Osmosis membrane bioreactor–microfiltration with magnesium-based draw solute for salinity reduction and phosphorus recovery

Osmosis membrane bioreactors (OMBRs), which integrate forward osmosis (FO) and a biological process, followed by reverse osmosis or membrane distillation (MD) have been receiving increasing attention for wastewater treatment and reuse. However, OMBR application in wastewater treatment is still hindered by the accumulation of inorganic and organic salts, which affects microbial activity in the OMBR. Therefore, in this study, a novel hybrid OMBR–MD system integrated with periodic microfiltration (MF) extraction was developed for simultaneous salinity reduction and phosphorus recovery using a magnesium-based draw solute by taking advantage of magnesium salt reversal. In the OMBR system, MgCl₂ was used as the draw solution to withdraw clean water passing through the FO membrane, whereby all contaminants and mineral salts, including phosphate, ammonia and magnesium reversed from the draw solution, were retained in the bioreactor. The MF membrane was used to bleed the water out of the bioreactor for salinity reduction and subsequent phosphorus recovery. The pH of the effluent from the MF containing high phosphorus was adjusted to 10 to precipitate struvite, and the amount of the produced struvite was quantitatively determined to be 41 mg per liter of the MF permeate. The MD process was used to recover the diluted MgCl₂ draw solution with an initial flux of 8.2 L/m² h under a temperature difference of 30 °C (55 °C in the feed and 20 °C in the distillate). Subsequently, the flux slightly decreased to 6.3 L/m² h after 6 h because of the decreasing vapor pressure in the salt solution based on Raoult's law.