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Development of predictive tools for membrane ageing

December 1, 2010

This collaboration hopes to confirm indicators of membrane ageing to help out Water Authorities in predicting when membranes will need replacement.

(Project 2008) Project Leader: Pierre — Le Clech, UNSW

The objectives of this project are to: develop analytical techniques to detect indicators of incipient changes due to membrane ageing conduct accelerated ageing studies for five membrane materials (hollow fibres and flat sheet) with four chemical agents under high temperatures and concentrations assess the long-term ageing propensity during cyclical fouling/cleaning experiments collect industrially-aged membrane from local filtration plants, to be examined with the same analytical tools. This will confirm findings obtained with lab-aged materials Microfiltration (MF) and ultrafiltration (UF) are being used more commonly to remove pathogens and salts and treat water and wastewaters.

Whether using flat sheet or hollow fibres, membranes are subject to a repetitive cycle of chemical cleaning, shear and mechanical agitation to remove material captured on the membrane surface during operation. Despite their widespread applications, little is known about the changes occurring at the molecular and structural level when membranes are subject to the individual or combined effects of chemical attack and mechanical strain. These changes (potentially resulting in membrane failure) can be due to a progressive build up of residue deposition, but loss of integrity (i.e. failure to separate critical components such as pathogens) is another cause driving membrane replacement. Among the main organic foulants are proteinous materials, polysaccharides, and humic substances commonly found in both water and wastewater treatment. Chemical treatment, with acids, bases, and oxidising agents, has commonly been used for these foulants, but assessment of long-term progressive degradation of performance due to accumulated residual deposition has not been studied in parallel with membrane aging and morphological changes.

Cyclical cleaning may result in slow changes in the pore size distribution as well as more heterogeneous surfaces due to residual macromolecules. This project focuses on the ageing effect of chemical agents (both oxidising and non-oxidising) commonly used in microporous membrane plants. By using a wide range of analytical techniques – mechanical, morphological and chemical – the complete assessment of the membrane state will be carried out before and after ageing. Challenge tests, run with model particles, will be conducted to assess the potential loss of membrane integrity. In a complementary series of experiments, the combined effect of fouling and cleaning will be assessed during consecutive cyclical runs.

This study will focus particularly on the application of a rigorous methodology to allow comparison between the different membranes and cleaning agents tested. This will allow a better understanding of the long-term performances of membrane systems. Critical assessment of the techniques used to assess nascent changes in membrane characteristics is based on a state-of-the-art literature review combined with the detailed assessment of the reproducibility and sensitivity of many potential techniques used to characterise membrane surface.

A report will be produced ranking membrane resistances to chemical cleaners (i.e. accelerated chemical ageing study). Experiments have been based on three commercially available symmetric hollow fibre membranes (outside diameter of 0.6, 1.2 and 2.3 mm) and two flat sheet membranes of different materials. The membranes all have nominal pore size ranging from 0.02 to 0.2 um. Accelerated ageing tests have been carried out initially using hydrochloric acid, sodium hydroxide, sodium dodecyl sulfate, and hypochlorite solutions (diluted in deionised water) and the membranes autopsied with short-listed techniques. Major outcomes expected from this project: Literature review manuscript to be submitted to peer-reviewed publication. Short list of recommended techniques to be used in the 2nd stage of the project (consecutive cyclical fouling/cleaning experiments). New automatic bench-scale experimental rig allowing consecutive cycles of fouling/cleaning of hollow fibre module with minimum human interaction.

The project has involved a collaboration with Dr Ho Kyong Shon, from the University of Technology, Sydney to characterise 10-year-old membranes used in Sydney Olympic Park. Results from this work will be presented at the upcoming AMS6/IMSTEC10 Conference in Sydney in late November 2010.