Effect of organic and inorganic draw solution on recalcitrant compounds build up in a hybrid ultrafiltration-osmotic membrane reactor treating refinery effluent

[Display omitted] •UF-OMBR allowed for salinity build-up control.•Recalcitrant build-up was not mitigated by UF in the UF-OMBR.•Recalcitrant compounds build up when NaCl was used as DS.•CH3COONa favored recalcitrant compounds degradation and improved process efficiency.•UF-OMBR salinity and flux simulations showed good fit with the experimental values. When osmotic membrane bioreactor (OMBR) is applied to the treatment of effluent with compounds that behave as slow degradable or recalcitrant compounds both salinity and organic build-up control is necessary. In this study salinity and organic build-up in a hybrid ultrafiltration-osmotic membrane bioreactor (UF-OMBR) treating oil refinery effluent was assessed. Furthermore, the effect of two different draw solutions (DS) over the biodegradation of recalcitrant compounds in the UF-OMBR treating oil refinery effluent was investigated. The operation was conducted in a continuous basis (505 days). Simulations showed that salinity build up in a conventional OMBR would be 10x and 5x times higher when sodium chloride (NaCl) and sodium acetate (CH3COONa) were used as DS, respectively, in comparison with the experimental UF-OMBR. The UF, however, was not able to relief recalcitrant build up in the reactor when operated with NaCl, which led to a decline in process efficiency. The use of CH3COONa as DS, favored the microbiological activity and enhanced recalcitrant biodegradation. This hypothesis was proven by the increase in mixed liquor volatile suspended solids, the decrease in residual dissolved organic carbon and N-NH3 concentration in UF and FO permeate, in addition to the absence of recalcitrant functional groups in FTIR spectra. However, a slightly higher flux decline, and consequently low permeate flux, occurred when CH3COONa was used (0.60 ± 0.15 L/m2·h) when compared with the permeate flux observed in the operation with NaCl as DS (1.07 ± 0.32 L/m2·h). For being a biodegradable compound, the presence of CH3COONa favored the biofouling layer formation.