The main objective of this study is to examine how the charge densities of four monovalent anionsfluoride (F–), chloride (Cl–), bromide (Br–), and nitrate (NO3 –)influence their Donnan (charge) ...exclusion by a charged nanofiltration (NF) membrane. We systematically studied the rejection behavior of ternary ion solutions containing sodium cation (Na+) and two of the monovalent anions as a function of the pH with a polyamide NF membrane. In the solutions containing F– and Cl– or F– and Br–, F– rejection was higher than Cl– or Br– rejection only when the solution pH was higher than 5.5, suggesting that F– (which has a higher charge density) was repelled more strongly by the negatively charged membrane. The order of change in the activation energy for the transport of the four anions through the polyamide membrane as a response to the increase of the membrane negative charge was the following: F– > Cl– > NO3 – > Br–. This order corroborates our main hypothesis that an anion with a smaller ionic radius, and hence a higher charge density, is more affected by the Donnan (charge)-exclusion mechanism in NF. We conclude with a proposed mechanism for the role of ionic charge density in the rejection of monovalent anions in NF.
Membrane distillation (MD) has become an area of rapidly increasing research and development since the 1990s, providing a potentially cost effective thermally-driven desalination technology when ...paired with waste heat, solar thermal or geothermal heat sources. One principal challenge for MD is scaling and fouling contamination of the membrane, which has gained growing attention in the literature recently as well. The present paper surveys the published literature on MD membrane fouling. The goal of this work is to synthesize the key fouling conditions, fouling types, harmful effects, and mitigation techniques to provide a basis for future technology development. The investigation includes physical, thermal and flow conditions that affect fouling, types of fouling, mechanisms of fouling, fouling differences by sources of water, system design, effects of operating parameters, prevention, cleaning, membrane damage, and future trends. Finally, numerical modeling of the heat and mass transfer processes has been used to calculate the saturation index at the MD membrane interface and is used to better understand and explain some of trends reported in literature.
•Review: temperature, flow rate, SI, feed type, specific salt effects in MD fouling•Module type, and temperature and concentration polarization greatly affect fouling.•High temperature prevents MD biofouling.•Particulate fouling can be easily avoided by microfiltration.•Reducing feed pH, membrane superhydrophobicity, and antiscalants prevent MD scaling.
Nanofiltration performance as a function of feed temperature is relevant to several industrial settings including pretreatment for scale control in thermal desalination. Understanding of solute ...transport as a function of temperature is critical for effective membrane and system design. In this study, nanofiltration is modeled at 22, 40 and 50°C using the Donnan Steric Pore Model with dielectric exclusion (DSPM-DE). This modeling includes the temperature dependence of the three modes of solute transport, namely the convective, electromigrative, and diffusive modes, and the three mechanisms of solute exclusion, namely Donnan, steric, and dielectric exclusion. The effect of temperature is captured through the variation of membrane parameters and solvent and ionic mobilities with temperature. We compare the most abundant ionic compound in natural water, sodium-chloride with magnesium-chloride to portray how the salt passage and rejection change for a 1:1 salt compared to a 2:1 salt, and we analyze Arabian Gulf seawater to understand how rejection of scale-forming ions, such as Mg2+ and Ca2+, is affected by feed temperature. In all cases, solute transport increases with temperature, attributed predominantly to the cumulative effect of membrane parameters and only to a small extent (up to 5%) to the solvent viscosity and ion diffusivity together.
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•Nanofiltration is modeled at different temperatures.•Membrane parameters as well as water and ion mobility change with temperature.•Each form of ion transport increases in magnitude with temperature.•Main contribution to temperature induced changes is due to membrane parameters.•Transport and rejection ratio of seawater ions with temperature is observed.
Conventional water resources in many regions are insufficient to meet the water needs of growing populations, thus reuse is gaining acceptance as a method of water supply augmentation. Recent ...advancements in membrane technology have allowed for the reclamation of municipal wastewater for the production of drinking water, i.e., potable reuse. Although public perception can be a challenge, potable reuse is often the least energy-intensive method of providing additional drinking water to water stressed regions. A variety of membranes have been developed that can remove water contaminants ranging from particles and pathogens to dissolved organic compounds and salts. Typically, potable reuse treatment plants use polymeric membranes for microfiltration or ultrafiltration in conjunction with reverse osmosis and, in some cases, nanofiltration. Membrane properties, including pore size, wettability, surface charge, roughness, thermal resistance, chemical stability, permeability, thickness and mechanical strength, vary between membranes and applications. Advancements in membrane technology including new membrane materials, coatings, and manufacturing methods, as well as emerging membrane processes such as membrane bioreactors, electrodialysis, and forward osmosis have been developed to improve selectivity, energy consumption, fouling resistance, and/or capital cost. The purpose of this review is to provide a comprehensive summary of the role of polymeric membranes and process components in the treatment of wastewater to potable water quality and to highlight recent advancements and needs in separation processes. Beyond membranes themselves, this review covers the background and history of potable reuse, and commonly used potable reuse process chains, pretreatment steps, and advanced oxidation processes. Key trends in membrane technology include novel configurations, materials, and fouling prevention techniques. Challenges still facing membrane-based potable reuse applications, including chemical and biological contaminant removal, membrane fouling, and public perception, are highlighted as areas in need of further research and development.
Membrane distillation (MD) is a rapidly emerging water treatment technology; however, membrane pore wetting is a primary barrier to widespread industrial use of MD. The primary causes of membrane ...wetting are exceedance of liquid entry pressure and membrane fouling. Developments in membrane design and the use of pretreatment have provided significant advancement toward wetting prevention in membrane distillation, but further progress is needed. In this study, a broad review is carried out on wetting incidence in membrane distillation processes. Based on this perspective, the study describes the wetting mechanisms, wetting causes, and wetting detection methods, as well as hydrophobicity measurements of MD membranes. This review discusses current understanding and areas for future investigation on the influence of operating conditions, MD configuration, and membrane non-wettability characteristics on wetting phenomena. Additionally, the review highlights mathematical wetting models and several approaches to wetting control, such as membrane fabrication and modification, as well as techniques for membrane restoration in MD. The literature shows that inorganic scaling and organic fouling are the main causes of membrane wetting. The regeneration of wetting MD membranes is found to be challenging and the obtained results are usually not favorable. Several pretreatment processes are found to inhibit membrane wetting by removing the wetting agents from the feed solution. Various advanced membrane designs are considered to bring membrane surface non-wettability to the states of superhydrophobicity and superomniphobicity; however, these methods commonly demand complex fabrication processes or high-specialized equipment. Recharging air in the feed to maintain protective air layers on the membrane surface has proven to be very effective to prevent wetting, but such techniques are immature and in need of significant research on design, optimization, and pilot-scale studies.
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•This review offers an extensive literature survey on the wetting phenomenon in MD.•Describes wetting fundamentals, equations, parameters, and measurement techniques.•Analyzes wetting conditions, effects, prevention, and reversal techniques.•This review lays the groundwork for future technological advances in MD.
Atmospheric water harvesting (AWH) has received tremendous interest because of population growth, limited freshwater resources, and water pollution. However, key challenges remain in developing ...efficient, flexible, and lightweight AWH materials with scalability. Here, we demonstrated a radiative cooling fabric for AWH via its hierarchically structured cellulose network and hybrid sorption–dewing mechanisms. With 8.3% solar absorption and ∼0.9 infrared (IR) emissivity, the material can drop up to 7.5 °C below ambient temperature without energy consumption via radiative cooling. Water adsorption onto the hydrophilic functional groups of cellulose is dominated by sorption at low relative humidity (RH) and dewing at high RH. The cellulose network provides desirable mechanical properties with entangled high-aspect-ratio fibers over tens of adsorption–extraction cycles. In the field test, the cellulose sample exhibited water uptake of 1.29 kg/kg at 80% RH during the night. The profusion of radiative cooling fabric features desirable cost effectiveness and allows fast deployment into large-scale AWH applications.
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•Heat transfer resistances of HX and gap fix relative performance of DCMD and PGMD.•DCMD/CGMD with a thick membrane is resistant to high salinity, similar to AGMD.•Method to ...simultaneously determine cost-optimal membrane thickness and area.•Dimensionless framework gives generalizable results across all MD designs.•Derived maximum allowable membrane area per feed flow rate for high salinity MD.
This study presents a comprehensive analytical framework to design efficient single-stage membrane distillation (MD) systems for the desalination of feed streams up to high salinity. MD performance is quantified in terms of energy efficiency (represented as a gained output ratio, or GOR) and vapor flux, both of which together affect the specific cost of pure water production. Irrespective of the feed salinity, permeate or conductive gap MD (P/CGMD) perform better than direct contact MD (DCMD) when the heat transfer resistance of the gap (in P/CGMD) is lower than that of the external heat exchanger in DCMD. Air gap MD’s (AGMD) better performance relative to the other configurations at high salinity and large system area can be explained in terms of its thicker ‘effective membrane’, which includes the air-gap region. CGMD and DCMD employing a thick membrane are also resilient to high salinity, similar to AGMD, while not being susceptible to the gap flooding that can harm AGMD’s performance. A method is described to simultaneously determine the cost-optimal membrane thickness and system size as a function of the ratio of specific costs of heat energy and module area. At low salinity and small system size, GOR rises and flux declines with an increase in membrane area. For salty feed solutions, there exists a critical system size beyond which GOR also begins to decline. Since both GOR and flux are lower, no economic rationale favors operation above this critical size, irrespective of the costs of thermal energy and system area. A closed-form analytical expression for this critical system area is derived as a function of the feed salinity and two dimensionless ratios of heat transfer resistances within the MD module.
Energy consumption in reverse osmosis has decreased substantially over the last few decades thanks to the implementation of energy recovery devices, high efficiency pumps, and improved membranes. ...However, to further increase the adoption of the technology, its energy needs must still be reduced. Batch and semi-batch reverse osmosis configurations have shown potential to be the most energy efficient reverse osmosis variants, but have remaining inefficiencies from brine mixing and high downtime. In this study, we modeled in new detail a further improved double-acting batch reverse osmosis system that uses a high-pressure tank with a reciprocating piston. A high-pressure pump is used to fill the inactive side of the high pressure tank with feed that pressurizes the reverse osmosis system during the permeate production stage; the feed is then used as the working fluid for the next cycle of the system. Additionally, to account for the dynamic behavior of the system under different operational conditions, a hydraulic simulation model was developed to accurately predict the evolution of the pressure and other important parameters over time. A specific energy consumption of 1.88 kWh/m3 was calculated for seawater with a salinity of 35 g/kg, recovery of 50%, and a permeate flux of 15 LHM, and lower values SEC were obtained for lower permeate fluxes. Moreover, the downtime was found to be below 10% of the cycle time, without significantly increase the energy consumption of the system. This design also showed the potential to reduce the start-up time compared to previously proposed batch RO configurations.
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•The proposed double-acting batch RO configuration has a reduced energy use compared to previously proposed systems.•Properly designed double-acting batch RO systems have lower downtimes and start-up times than others batch RO system.•Water hydraulic models help to accurately predict the evolution of the pressure in batch RO systems.
High salinity waters (>7 wt%) remain a dominant challenge for reverse osmosis desalination, as the required osmotic pressures can exceed the burst pressure of the membranes. Osmotically assisted ...processes have been designed to circumvent this challenge, but have reduced efficiencies and extensive requirements for specialty pumps, stages and other equipment. Here, we present the first batch osmotically assisted process, which dramatically reduces the energy needs and required components of past processes. To do so, we develop an improved time-varying model which can capture the time-varying concentration profiles, fully discretized across the membrane module. This new process, batch counterflow reverse osmosis (BCFRO), recirculates the fluid on both sides of a membrane. A double-acting piston tank enables this brine recirculation to increase pressure over time. The combination of the new process with batch reverse osmosis, using the same components, can lead to a second law efficiency of 60% for high recovery of seawater. The analysis includes detailed trade-off mapping of recovery ratio per pass, total water recovery, water flux and pressures. Such an approach would be the best-in-class for higher salinities where the cost of energy is relatively important.
•First batch osmotically assisted process for high salinity reverse osmosis.•Uses a reciprocating piston tank to achieve batch operation and reduce downtime•Spatiotemporal salinity profiles calculated by discretizing the membrane module•Energy: more efficient than existing processes operating with the same pressure.•Cost: requires fewer components than existing processes when multi-staging.
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