Real-time computational imaging of reverse osmosis membrane scaling under intermittent operation

Abstract

Small-scale photovoltaic-powered reverse osmosis (PVRO) could be a game-changer for off-the-grid water-stressed communities. However, cost and maintainability issues, due to membrane fouling, remain potentially major limitations of intermittently run PVRO. In particular, there is a common perception that this intermittent operation causes premature membrane degradation but has not been fully evaluated to date. This study seeks to understand the impacts of intermittent operation on mineral scaling propagation in PVRO through real-time imaging and post-operation morphological analysis of the scales. Lab-mixed synthetic groundwater, supersaturated with CaSO4, was used as the feedwater to scale BW30 RO membranes. Then, scaling growth on a statistically significant portion of a spacer-filled RO crossflow channel was imaged in real-time. Comparison between continuous (24 h) and intermittent (8 h/d × 3 d) experiments did not indicate any negative effect of periodic shutdowns on flux decline, which is contrary to the perception that intermittent operation decrease membrane permeability. In-situ imaging further supported this observation and highlighted a slower growth of intensive scaling spots under intermittent operation. After intermittent shut-off periods, the scaling distribution on the membrane appeared thinner and more uniform, hinting a momentary osmotic backflow at the end of each intermittent cycle. Scanning electron microscopy also confirmed the reduced CaSO4 scale growth and a different morphological structure of the scales under intermittent RO operation. This macro-scale imaging and postprocessing approach can aid in the development of optimized backwash and rinsing cycles, reducing fouling-related irreversible damage to the membrane and increasing the accessibility of PVRO for remote communities.