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 Ca2 +  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 Ca2 +  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.

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