| Summary: | 碩士 === 國立臺南大學 === 環境生態研究所碩士班 === 99 === The aim of this study is focus on the fouling of RO (Reverse Osmosis) membrane caused by actual high-sodium wastewater. Actual high-sodium wastewater before inlet into RO system in waste liquid treatment plant is used as environmental sample for this study. The elements of high-sodium salts sampled on RO membrane were analyzed by XRF (X-ray Fluorescence spectrometer) while cations and anions of actual wastewater were detected by IC (Ion Chromatography). The actual and synthesis wastewater (such as sodium chloride, sodium sulfate and sodium phosphate solution) were used for dead-end test which is operated under fixed 8kgf/cm2 pressure. The microstructure and microanalysis of membrane fouling before and after water cleaning were analyzed by SEM / EDS (Scanning Electron Microscopy / Energy Dispersive Spectrometer) analysis. Based on Gouy-Chapman theory of diffusion double layer, the effects of ion valence and electrolyte concentration on RO membrane operation parameters such as flux and removal rate of salts for RO memebrane after water cleaning were measured simultaneously. The diagnosis of the cause of RO membrane fauling would then be identified clearly by the methodology above.
Water quality analysis of actual wastewater before inlet into RO system showed high concentrations of EC (Electrical Conductivity) and TDS (Total Dissolved Solids) which means such actual wastewater is composed of soluble inorganic salts. The study shows that SDI (Silt Density Index) of actual wastewater is higher than 5, and LSI (Langelier Saturation Index) is higher than 0. Base on the indexes above, it is obviously that RO membrane would be easily blocked by actual wastewater. By using XRF analysis of the crystallization of soluble salts from actual wastewater, it shows Cl , S and P are the main elements. The anion and cation analysis for cleaning water of RO membrane fouling indicates that Cl- 、SO4 2-、PO4 3-and Na+ were the main ions in actual wastewater. It is clearly that actual wastewater sampled before inlet into RO system containing high amounts of sodium chloride, sodium sulfate and sodium phosphate.
In order to discuss the effects of different sodium salt on the fouling of RO membrane, electrolyte concentration and ion valence were used to be evaluated by conducting dead-end test. By using IC, SEM / EDS, flux analysis and comparison of removal rate simultaneously, RO membrane fouling by sodium sulfate solution showed that, as the electrolyte concentration increased, or as the longer the fouling caused, the higher amount of crystallization by concentration of wastewater containing soluble inorganic salts were observed. The pore of RO membrane would be enlarged after water cleaning of the soluble salts crystallization among RO membrane, and then the flux increased, but removal rate of salts decreased. Damage of RO membrane is mainly caused by sodium sulfate, instead of sodium chloride or sodium phosphate. Expansion caused by the crystallization of sodium sulfate among RO membrane is the main cause of RO membrane damage. Due to the sodium sulfate crystallization among RO membrane, expansion in the pore of RO membrane cause the flux increased and the removal rate of salts decreased .
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