Recent studies have shown that the osmotic membrane bioreactor (OsMBR), a novel concept integrating forward osmosis (FO) separation with biological wastewater treatment, holds promise for transforming wastewater into high-purity water with relatively low energy consumption. The OsMBR system has the potential to be implemented at the Department of Defense’s (DoD) Forward Operating Bases for on-site, cost-effective, and sustainable wastewater treatment. Robust membranes with high water flux and low fouling propensity for efficient operation of the OsMBR were developed. The fabricated membranes demonstrated high fouling resistance against both synthetic and real wastewaters containing high loadings of organic foulants.
Two different modification strategies were employed, i.e., in-situ fabrication and post-fabrication membrane modification. For in-situ fabrication modification, the thin-film composite (TFC) membrane was first prepared by interfacial polymerization with 1,3-phenylenediamine (MPD) and 1,3,5-benzenetricarbonyl trichloride (TMC). jeffamine, an amine-terminated poly(ethylene glycol) derivative, was then grafted to the nascent polyamide layer by reaction with surface acyl chloride groups.
For post-fabrication strategy, the surface of commercial TFC-FO membranes was modified by grafting three different hydrophilic materials, i.e., jeffamine, silica nanoparticles (SiNPs), and zwitterionic polymer brushes. jeffamine was bound to the membrane surface using a carbodiimide-mediated, amide coupling reaction. SiNP-based modification was performed first by functionalization of SiNPs using silane molecules with amine terminal groups or quaternary ammonium moieties, followed by dip-coating of the TFC membrane with the functionalized SiNPs. Zwitterionic polymer brush layer was grafted on the TFC membranes via surface-initiated atom-transfer radical-polymerization (ATRP).
The in-situ fabrication membrane modification, which involves the fabrication of polyamide TFC membrane via interfacial polymerization, exhibited a large variation of transport properties, i.e., water and salt permeabilities. This inconsistency made it difficult to ensure reproducible fouling resistance of the membranes prepared by the in-situ fabrication technique. Because commercial TFC-FO membranes possess consistent membrane transport properties, the development of post-fabrication modification techniques was focused on instead. Antifouling properties of the modified membranes were assessed using adsorption tests with representative foulants, including proteins and bacteria, as well as dynamic fouling experiments in forward osmosis with various organic foulants (i.e., bovine serum albumin, sodium alginate, and natural organic matter). Among the developed techniques, the zwitterionic polymer-based modification resulted in the most fouling resistant TFC membrane. The observed excellent fouling resistance was attributed to the exceptional water affinity, net-zero charge, and dense grafting of zwitterionic polymer brushes. Finally, the improved fouling resistance of the zwitterionic polymer modified TFC membrane was demonstrated in an osmotic membrane bioreactor using activated sludge.
The developed fouling resistant forward osmosis membranes for an OsMBR demonstrated excellent fouling resistance against synthetic and real wastewaters. These antifouling forward osmosis membranes can also be used in seawater and brackish water desalination, in addition to wastewater treatment and reuse. Ultimately, the use of OsMBR for onsite wastewater treatment at DOD forward operating bases will reduce the need to haul wastewater offsite for treatment and disposal, resulting in economic, environmental, and safety benefits.