Ozone membrane contactors for water and wastewater treatment: A critical review on materials selection, mass transfer and process design

Gas-liquid membrane contactors are frequently proposed as promising alternative for traditional ozone injection methods in water treatment. However, information on successfully implemented large-scale applications is scarce. This review discusses the state of research of ozone membrane contactors for water and wastewater applications with a focus on material stability, mass transfer performance and process design. It aims to identify favorable operating conditions, the benefits compared to traditional injection methods and critical aspects for upscaling. Reported experimental ozone mass transfer coefficients (K) in hollow fiber and single tube contactors were analyzed for relevant influential parameters and compared to calculations using the Lévêque solution and the Kreulen modification. Volumetric mass transfer coefficients (Ka_s) were used for comparison with other ozone delivery methods. Differences between experimental and calculated mass transfer coefficients increased towards lower mass transfer and liquid velocity, potentially due to enhanced membrane resistances and ozone decay. The highest mass transfer coefficients were found for hydrophobic polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) membranes, while polydimethylsiloxane (PDMS), hydrophilic PVDF, and inorganic membranes showed lower transfer efficiencies. Although mass transfer enhancement by fast ozone depletion in the liquid bulk and in the boundary layer can be significant during water treatment, this design-factor is mostly neglected for ozone membrane contactors examined in the peer-reviewed literature. PVDF and PTFE hollow fiber modules exhibit higher volumetric mass transfer coefficients compared to traditional injection methods including bubble columns and venturi injectors, but full-scale and economic studies are missing to assess potential benefits of this new ozone injection method. In addition, long-term material stability is uncertain for most materials. Overall, this study provides a comprehensive comparison of gas–liquid contactors, suggesting future areas of research.