Antimicrobial and fouling-resistant membranes for treatment of agricultural and municipal wastewater

Abstract

This research project introduced a novel membrane coating strategy to combat biofouling, which is a major problem for the membrane-based treatment of agricultural and municipal wastewaters. The novelty of the strategy is that the membrane coatings have the unique ability to switch reversibly between passive (antifouling) and active (antimicrobial) fouling control mechanisms. This dual-mode approach differs fundamentally from other coating strategies that rely solely on one mode of fouling control. The research project had two complementary objectives: (1) preparation, characterization, and testing of dual-mode polymer nanolayers on planar surfaces and (2) evaluation of these nanolayers as membrane modifiers. The first objective was designed to provide a fundamental understanding of how polymer nanolayer chemistry and structure affect bacterial deposition and to demonstrate the reversibility of chemical switching. The second objective, which focused on membrane development, characterization, and testing, was designed to demonstrate methods for the production of water treatment membranes that couple passive and active biofouling control mechanisms. Both objectives were attained through synergistic collaboration among the three research groups. Using planar silicon and glass surfaces, we demonstrated using infrared spectroscopy that this new polymer coating can switch reversibly between the anti-fouling, zwitterion mode and an anti-microbial, quaternary amine mode. We showed that switching could be done more than 50 times without loss of activity and that the kinetics for switching from a low fouling zwitterion surface to an antimicrobial quaternary amine surface is practical for use. While a low pH was required for switching in the original polymer, we illustrated that by slightly altering the chemistry, it is possible to adjust the pH at which the switching occurs. A method was developed for applying the new zwitterionic surface chemistry onto polyethersulfone (PES) ultrafiltration membranes. Bacteria deposition studies showed that the new chemistry performed better than other common anti-fouling chemistries. Biofilm studies showed that PESultrafiltration membranes coated with the new chemistry accumulated half the biomass volume as unmodified membranes. Biofilm studies also showed that PES membranes coated with the new chemistry in the anti-microbial mode attained higher biofilm mortality than PES membranes coated with a common, non-switchablezwitterionic polymer. Results from our research are expected to improve membrane performance for the purification of wastewaters prior to use in irrigation. Since reduction in flux due to biofouling is one of the largest costs associated with membrane processes in water treatment, using dual-mode nanolayer coatings that switch between passive and active control of biofouling and enable detachment of attached biofoulants would have significant economic and societal impacts. Specifically, this research program developed and tested advanced ultrafiltration membranes for the treatment of wastewaters. Such membranes could find use in membrane bioreactors treating municipal wastewater, a slightly upgraded version of what presently is used in Israel for irrigation. They also may find use for pretreatment of agricultural wastewaters, e.g., rendering facility wastewater, prior to reverse osmosis for desalination. The need to desalinate such impaired waters water for unlimited agricultural use is likely in the near future.