represents a promising direction for future sustainable water purification technology developments.
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 CODCr, 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.
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.
Membrane distillation is a promising approach to the fresh water crisis due to the low operating temperature and high rejection of non-volatile components. However, it suffers from the polarization effect and membrane fouling. This paper aims at developing a novel two-phase flow enhanced direct contact membrane distillation (TP-DCMD) system and investigating the effect of two-phase flow behaviors on the heat and mass transfer and membrane fouling comprehensively. First, a novel experimental system of TP-DCMD is developed and well validated using the mass and energy conservations. Then, the effect of flow parameters on the transmembrane permeate flux is discussed. The introduction of gas could improve the membrane distillation performance by as high as 27 %. The relationship between the performance enhancement and two-phase flow regimes is discussed. Both permeate flux and multiplier clouds are proposed based on two-phase flow regime map. The slug flow demonstrates superior enhancing effect to the other flow regimes due to the low possibility of gas penetration. Finally, the effect of two-phase flows on the membrane fouling is investigated. After 420 min of accelerating fouling test, the transmembrane permeate flux drops by 38.2 % in the conventional DCMD system, while it just decreases by 6.6 % in TP-DCMD system.
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.
This work studies a promising waste valorization approach involving recovery of fresh water from protein-rich waste water from the meat processing and rendering industry via membrane distillation (MD). Blood stick water samples from an industrial plant have been treated in batch and continuous tests with two different PTFE membranes (native hydrophobic and hydrophilic coated). Treated water coming from MD permeate was evaluated using various parameters including sodium (for salinity), total organic carbon (TOC) and total nitrogen (TN). Overall, MD was shown to yield concentrated high strength stick water up to a concentration factor of 20 for potential re-use and recover clean water rejecting >99% salinity and TOC, and >95% of TN. Compared to conventional native hydrophobic PTFE, the hydrophilic coated membrane showed higher constant rejection in continuous experiments, but performed with less total flux. Uniquely for the first time in MD studies, the transfer of volatile fatty acids through the membrane into the permeate was explored, showing concentrations of 30–160 mg/L in the permeate appearing more related to hydrophobic surface diffusion than volatile diffusion. While MD for valorization of meat industry wastewater appears viable with correct conditions and membranes, the value of recovered water due to fatty acid presence and other practical considerations, such as membrane cleaning, need to be considered.
The persistence of pathogenic microorganisms in treated wastewater effluent makes disinfection crucial to achieve wastewater reuse. Membrane processes such as ultrafiltration and reverse osmosis (RO) have shown promising results for virus and other contaminant removal from treated wastewater effluents for reuse application. However, RO produces a concentrate stream which contains high concentrations of pathogens and contaminants that often requires treatment and volume reduction before disposal. Membrane distillation (MD) is a treatment process that can reduce RO concentrate volume while augmenting the potable water supply. MD is also a dual barrier approach for virus removal as it operates at a high temperature and permeates only the vapor phase through the membrane interface. The effects of temperature on viable virus concentration and membrane rejection of viruses in MD are investigated in this study using two nonenveloped phages frequently used as enteric virus surrogates (MS2 and PhiX174) and an enveloped pathogenic virus (HCoV-229E). At typical MD operating temperatures (greater than 65 °C), viable concentrations of all three viruses were reduced by thermal inactivation by more than 6-log10 for MS2 and PhiX174 and more than 3-log10 for HCoV-229E. Also, membrane rejection was greater than 6-log10 for MS2 and PhiX174 and greater than 2.5-log10 for HCoV-229E.
Recovery on salinity gradient energy (SGE) usually existing at the junction of rivers and sea, have attracted a large amount of researchers. However, recovery and utilization of the SGE between desalinated seawater and seawater discharged from desalination plants, is relatively unexplored. In this work, a reverse electrodialysis (RED) approach is introduced to recover the SGE and convert it into electric energy. The objectives of this work are: i) exploring the feasibility to recover the SGE by adopting the RED method; ii) investigating the influence of different inlet parameter on the performance of a RED stack. On the basis of above aims, comparative experiments are implemented to verify the feasibility. Then the factors affecting the performance of the RED stack are analyzed. Results show that the power density of desalinated seawater (3.0 mol/L)/seawater (0.50 mol/L) is slightly higher than that of seawater/freshwater (0.060 mol/L), and lower than that of desalinated seawater/freshwater. A high concentration, temperature, and flow rate can improve efficiently the performance of the stack. For a RED stack composed of 10 membrane cells with desalinated seawater in the concentration range of 1.5 mol/L–3.0 mol/L, the open–circuit voltage and the maximum power density can reach 0.966 V and 0.372 W/m² respectively. This study can provide some guidance for the recovery of SGE between desalinated seawater and seawater.
Coal gasification brine (CGB) is a typical hypersaline organic wastewater and its proper treatment is a challenging task. In the study membrane distillation (MD) experiments were conducted with raw and pretreated CGB as the feeds. The flux evolutions and permeate quality in different processes were compared to study the interaction between organic and inorganic foulants and investigate the mixed fouling mechanism of membrane. In the MD process concentrating CGB, interaction between NaCl and HA-like materials resulted in the formation of a loose organic layer with small crystals wrapped and inlaid. The fouling layer caused insignificant flux decrease but induced membrane wetting, which finally brought drastic flux decrease. After the CGB was pretreated by extraction and Fenton oxidation processes, membrane fouling and wetting were effectively mitigated and the permeate of MD remained high quality. The HA-like substances were effectively removed and the hydrophilic low-molecular-weight organic compounds became the main organic component, promoting the formation of inorganic fouling layer composed of large CaSO4 crystals. The layer caused insignificant flux decline and could be easily removed by membrane rinse. The study suggests it is feasible to concentrate CGB to a high solid level with the combination of extraction, Fenton oxidation, and DCMD.
Humic acid (HA) removal research focuses on the global water treatment industry. In this work, efficient HA degradation with an ultra-high synergetic intensity is achieved by combined bubble discharge with activated carbon (AC). Adding AC to the discharge greatly improves HA removal efficiency and degradation speed; the synergetic intensity reaches 651.52% in the combined system, and the adsorption residual on AC is 4.52%. After 90 min of treatment, the HA removal efficiency reaches 98.90%, 31.29%, and 7.61% in the plasma-AC combined, solo bubble discharge, and solo AC adsorption systems, respectively. During the plasma process, the number of pore structures and active sites and the amount of oxygen-containing functional groups on the AC surface increase, resulting in a higher adsorption capacity to reactive species (H2O2 and O3) and HA and promoting interactions on the AC surface. For HA mineralization, the presence of AC greatly promotes the destruction of aromatic structures and chromophoric HA functional groups.
Polyfluoroalkyl and perfluoroalkyl substances (PFAS) are ecotoxic amphiphilic compounds containing alkyl-fluorinated chains terminated with weak acid moieties, and hence difficult to be degraded or removed from water sources. Direct contact membrane distillation (DCMD) was used for concentrating and removing of perfluoropentanoic acid (PFPeA) compounds from model contaminated water using commercially available poly (tetrafluoroethylene) (PTFE) membranes. The membranes were characterised for surface morphology, roughness, contact angle and pore size distribution before and after the DCMD test to investigate and evaluate membrane fouling. During the DCMD test performed for 6 h using 10 ppm PFPeA solution, the membrane exhibited progressive increased flux (from 17 to 43 kg m⁻² h⁻¹) and decreased PFPeA rejection (from 85 to 58%), as the feed temperature was increased from 50 to 70 °C. Further, the feed/retentate side showed a 1.8, 2.1 and 2.8-fold increase in PFPeA concentration tested at feed temperatures 50, 60, and 70 °C, respectively. The permeate side contained less than 1 ppm of PFPeA revealing that the PFPeA moved across the PTFE membrane during DCMD, which is attributed to progressive surface diffusion over time. This study opens a new route to concentrate and remove amphiphilic molecules, such as PFAS, from source points, relevant to landfill leachates or surface waters. The study also points at gaps in materials science and surface engineering to be tackled to deal with PFAS compounds efficiently.
Treating wastewater from textile plants using membrane distillation (MD) has great potential due to the high-salinity wastes and availability of waste heat. However, textile wastewaters also contain surfactants, which compromise the essential hydrophobic feature of the membrane, causing membrane wetting. To address this wetting issue, a custom-made membrane consisting of a hydrophilic layer coated on hydrophobic polytetrafluoroethylene (PTFE) was tested on textile wastewater in a pilot MD setup, and compared with a conventional hydrophobic PTFE membrane. The test was carried out with a feed temperature of 60 °C, and a permeate temperature of 45 °C. The overall salt rejection of both membranes was very high, at 99%. However, the hydrophobic membrane showed rising permeate electrical conductivity, which was attributed to wetting of the membrane. Meanwhile, the hydrophilic-coated membrane showed continually declining electrical conductivity demonstrating an intact membrane that resisted wetting from the surfactants. Despite this positive result, the coated membrane did not survive a simple sodium hydroxide clean, which would be typically applied to a membrane process. This brief study showed the viability of membrane distillation membranes on real textile wastewaters containing surfactants using hydrophilic-coated hydrophobic PTFE, but the cleaning process required for membranes needs optimization.
Depicted as large polymers by the traditional model, humic substances (HS) tend to be considered resistant to biodegradation. However, HS should be regarded as supramolecular associations of rather small molecules. There is evidence that they can be degraded not only by aerobic but also by anaerobic bacteria. HS presence alters biological transformations of organic pollutants in water and soil. HS, including humin, have a great potential for an application in aerobic and anaerobic wastewater treatment as well as in bioremediation. Black carbon materials, including char (biochar) and activated carbon (AC), long recognized effective sorbents, have been recently discovered to act as effective redox mediators (RM), which may significantly accelerate degradation of organic pollutants in a way similar to HS. Humic-like coating on the biochar surface has been identified. Explanation of mechanisms and possibility of applications of black carbon materials have only started to be explored. Results of many original and review papers, presented and discussed in this article, show an enormous potential for an interesting, multidisciplinary research as well as for a development of new, green technologies for biological wastewater treatment and bioremediation. Future research areas have been suggested.
Considerations including water scarcity in arid and semi-arid regions, water security concerns in areas where water demand exceeds water availability, and rigorous and costly requirements to remove nutrients and emerging contaminants from effluent discharge to surface waters have driven water reuse as an alternate water supply in some parts of the world. However, the potential of reusing treated wastewater has not yet been exploited in many areas. A transition to a circular economy could create significant synergies for the wide adoption of water reuse as an alternate water supply. This paper therefore examines opportunities and risks with the transition to such an economy. Findings show that although many of the barriers water reuse is facing, ranging from public perception to pricing and regulatory challenges, could be addressed more effectively through a wider circular economy perspective, care must be taken with regulating and monitoring levels of contaminants in the recycled water according to its use. A review of existing reuse schemes and regulations across the world, found variation, demonstrating the need for assessing benefits and risks on a case by case basis. Recycling and reuse are central to a circular economy approach and offer a strategy to improve water supply by managing wastewater better. Such strategy should also ensure the safety of water reuse, and therefore apply water quality standards appropriate to the specific use, but also ensure adequate and reliable operation of water reuse systems and appropriate regulatory enforcement.
In this study, a novel hydrophobic, microporous membrane was fabricated from styrene-butadiene-styrene (SBS) polymer using electrospinning and evaluated for membrane distillation applications. Compared to a commercially available polytetrafluoroethylene (PTFE) membrane, the SBS membrane had larger membrane pore size and fiber diameter and comparable membrane porosity. The fabricated SBS showed slightly lower water flux than the PTFE membrane because it was two times thicker. However, the SBS membrane had better salt rejection and most importantly could be fabricated via a simple process. The SBS membrane was also more hydrophobic than the reference PTFE membrane. In particular, as temperature of the reference water liquid increased to 60 °C, the SBS membrane remained hydrophobic with a contact angle of 100° whereas the PTFE became hydrophilic with a contact angle of less than 90°. The hydrophobic membrane surface prevented the intrusion of liquid into the membrane pores, thus improving the salt rejection of the SBS membrane. In addition, the SBS membrane had superior mechanical strength over the PTFE membrane. Using the SBS membrane, stable water flux was achieved throughout an extended MD operation period of 120 h to produce excellent quality distillate (over 99.7% salt rejection) from seawater.
Meat rendering operations produce stick water waste which is rich in proteins, fats, and minerals. Membrane distillation (MD) may further recover water and valuable solids, but hydrophobic membranes are contaminated by the fats. Here, commercial hydrophobic polytetrafluorethylene (PTFE) membranes with a hydrophilic polyurethane surface layer (PU-PTFE) are used for the first time for direct contact MD (DCMD) on real poultry, fish, and bovine stick waters. Metal membrane microfiltration (MMF) was also used to capture fats prior to MD. Although the standard hydrophobic PTFE membranes failed rapidly, PU-PTFE membranes effectively processed all stick water samples to colourless permeate with sodium rejections >99%. Initial clean solution fluxes 5-6 L/m²/h declined to less than half during short 40% water recovery tests for all stick water samples. Fish stick water uniquely showed reduced fouling and up to 78% water recovery. Lost flux was easily restored by rinsing the membrane with clean water. MMF prior to MD removed 92% of fats, facilitating superior MD performance. Differences in fouling between stick waters were attributed to temperature polarisation from higher melt temperature fats and relative proportions to proteins. Hydrophilic coated MD membranes are applicable to stick water processing but further studies should consider membrane cleaning and longer-term stability.
In the present study, theoretical and experimental investigations were carried out to examine the effect of changing the operating parameters of an air gap membrane distillation (AGMD) system on the performance of electrospun and commercial membranes. These parameters include feed, cooling water temperature and feed flow rate. Analytical models were used, with the aid of MATLAB, to predict the permeate flux of AGMD based on heat and mass transfer. Heat transfer was used to predict the temperature on the membrane surface on the feed side and the thin film layer in the cooling plate on the air gap side, which was used later to calculate the vapour pressure and the permeate flux. The molecular diffusion model corresponded well with the experimental measurements in terms of predicting the permeate flux by varying the feed temperature, whilst it was poor in term of coolant temperature and feed flow rate. The results also illustrate that high rejection rates of around 99% of heavy metals can be achieved by using superhydrophobic electrospun membranes. The electrospun membrane flux increased with increasing feed tank temperature and flow rate while it was reduced with an increase of cooling line temperature.
This paper presents a novel approach to fabricate superhydrophobic membranes by using environmentally friendly and cost effective superhydrophobic nanoparticles to enhance nanofibrous membrane performance in term of flux and rejection of heavy metals in Membrane distillation applications. Polyvinylidene fluoride (PVDF) membranes were fabricated using an electrospinning technique, in which electrospinning parameters such as polymer concentration, voltage, solvent ratio, and cationic surfactant were studied to optimize the membrane fibre diameters and produce beadless nanofibrous membranes. The nanofibrous membranes were characterized in terms of pore size, porosity, liquid entry pressure, contact angle, permeate flux and rejection percentage, and were compared to a commercial membrane. Air gap membrane distillation (AGMD) was used to demonstrate the improved ability of superhydrophobic PVDF membranes for removing heavy metals (such as lead) in comparison with pristine and commercial membranes. The results showed that pristine beadless membrane mat can be fabricated by using 15 wt% polymer concentration, 0.05 wt% cationic surfactant with 6:4 DMF to acetone ratio and 14 KV with lead rejection rate of 72.77 %, liquid entry pressure (17 psi) and water contact angle of 132º. In comparison, the composite 11 wt% PVDF membranes with 20 wt% of functionalized alumina (Al2O3) showed 150º WCA and 27 psi as liquid entry pressure which led to 99.36 % of heavy metal rejection and 5.9% increase in permeate flux.
Many countries including New Zealand, Australia, and South Korea discharge of farm effluents containing large reserves of plant nutrients into surface waters. Such discharge is currently considered a discretionary activity and requires legal consent that demands the effluent nutrient concentration to be minimized before entering surface waters. This can be achieved by land disposal or nutrient stripping of the effluent by tertiary treatment. Although the pond system (ie, biological treatment) is effective in removing suspended solids and carbon, there has been some debate about its efficiency in removing nutrients. Porous materials such as zeolite, a naturally occurring and electrically charged aluminosilicate material, can be used to adsorb nutrients from effluents. Then the nutrient-enriched material can be recycled as a soil conditioner or nutrient source. This chapter examines the potential of zeolite in nutrient stripping from wastewater streams and its value as a nutrient source.
Disinfection by-products (DBPs) in drinking water, including trihalomethanes (THMs) and haloacetic acids (HAAs), arise from reactions of natural organic matter (NOM) with chlorine and other disinfectants. The objective of this review was to investigate relationships between the molecular properties of NOM surrogates and DBP formation using data collated for 185 compounds. While formation of THMs correlated strongly with chlorine substitution, no meaningful relationships existed between compound physicochemical properties and DBP formation. Thus non-empirical predictors of DBP formation are unlikely in natural waters. Activated aromatic compounds are well known to be reactive precursors, in addition DBP formation from β-dicarbonyl, amino acid and carbohydrate precursors can be significant. Therefore effective DBP control strategies need to encompass both hydrophobic and hydrophilic NOM components, as well as consider data from NOM surrogates in the context of knowledge from representative treatment scenarios. In the future experiments employing surrogates of NOM are likely to remain a powerful tool in the search for unknown precursors and in understanding their response to various disinfection conditions.
Membrane Distillation (MD) is a thermally-driven separation process, in which only vapour molecules transfer through a microporous hydrophobic membrane. The driving force in the MD process is the vapour pressure difference induced by the temperature difference across the hydrophobic membrane. This process has various applications, such as desalination, wastewater treatment and in the food industry.This review addresses membrane characteristics, membrane-related heat and mass transfer concepts, fouling and the effects of operating condition. State of the art research results in these different areas will be presented and discussed.
The hydrophobic membranes used in membrane distillation (MD) are inherently prone to fouling by hydrophobic contaminants due to the long-range hydrophobic-hydrophobic interaction, which limits the application of this promising technology. In this study, a novel dual-layer composite membrane was fabricated through eletrospinning a hydrophilic poly(vinyl alcohol) (PVA) coating on a commercial hydrophobic polytetrafluoroethylene (PTFE) membrane and then cross-linking with glutaraldehyde for robust anti-oil-fouling MD. The prepared composite membrane showed an asymmetric wettability, the coating surface was underwater oleophobic with underwater oil contact angle of 148.7° and the PTFE substrate surface was hydrophobic with in-air water contact angle of 134.5°. The test results of the adhesive force between an oil droplet and membrane surface exhibited that the fabricated membrane had a desirable surface to resist oil adhesion. Direct contact MD experiments were conducted using a saline emulsion with 1000 mg/L crude oil to compare the performance of the modified membrane and the pristine PTFE membrane. The results demonstrated that the modified membrane can present stable performance with robust resistance to oil-fouling compared to the pristine PTFE membrane. It is believed that the fabricated composite membrane has great potential to be applied in MD process with high concentration of hydrophobic organics.
The performances of multistage air gap membrane distillation (MS-AGMD) and water gap membrane distillation (MS-WGMD) systems are experimentally investigated and compared using three-stages systems for different operating conditions. Parallel, series, and mixed stage-connections are considered. The flux of MS-WGMD is more than double the flux of MS-AGMD. The parallel stages-flow connections produced higher output flux than series connections (15% on average for MS-WGMD and 10% on average for MS-AGMD). The MS-AGMD system is found to be more sensitive to changes in the feed temperature and gap width than the MS-WGMD system. The effect of feed flow rate on permeate flux is higher than the coolant flow rate. The series stage connections are more sensitive to the change in hot and cold flow rates than the parallel connection; particularly the feed flow rate. The productivity of the three-stages system is almost three times the single stage system. The calculated specific electric energy consumption ranged from 5 to 10 kWh/m³ and it values for MS-WGMD system are lower than that of MS-AGMD system, and decreases with increasing the feed temperature.
The effect of different membranes, membrane modules, feed temperatures, flow rates and solute concentrations on the permeate flux and salt rejection in direct contact membrane distillation (DCMD) was first studied with synthetic seawater and compared to distilled water. After optimizing these operating conditions, DCMD was tested with real feed samples, namely river water (RW-R), seawater (SW-R), and a secondary effluent from a municipal wastewater treatment plant (MW-R). The permeate flux achieved with MW-R was significantly lower than those obtained with the other feed samples. Two membrane module configurations (H-cell and W-cell) were then studied using SW-S, spiking diphenhydramine (DP) as model organic pollutant in some experiments. The H-cell performed better in terms of permeate quality for the same volume of permeate collected. A long experiment (500 h) was conducted with SW-R employing a larger H-cell. Severe fouling was observed, but high rejections of ion species (>99%) were recorded together with complete rejections of pharmaceuticals (diclofenac, azithromycin, clarithromycin and erythromycin) detected in SW-R at 9.53–73.53 ng L⁻¹ (detection limits <0.16 ng L⁻¹). Colonies of Escherichia coli or enterococci were not detected in 100 mL of permeate (distillate) solution, complying with the European Directive for drinking water.
Thermally driven membrane distillation process, for desalination of a wide spectrum of water, has attracted both scientific and industrial attention for decades. However, membrane fouling by contaminants in the feed stream was of great concern. Mitigating membrane fouling requires insights from surface and material science. We reported an omniphobic polyvinylidene fluoride (PVDF) membrane with hierarchical structure which was created by spray coating of the nano/microspheres onto a commercial PVDF porous substrate. The multiscale microspheres were prepared based on electrostatic interactions between silica nanoparticles (SiNPs) and polystyrene (PS) microsphere. A chemical binding agent, polymer 3-methacryloxypropyltrimethoxysilane (PA174) synthesized via free radical polymerization, was utilized to enhance the adhesion of the particles to the membrane support, leading to a robust structure. The final fluorination step using 1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane (17-FAS) attributed the membrane omniphobicity. The water/hexadecane contact angle of the present membrane surface was 176°/138.4°, and the water sliding angle was 7°. Challenged with a hexadecane emulsion feed solution stabilized with the SDS surfactant in membrane distillation, the membrane showed a very stable flux and decrease in conductivity on the permeate side in contrast to decreased flux and increase in permeate conductivity for other benchmark membranes. This performance indicates that omniphobic membranes with hierarchical morphology are promising in addressing membrane wetting and fouling issues in the DCMD processes.
The thermal performance of air gap membrane distillation (AGMD) desalination is dominated by heat and mass transfer across the air gap between the membrane and the condensing surface. However, little is known about the impact of condensate flow patterns in some design variations of the air gap. In this study, air gap membrane distillation experiments were performed at various inlet temperatures, varying module inclination angle, condensing surface hydrophobicity, and gap spacer design to identify the effect of each on the permeate production rate and thermal efficiency of the system. Energy efficiency modeling was performed as well. Additionally, this study is one of the first with enhanced visualization of flow patterns within the air gap itself, by using a transparent, high thermal conductivity sapphire plate as the condenser surface. System-level numerical modeling is used to further understand the impact of these flow regimes on overall energy efficiency, including flux and GOR. A brief review of membrane distillation condensation regimes is provided as well. For tilting the AGMD flat-plate module, permeate flux was barely influenced except at extreme positive angles (>80ᵒ), and moderate negative angles (<−30ᵒ), where condensate fell onto the membrane surface. The surface with the hydrophobic coating (for dropwise condensation) was shown to have better droplet shedding (with very small nearly spherical droplets) and fewer droplets bridging the gap. Superhydrophobic surfaces (for jumping droplet condensation) were similar, with much smaller droplet sizes. Meanwhile, the hydrophilic surface for small gap sizes (< 3 mm) often had pinned regions of water around the hydrophilic surface and plastic spacer. Overall, the various results imply that the common assumption of a laminar condensate film poorly describes the flow patterns in real systems for all tilt angles and most spacer designs. Real system performance is likely to be between that of pure AGMD and permeate gap membrane distillation (PGMD) variants, and modeling shows that better condensing in air gaps may improve system energy efficiency significantly, with strong relative advantages at high salinity.
Transition metals Cd, Pd and Ag are toxic even at very low concentration. Cd is considered a priority substance; while, Pd and Ag are emerging pollutants. Membrane technologies have been applied for their extraction; however, they require important amounts of reagents, time and energy. Additionally, effective reagents for metal extraction in saline natural waters are limited. In this case, hollow fiber liquid phase micro-extraction with a configuration of solvent bar (SBME) using the ionic liquid Cyphos® 101 as extractant is proposed. Optimized conditions for SBME of Cd, Ag and Pd were 50% Cyphos® 101 in the organic solution, extraction time 30 min and 800 rpm stirring rate. Leaching was in all cases lower than 0.1%. Metallic concentrations were measured by flame atomic absorption spectroscopy. The method was applied to the extraction of Ag, Cd and Pd in natural water samples. Except for waste water, Pd extraction was higher than 90% in all cases. Cd (≈100%) and Ag (93-95%) offered their best results for saline samples. Concluding, the proposed system is a low cost and green methodology that allows a simple and fast extraction of trace pollutants such as Ag, Cd and Pd in different natural waters, including highly saline samples.
Increasing water consumption and diminishing fresh water resources have created the need for new water treatment technologies to supply safe water for domestic and industrial needs. The development of polymeric nanofiltration (NF) membrane technology led to water treatment at lower operating pressures than that of reverse osmosis. NF membranes reject particles and multivalent ions, however, monovalent ions pass through them along with water molecules. Factors such as selectivity and permeability, and fouling also limit their application. Incorporating suitable nanomaterials with polymer membranes has solved major problems, such as biofouling, scaling, low flux rate, selectivity, and degradation. Recent studies reveal that nanoporous single layer graphene and stacked graphene oxide (GO) membranes with desired spacing between layers are capable of rejecting monovalent ions, and are promising materials for future nanofiltration-based desalination. GO has antifouling properties that are highly advantageous for improving membrane properties. The basic understanding of the mechanism of graphene-based nanofiltration have been reported mainly based on computational studies. Hence, a great deal of experimental research is essential to develop efficient graphene membrane-based desalination methods for practical use. In this review, we highlight various properties of graphene and its derivatives that are essential for improving salt rejection, flux, and antifouling.
Colloidal silica involved fouling behaviors in direct contact membrane distillation (DCMD), vacuum membrane distillation (VMD) and sweeping gas membrane distillation (SGMD) were studied. Three foulants were used in the experiments, including colloidal silica as representative of particulate foulants, calcium bicarbonate as dissolved inorganic foulant, and NOM (humic acid + alginate + BSA) as the dissolved organic foulant. The three types of fouants were combined to produce four different feed waters: silica alone; silica + calcium bicarbonate; silica + NOM; and silica + calcium bicarbonate + NOM. With 25% feed recovery, it was found that VMD showed the worst performance for most of the foulant combinations due to turbulence dead zones caused by the membrane deformation that increased foulant deposition. For the silica + calcium bicarbonate + NOM feed DCMD had the greatest fouling rate, although DCMD also had the highest flux of all configurations. SGMD showed the best fouling resistance of all configurations, although it was inclined to calcium carbonate fouling because carbon dioxide was removed in the permeate leading to calcium carbonate precipitation and could be alleviated by using air as sweeping gas. For feeds containing high-concentration calcium bicarbonate or carbonate foulants, VMD should be avoided to lower the formation of carbonate precipitants on the membrane surface if scale inhibitors are not used.
In this study, we have evaluated the occurrence and distribution of 78 pharmaceuticals in different aquatic marine environments from the Gulf of Cadiz (SW Spain) for the first time. The obtained results revealed that pharmaceuticals were present in seawater at total concentrations ranging 61-2133 and 16-189ngL(-1) in coastal and oceanic transects, respectively. Potential marine pollution hotspots were observed in enclosed or semi-enclosed water bodies (Cadiz Bay), showing concentrations that were one or two orders of magnitude higher than in the open ocean. The presence of these chemicals in local sewage treatment plants (STPs), one of the main contamination sources, was also assessed, revealing total concentrations of up to 23μgL(-1) in effluents. PhACs with the highest detection frequencies and concentrations in the sampling region were analgesics and anti-inflammatories followed by antibiotics in the case of samples from Cadiz Bay or caffeine in oceanic seawater samples. Risk quotients, expressed as ratios between the measured environmental concentration (MEC) and the predicted no-effect concentrations (PNEC) were higher than 1 for two compounds (gemfibrozil and ofloxacin) in effluent of Jerez de la Frontera sewage treatment plant (STP). No high environmental risk was detected in both coastal and oceanic sampling areas, although the information available about the effects of these chemicals on marine biota is still very limited and negative effects on non-target species cannot be discarded.
Membrane distillation (MD) is a promising means for high-purity separations like those needed for potable water. The benefits of MD have not been realized for the treatment of surface or ground water, whereby the presence of both humic acid and calcium are detrimental to other membrane-based processes. Accordingly, this study investigated the efficacy of MD in rejecting various contaminants, namely, ibuprofen, boron and arsenic, in the presence of typical feeds comprising humic acid, calcium chloride and sodium chloride. Feeds investigated ranged from DI water to synthetic feeds mimicking surface water to NEWater brine (i.e., reverse osmosis concentrate from a local wastewater treatment plant in Singapore) brine. Results consistently indicate constant permeate fluxes and low conductivities throughout the experiments with varying concentrations of humic acid and calcium, along with varying pH values, which implies negligible membrane fouling and wetting. Also, complete rejections of boron and arsenic were achieved, while rejection of ibuprofen was approximately 90%. The detection of the non-volatile ibuprofen and humic acid in the permeate suggests some hydrophobic interactions with the membrane. Finally, a cost analysis was carried out to evaluate MD against the conventional nanofiltration (NF) process. Although MD is currently at a higher cost than NF, (i) the availability of improved membranes tailored for MD and low-cost heat would decrease costs to at least those of NF, and (ii) the performance of MD is superior in terms of sustained flux over prolonged periods, capacity to treat feeds with higher concentrations of foulants, and better permeate quality.
An experimental study is used to examine the effect of high concentration of several salts, i.e., NaCl, MgCl2, Na2CO3 and Na2SO4 on permeate flux and rejection factor by air gap membrane distillation (AGMD). A comparative study involving three different membrane pore sizes (0.2, 0.45 and 1.0μm) were performed to investigate the influence of pore size on energy consumption, permeate flux and rejection factor. The permeate flux decline is higher than that predicted from the vapour pressure reduction. Furthermore, the energy consumption was monitored at different membrane pore size and was found to be increased when the concentration increased.
Membrane scaling and mitigation techniques during air gap membrane distillation (AGMD) of seawater were investigated. The results showed a strong influence of AGMD operating temperature on not only the process water flux but also membrane scaling and subsequent cleaning efficiency. Elevating feed/coolant temperature from 35/25 to 60/50 °C increased water flux, but also escalated membrane scaling of the AGMD process. Membrane scaling was more severe, and occurred at a lower water recovery (68%) when operating at 60/50 °C compared to 35/25 °C (78%) due to increased concentration polarisation effect. Operating temperature also affected the efficiency of the subsequent membrane cleaning. Membrane scaling that occurred at low temperature (i.e. 35/25 °C) was more efficiently cleaned than at high temperature (i.e. 60/50 °C). In addition, membrane cleaning using vinegar was much more efficient than fresh water. Nevertheless, vinegar cleaning could not completely restore the membrane surface to the original condition. Scaling material remaining on the membrane surface facilitated scaling in the next operation cycle. On the other hand, anti-scalant addition could effectively control scaling. Membrane scaling during AGMD of seawater at 70% water recovery and 60/50 °C was effectively controlled by anti-scalant addition.
The reuse of wastewater is a key factor in a closed water cycle approach, in which wastewater is treated and then reused. This approach is both mandatory for the development of dry areas and necessary for the sustainability of industrialized countries in terms of environmental impacts and resource preservation. Although there are some virtuous examples of water reuse projects in the world, there is still much to be done, especially in terms of incentives and economic viability. Aim of the present paper is to give thermodynamic and engineering elements in order to develop an economic incentive to promote wastewater reuse and to adopt the closed water cycle approach. At this scope a techno-economic analysis of the civil wastewater depuration and reverse osmosis treatment of the secondary effluent is presented, by using the typical approach of the chemical engineering. The cost of the treated water in relation to the fundamental parameters of the plant is calculated together with an “energy based” incentive, evaluated through the efficiency of the state-of-the-art desalination process. This last can make a reuse project economically feasible on the basis of rigorous thermodynamic considerations. These latter give a universal character to the incentive calculation and also reward the process optimization towards the goal of lowering the carbon emissions. The validity of the proposed incentive method is evaluated through the analysis of three wastewater treatment and reuse projects at different scale. The results show how it is possible to obtain a positive Earnings Before Interests and Taxes (EBIT) for plant productivity above the 200 m3/day, by including the proposed incentive in the Business Plan of the integrated plant of Water Treatment and Reuse.
Direct contact membrane distillation (DCMD) supplied with waste heat was demonstrated for water recovery from saline demineralisation regeneration waste. The pilot plant was located at a gas fired power station which provided the < 40 °C waste heat and wastewater to the DCMD system with 0.67 m2 of membrane area. The trial was operated over three months without replacing the membranes or module and achieved 92.8% water recovery. Flux was approximately 3 L/(m2·h) and was dependant mostly on the waste heat temperature being supplied. Membrane fouling affected flux and thermal energy demand only at the very end of the trial. The system produced a high quality distillate product with average 99.9% dissolved solids rejection. Small amounts of ammonia and carbon dioxide however were found in the permeate. Membrane analysis post-trial revealed fouling was principally inorganic scale but organic matter on the membrane was also evident. Permeate side fouling was also observed, attributed to corrosion of the cooling heat exchanger. Based on the available energy for a continuously operating 500 MW (electric) rated power station, the treatment potential was estimated at up to 8000 kL/day, which is practical for supplying water to numerous industrial, residential or agricultural sites.
In this article, two methods for in-depth analysis of humic substances fluorescence are presented. The first one allows the combined analysis of fluorescence excitation emission matrix (EEM) with chromatography technique. The main issue is the coupling of size exclusion chromatography (SEC) with spectroscopy by the use of an absorption and a fluorescence spectrometer as additional detectors. These allow a detailed characterization of humic substances depending on their molecular size, concentration and optical properties. For the evaluation of the resulting complex data, a model based on non-negative matrix factorization, which is also presented in this article, was developed. From the results of the examined humic substances standards, the second method was developed. It allows the characterization and quantification of humic substances fluorescence of a natural water sample solely on the basis of an excitation-emission matrix (EEM). The validation of the model is carried out within the framework of extensive analysis of real water samples.
In 2013, around 1336 desalination plants in the United States (US) provided purified water mainly to municipalities, the industry sector and for power generation. In 2013 alone, ∼200 million m(3) of water were desalinated; the amount that could satisfy annual municipal water consumption of more than 1.5 million people in the US. Desalination has proven to be a reliable water supply source in many countries around the world, with the total global desalination capacity of ∼60 million m(3)/day in 2013. Desalination has been used to mitigate water scarcity and lessen the pressure on water resources. Currently, data and information about desalination are still limited, while extensive socio-economic analyses are missing. This paper presents an econometric model to fill this gap. It evaluates the impact of selected socio-economic variables on desalination development in the US in the time span 1970-2013. The results show that the GDP and population growth have significantly impacted the desalination sector over the analyzed time period. The insights into the economics of desalination provided with this paper can be used to further evaluate cost-effectiveness of desalination both in the US and in other countries around the world.
The study of nonisothermal flux of air through porous media dates back to 1873, whereas the existence of a nonisothermal liquid transport through membranes was first described in 1907. This phenomenon termed thermo-osmosis (TO) did not involve any liquid/vapor phase transition and was carried out through both dense and porous hydrophilic membranes. About 50. years later, when porous hydrophobic membranes were used and the nonisothermal vapor transport was studied through dry pores, the phenomenon was known as membrane distillation (MD). This nonisothermal membrane separation process is applied mostly in desalination and for the treatment of different types of wastewaters including brines for water production. It was known 50. years ago but has only recently made its way toward industrial applications. It still needs to be improved further in various key aspects. Compared with TO, much more interest is being devoted to MD. The total number of published articles on MD, as of December 31, 2013, is more than seven times greater than that of TO. A significantly increased interest in membrane distillation (MD) technology has been observed during the past 13. years for both its experimental and theoretical aspects, including MD membrane engineering. More than 58% of the research studies were performed using the direct contact membrane distillation (DCMD) configuration. A sharp increase of investigations on fabrication and modification of membranes for MD has been seen during the past 10. years. However, this amounts to only 16.8% of the total published studies on MD. This chapter provides a comprehensive historical perspective of thermo-osmosis (TO) and MD, important key characteristics of MD, membranes used in MD and possible MD technological configurations, different transport mechanisms through MD membranes and developed theoretical models, different fields of applications of MD, and future trends related to interesting and promising research fields in MD.
In this work, desalination tests were carried out by vacuum membrane distillation, with the aim of investigating the purity (in terms of conductivity) of the produced permeate. Consecutive experiments, with and without cleaning in between, were made on lab-prepared modules equipped with commercial capillary polypropylene membranes. In particular, by working at fixed operating conditions (feed flow rate and composition, temperature and vacuum pressure) the effects of a partial and of a total cleaning procedure on the permeate conductivity were analyzed and compared. Depending on the used approach, pure water or high-purity water can be obtained as permeate.
In this work, we first show the findings of the autopsy performed on the membranes used in the
Scarab AB ® membrane distillation (MD) system at the solar MD pilot plant in Plataforma Solar
de Almeria (PSA) in Spain. The fouling and the damage endured by the MD membranes during
intermittent long-term (2010-2013) solar-powered operation in the pilot plant were assessed and
characterized. Different cleaning strategies were used to remove the fouling layer and restore the
membrane properties. Data regarding relevant membrane characteristics for the MD process,
such as; contact angle, gas permeability, porosity, liquid entry pressure, mechanical strength,
etc., and their relationship with the membrane performance under MD operation were discussed
and analyzed. Scanning electron microscopy (SEM) was employed to study the morphology of
the fouled and cleaned membranes and characterize the membrane damage. The identified best
cleaning procedure was then applied in the MD plant system at PSA. Results suggested that
cleaning effectively removed great part of the fouling and reduced the wetting of the membranes.
However, this improvement was offset by the effect of inactive periods during which wetting
processes were favored.
The main objective of this work (Part I) is to conduct a comprehensive structural characterization of humic substances, using all the current fluorescence techniques: emission scan fluorescence (ESF), synchronous fluorescence spectroscopy (SFS), total luminescence spectroscopy (TLS or EEM) through the use of both 2-D contour maps and 3-D plots, fluorescence index and the λ0.5 parameter. Four humic substances were studied in this work: three of them were provided by the International Humic Substances Society (Suwannee River Fulvic Acid Standard, Suwannee River Humic Acid Standard and Nordic Reservoir Fulvic Acid Reference) and the other one was a commercial humic acid widely used as a surrogate for aquatic humic substances in various studies (Aldrich Humic Acid: ALHA). The EEM spectra for the three natural aquatic substances were quite similar, showing two main peaks of maximum fluorescence intensity: one located in the ultraviolet region and centered at around Ex/Em values of 230/437nm (peak A) and another one in the visible region, centered at around 335/460nm (peak C); however, the EEM spectrum of ALHA is completely different to those of natural aquatic humic substances, presenting four poorly resolved main peaks with a high degree of spectral overlap, located at 260/462, 300/479, 365/483 and 450/524nm. The synchronous spectra at Δλ=18 and 44nm (especially at Δλ=18nm) allowed the identification of a protein-like peak at λsyn around 290nm, which was not detected in the EEM spectra; as it happened with EEM spectra, the synchronous spectra of ALHA are quite different from those of the aquatic humic substances, presenting a higher number of bands that suggest greater structural complexity and a higher degree of polydispersity. Good correlations were achieved between (13)C NMR aromaticity and both fluorescence index and λ0.5 parameter. The different spectra presented by ALHA compared to those shown by the natural aquatic humic substances for all the fluorescence techniques studied suggest an important structural difference between them, which cast doubt on the use of commercial humic acids as surrogates for natural humic substances.
The effects of temperature on the stability of a soil humic acid were studied in the present work. Solid samples of Gohy-573 humic acid (HA) and dissolved ones in aqueous solution (pH 6.0, 0.1 mol L−1 NaClO4) were investigated in order to understand the impact of temperature on the chemical properties of the material. The methods applied to solid samples in the present investigation were thermogravimetric analysis (TGA), temperature-programmed desorption coupled with mass spectrometry (TPD–MS), and in situ diffuse reflectance infrared Fourier transformed spectroscopy (in situ DRIFTS). Humic acid samples were studied in the 25–800 °C range, with focus on thermal/chemical processes up to 250 °C. The reversibility of the changes observed was investigated by cyclic changes to specified temperature ranges (40–110 °C). All measurements were conducted under inert-gas atmosphere in order to avoid samples combustion at increased temperatures. Aqueous solutions were analyzed by UV–vis absorption spectroscopy after storage at temperatures up to 95 °C, and storage times up to 1 week. For temperatures below 100 °C experiments on solid and aqueous samples have shown results which were consistent to each other. The amount of water desorbed is temperature dependent and up to 70 °C this process was totally reversible. Above 70 °C an irreversible loss of water was also observed, which according to UV–vis spectroscopy corresponds to water produced by condensation leading to more condensed polyaromatic structures. The water released up to 110 °C was about 7 wt% of the total mass of the dried humic acid, where less than 50% corresponded to reversibly adsorbed water. At higher temperatures (>110 °C), gradual decomposition resulting in the formation of carbon dioxide (110–240 °C), and carbon monoxide (140–240 °C) takes place. Hence, thermal treatment of Gohy-573 humic acid above 70 °C results in irreversible structural changes, that could affect chemical properties (e.g., complex formation) of the material.
Desalination capacity has rapidly increased in the last decade because of the increase in water demand and a significant reduction in desalination cost as a result of significant technological advances, especially in the reverse osmosis process. The cost of desalinated seawater has fallen below US1.00/m3) for a similar facility. In addition to capital and operating costs, other parameters such as local incentives or subsidies may also contribute to the large difference in desalted water cost between regions and facilities. Plant suppliers and consultants have their own cost calculation methodologies, but they are confidential and provide water costs with different accuracies. The few existing costing methodologies and software packages such as WTCost© and DEEP provide an estimated cost with different accuracies and their applications are limited to specific conditions. Most of the available cost estimation tools are of the black box type, which provide few details concerning the parameters and methodologies applied for local conditions. Many desalination plants built recently have greater desalinated water delivery costs caused by special circumstances, such as plant remediation or upgrades, local variation in energy costs, and site-specific issues in raw materials costs (e.g., tariffs and transportation). Therefore, the availability of a more transparent and unique methodology for estimating the cost will help in selecting an appropriate desalination technology suitable for specific locations with consideration of all the parameters influencing the cost. A techno-economic evaluation and review of the costing aspects and the main parameters influencing the total water cost produced by different desalination technologies are herein presented in detail. Some recent developments, such as the increase of unit capacity, improvements in process design and materials, and the use of hybrid systems have contributed to cost reduction as well as reduction in energy consumption. The development of new and emerging low-energy desalination technologies, such as adsorption desalination, will have an impact on cost variation estimation in the future.
A new module design for membrane distillation, namely material gap membrane distillation (MGMD), for seawater desalination has been proposed and successfully tested. It has been observed that employing appropriate materials between the membrane and the condensation plate in an air gap membrane distillation (AGMD) module enhanced the water vapor flux significantly. An increase in the water vapor flux of about 200–800% was observed by filling the gap with sand and DI water at various feed water temperatures. However, insulating materials such as polypropylene and polyurethane have no effect on the water vapor flux. The influence of material thickness and characteristics has also been investigated in this study. An increase in the water gap width from 9 mm to 13 mm increases the water vapor flux. An investigation on an AGMD and MGMD performance comparison, carried out using two different commercial membranes provided by different manufacturers, is also reported in this paper.
http://www.sciencedirect.com/science/article/pii/S0376738813006595
Air Gap Membrane Distillation, using a high concentration of NaCl, MgCl2, Na2CO3, and Na2SO4, is implemented in this study. Permeate fluxes are measured for different feed concentrations and membrane pore sizes (0.2 and 0.45μm). The flux declines as the concentration of salt increases, and increases as the pore size increases. The TF200 membrane showed excellent hydrophobicity compared to TF450. Moreover, the energy consumption was measured at different salt concentrations for the different membrane sizes, and was found to be independent of membrane pore size, salt type and salt concentration in the feed solution.
Novel composite membrane distillation membranes were prepared by blending the hydrophilic polysulfone with hydrophobic surface modifying macromolecules (SMMs). Three different types of SMMs were tested. These SMMs were synthesized and characterized for fluorine content, molecular weights and glass transition temperature. Phase inversion method in a single casting step was used to prepare the composite membranes. The membranes were characterized by means of different techniques such as contact angle measurement, gas permeation test, liquid entry pressure of water and scanning electron microscopy. Finally, these membranes were tested for desalination by direct contact membrane distillation (DCMD). Different membrane preparation conditions affecting membrane morphology, structure and DCMD performance were investigated. The parameters studied were the SMMs type, polysulfone concentration, solvent type and non-solvent additive concentration in the casting solution. Attempts linking the membrane morphology to its DCMD performance have been made. It was found that increasing the polymer concentration or the non-solvent additive concentration decreased the permeate flux of the porous composite hydrophobic/hydrophilic membranes since the liquid entry pressure of water increased and the ratio of the membrane pore size time the porosity over the effective pore length (rɛ/Lp) decreased. Furthermore, the stoichiometric ratio of the SMMs, type of SMMs, was found to affect considerably the characteristics and permeate flux of the composite membranes. In general, the composite membranes with higher liquid entry pressure of water exhibited smaller permeate fluxes. Moreover, the obtained results were compared to those of a commercial polytetrafluoroethylene membrane and it was observed that some of the SMMs blended polysulfone membranes achieved better DCMD fluxes than those of the commercial membrane. A permeate flux 43% higher than that of the commercial membrane was achieved with 99.9% NaCl separation factor.
Extensive organic characterisation of a wastewater using liquid chromatography with a photodiode array and fluorescence spectroscopy (Method A), and UV(254) and organic carbon detector (Method B) was undertaken, as well as with fluorescence excitation emission spectroscopy (EEM). Characterisation was performed on the wastewater before and after ion exchange (IX) treatment and polyaluminium chlorohydrate (PACl) coagulation, and following microfiltration of the wastewater and pre-treated wastewaters. Characterisation by EEM was unable to detect biopolymers within the humic rich wastewaters and was not subsequently used to characterise the MF permeates. IX treatment preferentially removed low molecular weight (MW) organic acids and neutrals, and moderate amounts of biopolymers in contrast to a previous report of no biopolymer removal with IX. PACl preferentially removed moderate MW humic and fulvic acids, and large amounts of biopolymers. PACl showed a great preference for removal of proteins from the biopolymer component in comparison to IX. An increase in the fluorescence response of tryptophan-like compounds in the biopolymer fraction following IX treatment suggests that low MW neutrals may influence the structure and/or inhibit aggregation of organic compounds. Fouling rates for IX and PACl treated wastewaters had high initial fouling rates that reduced to lower fouling rates with time, while the untreated Eastern Treatment Plant (ETP) wastewater displayed a consistent, high rate of fouling. The results for the IX and PACl treated wastewaters were consistent with the long-term fouling rate being determined by cake filtration while both pore constriction and cake filtration contributed to the higher initial fouling rates. Higher rejection of biopolymers was observed for PACl and IX waters compared to the untreated ETP water, suggesting increased adhesion of biopolymers to the membrane or cake layer may lead to the higher rejection.
Direct contact membrane distillation process has been conducted for the treatment of humic acid solutions using microporous polytetrafluoroethylene and polyvinylidene fluoride membranes. The membranes were characterized in terms of their non-wettability, pore size and porosity. Water advancing and receding contact angles on the top membrane surfaces were measured. Experiments were also carried out employing pure water as feed at different mean temperatures and the water vapor permeance of each membrane was determined. Different humic acid concentrations in the feed solution, pH values and transmembrane temperature difference were tested. The direct contact membrane distillation technique is more adequate for the treatment of humic acid solutions than the applied pressure-driven separation processes, as lower membrane fouling was detected.
Membrane distillation (MD) is one of the non-isothermal membrane separation processes used in various applications such desalination, environmental/waste cleanup, food, etc. It is known since 1963 and is still being developed at laboratory stage for different purposes and not fully implemented in industry. An abrupt increase in the number of papers on MD membrane engineering (i.e. design, fabrication and testing in MD) is seen since only 6 years ago. The present paper offers a comprehensive MD state-of-the-art review covering a wide range of commercial membranes, MD membrane engineering, their MD performance, transport mechanisms, experimental and theoretical modeling of different MD configurations as well as recent developments in MD. Improved MD membranes with specific morphology, micro- and nano-structures are highly demanded. Membranes with different pore sizes, porosities, thicknesses and materials as well as novel structures are required in order to carry out systematic MD studies for better understanding mass transport in different MD configurations, thereby improving the MD performance and looking for MD industrialization.
Humic substances (HS) perform a fundamental role in aquatic environments, exhibiting different levels of reactivity in retaining metal ions and organic pollutants. Also, they control the primary production of these ecosystems and act in the carbon sequestering process. In order to improve our understanding vis-à-vis the structural and functional features of HS from aquatic systems, this study aimed to chemically and spectroscopically characterize humic acids (HA) isolated from bottom sediment samples of a stream in a Brazilian subtropical microbasin by elemental analysis, and infrared (FT-IR), ultraviolet and visible (UV–Vis) and solid-state 13C nuclear magnetic resonance (CP–MAS 13C NMR) spectroscopies, thermogravimetry (TG), and scanning electron microscopy (SEM). Although all samples originated from the same environment, the data showed that the HA have distinct chemical and spectroscopic properties, and that the location and characteristics of the sampling points from which the sediments were collected played an important role in the differences observed. Furthermore, vascular plant matter is probably the main contributor to these samples.