An Investigation on the Fouling of Humic Acid in Reverse Osmosis using Optical Coherence Tomography

Abstract

Desalination is frequently adopted nowadays to overcome the freshwater shortage in some areas of the world where salt water or brackish water is available. Currently, reverse osmosis (RO), which operates through a pressure gradient across a semi-permeable, non-porous membrane, is considered the most promising technology for desalinating seawater owing to its low energy consumption and high efficiency of salt rejection. One of the main problems facing RO technology is fouling, which is the deposition formed on the surface of the membrane. Fouling alters both the performance and durability of the membrane. The most common types of fouling found in RO include inorganic fouling, organic fouling, colloidal fouling, and biofouling. This thesis focuses on organic fouling in RO under hydrodynamic conditions, in which Humic Acid (HA) is chosen as the organic foulant due to its frequent presence in the RO feed water. In addition, an optical coherence tomography (OCT) was implemented to monitor the formation of HA fouling on RO under hydrodynamic conditions to provide insights into mechanisms of HA formation and growth. Furthermore, a digital image processing technique was applied on OCT image series to extract geometrical parameters of HA fouling, including covered surface proportion, total volume, and porosity, for investigating the relationships of permeate flux and fouling geometry over time. The fouling study was operated under various hydrodynamic conditions of pressure (10 bar and 15 bar), temperature (20 °C, 35 °C, and 45 °C), and composition of the solution with different NaCl concentrations (0, 5000 mg/L, and 10,000 mg/L) while the conditions of cross-flow velocity (23.92 m/h), pH 8, Ca2+ (400 mg/L), and HA concentration (total organic carbon concentration (TOC) of 12.55 mgC/L) were maintained constants. On the other hand, the morphology of the HA fouling on the membrane after drying at 60 ºC was examined by Scanning Electron Microscopy (SEM). In addition, the changes in TOC and turbidity were monitored as reference parameters to assess the fouling progress. The OCT results exhibit the effects of salinity, pressure, and temperature on the HA fouling formation and evolution, which impact the interactions of HA and membrane surface as well as the HA aggregation. The morphology of HA fouling observed from OCT is confirmed by the SEM analysis, which is a cake layer with the presence of HA aggregates as spherical particles of different sizes depending on the hydrodynamic conditions. The results of TOC and turbidity are well coincided with the OCT images in which the drop of TOC values and the increases of turbidity imply the formation of HA fouling and aggregation, respectively. Hydrodynamic conditions and salinity affect HA fouling formation and its structure, and as a result, they are associated with the drop of permeate flux at different rates. The permeate conductivity reflecting the salt rejection of the membrane varies depending on the HA fouling structure and its charge surface due to the mechanisms of Donnan exclusion and size exclusion. The relationships of permeate flux and geometrical fouling parameters of surface coverage proportion, total volume, and porosity exhibit good correlations with high R2. The best correlation is for the permeate flux and the covered surface regardless of the effects of hydrodynamic conditions and salinity, which implies the mechanism of surface blockage due to HA fouling for the permeate flux reduction. In addition, the cake filtration mechanism of the HA fouling also plays a significant role in the drop of permeate flux after the fouling fully covers the surface and continues growth thickening. The findings of this study highlighted the significant potential of merging OCT acquisition and digital image processing in facilitating direct observations of HA fouling mechanisms and correlations of permeate flux with geometrical fouling parameters in RO under various hydrodynamic conditions and salinities, which are beneficial for further developed strategies in prevention of HA fouling in industry.

Date of Award	7 May 2024
Original language	American English
Supervisor	Emad Alhseinat (Supervisor)