Monitoring of CaSO₄ deposition behaviors on biofilm during nanofiltration via ultrasonic time-domain reflectometry (UTDR)

The ultrasonic time-domain reflectometry (UTDR) as a non-destructive real-time method was employed to monitor the CaSO₄ deposition behaviors on biofilm during nanofiltration (NF). Two parallel experiments were performed to compare the different behaviors of CaSO₄ deposition with and without biofilm on the membrane. Results showed that the flux decline during combined fouling was slower than that in case of CaSO₄ fouling alone. The Ca^2 + rejection obtained with biofilm was higher than that without. A larger acoustic differential signal obtained by UTDR in the combined fouling revealed a denser and thicker layer formed on the membrane surface. Furthermore, the amount of CaSO₄ deposition on the biofouled membrane was more than that on non-biofouled membrane as a result of microorganisms as crystal nucleus to induce CaSO₄ crystallization and deposition. SEM images indicate that the CaSO₄ crystals deposited in order on the non-biofouled membrane, whereas on the biofouled membrane they were embedded in the biofilm. The denser and thicker fouling layer formed with biofilm was impermeable, resulting in a high Ca^2 + rejection. The complexation of Ca with polysaccharide in biofilm would eliminate the cake-enhanced osmotic pressure effect leading to a slow flux decline. To sum up, the independent measurements corroborate the ultrasonic measurements.