In-situ monitoring of biofouling on reverse osmosis membranes: Detection and mechanistic study using electrical impedance spectroscopy

Abstract

Electrical impedance spectroscopy (EIS) was employed to monitor biofilm formation on the membrane surface in-situ and non-invasively. An EIS-derived parameter, the normalized conductance of the diffusion polarization (GDP) layer showed two stages of biofilm formation. The first stage was related to the accumulation of bacterial cells and the formation of the respiration products from the bacteria. The second stage referred to the accumulation of the extracellular polymeric substances (EPS) which was the main component for the formation of the biofilm matrix. The effect of a biostat, sodium azide was also investigated and its presence slowed down the growth of bacteria and caused the partial detachment of bacteria from the membrane surface. Sodium azide's effects were also reflected in the normalized GDP plot. The sustainable flux, at which the fouling was minimal, could be estimated from the rate of change of the initial increase of the normalized GDP with respect to flux. Conventional monitoring methods such as transmembrane pressure (TMP) or autopsy by confocal laser scanning microscopy (CLSM) on their own gave little insights into the mechanisms of the biofilm formation. This study demonstrates the ability of EIS to be incorporated into a “canary” cell located in a side stream of the high pressure membrane vessel for the monitoring of biofouling as well as for the assessment of the cleaning efficiency in a water treatment plant.