Spiral-Wound RO Elements Performance Tuning with Surface Nano-Structured Polyamide TFC Membranes

Membrane surface modification is a popular approach to mitigate membrane fouling and, in some cases, can be also utilized to tailor desalination performance of polyamide (PA) based thin film composite (TFC) reverse osmosis (RO)/nanofiltration (NF) membranes. However, scalability of membrane surface modification approaches that are suitable for producing commercial-scale spiral-wound elements is seldomly reported. To date, scalability of membrane surface modification has been only demonstrated for in situ surface modification of commercial spiral wound PA RO/NF membrane elements, accomplished via either redox-initiated graft polymerization of a hydrophilic monomer (e.g., glycidyl methacrylate (GMA)) or surface coating of a polydopamine layer. In contrast to the above, membrane surface nano-structuring (SNS) via atmospheric pressure plasma-induced graft polymerization (APPIGP) can be easily implemented in the downstream process of PA TFC flat sheet membrane manufacturing. Therefore, the SNS polyamide (SNS-PA) sheets can be produced prior to fabrication of spiral wound elements. Moreover, the previous study has shown that SNS of a PA TFC brackish water RO (BWRO) base membrane with a tethered poly(acrylic acid) (PAA) layer (SNS-PAA-PA) can achieve a seawater RO (SWRO) membrane performance. In the present study the scale-up potential of APPIGP was evaluated by synthesizing large SNS-PAA-PA flat sheet membranes (30” x 24”) that are suitable for fabricating commercial scale spiral wound membrane elements (2.5” in outer diameter and 21” in length). The scaled-up synthesis of the SNS-PAA-PA membrane sheets was accomplished by utilizing an APP source mounted on an XYZ robot capable of scanning a large membrane sheet area, followed by graft polymerization of a hydrophilic monomer (i.e., acrylic acid) in a custom-made reactor designed for the large membrane sheet. The synthesized SNS-PAA-PA membrane sheets were then used to fabricate 2.5 x 21” spiral-wound RO elements. Compared to the commercial SWRO membrane element of the same size (Dow SW30-2521), the SNS-PAA-PA spiral-wound elements had up to ~2 times greater L_p and similar or lower B. Moreover, the results of a fouling study with model foulants (e.g., bovine serum albumin and alginic acid) indicate that the surface tethered PAA layers reduce membrane fouling and increase membrane cleaning effectiveness.