Evaluation for the distribution of fouling deposition on the microfiltration membrane using high frequency ultrasound

Distribution of fouling may largely affect flux of the microfiltration membrane, in which its formation is related to several factors such as operating pressure, flow rate, concentration/composition of feed solution, and membrane material. It therefore is essential to evaluate fouling distribution on microfiltration membranes before they will be further applied. In this study, an attempt was explored to develop methods and techniques for measuring the fouling distribution using a 35 MHz high frequency ultrasound system. The experiments were carried out from a waste water treatment system that is composed of polyvinylidene fluoride (PVDF) membrane with nominal pore size of 0.22 μm and feed solutions with the humic acid solution of 4 ppm. The operating pressure and flow rate were respectively maintained at 1 bar and 0.22 L/min. Areas of those filtration membranes after 5, 15, 30, 60, and 100 minutes filtration durations were raster scanned by the high frequency ultrasound system. The peak-to-peak voltage (Vpp) of ultrasonic signals reflected from the surface of membrane was calculated for the reconstruction of C-scan images. The average Vpp and flux of filtrate were found to decrease exponentially with the increase of filtration duration. In accordance with the flux of filtrate approached to saturate, the variation of average Vpp tended to be nearly minimum. It can be readily observed for the changes of average Vpp that decreased from 3.02±0.05 V at the beginning of filtration to 1.73±0.25 V at 100 min duration after filtration with the humic acid solution of 4 ppm. Moreover, the fouling of the humic acid on the membrane was not homogeneously distributed. The acoustic impedance mismatch between the fouling of humic acid and PVDF membrane led the amplitude of ultrasonic signals reflected from the membrane surface to decrease with the increase of fouling deposition. C-scan results indicated that fouling deposition is a both temporal- and spatial-dependent process and that may be feasible to be sensitively and rapidly evaluated by high frequency ultrasound image incorporated with the analysis method.