Design, Construction and Optimization of Forward Osmosis Membrane Bio-Reactor

Abstract

The forward osmosis membrane bioreactor (FO-MBR) is an emerging alternative to conventional MBR, which can be used for both potable and non-potable reuse applications. FO-MBR combines the conventional MBR and forward osmosis (FO) processes. FO membrane has the capability of high range of contaminants removal, and has a lower fouling tendency than pressure driven membranes which results in less frequency of membrane cleaning. Due to low hydraulic pressure required for operation, the energy demand is exceptionally low, which makes FO-MBR system cost effective technology for wastewater treatment. In this study laboratory scale FOMBR batch setup was developed using external hollow fiber membrane module (SMTC, Singapore) to optimize different operational parameters and their effect on the process was investigated. Operational parameters include temperature, osmotic backwashing, cross flow velocity of the system and draw solution (DS) volumes and concentrations were optimized. This system was modified to semi-continuous FO-MBR after all the parameters were optimized on batch setup. All the optimized parameters were cross checked on the semi-continuous system and was further improved to a continuous FO-MBR. Different salts were studied to investigate reverse solute transport and compared to select the most appropriate draw solution in FO-MBR for wastewater treatment. Internal concentration polarization is a major issue in FO-MBR, also discussed in this study. Results showed that change in initial volume of draw solution (DS) has no effect on the process. Increase in cross flow velocity increased the flux, but system cannot accommodate higher velocities beyond 150 ml/min, therefore cross flow velocity was optimized to a value of 150 ml/min. Increase in DS concentration from 0.5M to 2.0M increase the flux but further increase in molar concentration from 2M to 3M, the conductivity drop increases abruptly. Beyond 3M, change is almost constant which causes reduction of average flux, so optimized value was 2M. For osmotic backwashing draw solution was replaced by DI water. It was noted that osmotic backwashing was ineffective for change in flux. The flux was found to be 7.24 LMH at 220C and 7.18 LMH at 300C. This revealed that the temperature effect on flux is negligible. FO-MBR system was operated with MgCl2 and KCl as draw solutions and MLSS of 6 g/L, it was observed that KCl has higher flux of 4.61 LMH than MgCl2 which has a flux of 3.95 LMH. Although KCl has higher flux than MgCl2 but its reverse solute transport is significantly higher which affect the system process severely. MgCl2 was the most appropriate salt for wastewater treatment in FO-MBR setup.