A Molecular Marker Approach for Characterization of RO Membrane and Module Solute Transport

The use of reverse osmosis (RO) processes has grown significantly over the past few decades in water treatment and reuse applications to safeguard water supplies against harmful pathogens and impurities as well as reducing water salinity to enable indirect potable water reuse. Maintaining membrane transport characteristics (e.g., flux and rejection) are paramount to ensuring process performance, particularly with respect to the passage of organics and nanosize enteric viruses that may pose a potential health hazard. Loss of membrane integrity (e.g., due to exposure to oxidants such as chlorine) is of particular concern in the case of water reuse applications where membrane integrity breaches can result in the passage of nanosize contaminants (e.g., enteric viruses). Clearly, in order to ensure that RO membranes operate reliably, there is a need for online monitoring of membrane performance with respect to its rejection performance. RO plant performance would also benefit from real-time information regarding hydrodynamic conditions in membrane modules as quantified, for example, via residence time distribution concentration which can serve to assess the overall level of mixing within the plant elements as impacted by structural changes within elements (e.g. due to telescoping and embedding of spacers), as well as membrane fouling and mineral scaling. Accordingly, given the need for online membrane performance monitoring, an online fluorescent marker-based approach was developed for: (a) assessing membrane mass transfer coefficient on the feed-side channel (thus enabling assessment of mixing and thus concentration polarization conditions); and (b) membrane integrity detection/characterization. The approach involved dosing the feed stream with the marker and monitoring the marker concentration in the permeate and concentrate streams. The change in marker concentration in the effluent streams in response to a controlled introduction of a marker dose to the feed was then monitored online. Application of the relevant membrane solute flux models to the marker, along with analysis of the marker’s residence time distribution, enabled assessment of the feed-side mass transfer coefficient and detection of membrane integrity breaches. Detection and quantification of the effect of membrane breaches (induced by controlled exposure to sodium hypochlorite) on membrane transport characteristics was evaluated for commercial polyamide RO membranes. Membrane integrity breaches were first characterized by imaging via scanning electron microscope (SEM) and atomic force microscope (AFM). The characteristics of the integrity breach were correlated with the level and mode of chlorine exposure. Experimental studies were then carried out, with a specially-designed online fluorescent marker detection system, using both flat-sheet membranes (in a plate-and-frame RO cell) as well as in a spiral-wound membrane pilot. Results from the present study demonstrate the technical feasibility for real-time online detection of RO membrane integrity breaches and assessment of RO module hydrodynamics performance.