Characterization of colloidal fouling in forward osmosis via ultrasonic time- (UTDR) and frequency-domain reflectometry (UFDR)

Ultrasonic time- (UTDR) and frequency-domain reflectometry (UFDR) were applied to characterize internal and external fouling in the forward osmosis (FO) process. Unlike RO applications, in which the transducer was placed above the membrane to detect external fouling on the top surface of membrane, the transducer was placed below the membrane to enable detection of internal and external fouling in an FO application. The amplitude changes and arrival-time shifts in UTDR were more sensitive than the flux decline as an indicator of FO fouling. In the active-layer facing the feed solution (AL-FS) mode, the cake-layer thickness obtained via the differential UTDR signal agreed well with the mass-balance data. The Short-Time-Fourier-Transform (STFT) analysis permitted decomposing the non-overlapping peaks that revealed a significant decrease in the amplitudes of the high-frequency components due to the grain-scattering effect in the active-layer facing the draw solution (AL-DS) mode. In contrast, the frequency distribution remained unchanged in the AL-FS mode. The total reflected power (TRP) calculations for the arrival-times and principal peaks in the frequency-domain correlated well with the overall fouling and internal fouling, respectively.