Submerged or sidestream? The influence of module configuration on fouling and salinity in osmotic membrane bioreactors

Abstract

The role of submerged and sidestream forward osmosis (FO) membrane module configuration in osmotic membrane bioreactors (OMBRs) was investigated. Experiments were performed under identical conditions (solids retention time, bioreactor volume, feed solution, draw solute, and draw solution concentration) to isolate the effect of FO module configuration and associated hydrodynamics on water flux, reverse salt flux, and membrane fouling. Steady-state water flux of fouled membranes was the same for submerged and sidestream configurations and two draw solution concentrations, leading to the concept of a homeostatic flux in OMBRs similar to the critical flux in conventional membrane bioreactors. For the 35g/L NaCl draw solution, specific reverse salt flux (SRSF) was 1.61 ± 0.01 and 0.59 ± 0.07g/L for submerged and sidestream configurations, respectively and for the 100g/L NaCl draw solution, SRSF was 2.22 ± 0.25 and 1.05 ± 0.35g/L for submerged and sidestream configurations, respectively. Despite a significant increase in driving force, fouled membranes did not have higher steady-state water flux; instead, the 100g/L draw solution resulted in greater membrane fouling; foulant cake layers were 2 to 4 times thicker, likely due to higher initial water flux that resulted in more foulants being transported to the membrane surface. Experimental results were used as model inputs to predict results for a larger scale system. Model results predicted lower steady-state bioreactor salinities in the sidestream configuration, particularly when longer solids retention times were used.