Our batch systems operate in cycles consisting of three steps.
-Permeate production: The system is pressurized as the high-pressure pump forces a make-up fluid (freshwater or salty water) into the bladder, forcing product to exit the system through the membrane. The bladder grows over the course of a 5-10 minute cycle, the feedwater becomes more concentrated, and the system pressure rises.
-Brine flush: When permeate production is complete (based on hitting the desired recovery, pressure limit, or bladder capacity), actuated valves (not shown here) are opened and the system depressurizes. The membranes are backwashed while new feed is pumped into the system, flushing out the brine.
-Bladder recharge: When the desired amount of brine has been flushed, the brine discharge valve is closed and the bladder discharge valve opens. New feed continues to enter the system and the bladder is emptied out.
-Rinse and repeat: The cycle repeats as long as water production is needed.
Here we show data from a single batch cycle. We have measured:
-Feed flow: The flush and recharge steps are quick, maximizing permeate production time. No feed enters the system during permeate production.
-Backwash and permeate flux: Regular membrane backwashing is built-in to the batch cycle, reducing fouling and scaling. During this permeate production cycle, permeate flux was kept constant.
-System pressure: Over the course of the cycle, system pressure starts out low (just above the feed osmotic pressure) and gradually rises to reach the final brine osmotic pressure.
-Differential pressure: We measure differential pressure across the bladder. There is a sharp rise towards the end when the bladder is nearly full, indicating the end of permeate production. The differential pressure is low (~7 psi), so bladder fatigue is not a major concern.