Perspectives of membrane fouling monitoring by 3ω sensing

Fouling remains a challenge for efficient membrane operation. The optimization of performance is limited by the low resolution and indirect measure of fouling through flux and pressure measurements, calling for direct, local in situ fouling measurement techniques. The 3ω method was initially developed for measurement of thermal conductivity but can, nonetheless, also be used to measure deposit thicknesses and has potential for membrane fouling monitoring. In this project, 20 µm thick platinum wires are integrated into membranes and an alternating current (AC) is applied to heat the wire. The measured oscillating voltage contains the fundamental and a third harmonic component. The amplitude of the third harmonic wave, i.e. the U3ω signal, is inversely proportional to the heat conductance, which decreases as the fouling layer builds up. Crossflow filtration tests demonstrate that accurate measurement of the U3ω signal can reveal changes in crossflow velocity along the membrane, as an increasing crossflow velocity enhances heat convection from the Joule heated platinum wire. This effect increases with lower AC frequency, caused by a larger and frequency penetration penetration depth of the thermal wave out from the stagnant boundary layer, as illustrated in Fig. 1. U3ω signals measured on membranes coated with layers of acrylic varnish to form an artificial fouling layer show increasing amplitude with increasing layer thickness due to the lower thermal conductivity, i.e. insulating effect, of acrylic compared to surrounding water. By measuring U3ω signals at high frequencies, the signal will reflect the thermal conductivity of the material, while by reducing the AC frequency, i.e. increasing penetration depths, the signal will reflect also the layer thickness. This can serve to distinguish between type and amount of fouling. The same tendency is observed by in situ measurements of U3ω signal during fouling of an ultrafiltration membrane by filtration of a dilute milk suspension. Here, the signal amplitude increases as the membrane is fouled, but is reduced again after membrane cleaning. Hence, measurements of U3ω signals can serve as a valuable tool to monitor membrane fouling and cleaning locally during membrane filtration. In the ongoing project collaboration, it will be investigated how the method can be used to monitor different types of fouling, while investigating how the method can feasibly be implemented in industrial membrane modules.