Benefits of Organic Ionic Salts As Draw Solutions for Osmotic Membrane Bioreactors

The osmotic membrane bioreactor (OMBR) system represents a unique and innovative combination of forward osmosis (FO) and biological processes to enhance the quality of wastewater effluent for potable reuse applications and for discharge to the natural environment. Through osmosis, water diffuses from the bioreactor, across a semi-permeable membrane, and into a draw solution, a high concentration solution with high osmotic pressure. The FO membrane submerged in the bioreactor separates the mixed liquor from the draw solution, which extracts water from the bioreactor and drives this natural process. Water transport from the bioreactor results in dilution of the draw solution. The diluted draw solution is sent to a reconcentration process (such as reverse osmosis (RO) or membrane distillation (MD)), which reconcentrates the draw solution and generates a high-quality product water.

The type of draw solution plays a key role in the OMBR process, affecting nutrient removal potential and FO and bioreactor performance efficiencies; for this reason, selection of an optimal draw solution is an important consideration in the OMBR system. OMBR performance is affected by reverse salt flux, or passage of small amounts of draw solute from the draw solution into the feed solution. Reverse salt flux reduces the driving force and necessitates salt replenishment and varies for depending on the selected draw solution. Organic ionic salt solutions as draw solutions for specific use in OMBRs have the potential to degrade in the bioreactor as a carbon source to aid in nutrient removal. Exploring new organic draw solutions could also increase process performance (lower reverse salt flux) when compared to currently implemented inorganic ionic salt draw solutions.

The objective of this presentation is to evaluate a selection of organic ionic salt draw solutions and their potential benefits over currently implemented inorganics in OMBR systems. Results from bench-scale experiments show that considering specific salt flux, biodegradation quantification, and replenishment cost, magnesium acetate and sodium propionate performs best overall and are the most suitable draw solutions for an OMBR system.