Optical coherence tomography and digital image processing for scaling and Co-precipitation investigation on reverse osmosis membrane

Scaling known as the precipitation of inorganic salts is a main concern in the reverse osmosis (RO) process due to its effects on the process performance and life-time of membranes. Although single salt precipitation has been continuously studied, the co-precipitation of salts is rarely investigated due to its complexity, especially under process hydrodynamic conditions. Recently, optical coherence tomography (OCT) has been adopted as an in situ and rapid technique for monitoring the fouling in membrane processes. This study aims to develop an approach using OCT and digital image processing for qualitative and quantitative analysis of single salt precipitation and co-precipitation on RO membrane under hydrodynamic conditions. The effects of supersaturation, sodium chloride (NaCl), flow rate, and applied pressure are investigated. The developed approach enables the detection of crystal growth under the OCT. The morphology of precipitated salt, i.e CaSO_4, including crystal shape and size obtained from OCT acquisition, agrees with the images from a scanning electron microscope (SEM). The growth of each crystal in co-precipitation of CaSO₄ and CaCO₃ is well detected in appearance time and growth pattern. This is beneficial to differentiate crystal morphology in the co-precipitation system. The quantitative surface coverage proportion and total volume are in well correlated the changes of RO performance parameters of permeate flux, conductivity (feed, concentrate, and permeate), and Ca²⁺ concentrate profile by time, which allows the scaling progress to be directly evaluated over time. This study reveals the high potential of combining OCT acquisition and digital image processing for direct observations and studying single salt and co-precipitation under RO hydrodynamic conditions, which can be extended for another type of composite fouling.