Reverse solute transport, microbial toxicity, membrane cleaning and flux of regenerated draw in the FO-MBR using a micellar draw solution

Micellar draw solutions containing either sodium dodecyl sulphate (SDS), tetraethyl ammonium bromide (TEAB), 1-Octane sulphonic acid sodium salt (1-OSA) or trimethyloctyl ammonium bromide (TMOAB) were tested in the FOMBR system. After 48 h, reverse transported draw solute concentrations with 1 M draw solution concentrations for NaCl, SDS, TEAB, TMOAB and 1-OSA reached 3.22 mg/l (55 μM), 0.012 mg/l (0.04 μM), 0.067 mg/l (0.32 μM), 0.057 mg/l (0.23 μM) and 0.062 mg/l (0.29 μM), respectively. Thus, reverse transport of surfactant solute was minimal. The toxicity of each draw solution was tested using Pseudomonas aeruginosa specie and mixed (activated) sludge. The percentage growth for P. aeruginosa at the highest tested draw solute concentration of 0.05 M was 60% for SDS and 49% for TEAB, while the other surfactants proved to be more toxic. Similar to bacterial growth, the TOC reduction was higher for P. aeruginosa when compared to the mixed sludge consortium. For membrane cleaning, 0.0005 M EDTA with 0.125 g/l NaOH (a pH of 11.6) for 0.5 h was found to be an effective cleaning strategy, along with prior- and post-cleaning of the membrane using DI water. Reuse of the regenerated draw solutes indicated that the SDS flux returned close to the original flux, while the TEAB flux reached 50% of the original. Overall, SDS and TEAB are recommended for future investigation, because of minimal reverse solute transport, low in-situ microbial toxicity, good flux recovery after membrane cleaning, and effective draw solute regeneration.