Impacts of different draw solutions on a novel anaerobic forward osmosis membrane bioreactor (AnFOMBR)

Two anaerobic forward osmosis (FO) membrane bioreactors (AnFOMBRs), R_chloride and R_sulfate, were operated for 100 days using NaCl and Na₂SO₄ as the draw solution, respectively. The operating conditions were identical for both systems, with a solids retention time of 30 d, hydraulic retention time of 8 h and using cellulose triacetate FO membrane. High rejection performance of FO membranes resulted in salinity accumulation in the bioreactors. R_chloride and R_sulfate reached a stable conductivity of about 35 and 11 mS/cm, respectively, at the end of the experimental run. Hypersalinity of R_chloride undesirably impacted biological growth; mixed liquor volatile suspended solids in R_chloride was much lower at 376 mg/L, whereas that of R_sulfate was 1,170 mg/L. Organic removals were excellent due to reduced organic loadings at low fluxes and thus, R_sulfate and R_chloride achieved secondary total organic carbon (TOC) removal efficiencies of at least 75%. Both AnFOMBRs started with an initial flux of 5 LMH. Flux for R_chloride stabilized at 0.25 LMH, while R_sulfate at 0.96 LMH. The high salinities of both reactors negatively impacted methanogenic growth. Application of the fluorescence in-situ hybridization (FISH) technique confirmed the ousting of methanogens by sulfate reducing bacteria from the anaerobic consortium. Sparsely located methanogens were detected in R_chloride but none were detected in R_sulfate.