Development of Micellar Draw Solution and Process Optimization of the Forward Osmosis Membrane Bioreactor

Abstract

The wide spread application of forward osmosis (FO) awaits substantial investigation in the development of novel draw solutions. When surfactant monomers aggregate above the critical micelle concentration (CMC) and Krafft temperature (Tk), micelles are formed. Micellar solutions were evaluated as novel draw solutions for FO systems. It was found that the micellar solutions generate stable water fluxes, independent of declining draw solution concentration, around their CMCs. The reverse transport of different micellar solutions was found to be 53-268 times lower as compared to the sodium chloride, at similar concentrations. Upto 95 % recovery of micellar solutions with energy efficient methods like ultra filtration (UF) or Krafft temperature swing is another advantage. Microbial toxicity effects of the reverse transported micellar draw solutions were studied using isolated microbial species and mixed activated sludge. Micellar solutions were recommended for use in the forward osmosis membrane bioreactor (FO-MBR) but with concentrations above CMC. Different operational parameters for improving the efficiency of the FO-MBR were also optimized. A draw solution concentration of 2 M was found most suitable since the higher concentrations invited extensive concentration polarization (CP) which caused a net water flux decline. Cross-flow velocity of 300 ml/min was optimized to circulate the feed solution (sludge) in FO-MBR, because greater velocities caused a breakdown of flocs to encourage severe membrane fouling. Micellar fouled FO membranes were found to show 100 % flux recovery after cleaning with Ethylenediaminetetraacetic acid (EDTA) solution. It was discovered that the major transport of organic contaminants and draw solute, across the FO membrane, occurs in the first hour of operation of the FO-MBR. In short, micellar draw solutions are a good addition towards the development of FO.