Development of novel monitors for early detection of fouling in reverse osmosis systems
Sim, Victor Siang Tze
Date of Issue2014
School of Civil and Environmental Engineering
Singapore Membrane Technology Centre
Nanyang Environment & Water Research Institute (NEWRI)
Today, the leading technology for new desalination installations uses reverse osmosis (RO) membranes. The current generation of RO membranes is of sufficiently high permeability that further significant improvement in RO membrane permeability is less likely to be the primary driver for a major reduction in the cost of desalination but should arise from a variety of other process improvements. As the primary energy use in an RO system is the power required for the high pressure to pump the feed water and is directly related to the feed pressure and flow rate, savings can come in the energy costs in pre‐empting fouling problems. In this project, a novel sensor has been developed to provide more reliable information for process monitoring based on a side-stream RO fouling monitor cell simulating flows in the plant termed the ‘canary cell’. The canary cell is found to be representative of the spiral wound module (SWM), which is the most commonly used configuration in the water desalination industry that can serve as an early warning system. Using a non‐invasive method of Ultrasonic Time Domain Reflectometry (UTDR), foulants on the RO membrane can be detected. The UTDR sensor applies an acoustic signal that reflects from interfaces such as membrane and fouling layer in the cell. The method can detect changes in properties over a few microns and are well suited for integration with the ‘canary cell’ to detect incipient fouling. The UTDR sensor has been shown to monitor colloidal fouling in-situ, in real-time and non-destructively that provides evidence for the metastability of colloidal fouling. Various operational parameters are further evaluated to understand and potentially control the onset of metastability. With the usage of acoustic enhancers, the UTDR sensor was further adapted for the detection of the biofouling, which would otherwise be incapable of reflecting the ultrasound wave for detection due to insufficient difference in acoustic impedance since the bulk of biofilms are water medium. The combination with another online sensor, the Feed Fouling Monitor (FFM), further expanded the application of the UTDR sensor to predicting RO fouling through the measurements of foulant thickness and mass-based specific cake resistance. FFM is an adaptation of the Integrity Sensor (IS) that operates by measuring the transmembrane pressure (TMP) across a membrane of a low hydraulic resistance relative to a reference pressure differential.
DRNTU::Engineering::Environmental engineering::Water supply