Membrane fouling in seawater reverse osmosis (SWRO) desalination process

• Main Author: Yin, Wenqiang
• Other Authors: Chong Tzyy Haur
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Environmental engineering
• Online Access: https://hdl.handle.net/10356/136781
• Institution: Nanyang Technological University

Seawater reverse osmosis (SWRO) desalination technology is an important technology for providing potable water for industries and human daily life due to its lower-energy consumption compared to thermal based desalination technology. However, membrane fouling is still a persistent problem in SWRO desalination plants. Organic fouling and biofouling, in particular the interplay between them, are the challenges that require attention. In the first part of my study, the focus was on organic fouling in SWRO desalination process. Classifying organics in seawater will provide an in-depth understanding of the important fraction on SWRO organic fouling. The dissolved organic matter (DOM) in seawater was fractionated and concentrated by a membrane-based technique into three major fractions according to their molecular weight (MW) as defined in the liquid chromatography with organic carbon detection (LC-OCD) method, namely the fraction of biopolymer (F.BP, MW > 1000 Da), fraction of humic substance with building block (F.HS&BB, MW 350 – 1000 Da), and fraction of low molecular weight compounds (F.LMW, MW < 350 Da). An overall recovery of >80% of the organics in seawater was attained. Compared with model foulants such as sodium alginate (SA), bovine serum albumin (BSA), and humic acid (HA), all isolated organic fractions showed lower fluorescence intensities, and had lower fouling potentials than common model foulants used in SWRO fouling studies, thus the model foulants were not good representatives of the natural organics in seawater. In addition, results from atomic force microscopy (AFM) and extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory showed that initial fouling (i.e., foulant-membrane interaction) was the main driver in SWRO organic fouling with F.BP as the major contributor followed by F.LMW. In addition, divalent ions were found to enhance the RO fouling by increasing the adhesion and cohesion forces between foulant-membrane and foulant-foulant. In second part of my study, the focus was on biofouling and the interplay between organic fouling and biofouling in SWRO desalination process by using the isolated dissolved organic fractions. From the assimilable organic carbon (AOC) results, the AOC/DOC ratio was in the order of F.LMW (~35%) > F.BP (~19%) > F.HS&BB (~8%); AOC/DOC of seawater was ~20%; organic compositions of seawater were BP ~6%, HS&BB ~52% and LMW ~42%; thus LMW accounted for >70% of AOC in seawater. Their impact on SWRO biofouling in term of flux decline rate was in the order of F.LMW (~30%) > F.BP (~20%) > F.HS&BB (<10%). Despite being the major organic compound in seawater, HS&BB showed marginal effect on biofouling. The role of indigenous BP was less critical owing to its relatively low concentration. LMW, which was the major AOC contributor, played a significant role in biofouling by promoting microbial growth that contributed to the build-up of soluble microbial products and exopolymeric substances (i.e., in particular BP). Therefore, seawater pretreatment shall focus on the removal of AOC (i.e., LMW) rather than the removal of biopolymer. In third part of my study, the focus was on the LMW removal by using low-pressure NF pretreatment process and the subsequent impact on SWRO fouling. Three different membranes were evaluated: NF270 was a commercial membrane based on polyamide thin film composite with negatively charged surface, NF1 and NF2 were glutaraldehyde cross-linked layer-by-layer polyelectrolyte membranes with positively charged surface (i.e., NF2 contained more polyelectrolyte layers than NF1). The organic/inorganic rejection of NF membranes followed the order of NF2 > NF1 > NF270 while pure water permeability (PWP) was in the order of NF1 > NF270 > NF2. Meanwhile the degree of RO fouling in term of flux decline and foulant amount was in the order of NF270 > NF1 > NF2. In addition, the results suggested that NF membrane can be optimized to achieve excellent removal of LMW, but a balance between permeability, rejection and fouling shall be considered when selecting the membrane for seawater pretreatment. For instance, NF2 membrane with the lowest permeability (~1.9 LMH/bar) was less competitive even though it showed the lowest SWRO fouling (i.e., flux decline ~3%) as compared to NF 1 (permeability of 4.0 LMH/bar, flux decline ~10%).