Real-time monitoring of organic fouling in reverse osmosis under hydrodynamic conditions: A comprehensive analysis

This study thoroughly examines how humic acid (HA) fouling impacts reverse osmosis (RO) performance under varying conditions using advanced imaging and analytical techniques. Optical coherence tomography (OCT) was employed to capture three-dimensional (3D) images of HA fouling. MATLAB was utilized for digital image processing to extract geometric parameters of fouling, including covered surface proportion, total volume, thickness, and porosity. The study visualizes the “shielding” effects of NaCl, with 5000 and 10,000 mg/L NaCl, on calcium (Ca²⁺) and humic acid (HA) by the formation of a more homogeneous fouling layer with fewer and smaller aggregates and a delay in HA fouling progression. However, the combination of high temperature (45 °C) and high operating pressure (15 bar) surpasses the “shielding” effects of 10,000 mg/L NaCl, leading to more extensive fouling and aggregation. The observations are consistent with the results from monitoring of the chemical parameters of the feed, including total organic carbon concentration (TOC), Ca²⁺ concentration, and turbidity, as well as the fouling morphology examined using Scanning Electron Microscopy (SEM). The reduction in permeate flux shows a linear relationship with both the surface coverage proportion and the thickness of the fouling layer. Faster coverage and a thicker fouling layer result in a more significant decrease in permeate flux. We observed a stronger correlation between the permeate flux and fouling thickness during the latter fouling stage, compared to those that correlated the permeate flux with surface coverage in the initial stage. This suggests that the filter cake layer mechanism has a more significant impact on the decline in permeate flux than the surface blockage mechanism. Nevertheless, the membrane's surface characteristics are altered by the complete coverage of the membrane surface by HA fouling, which in turn affects the efficacy of salt rejection. Following the Donnan exclusion effect and size exclusion mechanism, this variation is contingent upon the charge density and porosity of the HA fouling layer.