Leaching of dissolved organic carbon from commercial reverse osmosis membranes exacerbates interferences in organic solute rejection during bench-scale filtration

Abstract

Materials in contact with purified (e.g., Drinking water, ultrapure-water) water are sources of dissolved organic carbon which can increase the regrowth potential. Reverse osmosis (RO) membrane materials are composed of flexible polymers that are also in contact with treated water, but their contribution to DOC and microbial regrowth in bench-scale filtration studies is not reported. Bench-scale filtration studies use less volumetric throughput compared to pilot and full-scale filtration systems, which likely exacerbates the influence of leachates on for DOC measurement in permeates. This limits our ability to determine if standard membrane preparation procedures ensure sufficient removal of leachable organics. Migration of DOC from 8 different reverse osmosis membrane filters was assessed by two different experimental approaches; (1) Immersion assay—membranes were soaked in a pasteurized, isotonic water solution with a low and constant concentration of DOC. (2) Volume throughput (VT) test—Permeate was collected at defined VT steps to determine the impact of VT on the washout of DOC during system start-up. Test water from the immersion assay and the filtration test were analyzed with respect to (i) DOC (mg/L), (ii) microbial regrowth potential (% increase in bacterial activity), (iii) DOC fractional composition via LC-OCD. The presented study identified variations in the leachability of DOC between commercial RO membranes. Leaching of DOC from RO membranes varied by supplier and membrane composition. Alfa Laval RO98 exhibited the highest release of DOC compared to the other membranes tested during the immersion assay, suggested by an increase in test water DOC from 0.3 mg/L to 7.43 ± 1.54 mg/L, (n = 3) within 48 h. Aquaporin biomimetic-based membranes had the lowest release of DOC after 48 h of immersion (0.702 ± 0.17 mg DOC/L, n = 3). Higher concentrations of leached DOC did not lead to a higher regrowth potential on the released solute. Results of the filtration test demonstrate that DOC leaching from the membrane can exceed the point of flux-stabilisation, which is the common endpoint of membrane preparation. In all membranes tested, the chemical composition of DOC was dominated by low molecular weight acids, indicative of monomers used during membrane interfacial polymerization. The implication of this study suggests that on bench-scale filtration less volumetric throughput compared to pilot-full scale filtration exacerbates the influence of leachates on for example DOC measurement in permeates; promoting interferences in DOC permeate measurement and corresponding rejection coefficient. Relevance for full-scale filtration systems is limited due to higher volume throughputs applied during system start-up and commissioning.