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Water scarcity is a global issue, and desalination is an alternative to providing fresh water. Renewable energies could be used in thermal desalination to produce freshwater from high saline concentration solutions. In this paper, the experimental performance of an air-injection-Air Gap Membrane Distillation (AGMD) module is presented. The effect of the operation parameters (saline solution temperature, air flow, and salt concentration) on the distilled water rate was evaluated. The air injection enhanced the distilled water rate by 22% at the highest air flow and a solution flow rate of 80 °C, compared to the conventional condition (without air injection) at a salt concentration of 100,000 ppm. Under the same operating conditions, the increase was 17% at a salt concentration of 70,000 ppm. The maximum distilled water rate was 14.10 L/m2·h at 80 °C and an airflow of 1.5 L/min with the highest salt concentration, while it was also 14.10 L/m2·h at the lower salt concentration was 14.10 L/m2·h. The distilled water quality also improved as the air flow increased, since a conductivity reduction of 66% was observed. With the described mathematical model, 94% of the calculated values fell within ±10% of the experimental data for both salt concentration conditions.

Water can be purified with a hydrophobic membrane in an air gap membrane distillation (AGMD) configuration. The present study presents experimental data for such a system and comparison with model predictions from simulations in a Matlab environment. The importance of the respective roles of feed water temperature and flow rate, cold water temperature and flow rate, and the extent of air gap on water production rate is studied, both from experiments and simulations. The comparison is found to be quite good, thus validating the choice of the mathematical model. In addition, possible improvements in the device performance are examined by using a surface modification of the hydrophobic membrane. In this respect, a layer of lead from a pencil is coated over the membrane. Evaporation rates through the pencil-coated membrane were measured in a solar simulator with 1 sun, 2 sun, and 3 sun settings. A distinct improvement in the evaporation rate was obtained when a pencil coated membrane was used. Following this observation, an augmented two-sided AGMD system is proposed, which carries a coated membrane exposed to solar radiation on one side and an uncoated membrane on the other. [ABSTRACT FROM AUTHOR]

The challenge for water recovery from dye manufacturing wastewaters that contain high salinity and high dye concentration is recovery efficiency and water quality. The permeate flux and water quality mainly reflect the membrane distillation performance in treatments of these wastewaters. This study investigated the effects of Air Gap Membrane Distillation's (AGMD) operational parameters and its interactions on permeate fluxes and recovered water quality in treatments of real dye manufacturing wastewaters using design of experiments (DOE). The DOE method used the response surface methodology (RSM) of central composite design (CCD) to predict and optimize the permeate flux and recovered water quality. The results showed a reliable predictive model of actual permeate flux (15.5 kg.m−2.h−1) with the adjusted R2adj of 0.995 and only 34.4%–73.8% of the theoretical permeate flux because of salt contents' main effect. The rejection rate of chloride ion (Cl−), sulfate ion (SO42-), and ADMI (American Dye Manufacturers' Institute) color index from real dye manufacturing wastewaters with high salinity was more than 99,9%. The COD removal rate of raw wastewater (C1) from dye manufacturing was 99,4%. This study help in understanding theoretical and actual membrane flux and the scale-up of a membrane treatment plant. [ABSTRACT FROM AUTHOR]

This work aimed to investigate temperature polarization (TP) and concentration polarization (CP), which affect solar-powered air-gap membrane distillation (SP-AGMD) system performance under various operating conditions. A mathematical model for the SP-AGMD system using the experimental results was performed to calculate the temperature polarization coefficient (τ), interface temperature (Tfm), and interface concentration (Cfm) at various salt concentrations (Cf), feed temperatures (Tf), and flow rates (Mf). The system of SP-AGMD was simulated using the TRNSYS program. An evacuated tube collector (ETC) with a 2.5 m2 surface area was utilized for solar water heating. Electrical powering of cooler and circulation water pumps in the SP-AGMD system was provided using a photovoltaic system. Data were subjected to one-way analysis of variance (ANOVA) and Spearman’s correlation analysis to test the significant impact of operating conditions and polarization phenomena at p < 0.05. Statistical analysis showed that Mf induced a highly significant difference in the productivity (Pr) and heat-transfer (hf) coefficients (p < 0.001) and a significant difference in τ (p < 0.05). Great F-ratios showed that Mf is the most influential parameter. Pr was enhanced by 99% and 146%, with increasing Tf (60 °C) and Mf (12 L/h), respectively, at a stable salt concentration (Cf) of 0.5% and a cooling temperature (Tc) of 20 °C. Also, the temperature increased to 85 °C when solar radiation reached 1002 W/m2 during summer. The inlet heat temperature of AGMD increased to 73 °C, and the Pr reached 1.62 kg/(m2·h).

The impact of the membrane module length on the air gap membrane distillation (AGMD) process performance was investigated experimentally under various operating parameters. In this work, two different lengths of the AGMD module (i.e., 0.59 m and 0.28 m) were tested separately to get the optimum in terms of lower energy consumption and higher water output. The mechanism of mass and heat transfer in the AGMD process is considered as well. The experimental results were subjected to the two-way analysis of variance (ANOVA) to examine the significant influence of operating parameters and modules at p < 0.05. Also, the results were subjected to simple linear regression analysis to measure the relationship between the operating parameters (independent variables) and the performance indicators (dependent variables). Statistical analysis carried on the experimental data manifested that the feed temperature parameter brought a highly significant difference (p < 0.001*) on the water output (Pw) and temperature difference (∆Tcross). However, feed temperature had a non-significant effect (p > 0.05) on the gained output ratio (GOR), energy consumption (STEC), and heat input (QH.I). Under optimal operating parameters of a flow rate of 18 L/h, feed temperature of 60 °C, coolant temperature of 20 °C, and feed salinity of 5000 mg/L, the Pw and GOR were increased by 64.50% and 50.00%; the STEC and QH.I were reduced by 57.78% and 34.09%, respectively, from the long module to the short one. Thence, changing the length of the membrane module from 0.59 to 0.28 m promoted remarkably the performance of the AGMD process. [ABSTRACT FROM AUTHOR]

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

The single stage air gap membrane distillation (AGMD) has been successfully realized to desalt simulated and natural seawater. The effect of different operating parameters including feed temperature, width of air gap, feed salt concentration and feed and permeate flow rates on the performance of AGMD process has been investigated. Two different commercial polyvinylidene fluoride (PVDF) membranes, with different pore size (0.45 and 0.22 µm) were tested and compared for different operating parameters. Experimental results displayed that the larger pore size membrane showed a better mass transfer performance but led to a slight decrease of water production quality. However, AGMD process can produce water with high salt rejection, more than 99% and a maximum permeate flux of 9.06 kg m–2 h–1 was obtained at a feed temperature of 77°C. The comparison between predicted model and experimental AGMD permeate fluxes shown a good agreement, their average deviation was around 6.9%. [ABSTRACT FROM AUTHOR]

As freshly available water around the world becomes scarcer, schemes to reuse and rectify contaminated water sources are becoming a necessity. The implementation of conventional treatment processes increases stress on existing infrastructure resources, requiring significant quantities of energy and/or chemicals, including pre-treatment processes and ongoing maintenance. An unconventional alternative to these processes is air-gap membrane distillation (AGMD), an emerging technology delivering excellent rejection of contaminants over a broad range of operating conditions. While showing great promise, the size of membrane distillation systems in existing literature is not readily scaled to industrial levels. In this paper, we present the results of our research in terms of permeate quality, rejection efficiency and scalability of a large laboratory scale AGMD system, with effective area of approximately 25 times larger than those presented in previous studies. This study found a large discrepancy in flux production when compared with small scale results, with experimental data analysed using normality and residual analysis tests. Statistical analysis of the AGMD process data provides insight into the key driving forces and interactions of feedwater temperature, concentration and flowrate on flux production. Results showed excellent rejection of contaminants (>98%) along with some fouling evident after approximately 25 h of operation. [ABSTRACT FROM AUTHOR]

Cruise tourism industry is expanding worldwide. An average cruise ship produces over 1000 m 3 per day of fresh water. To date, multi-stage flash distillation (MSF) and seawater reverse osmosis (SWRO) are the most used desalination technologies on board. However, their application has some drawbacks: a great volume needed on board (MSF), increases of fuel consumptions (SWRO), carbon dioxide emissions and total costs. In this way, membrane distillation (MD) is proposed to overcome limitations of existing conventional technologies. Different configurations (DCMD, AGMD and WGMD) and two membranes (0.45 and 0.20 μm) were tested and compared with each other in order to study potential use of MD on board cruise ships, taking advantage of waste heat generated in the engine cooling system. Operational conditions were established based on an approximation of real conditions encountered in a cruise vessel. Results obtained for simulated and real seawater tests showed advantages for WGMD configuration using PTFE 0.45 μm pore size membrane, reaching a high permeate quality (99.99% of salt rejection) and average fluxes around 13 L m − 2 h − 1 , being these similar to those produced by SWRO. In conclusion, use of MD, in the cruise industry, can be a sustainable water production choice that should be considered. [ABSTRACT FROM AUTHOR]

Membrane distillation is a novel process that could be adapted effectively for many water purification applications. In recent years, several bench, pilot and commercial scale membrane distillation systems with production capacities ranging from 20 L/d to 50 m3/d were developed and tested. In this work, a single cassette air-gap membrane distillation (AGMD) module was characterized to identify the effect of process parameters on distillate flux and thermal efficiency. Favorable conditions to obtain distillate flow rate of 1.5-3 kg/h were determined on a bench scale experimental setup. Factorial design of experiments was conducted and response surface methodology (RSM) was applied to develop an empirical regression model relating operating parameters with AGMD system performance indicators. Operating parameters including hot feed inlet temperature (THin), cold feed inlet temperature (TCin), feed flow rate (Vf) and feed conductivity (Cf) were considered. Distillate flux (Jd) and specific performance ratio (SPR) were selected as the performance indicators for the modeling. The developed regression model using RSM was tested by analysis of variance. Regression analysis showed agreement with the experimental data fitted with second-order polynomial model having determination coefficient (R2) values of 0.996 and 0.941 for Jd and SPR, respectively. Numerical optimization has been carried out to identify optimal set of operating conditions for achieving desired operation. Also, dynamic simulation of the membrane distillation module integrated solar thermal system has been reported along with validation of the system model by comparing with the experimental data obtained from a pilot scale setup located in UAE. [ABSTRACT FROM AUTHOR]

In membrane distillation, the liquid phase including dissolved components is retained by a hydrophobic membrane, while the microporous structure allows transport of vapor through the membrane. The vapor pressure difference over the membrane is the driving force and is applied using a variety of configurations. This paper directly compares the flux and energy efficiency of a lab scale direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), permeate gap membrane distillation (PGMD) and vacuum membrane distillation (VMD) using the same bulk driving force. The highest flux was observed for VMD, followed by DCMD > PGMD > AGMD. Furthermore, it was observed that the different configurations are not equally sensitive to the applied process conditions, including temperature difference, flow velocity and salinity. For the first time, also the importance of the specific requirements for the membrane for each configuration was investigated. [ABSTRACT FROM AUTHOR]

A comparative study was conducted to evaluate the performance of two membrane types of electrospun poly(vinylidene fluoride) (PVDF) and commercial ploytetrafluoroethylene (PTFE). The optimized needleless electrospinning technique was used to prepare PVDF membranes. Scanning electron microscopy (SEM), wettability tests, water flux, mechanical strength and liquid entry pressure (LEP) measurements were performed to evaluate the prepared membrane. Air gap membrane distillation (AGMD) experiments were carried out to investigate the salt rejection performance and the durability of membranes. The results show that our nanofibrous PVDF membrane presents higher water permeation flux (>20 kg/m h) compared to commonly used PTFE. In addition, the experimental data confirms that competitive salt rejection efficiency (>99.8%) was obtained in this new membrane. [ABSTRACT FROM AUTHOR]

In this paper, the experiments of desalinating the NaCl aqueous solution were carried out using an air gap membrane distillation module with energy recovery. Response surface methodology (RSM) was applied to develop a regression model relating operating parameters with performance indicators of the AGMD system. Operating parameters including cold feed inlet temperature (T 1 ), hot feed inlet temperature (T 3 ) and feed-in flow rate (F) were considered and distillate flux (J) and gained output ratio (GOR) were selected as the performance indicators. The regression model developed by RSM was tested by analysis of variance (ANOVA). Based on the regression model, it was found that T 3 has the highest positive effect on both J and GOR, and there was a trade-off between J and GOR when T 1 and F varied. Non-dominated sorting genetic algorithm II (NSGA-II) was firstly used to maximized J and GOR simultaneously. Under the optimum condition, the maximum GOR and J could reach 8.78 and 5.07 L/m 2 ·h respectively. [ABSTRACT FROM AUTHOR]

Abstract: Membrane distillation (MD) has shown potential as a desalination process. As a thermally driven membrane technology which runs at relatively low pressure and can tolerate high salinity feed, MD may be useful for desalination under condition for which reverse osmosis is not a good option. However, improved energy utilization and reduced water production cost in MD systems remains an important technical challenge. In this paper, we examine how optimization of the heat and mass transfer properties of the MD membrane can raise water flux and reduce unit energy consumption. Numerical modeling tools and factorial analysis are used to examine the effect of five MD membrane characteristics: porosity, tortuosity, thermal conductivity, pore diameter, and thickness. The water production cost (in USD/m3 of purified water) is calculated as a function of these parameters for a small-scale, single-stage MD process driven by waste heat. Two MD configurations were considered: Direct Contact MD (DCMD) and Air Gap MD (AGMD). All five membrane properties examined were found to have a significant effect on water production cost, although the magnitude of this effect was greatly dependent on the MD configuration. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: Solar-powered membrane distillation (SP-MD) desalination systems have been proposed as promising technology in off-grid areas with strong solar radiation. However, despite the various SP-MD pilot systems constructed and tested over the past two decades, such systems have not been commercialized or implemented on a large scale yet. The objective of this study is to provide a comprehensive review and assessment of established SP-MD systems and to identify common and contrasting features among them. The study focuses mainly on SP-MD systems that had been tested under field conditions for at least several days and which are either fully or partially powered by solar energy. The systems were first reviewed in terms of their physical properties and operating conditions. The systems were then evaluated in terms of several performance indicators, such as membrane flux achieved, energy consumption, gained output ratio (GOR), performance ratio, and the energy recovery scheme applied. A numerical model was constructed and run to evaluate how a key performance indicator, GOR, can be enhanced by certain operating parameters. The results of this study are used to suggest new areas of improvement for future SP-MD projects. [Copyright &y& Elsevier]

• Improvement in water productivity of a solar still with a hydrophobic membrane. • Validation experiments with a single and multi-stage AGMD. • Evaporation rate with and without hydrophobic porous membrane. • Dropwise condensation patterns. • Exergy analysis when the cold surface temperature is below ambient. While a low temperature solar still for water production is simple in design, the accompanying productivity is quite low. This factor has necessitated development of enhancement techniques for water production. Alternatives suggested are the use of porous media in a solar still such as sponge, blackened jute cloth, pebble pieces, and floating carbonaceous material for absorbing solar energy. The use of a hydrophobic porous membrane in a solar still has not been investigated systematically thus far. The formation of a convex liquid meniscus alters vapor pressure across the pores of the membrane and is the origin of increased evaporation rates. The goal of the present study is to examine the performance of a solar still configuration towards water production in the presence of a surface-mounted hydrophobic membrane. Vertical arrangement is possible in a prototype but is inconvenient to setup in a laboratory-scale experiment. Hence, the device is compared with another without a membrane in a horizontal solar still configuration. Variable air gap is necessary to drain the condensed drop from an inclined condensing surface. The air gap membrane distillation system (AGMD) thus configured is selected for investigation and further comparison with a mathematical model. Experiments carried out in the present work show that a 40–70% increase in water production is obtained with the use of a membrane. The increase in evaporation rate is visible in the dropwise condensation patterns over the condensing surface. The present study thus confirms the utility of using a hydrophobic membrane in increasing water productivity of a solar still apparatus. Exergy analysis of the solar still when a chiller is used for the condensing surface is also presented. [ABSTRACT FROM AUTHOR]

• Air-gap membrane distillation was used to treat RO reject and natural ground water. • Various feed temperature and flow rate were tested with two commercial membranes. • Low ultrasonic power was directly applied through spacers. • Ultrasound improved the flux by 100% for fouled membranes. • Fouling was decreased without membrane damage. Air Gap Membrane distillation (AGMD) is a thermally driven separation process capable of treating challenging water types, but its low productivity is a major drawback. Membrane fouling is a common problem in many membrane treatment systems, which exacerbates AGMD's low overall productivity. In this study, we investigated the direct application of low-power ultrasound (8–23 W), as an in-line cleaning and performance boosting technique for AGMD. Two different highly saline feedwaters, namely natural groundwater (3970 μS/cm) and RO reject stream water (12760 μS/cm) were treated using Polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) membranes. Theoretical calculations and experimental investigations are presented, showing that the applied ultrasonic power range only produced acoustic streaming effects that enhanced cleaning and mass transfer. Attenuated Total Reflection Fourier-Transform Infrared Spectroscopy (ATR FT-IR) analysis showed that ultrasound was capable of effectively removing silica and calcium scaling. Ultrasound application on a fouled membrane resulted in a 100% increase in the permeate flux. Cleaning effects accounted for around 30–50% of this increase and the remainder was attributed to mass transfer improvements. Contaminant rejection percentages were consistently high for all treatments (>99%), indicating that ultrasound did not deteriorate the membrane structure. Scanning Electron Microscopy (SEM) analysis of the membrane surface was used to confirm this observation. The images of the membrane surface demonstrated that ultrasound successfully cleaned the previously fouled membrane, with no signs of structural damage. The results of this study highlight the efficient and effective application of direct low power ultrasound for improving AGMD performance. [ABSTRACT FROM AUTHOR]