Integration of Forward Osmosis Membrane Bioreactor (FO-MBR) and Membrane Distillation (MD) units for water reclamation and regeneration of draw solutions

Abstract

In the current lab scale study, the two treatment technologies of FO (Forward osmosis) and DCMD (Direct contact membrane distillation) were used in combination to create a continuous treatment system for water reclamation using, inorganic NaCl, surfactant (Tetraethyl ammonium bromide, TEAB) and polyelectrolyte (Polydiallyldimethylammonium chloride, PDAC) as FO draw solutes with both DI water (FO-MD), and monoculture municipal synthetic wastewater with Bacillus subtillis inoculum as feed (FOMBR-MD).Each of the units was optimized separately at bench scale, and then combined to form a Forward Osmosis Membrane Bioreactor – Membrane Distillation (FOMBR-MD) hybrid for water reclamation from wastewater. In this hybrid system, the diluted draw solution was fed to the MD unit for reconcentration. The MD unit was tested at various operating temperatures; for FO-MD, a temperature difference of 15 °C was chosen, with a feed temperature of 35 °C and a permeate temperature of 20 °C. For the FOMBR, the temperature difference was increased to 25 °C. When the cross flow velocity was kept constant, the MD flux increased significantly with increasing temperature. Low grade waste heat from industry can potentially be utilised for the FO-MD process. The increase in flux was higher for NaCl and TEAB (60–80 %) when compared to PDAC (20–50 %), as the temperature difference increased to 35 °C. A greater feed flow velocity increased the turbulence in the flow channel, and increased the MD flux. Up to a 60 % increase in flux was observed when the feed velocity was increased from 0.12 to 0.17 m/s, while the increase in flux was not as high when the flow was increased from 0.17 to 0.21 m/s and thus 0.17 m/s was chosen as the cross- flow velocity for MD. Higher feed concentrations tended to reduce vapour pressures and hence decreased the MD water fluxes.The flux for FO-MBR and MD were well balanced for lower molecular weight draw solutes, but lower FO fluxes were observed for higher molecular weight draw solutes (TEAB, PDAC). This greater imbalance reflected their lower diffusivities and greater tendencies toward concentration polarization. Reverse solute transport (RST) values leading to salinity or toxicity were not reflected by the observations, and therefore it was feasible to run an FOMBR-MD Hybrid for extended times. However, a greater decline in flux versus time was observed for NaCl than for TEAB. PDAC showed the lowest but most stable flux, and a lesser fouling tendency compared to that of TEAB. Overall, The FO-MD hybrid enabled pure water production and a non-volatile component rejection of nearly 99 %, with an RST of <0.1 GMH for the draw solutions back to the FO feed and a much lower solute transport from the MD feed to the permeate. NaCl and other low molecular weight solutes are recommended when high fluxes and system stability are priorities and where high reverse solute transport (RST), leading to salinity or solute toxicity in the MBR feed tank, is not an issue. Higher molecular weight solutes, such as TEAB or PDAC, are recommended when high flux is less critical and when the sensitivity of the microbial consortium to RST and solute toxicity is a concern. In all cases, some degree of process control is recommended in order to bring the systems more quickly to a stable operating state; periodic membrane cleaning is deemed necessary to maintain this state.