Nanofiltration/Reverse osmosis as pretreatment technique for water reuse: ultrafiltration versus tertiary membrane reactor

Abstract

The potential for nanofiltration (NF) and reverse osmosis (RO) in secondary effluent reclamation is noteworthy, but both processes require extensive pre‐treatment to prevent membrane fouling and promote system longevity. In this study, performance of direct ultrafiltration (UF) and tertiary membrane bioreactors (tMBRs) as feed pre‐treatments were assessed on a pilot scale. Both technologies achieved high organic matter removal (87 and 93% for UF and tMBR, respectively). Differences in efficiencies were related to suspended biomass in the tMBR, which enhanced degradation of the dissolved organic substances (<5 mg/L) and also stabilized process performance by minimizing residual fouling. Whichever pre‐treatment was applied, RO reached higher rejection coefficients and low fouling levels. Nevertheless, lower normalized flux (12%) was achieved when the UF effluent was fed due to high concentration of organic substances. Membrane autopsies by scanning electron microscopy with dispersive X‐ray and Fourier transform IR spectroscopy revealed that organic fouling appears to be predominant on both NF/RO membranes. The main organic foulants identified were polysaccharides and proteins. Inorganic foulants mainly consisted of calcium, phosphorus, and iron. Complete removal of pharmaceutical active compounds was observed for RO, while NF presented moderate retention coefficients (94, 85, 70, and 27% for atenolol, paraxanthine, caffeine and clofibric acid, respectively).