Osmotic membrane bioreactor (OMBR) in refinery wastewater treatment: The impact of a draw solute with lower diffusivity in the process performance

Abstract

The present work considered an osmotic membrane bioreactor (OMBR) for the treatment of a complex and recalcitrant refinery wastewater. Magnesium chloride (MgCl2) was employed as draw solution (DS) to minimize the effects of salinity build up within the mixed liquor (ML). The flux decrease was mainly attributed to the salinity build up. Due to the higher contaminant retention time compared to conventional membrane bioreactor or hybrids UF-OMBRs, the recalcitrant compounds started to be degraded. Moreover, the reverse flux of magnesium favored a higher granulation of the sludge, which in turn enhanced nitrification and denitrification processes. The increase in fouling resistance was attributed to deposits formation, as showed in SEM micrographs and EDX spectra, and the pore blockage by low molecular weight compounds originated by the biological degradation. The increase in salinity (maximum ~10 g/L) lead to a higher production of SMP and EPS protein-like substances, which have lower effect in fouling propensity. Furthermore, even though the ML volatile suspended solids (MLVSS) and ML suspended solids (MLSS) decreased, the mean MLVSS/MLSS ratio was 0.75 ± 0.15, which shows that the salinity build up did not degrade the biological activity. In this sense, when the OMBR is operated with MgCl2 as DS, the permeate shows good physical–chemical quality due to increased contaminants retention time, demonstrating that the system can be used even for applications that demands elevated physical–chemical quality.