Organic Ionic Salt Draw Solutions for Osmotic Membrane Bioreactors

Abstract

This investigation evaluates the use of organic ionic salt solutions as draw solutions for specific use in osmotic membrane bioreactors. The osmotic membrane bioreactor is an innovative system, representing a combination of forward osmosis and membrane bioreactor technology to improve the quality of wastewater effluent for potable reuse applications. Selecting an optimal draw solution for forward osmosis applications is imperative for efficient system performance. A selection of organic ionic draw solutions underwent a desktop screening process before being tested in the laboratory and evaluated for performance using specific salt flux (reverse salt flux per unit water flux), biodegradation potential, and cost. All of the salts tested have organic anions with the potential to be degraded in the bioreactor as a carbon source and aid in nutrient removal. Two of the salts (sodium propionate and magnesium acetate) were found to have specific salt fluxes three to six times lower than two commonly used inorganic draw solutions, NaCl and MgCl2. Magnesium acetate had the lowest specific salt flux, indicating the highest process efficiency, but also required the most salt to achieve desired osmotic pressures, contributing to higher costs than sodium propionate. Sodium propionate was tested in batch biological experiments; it found to increase the rate of denitrification in an anoxic environment and to degrade completely in an aerobic environment. Thus, sodium propionate appears to be a highly suitable draw solution for osmotic membrane bioreactor systems.