Potential application of osmotic backwashing to brackish water desalination membranes

Brackish water Reverse Osmosis (RO) and Nanofiltration (NF) are an attractive technology for potable water production and wastewater reclamation. However, scaling and fouling remain common problems that limit the optimal use of membranes, cause flux decline, increase energy demand and require periodic cleaning by chemical means. Herein, we consider the use of osmotic backwashing, already reported in the literature for seawater RO, as a cleaning method for brackish RO and NF. Three commercial membranes were used in a bench-scale system along with three draw solutions, NaCl, Na2SO4 and MgSO4, to assess the osmotic flux induced during a backwash cycle. The backwash flux was found to be in good agreement with calculations made using a computational model, which was then used to reveal the role of solution chemistry as well as membrane support properties in controlling the backwash intensity and duration. Specifically, sulphate-based solutions showed a good ability to maintain an osmotic flux, better than NaCl, for which lower membrane selectivity restricts use as a draw agent. While preliminary experiments demonstrated the ability of an osmotic backwash to remove CaPO4 scale, achieving nearly complete flux restoration for some cases, the process requires further optimization.