Multiple benefits of MBR and absolute-rated cartridge filters for RO pretreatment: Results from a full-scale facility

ABSTRACT There is interest in processes that can both contribute to pathogen removal and provide effective pretreatment for reverse osmosis (RO) in potable reuse systems. The most familiar reuse treatment train employs micro-/ultra-filtration (MF/UF) along with cartridge filters (CFs) upstream of RO. However, there are some applications for which other pre-RO processes, such as membrane bioreactors (MBRs), are desirable, and more research is needed on MBR for RO pretreatment. In addition, although CFs are widely used, the literature does not elucidate their capability for pathogen removal and fouling control. In this study, MBR along with absolute-rated 1 µm CFs were demonstrated for RO pretreatment at full scale. This plant included two identical trains, which provide additional robustness to the results. Online MBR and CF data confirmed performance within critical control point limits, and breach testing of the CFs demonstrated a consistent response pattern in differential pressure (dP) data and with novel derived parameters. These observed breach responses and novel parameters are a useful contribution to the online monitoring of CFs as a pathogen barrier. The combined pretreatment processes also achieved remarkably low fouling rates at the downstream RO system, lower than other recently-reported MBR-RO facilities and consistent with 5 -7 month clean-in-place intervals. The observation of significant increase in dP over 2 weeks showed that the CFs were capturing colloidal particles even from low-turbidity influent, and this resulted in lower-than-expected fouling rates at the downstream RO. This study therefore provides a fundamental insight into the into RO fouling propensity of MBR filtrate: namely, colloidal particles in the size range of 1 µm and above are an important contributor to fouling propensity when they are not removed by pre-RO treatment. Overall, these results demonstrated the multiple benefits of a novel MBR-CF pre-RO treatment train.