PhD dissertation “From ageing analysis to second life: a sustainable approach to osmosis membrane recycling”, by Bianca Zappulla

Desalination is steadily increasing worldwide as a response to water scarcity, leading to a proportional rise in the production and consumption of membranes, particularly those used in reverse osmosis (RO) systems. Some sources estimate that there are more than 7 million of RO membranes installed worldwide. As the manufacture of one single RO membrane module demands 12 kg of plastic and emits 85 kg of carbon dioxide equivalent, promoting membrane reuse and extending their life cycle –currently, between 5 and 10 years–is essential to achieve a more sustainable and circular water treatment model.

In this doctoral thesis, constraints during operational RO membrane life such as drying, industrial use, and chemical exposure were studied to i) better understand their impact on membrane performance and ii) evaluate whether these factors pose limitations for membrane recycling. This approach was also applied to forward osmosis (FO) membranes. Results showed that:

• Drying significantly reduces membrane permeabilities (up to 85%). However, after ethanol-soaking, membranes partially recover, indicating that the PA layer damage may be reversible. In contrast, for the PSf layer, drying causes irreversible damage (up to 90% permeability loss).
• Industrial use leads to performance degradation, mainly due to pore compaction in the PSf layer and partial degradation of the PA layer, confirming the impact of long-term use on membrane integrity.
• A three-step free chlorine protocol to selectively degrade the PA layer successfully produces nanofiltration (NF)- and ultrafiltration (UF)-like membranes, confirming the feasibility of transforming end-of-life (EoL) RO membranes into membranes with different separation properties.
• While membranes exhibit good resistance at acidic, neutral and basic pH values, the addition of 10 ppm free chlorine triggers degradation, particularly under neutral and basic conditions.

Bianca Zappulla also explored an alternative oxidative chemical treatment with ozone for membrane recycling. At high exposure, ozone could remove not only the PA but also the PSf layer, a behavior distinct from free chlorine, thereby providing a promising strategy for the reuse of the polyester support layer. At low ozone exposures, the method proved effective in converting fully aromatic membranes, while semi-aromatic membranes demonstrated higher resistance.

Finally, the researcher carried out pilot-scale tests to validate the reuse potential of transformed membranes. New but discarded membranes and EoL RO membranes were converted into NF- and UF-like membranes and tested in a gravity-driven membrane (GDM) system treating effluent from a municipal wastewater treatment plant. Results demonstrated that transformed RO membranes (repurposed into UF-like membranes) produced permeate water suitable for non-potable water reuse. Moreover, the study highlighted the potential of NF-like membranes for use in GDM systems.

Altogether, this research work gathers new scientific knowledge on RO/FO membrane operation and recycling. This can contribute to extend membranes’ lifetime and increase their recycling potential. The thesis was directed by Dr Gaëtan Blandin, Dr Hèctor Monclús and Dr Ignasi Rodriguez-Roda from LEQUIA research group of University of Girona – which has a long-track record of research and development within the membrane field.