Batch counterflow reverse osmosis

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.