Recovery of Volatile Fatty Acids Using Forward Osmosis : Influence of Solution Chemistry, Temperature, and Membrane Orientation

This study investigates the suitability of forward osmosis (FO) for recovery of volatile fatty acids (VFAs) from anaerobic digesters (AD) and identifies the conditions favorable for commercially viable maximum recovery of VFAs. The recovery efficiency of VFAs is evaluated using a polyamide (PA)-based thin-film composite (TFC) membrane. The pH (3, 5, 7, and 9), temperature (20°C and 40°C), and membrane orientation active-layer [AL]-facing FS and AL-DS) were changed, and water flux, reverse salt flux (RSF), rejection rate, and concentration factor (CF) were evaluated for five VFAs. The water flux and RSF were higher at a higher pH, temperature and in AL-DS mode. A low rejection rate of 23 to 36% and a CF of 0.20 to 1.90 were observed at a pH below the pKa due to the solubility of molecular VFAs, while rejection rates of 80 to 97% and concentration increase by to 1- to 4.8-fold at a pH above the pKa values were achieved due to deprotonation of VFAs and changes in membrane surface charges. With an equal increase in temperature of FS and DS from 20 to 40°C, the rejection rate decreased by almost 20%. With a transmembrane temperature change, a decrease in rejection rate of 20% was observed compared with baseline experiments due to decreases in viscosity and high diffusivity. In AL-DS mode, VFAs were rejected at a rate of 20% lower than that in AL-FS mode due to internal concentration polarization and membrane properties. These findings provide useful information on the factors that can influence optimal recovery rates of VFAs