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.
As reverse osmosis (RO) desalination capacity increases worldwide, the need to reduce its specific energy consumption becomes more urgent. In addition to the incremental changes attainable with ...improved components such as membranes and pumps, more significant reduction of energy consumption can be achieved through time-varying RO processes including semi-batch processes such as closed-circuit reverse osmosis (CCRO) and fully-batch processes that have not yet been commercialized or modelled in detail. In this study, numerical models of the energy consumption of batch RO (BRO), CCRO, and the standard continuous RO process are detailed. Two new energy-efficient configurations of batch RO are analyzed. Batch systems use significantly less energy than continuous RO over a wide range of recovery ratios and source water salinities. Relative to continuous RO, models predict that CCRO and batch RO demonstrate up to 37% and 64% energy savings, respectively, for brackish water desalination at high water recovery. For batch RO and CCRO, the primary reductions in energy use stem from atmospheric pressure brine discharge and reduced streamwise variation in driving pressure. Fully-batch systems further reduce energy consumption by not mixing streams of different concentrations, which CCRO does. These results demonstrate that time-varying processes can significantly raise RO energy efficiency.
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•Numerical modelling of energy use in batch, semi-batch (CCRO), and continuous RO.•Contour maps of efficiency vs. recovery ratio (40–90%) and feed TDS (1–40 g/kg).•Two novel batch RO systems are more efficient than semi-batch or continuous RO.•For seawater at 50% recovery, batch RO requires 25% less energy than continuous RO.•Greatest energy savings (up to 64%) in brackish water at high recovery.
On-site treatment and reuse is an increasingly preferred option for produced water management in unconventional oil and gas extraction. This paper analyzes and compares the energetics of several ...desalination technologies at the high salinities and diverse compositions commonly encountered in produced water from shale formations to guide technology selection and to inform further system development. Produced water properties are modeled using Pitzer's equations, and emphasis is placed on how these properties drive differences in system thermodynamics at salinities significantly above the oceanic range. Models of mechanical vapor compression, multi-effect distillation, forward osmosis, humidification–dehumidification, membrane distillation, and a hypothetical high pressure reverse osmosis system show that for a fixed brine salinity, evaporative system energetics tend to be less sensitive to changes in feed salinity. Consequently, second law efficiencies of evaporative systems tend to be higher when treating typical produced waters to near-saturation than when treating seawater. In addition, if realized for high-salinity produced waters, reverse osmosis has the potential to achieve very high efficiencies. The results suggest a different energetic paradigm in comparing membrane and evaporative systems for high salinity wastewater treatment than has been commonly accepted for lower salinity water.
•Several desalination systems were modeled at high salinity.•The effect of salinity on desalination system energy consumption was analyzed.•The least energy required to desalinate produced water is much higher than seawater.•Evaporative systems analyzed tended to have higher efficiencies at higher salinity.•If realized, high salinity reverse osmosis could provide significant energy savings.
Enhanced fouling resistance has been observed in recent variants of reverse osmosis (RO) desalination which use time-varying batch or semi-batch processes, such as closed-circuit RO (CCRO) and pulse ...flow RO (PFRO). However, the mechanisms of batch processes' fouling resistance are not well-understood, and models have not been developed for prediction of their fouling performance. Here, a framework for predicting reverse osmosis fouling is developed by comparing the fluid residence time in batch and continuous (conventional) reverse osmosis systems to the nucleation induction times for crystallization of sparingly soluble salts. This study considers the inorganic foulants calcium sulfate (gypsum), calcium carbonate (calcite), and silica, and the work predicts maximum recovery ratios for the treatment of typical water sources using batch reverse osmosis (BRO) and continuous reverse osmosis. The prediction method is validated through comparisons to the measured time delay for CaSO4 membrane scaling in a bench-scale, recirculating reverse osmosis unit. The maximum recovery ratio for each salt solution (CaCO3, CaSO4) is individually predicted as a function of inlet salinity, as shown in contour plots. Next, the maximum recovery ratios of batch and conventional RO are compared across several water sources, including seawater, brackish groundwater, and RO brine. Batch RO's shorter residence times, associated with cycling from low to high salinity during each batch, enable significantly higher recovery ratios and higher salinity than in continuous RO for all cases examined. Finally, representative brackish RO brine samples were analyzed to determine the maximum possible recovery with batch RO. Overall, the induction time modeling methodology provided here can be used to allow batch RO to operate at high salinity and high recovery, while controlling scaling. The results show that, in addition to its known energy efficiency improvement, batch RO has superior inorganic fouling resistance relative to conventional RO.
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•Inorganic fouling (scaling) propensity was modeled for batch and continuous RO.•Predictive framework compares nucleation induction time and residence time.•Each batch, salinity cycles from low to high, yielding very short residence time.•Fouling compared for CaSO4, CaCO3 & silica in seawater, groundwater, & RO brine.•Batch and semi-batch RO enable much higher maximum water recovery ratio and salinity.
Batch operation of reverse osmosis (RO) has been proposed as a method to reduce seawater RO (SWRO) energy consumption and fouling propensity. In this paper, we use a transient numerical model of the ...RO process to investigate the impact of several practical loss mechanisms on the overall energetic performance of batch SWRO compared to a conventional continuous system. A critical variable that controls the energetic advantage of batch RO is the reset time between cycles. A large reset time necessitates higher operating flux and therefore results in increased energy consumption. On the other hand, ensuring a low cycle reset time requires higher energy for the refilling process. A batch SWRO design with an atmospheric pressure feed tank and pressure exchangers for energy recovery does not show promise for energy savings. Batch SWRO must be designed with a large number of short pressure vessels (with fewer membranes each) and lower energy recovery losses (e.g., by using pressurized feed storage) in order to reduce energy consumption by up to 8%. These modifications are more complex and hence capital expenses would determine the overall feasibility of such designs to improve seawater desalination.
•Batch RO design should minimize volume of tank and tubes.•Flux during cycle time must be high to compensate for idle time during cycle reset.•Faster flow during cycle reset is necessary but this consumes significant energy.•A retrofitted continuous SWRO plant operated in batch mode cannot save energy.•Designs with pressurized feed storage and short channel length are promising.
Resistance to fouling is often cited as an advantage of emerging desalination technologies such as forward osmosis and membrane distillation over the widely-used reverse osmosis process. However, the ...nature and magnitude of differences in fouling behavior between these three processes are not well characterized. This study directly compares the fouling and scaling behavior of reverse osmosis (RO), forward osmosis (FO), and direct contact membrane distillation (MD) in the same membrane module under identical hydrodynamic conditions (flux and cross-flow velocity). Fouling experiments were conducted using calcium sulfate as a model inorganic foulant and alginate as a model organic foulant. Although all three processes tolerated some degree of feed supersaturation for 36 h without inorganic fouling (scaling), FO exhibited the greatest scaling resistance, withstanding a feed of 33± 2 mM CaSO4 (approximately twice saturation) without significant flux decline. Scaling occurred at similar concentrations at the membrane between MD and RO; however, while MD tolerated a more concentrated bulk feed due to reduced concentration polarization, flux decline after fouling was considerably more severe in MD. In contrast, MD tolerated organic fouling much better than FO or RO: despite accumulating a similar quantity of alginate gel over 18 h of operation, flux declined only 14% in MD versus 46–47% in RO and FO. These results are explained with respect to differences in temperature, membrane materials, and transport mechanisms between the three processes. Although FO and MD each exhibited superior resistance to one type of foulant, neither process outperformed RO in resistance to both organic and inorganic fouling. These findings inform a more nuanced approach to process selection for the treatment of complex water sources.
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•Three desalination processes were compared under identical hydrodynamic conditions.•Membrane distillation (MD) showed the greatest resistance to organic fouling.•Forward osmosis (FO) showed the greatest resistance to inorganic fouling (scaling).•Neither MD nor FO outperformed reverse osmosis (RO) with both types of fouling.•All three processes tolerated supersaturated feeds (SI > 0.1) without scaling.
A layered model is developed to describe mass transport through fouled membranes in pressure retarded osmosis (PRO) and forward osmosis (FO) processes. This resistance-in-series model accounts for ...salt and water transport through the active layer, support layer, external boundary layers, and the cake layer formed by foulants. Closed-form algebraic expressions for the water flux, salt flux, salt concentration profile, and overall transport coefficients are presented for FO membranes in both membrane orientations. Organic fouling experiments using alginate are used to validate the model and observe the effects of feed salinity, cross-flow velocity, and membrane orientation on foulant accumulation rates. Increasing feed salinity and cross flow velocity both lead to a decrease in foulant accumulation in PRO orientation. Under identical operating conditions, foulant accumulation rates are comparable in FO and PRO modes. The model is then evaluated to show how foulant accumulation affects FO system performance in both membrane orientations. The model is also used to elucidate the effect of fouling on power production in PRO: fouling affects the optimal PRO operating pressure, and a 0.5 mm-thick cake layer can reduce power production by 50%. The mathematical expressions developed serve as a simple tool to predict the performance of fouled membranes in both PRO and FO processes.
•New transport models were developed for FO and PRO in presence of cake layer.•The solute transfer rate for the cake layer was integrated in the transport model.•The interface concentrations were expressed in closed forms.•The water flux significantly lowers for few hours experimental time in presence of foulant.•The cake layer thickness strongly reduces the maximum power density.
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Fouling models rely on knowledge of foulant accumulation and removal mechanisms. In this study, a fouling visualization apparatus is developed to elucidate centimeter-scale mechanisms of organic ...fouling and cleaning in reverse osmosis (RO) and forward osmosis (FO). Alginate is used as a model organic foulant and dyed with methylene blue, which is shown not to affect fouling or cleaning, and to sufficiently highlight the gel for visualization at low salinity (up to 1% NaCl). When cleaning by increasing the cross-flow velocity, with or without reverse permeation, foulant peels off the membrane in discrete pieces in both RO and FO. Videos of cleaning show that foulant cake swelling and wrinkling can facilitate gel detachment and removal. Despite their effectiveness in slowing fouling, spacers can hinder removal of detached foulant pieces by obstructing their path. Finally, photographs point to a new mechanism of internal fouling in FO: vapor formation due to sub-atmospheric pressure within the membrane. Awareness of these mechanisms allows for better modeling of fouling and motivates optimization of swelling-inducing cleaning procedures.
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•Alginate gel fouling and cleaning mechanisms are visualized in RO and FO.•Methylene blue stains low-salinity gel without affecting fouling or cleaning.•When cleaning with high feed velocity, gel peels off in macroscopic pieces.•Changing solution ionic composition causes gel to swell, wrinkle and detach.•Images provide evidence of vapor formation inside FO membrane in PRO orientation.
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