Critical Wetting Moment and Effective Cleaning of Gypsum Scaling in Membrane Distillation

Abstract

Wetting caused by mineral scaling significantly hinders the application of membrane distillation (MD), a promising technology for water recovery from saline streams. However, the wetting dynamics and the adequate control solution remain poorly addressed. Herein, we conducted a non-invasive, real-time electrochemical impedance spectroscopy (EIS) analysis for gypsum scaling in MD to seize the critical wetting moment, hypothesizing a timely cleaning performed then could effectively prevent membrane wetting. Our results showed that single-frequency impedance experienced a wave-peak trend during gypsum scaling near the membrane interface. The full-frequency EIS analysis with an equivalent circuit demonstrated that the transition of fouling from the exterior to the interior was marked by the impedance peak, in line with the modeled characteristic data. This result agrees with the crystal penetration depth revealed in scanning electron microscopy data and is confirmed by in-situ characterization of gypsum nucleation via optical coherence tomography. Under critical wetting moment, an in-situ water rinse extended membrane lifespan in a five cyclic scaling-cleaning experiment, with flux recovery rate over 95% and negligible increases in permeate conductivity, while cleaning missing that moment resulted in a failed membrane recovery. The proposed intelligent and effective anti-fouling/wetting strategy could insight into various membrane processes dealing with complex streams.

Keywords: electrochemical impedance spectroscopy, Gypsum scaling, wetting, Membrane cleaning；Membrane distillation