Batch Reverse Osmosis Desalination Modeling under a Time-Dependent Pressure Profile

Inorganic fouling mitigation by salinity cycling in batch reverse osmosis

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

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.

Reverse osmosis (RO) technologies have been widely implemented around the world to address the rising severity of freshwater scarcity. As desalination capacity increases, reducing the energy consumption of the RO process per permeate volume (i.e., specific energy consumption) is of particular importance. In this study, numerical models are used to characterize and compare the energy efficiency of one-stage continuous RO, multi-stage continuous RO, and closed-circuit RO (CCRO) processes. The simulated results across a broad range of feed salinity (5,000–50,000 ppm, i.e., 5–50 g kg−1) and recovery (40%–95%) demonstrate that, compared with the most common one-stage continuous RO, two-stage and three-stage continuous RO can reduce the specific energy consumption by up to 40.9% and 53.6%, respectively, while one-stage and two-stage CCRO can lead to 45.0% and 67.5% reduction, respectively. The differences in energy efficiencies of various RO configurations are more salient when desalinating high-salinity feed at a high recovery ratio. From the standpoints of energy saving and capital cost, the simulated results indicate that multi-stage CCRO is an optimal desalination process with great potential for practical implementation.

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

Batch RO technologies can help fulfil the growing need for high-recovery desalination. Several experimental studies have given results for batch and semi-batch RO processes individually, but under differing conditions. There is a lack of side-by-side comparisons. Here we report, for the first time, experiments with a high-pressure RO system that can be operated in batch, semi-batch, or hybrid semi-batch/batch mode. Using the same membrane and supply pump at pressures up to 110 bar, we compare performance against semi-batch mode which is used as the baseline throughout. With seawater feed, batch RO reduces electrical SEC by 10 % at recovery of 0.655. With brackish water feed, we use hybrid mode to avoid an excessively large work exchanger volume. Working at recovery of 0.94, a work exchanger volume of 20 L reduced SEC by 23 %. A work exchanger volume of 40 L reduced SEC further, resulting in SEC of 1.81, 1.89 and 2.02 kWh/m3 at recoveries of 0.9, 0.92 and 0.94 respectively, thus representing a 22–32 % reduction against the semi-batch baseline. The results also showed permeate conductivity reduced by up to 50 %. The study confirms the advantages of batch and hybrid RO over semi-batch RO, especially for brackish water desalination at high recovery.

Semi-closed reverse osmosis (SCRO): A concise, flexible, and energy-efficient desalination process

Novel Tri Hybrid Desalination Plants

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

A desalination system with efficiency approaching the theoretical limits

Membrane compaction in batch reverse osmosis operation and its impact on specific energy consumption

Pressure exchanger batch reverse osmosis with zero downtime operation

Reverse Osmosis (RO) process is being engaged to yield fresh water from brackish water sources. However, the RO process is characterized by its high specific energy consumption (SEC) owing to high-pressure pumps. The current study focuses on reducing the SEC of the brackish water RO desalination plant using model-based optimization practice. The inlet conditions of RO process such as the feed pressure, flow rate (individual membrane module and total plant) and temperature, have a substantial influence on the performance indicators namely, water productivity, product concentration and SEC. Therefore, the optimisation of this study has been directed to determine optimal inlet conditions within feasible limits to minimise SEC. Arab Potash Company (APC) brackish water RO desalination plant has been considered as the case study. The optimal inlet conditions have resulted in a significant energy saving of 27.97% depending on the set of decision variables being considered at a fixed brackish water feed concentration.

Batch RO is designed to achieve high energy efficiency and high recovery in desalination. However, so far relatively few experiments on batch RO have been reported. Here we present an extensive experimental study of a single-acting, free-piston batch RO system using an 8-inch spiral wound membrane. The system was tested in the laboratory with brackish feed water containing up to 5 g/L NaCl. The objective was to quantify system performance in terms of Specific Energy Consumption (SEC), recovery, rejection, and output. Sensitivity to permeate flux and recirculation flow rate was also investigated. Performance was compared against the predictions of a theoretical model that accounts for salt retention, concentration polarization, and longitudinal concentration gradient in the RO module. For the first time, osmotic backflow was measured and incorporated into the model. For feed concentrations ranging from 1 to 5 g/L and recovery of 0.8, hydraulic SEC was measured in the range 0.22–0.48 kWh/m3 and electrical SEC in the range 0.48–0.83 kWh/m3. With improvements to the membrane permeability from 4.4 to 8 LMH/bar, selection of more efficient pumps, and reduction of valve friction losses, the model predicts that hydraulic SEC will be lowered to 0.14–0.39 kWh/m3.

Batch RO desalination is a new approach to high-recovery, energy-efficient desalination. So far, however, batch RO has been tested mostly with pure sodium chloride solutions. An important application of batch RO is desalination of brackish groundwater which, besides sodium chloride, contains sparingly soluble salts. In this experimental study of a batch RO system, we used simulated groundwater (with total dissolved solids ranging from 1180 to 3637 mg/L) following compositions of samples taken from a location in Egypt and a location in India. The groundwater contained high levels of salts which could be expected to cause scaling on the RO membrane surface. For example, the Langelier Saturation Index (LSI) for calcite reached 2.6 in the brine. Nonetheless, the system resisted scaling throughout >100 h of operation. Membrane permeability remained almost unchanged, as demonstrated by tests conducted before and after the experiments. Induction time calculations showed that salinity cycling did not fully explain the scaling inhibition. Other anti-scaling mechanisms – such as periodic flushing, osmotic backwash, and feed flow reversal – were also likely contributors. At recovery of 0.8, hydraulic specific energy consumption (SEC) was <0.5 kWh/m3 and close to that obtained with sodium chloride solution at equivalent osmotic pressure.