Backwashable hollow-fiber membranes enable ultrahigh recovery ratio effluent nanofiltration

Applying nanofiltration (NF) membranes to treat tertiary wastewater effluent can significantly upgrade water quality with minimal energy and chemical requirements. However, membrane fouling limits water recovery, typically under 85 %, and generates a large, costly-to-treat retentate. Minimizing fouling is key to significantly increasing recovery and reducing environmental impact. Here, we studied the application of physical hydraulic backwash to achieve ultra-high recovery effluent filtration by controlling fouling, minimizing concentrate volume, and improving water quality. We conducted lab-scale semi-batch effluent NF with/without backwash, targeting over 95 % recovery using backwash-compatible hollow-fiber polyelectrolyte multilayer membranes. Two backwash protocols were tested: 10 % TMP increase and Time (2 h) + 10 % TMP increase. The latter showed better performance and was selected for comparison vs filtration without backwash. Without backwashing, we reached 94 % water recovery before the trans-membrane pressure increased by 40 %. When applying automatic backwash with the permeate, we achieved up to ∼ 98 % water recovery, reducing the concentrate volume by ∼ 300 % compared to filtration without backwash, with a low energy demand increase (5–7 %). Backwashed filtration consistently produced better water quality that meets the Israeli effluent reuse regulations for unrestricted irrigation and river discharge. Furthermore, we found that the membrane performances can be fully restored and maintained in multiple filtration cycles by applying a standard chemical cleaning-in-place protocol. Our results indicate that backwashing nanofiltration membranes is a practical strategy to control fouling in ultra-high recovery nanofiltration, significantly reducing concentrate volume and increasing the production of high-quality permeate.