(324a) Membrane Integrity Monitoring in Multi-Element Spiral-Wound RO/NF Plant

Reverse osmosis (RO) and nanofiltration (NF) processes have been promoted in recent years as the separation process of choice providing a final barrier in the treatment of wastewater for indirect (as well as direct) potable water reuse. RO membranes are effective in rejecting species as small as monovalent ions and many organic microcontaminants, as well as in providing essentially complete rejection of pathogens (e.g., various bacteria and viruses; [3]). In order to ensure public health protection and meet regulatory constraints, however, RO/NF treatment in water reuse applications requires reliable real-time methods for membrane integrity monitoring (MIM). Membrane integrity breaches may occur due to various factors including membrane element manufacturing defects, insufficient or improper pretreatment or pre-filtration, chemical attack (e.g., oxidation), faulty installation and maintenance, failure of module assembly components (o-rings), and stress/strain on membranes associated with operating conditions (e.g., water hammer, passage of sharp debris, cleaning of fouled/scaled membranes). At present, however, reliable real-time RO/NF MIM methods are still lacking for online detection of membrane element integrity breaches. In the present work, a patent-pending Pulsed-Marker Membrane Integrity Monitoring (PM-MIM) approach has been extended to enable online membrane breach detection in multi-element spiral-wound RO/NF membrane plants. The present study demonstrated that the presence of a compromised membrane element in a multi-element RO/NF membrane bank can be identified by resolving the convective transport of a molecular marker through each individual membrane element in the bank. In monitoring membrane plant integrity, the extended PM-MIM approach involves single-point monitoring of the dynamic concentration change of the marker concentration in the membrane bank (combined) permeate stream, in response to a pulsed marker injection in the RO/NF feed. Data fusion of marker permeate concentration with online water flow, salinity, and pressure data, combined with considerations of fluid residence time and longitudinal marker dispersion in the RO/NF retentate channels, allows for assessments of marker convective transport through each individual membrane element in the membrane bank. Laboratory evaluations of the extended PM-MIM approach were conducted using a small-scale, multi-element (2.5”) RO system. Utilizing a pre-breached RO element, it was demonstrated that clear discrimination between intact and compromised RO elements can be identified from the differences in the level of marker convective transport, as resolved from dynamic marker concentration data and data fusion. Subsequent implementation of the PM-MIM approach was evaluated in a field-deployed RO plant (21x 4” RO elements, 35,000 gallon/day feed capacity) treating agricultural drainage water. The extended PM-MIM capability for online characterization of individual elements’ marker convective mass transport was verified in the pilot-scale, demonstrating real-time integrity monitoring of a field-deployed multi-element membrane plant.