(156d) Municipal Wastewater Treatment By An Osmotic Membrane Bioreactor: Use of Sodium and Magnesium Chloride As Draw Solution

Osmotic membrane bioreactor (OMBR) technology, which integrates forward osmosis (FO) and biological treatment process, has received increasing attention. In this study, an OMBR was developed for municipal wastewater treatment, using two draw solutes (DS), i.e. MgCl₂ and NaCl. The potential and limitations of this innovative treatment process was investigated with respect to pollutant removal efficiencies, flux stabilities and membrane fouling. During a 100-day operation, high removal of organic matter (98%), nitrogen (98%) and PO₄³⁺-P (99%) were achieved with both DS, with the effluent TOC, NH₄⁺-N, total nitrogen (TN) and PO₄^3--P being lower than 3.0 mg/L, 0.5 mg/L, 1.5 mg/L and 0.1 mg/L, respectively. Significant aerobic denitrification (up to 98%) was observed in the OMBR, owing to the high rejection of the FO membrane and the prolonged retention of NO₂^--N and NO₃^--N within the bioreactor which facilitated their aerobic removal. However, salt leakage from the DS and the retention of electrolytes from the feed wastewater resulted in significant salt accumulation in the bioreactor, which not only reduced the water flux considerably but also adversely affected pollutant removal efficiencies. Nitrification was affected acutely with both the DS. Marginal inhibition of denitrification was also observed with NaCl. However, for both DS, TOC removal was not evidently affected. Compared to MgCl₂, NaCl gave 20% higher initial water flux but resulted in more significant water flux decline (about twice), due to a more severe reverse transport of NaCl as the DS (about 6.5 times). Membrane fouling was controlled by periodically soaking and washing with tap water, and resulted in only 5% reduction in water flux decline over 60 days. Different rejection and transportation behaviors of NH₄⁺-N, NO₂^--N and NO₃^--N were also observed with different DS, which was found to be governed by electrostatic interactions and the Gibbs-Donnan effect.