Batch Reverse Osmosis Research Articles

Double-acting batch reverse osmosis configuration for best-in-class efficiency and low downtime

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.

A compact hybrid batch/semi-batch reverse osmosis (HBSRO) system for high-recovery, low-energy desalination

Batch reverse osmosis (RO) is a promising approach to high-recovery desalination. It has low energy consumption, but system size increases sharply with recovery because of the need for a large work exchange vessel. In this study, we propose a compact hybrid batch/semi-batch reverse osmosis (HBSRO) system incorporating aspects of each approach. HBSRO works in three phases, i.e. semi-batch pressurisation phase, batch pressurisation phase, and finally purge-and-refill phase. We analyse ideal and practical cases of HBSRO to gain understanding about the specific energy consumption (SEC) and size of the system. In the ideal analysis, HBSRO can halve the size of work exchange vessel while incurring just a 5% energy penalty compared to batch RO at all recoveries. In the practical case, accounting for non-idealities, HBSRO has lower SEC than batch RO at recovery over 0.9, because a smaller volume of work exchange vessel minimises the energy penalty of the purge-and-refill phase in HBSRO. The reduced volume not only makes HBSRO more practical, but also improves energy-efficiency through reduced losses. Thus, our study highlights that HBSRO is highly flexible, achieving high recovery, compact size, and low SEC – advantages that are especially important in minimal or zero liquid discharge applications.

Pressure profile impact on batch reverse osmosis desalination: investigation and optimization

Pressure profile impact on batch reverse osmosis desalination: investigation and optimization

Effects of finite flux and flushing efficacy on specific energy consumption in semi-batch and batch reverse osmosis processes

This work focuses on Specific Energy Consumption (SEC) in batch and semi-batch reverse osmosis (RO) processes. It is proved from optimal control theory that the minimal SEC in semi-batch RO occurs at constant flux conditions, similar to conclusions in both batch RO and continuous, infinite-stage RO. While semi-batch RO is comparable to two- or three-stage RO at the thermodynamic limit (i.e. zero flux), its performance may be severely compromised by finite flux applied in desalination and efficacy of flushing before the next filtration step. Batch RO is even more susceptible to salt retained during flushing. On the basis of typical flux used in industrial desalination and a 95% flushing efficacy, semi-batch RO at the cyclic steady state is only similar to one-stage RO at low recoveries (e.g. 30 and 40%) and excels slightly at high recoveries (e.g. 50 and 60%). Batch RO may outperform one-stage RO but is still not as good as two-stage RO. Reducing salt retention and operating at a reduced flux are ways to make batch and semi-batch ROs energy advantageous over their continuous counterparts.

Design, modelling and optimisation of a batch reverse osmosis (RO) desalination system using a free piston for brackish water treatment

Batch RO is a concept for achieving the minimum possible energy consumption in desalination, even at high recoveries. We present a batch RO design that operates cyclically in two alternating phases. The system uses a free piston, housed in a pressure vessel, to transfer pressure from the feed fluid to the recirculating fluid. No complete design procedure for this configuration currently exists. To fill this gap, we present a systematic model based on justified assumptions. The specific energy consumption (SEC) is broken down into contributions from the feed pump, recirculating pump, and auxiliary loads. The calculation of feed pump SEC includes three non-ideal correction factors: concentration polarisation, longitudinal concentration gradient, and salt retention. The model requires only the solution of explicit algebraic equations, without need of specialised numerical techniques, and is implemented in a simple 3-step procedure. The model is applied to an example involving desalination of brackish water using an 8-inch spiral-wound RO module. The design parameters are explored and optimised in a sensitivity analysis. The results show that the optimised batch RO at 80% recovery can produce fresh water with low-energy consumption, achieving 2nd law efficiency of 33.2% compared to 10–15% for conventional brackish water RO.

Batch reverse osmosis: a new research direction in water desalination

Batch reverse osmosis: a new research direction in water desalination

Impact of salt retention on true batch reverse osmosis energy consumption: Experiments and model validation

In theory, the batch reverse osmosis (RO) process achieves the lowest practical energy consumption by varying pressure over time. However, few batch RO systems have been built and operated. We have designed, built, and operated the first “true” batch RO prototype using a flexible bladder. The flexible bladder serves as the high-pressure variable-volume tank that is inherent to true batch RO designs (as opposed to batch RO with energy recovery devices). We experimentally validated a model of batch RO energy consumption (≤2.7% difference) by measuring the hydraulic work of the high pressure and circulation pumps. We find that batch RO energy consumption will be greater than expected mostly due to salt retention, a problem neglected by most previous studies. However, despite operating at elevated salinity and flux conditions, batch RO can still save energy relative to single-stage and multi-stage continuous systems. For a seawater desalination plant (35 g/kg intake, 50% recovery, 15 L m−2 h−1), our newly-validated model predicts that batch RO would save 11% energy compared to a single-stage continuous RO plant. Our work demonstrates that batch RO is an energy-efficient process with the potential to reduce the cost of water desalination.

Dynamic Operation of Batch Reverse Osmosis and Batch Pressure Retarded Osmosis

Optimal control theory is applied to dynamic operation of batch reverse osmosis (RO) for seawater desalination and batch pressure retarded osmosis (PRO) for power generation. It is proved that the water flux should be constant during the entire operation in order to minimize specific energy consumption (SEC) in RO or to maximize specific energy production (SEP) in PRO. A dimensionless parameter $\gamma = A_m L_p\pi_0t_f/V_0$, similar to the one used in continuous osmotic membrane processes, is proposed to characterize the energy performance. While the SEC in RO is a monotonic function of recovery and $\gamma$, the SEP in PRO is solely a monotonic function of $\gamma$. It is shown that batch RO/PRO excels continuous RO/PRO because it mitigates the spatial variation of flux in a long pressure vessel observed in the latter. The batch operation provides an elegant way to implement continuous multi-stage RO with interstage pumps and an energy recovery device (ERD) as well as continuous multi-stage PRO with int...

Practical aspects of batch RO design for energy-efficient seawater desalination

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.

Off-grid desalination for irrigation in the Jordan Valley

Groundwater resources in many regions of the world are becoming increasingly depleted and salinized. With many aquifers straddling political boundaries, on-going depletion presents both a flash-point for conflict and an opportunity for cooperation. A salient example is that of transboundary groundwater resources in the Jordan Valley. These are shared among Israeli, Jordanian and Palestinian residents. Here we describe a collaborative project aiming to develop a desalination system for use by Palestinian farmers in the West Bank. Students have collaborated across borders in a programme of training and research, in which they have constructed desalination prototypes. These are based on a simple but efficient batch-reverse osmosis (RO) technology that incorporates energy recovery and brine recirculation to achieve 70%–76% recovery and specific energy consumption <1.3 kWh/m 3. The technology can be solar powered with minimal PV footprint. Being built almost entirely from off-the-shelf parts, the system is readily implemented with levels of engineering expertise available in many areas of the world. To test and upscale the technology, and to propagate the knowledge about it, it is being trialled at centres in the UK, Israel and soon in Palestine. It is concluded that the project demonstrates a valuable approach in regions facing transboundary groundwater challenges, and that further learning resources should be developed for free access to foster collaboration across borders.

Isothermal Organic Rankine Cycle (ORC) driving Reverse Osmosis (RO) desalination: Experimental investigation and case study using R245fa working fluid

In many regions of the world, groundwater salinity contributes to the growing fresh water deficit. Desalination of saline water via reverse osmosis (RO) could be driven by Organic Rankine cycle (ORC) engines, exploiting readily available low-grade heat (e.g. solar or waste heat). However, the specific energy consumption (SEC) of conventional ORC-RO systems is quite high, while the ORC efficiency is significantly low at low temperatures. To improve on the efficiency and SEC of brackish ground water desalination processes, a novel isothermal ORC driven batch RO desalination system was experimentally investigated, using R245fa working fluid. Results showed about a half of the energy requirement of conventional ORC-RO desalination systems. A case study indicated that the system can be potentially employed in recovering waste heat from a bakery facility to produce about 0.4 L of fresh water per kg of baked food.

Inorganic fouling mitigation by salinity cycling in batch reverse osmosis

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.

Energy efficiency of batch and semi-batch (CCRO) reverse osmosis desalination

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.

On the asymptotic flux of ultrapermeable seawater reverse osmosis membranes due to concentration polarisation

Just as thermodynamic considerations impose a finite limit on the energy requirements of reverse osmosis, concentration polarisation imposes a finite limit on flux, or equivalently, on system size. In the limit of infinite permeability, we show the limiting flux to be linearly dependent on the mass transfer coefficient and show this to be true for low recovery systems just as well as moderate and high recovery single stage and batch reverse osmosis system designs. At low recovery, the limiting flux depends on the logarithm of the ratio of hydraulic to bulk osmotic pressure and at moderate or higher recovery, the relationship with this pressure ratio is a little more complex but nonetheless can be expressed as an explicit analytical formula. For a single stage seawater reverse osmosis system operating at a hydraulic pressure, recovery ratio, and value of mass transfer coefficient that are typical today, the flux asymptote is roughly 60Lm−2h−1 – roughly four times where average fluxes in seawater reverse osmosis systems currently stand.

Operation and modeling of RO desalination process in batch mode

The performance of a batch reverse osmosis (RO) desalination process in terms of permeate quantity and salinity as a function of feed pressure and feed salinity is evaluated by using laboratory experiments and process modeling. Special attention is paid to the water and salt permeability constants (Kw, Ks) which affect the permeate and salt flux across the membrane. Kw and Ks are found to be strongly pressure-dependent for the batch system which is in-line with earlier observations for continuous RO systems. However, the most important findings of this work are the dependence of Kw and Ks on feed salinity, something that have never been observed or reported in the literature. In order to better qualify these observations, further experiments with the batch system are conducted with a constant feed salinity so that the operating condition resembles that of a continuous RO process.

Threshold performance of a spiral-wound reverse osmosis membrane in the treatment of olive mill effluents from two-phase and three-phase extraction processes

A reverse osmosis (RO) treatment stage was examined for the complete depuration of the different effluents exiting the olive mill factories (OMW) working with diverse extraction procedures, that is, the two-phase and the three-phase extraction processes, respectively. In the present work, the modelization of batch RO purification of OMW by means of the relevant equations of the threshold flux theory for fouling control and plant dimension is addressed. Results show that higher threshold flux values (20.2–22.1% increase) and major feed recovery rates (80.2–85.0%) as well as very significant reduction of the long-term fouling index (27.3–52.7%) were achieved by using as pretreatment steps the following series of processes: pH-T flocculation, UV/TiO2 photocatalysis, UF and NF in series. This leads to both lower energy and capital costs, in particular a reduction of the required membrane area in case of batch membrane processes equal to 22.3–44.8%. Accurate prediction of the rejection behavior was attained by the used leaky solution-diffusion model in all cases, with reflection coefficients (σCOD) ranging from 0.86 to 1.0. The purified effluent streams are finally compatible with irrigation water quality standards (COD values below 1000mgL−1).

Permeate recovery and flux maximization in semibatch reverse osmosis

Most reverse osmosis (RO) and nanofiltration (NF) processes operate as once through or plug flow (PF) systems in which permeate recovery, flux and cross-flow are coupled. Such processes are well suited for applications where permeate recovery percentages are limited and where well pretreated source waters of uniform composition are available. Some membrane filtration applications, such as ultrafiltration, operate as batch systems. Batch operations allow independent manipulation of permeate recovery, flux and cross-flow, and are therefore well suited for high recovery and/or problematic source waters. However, the application of batch operation to RO and NF has been limited because of the increase in energy requirements and reduction in permeate quality that can result. In addition, batch systems can require higher capacity components and more membrane elements and housings than comparable continuous or plug flow systems to compensate for downtime between batches. New semibatch or continuous batch processes for RO or NF applications are emerging. These processes provide high adjustable recovery rates, independently adjustable cross-flow and resistance to and even reversal of fouling and scaling. They can consume less energy and require fewer membrane elements than PF systems. High recovery operation reduces both concentrate production and source water pumping and pretreatment requirements. High cross-flow, reduced lead element flux, more even flux distribution and salinity cycling can reduce the effects of fouling and scaling and the associated chemical and cleaning requirements. These features are particularly beneficial for inland desalination, wastewater concentration and water reuse applications. This paper considers process designs and projected membrane performance of semibatch RO systems for brackish and low salinity source waters. The potential advantages of new process design features that can be employed in batch RO but are not practical for conventional RO, such as short membrane arrays and high recovery single stage designs, are evaluated. Particular focus is given to the operating conditions and performance of individual membrane elements in multi-element membrane arrays. Comparisons are made to the performance of conventional PF RO designs. The analysis will help engineers of RO and NF systems to understand the new capabilities, optimization considerations and potential new applications for this emerging process technology.

Low-Energy Consumption With Closed-Circuit Desalination

AbstractSince the inception of reverse osmosis (RO) for desalination in the late 1950s, basic membrane desalination processes have remained essentially unchanged, despite improved membranes and energy recovery devices. Conventional RO is a hydrodynamic plug-flow process with pressurized feed to semipermeable membranes split into two outlet streams—one of pressurized brine and the other of unpressurized permeate. The energy that remains in the pressurized brine stream must be recovered with energy recovery devices or lost across a brine valve. A new desalination technique—a continuous closed-circuit desalination (CCD) process—operates on the basis of relatively low average feed pressures and other operating advantages of batch RO with the continuous flow rates of feed and permeate of conventional RO. This article provides an overview of the CCD process, presents experimental data from operating CCD processes, and establishes the principles on which the technology can be applied to satisfy a wide range of d...

Evaluation of coagulation (FeCl3) and anion exchange (MIEX) for stabilized landfill leachate treatment and high-pressure membrane pretreatment

Laboratory-scale experiments were conducted to assess the effectiveness of coagulation (ferric chloride – FeCl3) and magnetic ion exchange (MIEX) as a pretreatment option for treating stabilized landfill leachate using reverse osmosis (RO) and nanofiltration (NF) membranes. Leachate from three different landfills was collected and characterized as stabilized leachate with a ratio of biochemical oxygen demand to chemical oxygen demand in the range of 0.02–0.11. Batch jar-tests were conducted to determine the effectiveness of FeCl3 and MIEX. A maximum reduction of 71% and 34% dissolved organic carbon (DOC) and 94% and 48% UV-254 absorbing organic matter (OM) was observed using FeCl3 and MIEX, respectively. Effective doses of FeCl3 (22mmolL−1) and MIEX (5mLL−1) were selected for leachate pretreatment before membrane operation. Batch RO and NF membrane experiments were conducted with raw and pretreated leachates to study the change in permeate flux for each type of membrane and leachate. Even though the pretreated leachate contained over 45% less UV-254 absorbing OM than raw leachate, leachate pretreatment did not increase the permeate flux as compared to raw leachate during RO and NF membrane treatment. The potential cause of the decrease in permeate flux in pretreated leachate as compared to raw leachate was hypothesized as the change in characteristics of pretreated leachate and membrane surfaces caused by the continuous increase in pH of pretreated leachate.

Measurement of Concentration Profiles by Holographic Interferometry and Modelling in Unstirred Batch Reverse Osmosis

An optical method (real-time holographic interferometry) has been used to visualize concentration changes in the vicinity of the membrane surface during the dead-end reverse osmosis of salt solutio...