Raman micro-spectroscopy for the study of concentration polarization in the presence of biofouling in pressure driven membrane systems

In nanofiltration (NF) and reverse-osmosis (RO) membrane systems accumulation of biomass on the membrane surface increases the solute concentration next to the membrane, thus resulting in an enhanced concentration polarization layer. In this study the impact of biofilm on the solute concentration in a nanofiltration experimental setup has been investigated by means of optical coherent tomography (OCT) and confocal Raman micro-spectroscopy (RMS) and mathematical modelling. A B. subtilis biofilm was grown for six days in a NF flow cell. The morphological properties of the biofilm have been resolved by OCT. The concentration polarization layer was evaluated prior and after the biofilm growth in the experimental setup. The results provide confirmation that biofouling enhanced osmotic pressure (BEOP) is indeed a mechanism responsible for flux decline in spiral wound membrane systems facing biofouling issues. According to the measurements, there seems to be a critical biofilm thickness threshold (∼60 μm) that must be reached before the osmotic pressure increase 15 percent or more due to the presence of biofilm. The developed mathematical model, utilizing fully developed laminar flow and, accurately represents the solute concentration profile in the examined scenarios in the presence of biofilm.