Physico-chemical pretreatment to seawater reverse osmosis (SWRO): organic characterization and membrane autopsy

Abstract

In this study, different pretreatment methods such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), powdered activated carbon (PAC) adsorption and ferric chloride (FeCl 3) flocculation were evaluated in terms of their capability in removing seawater organic matter (SWOM) and the characteristics of the foulants on the seawater reverse osmosis (SWRO) membranes. A detailed experiment with a crossflow SWRO filtration unit was conducted with SR membrane (MWCO 100 Da) at 60 bar with seawater (conductivity = 48.9 mS/cm) drawn from south-western Korea. The SWOM removal by UF, NF, PAC adsorption and FeCl 3 flocculation was 20.3, 28.9, 46 and 23.3%, respectively. SWOM used in this study predominantly consisted of small size organic matter (<1000 Da). A large amount of the hydrophobic fraction present in SWOM was removed by PAC adsorption. The SDI 5min significantly decreased from 12.7 (without any pretreatment) to 3.2 (MF), 1.3 (UF), 1.0 (NF) and 4.4 (PAC adsorption). RO filtration of seawater with and without pretreatment showed significant flux decline (normalized flux decline ( J/J0) = 0.17 ± 0.02) within 20-h operation. The elemental analyses made on the RO surface after direct RO filtration showed that the relative fraction of the carbon decreased, while sodium (Na), magnesium (Mg), chlorine (Cl) and iron (Fe) elements were found in the foulants extracted from the fouled membrane surface. The average roughness of the clean membrane surface was 41.5 nm. After MF and UF pretreatment, the roughness slightly increased to 54.8 and 55.6 nm, respectively. On the other hand, without any pretreatment, with PAC adsorption and with FeCl 3 flocculation, the roughness increased up to 69.7, 66.4 and 110 nm, respectively. It can be concluded that the pretreatment by MF and UF could relatively preserve the RO membrane surface.