Use of Ultrasonic Sensors for Characterization of Membrane Fouling and Cleaning

The modern challenges for membrane separations in a wide range of processes require more sophisticated approaches for the detection and remediation of fouling, i.e., the association of solutes, particulate matter, and colloids on and/or within a membrane. Most commonly, fouling is assessed from inferred measurements of permeation rate and/or permeate quality. The use of acoustic techniques for direct observations of membrane fouling was introduced over 10 years ago. We summarize here, recent developments in ultrasonic reflectometry that use both time-domain and frequency-domain spectra for noninvasive, real-time assessments of fouling in a variety of module configurations and geometries. In addition, we describe recent developments and applications of scanning acoustic microscopy (SAM) for post-mortem characterization of membranes with particular emphasis on biofouling. INTRODUCTION Many different filter media are currently available for use in a broad variety of liquidand gas-based separations. Of these media, nonwovens and membranes have been gaining increased attention, where the latter include polymeric, ceramic, metallic or composite materials. Although microporous membranes have been successfully utilized in industrial microand ultrafiltration applications, membrane fouling continues to be the key challenge which limits the operations of membrane-based liquid separations. Fouling occurs when the component(s) filtered from the feedstream collect near the membrane/fluid interface. The earliest stage of the fouling process is characterized by concentration polarization (CP) associated with a boundary layer, in which a gradient of excluded products forms near the membrane surface [1]. Under some conditions the excluded product can associate with the membrane surface or membrane pores, forming what is generically known as a fouling layer (Figure 1). Membrane fouling is a complex phenomenon that depends upon the type of foulant(s), the feed concentration, oxidation/reduction conditions, temperature, pH, ionic strength, and separation system hydrodynamics [2]. FIGURE 1. Schematic representing the development of a concentration polarization boundary layer (CPBL) and subsequent fouling of a membrane during a cross-flow filtration. This fouling layer usually has an adverse effect on separation performance by decreasing flux and selectivity as well as serving as a source of contamination. Inorganic fouling or scaling occurs when local concentrations exceed saturation and solid phases precipitate from solution near the membrane surface, or in the membrane pores [3]. Other common types of fouling often occur during the separations of select biopolymers such as proteins [2], as well as with the retention and selection of microorganisms which that form biofilms [4]. This review summarizes developments regarding the use of non-destructive acoustic testing (NDT) as a novel in-situ method to monitor inorganic fouling and biologically associated fouling (commonly referred to as “biofouling”) of membranes during liquid separation processes. Recent acoustic studies have reported considerable success in real-time monitoring of inorganic scaling processes on a variety of membranes. In contrast, monitoring of biofouling by Permeate Fouling Layer Concentration Polarization Boundary Layer Bulk Concentration