Real-time monitoring of membrane fouling caused by phenolic compounds

Abstract

Following the booming blooming of the bio-economy, membrane-based processes have gained increasing interest as a selective, energy efficient, and environmentally friendly option for the separation and purification of organic compounds originating in bio-based streams. However, the stability of membrane-based systems is often greatly limited by membrane fouling problems, especially in industrial scale processes. The effective prevention and mitigation of membrane fouling requires a deep understanding of underlying fouling phenomena. It is therefore of great importance to develop techniques to investigate membrane fouling mechanisms and collect detailed and dynamic information about the formation of fouling layers.

The first part of this study offers insights into the current state of real-time in situ membrane fouling monitoring in the biotechnology, biorefinery, and food sectors, through a comprehensive literature review and interviews with industrial users of membranes and suppliers of analytical instruments. Working principles, capabilities, strengths, weaknesses, and views on the importance and potential usability of a variety of real-time monitoring techniques are identified. The interviews revealed that an ideal universally applicable monitoring tool should enable the simultaneous monitoring of both fouling layer thickness and composition, but the results of the literature review showed that the currently available monitoring techniques enable either the monitoring of fouling layer thickness, and the distribution or monitoring of composition. Complementary methods which can gain both morphological and molecular information are thus required to gain a deeper insight into fouling.

Membrane fouling caused by phenolic and ligneous compounds is known to cause major challenges in several membrane-based processes related to biorefineries and water purification. The aim of the second part of this study was therefore to study whether adsorptive fouling of phenolic model compounds could be monitored in real-time with the combination of normal Raman spectroscopy and the simplest chemometrical technique, i.e. principal component analysis (PCA). Adsorption of vanillin on the surface of the membrane was monitored at different concentrations and pH values, and in mixed binary and ternary solutions of vanillin and other phenolics ferulic acid and eugenol. It was found that the developed real-time membrane fouling monitoring method is usable even in flow conditions. The results of the single-compound experiments showed that a combination of Raman spectroscopy and PCA could enable fast, robust, and simple monitoring of early-stage membrane fouling. However, PCA failed to be a sufficient technique for the quantification and visualization of multicomponent adsorption processes because of the significant overlap of the peaks of the different phenolics with similar structures. The development of more advanced data analysis methods is therefore needed in future.

The second part of this study also highlights the importance of using complementary methods for the detection of fouling. In this study, a deep insight into membrane-foulant interactions was achieved with a novel combination of Raman spectroscopy, surface plasmon resonance and molecular dynamics simulation. This study also emphasizes the significance of adsorptive fouling as the initiator of fouling layer formation, and underlines that conventional membrane fouling monitoring methods (based either on observing a decrease in membrane flux or an increase in transmembrane pressure) are not necessarily even capable of verifying the formation of an adsorptive fouling layer, which might cause an increase in the membrane’s permeability.