Thesis (M.Sc.)--Chulalongkorn University, 2008

Ultra low pressure reverse osmosis membrane is a new technology, which apply to defluoridation from groundwater. This membrane can operated at very low pressure. However, the major problem of reverse osmosis is membrane fouling. Membrane fouling can effect on fluoride rejection rate and cause of membrane degradation. In this study, the membrane fouling of UTC-70 ultra low pressure reverse osmosis membrane in groundwater defluoridation was investigated. Chiang mai basin was selected as study area. In this area, groundwater contains high fluoride concentration. Two sites of groundwater in this area were selected as sampling point. Fisrt site is groundwater from Pra Too Khong, Lamphun, wheareas, the second site is San Kam Pang hot spring well, Chiang mai. Groundwater from two sites was filtrated with ULPRO membrane, which operated in cross-flow mode. The operating transmembrane pressure was varied at 0.3 and 0.5 MPa. Furthermore, the concentration polarization phenomenon was investigated. The NaF concentration varied at 0, 10, 25, and 50 mM was used as feed solution. From the results, it was found that the mass transfer coefficient in solution was 2.73x10-5 and 1.75x10-5 m/s at 0.3 and 0.5 MPa, respectively. While, the average solute mass transfer coefficient through the membrane was 4.5x10-7 m/s under 0.3 and 0.5 MPa, respectively. For the groundwater defluoridation experiment, it was found that the high fluoride rejection rate more than 90% was obtained. In Pra Too Khong groundwater, the fluoride rejection rate was 95.7-96.7%, wheareas, in San Kam Pang hot spring well was 96.7-97.5%. From the results, it can be indicated that the higher fluoride rejection rate was obtained at higher transmembrane pressure (0.5 MPa). From the investigation of membrane fouling, it was found that the membrane was fouled with dense-gel layer (polymerized fouled layer) in all case of experiment. Furthermore, the effect of silica fouling on groundwater defluoridation was investigated. The results showed that silica can effect on permeate flux and fluoride rejection rate in groundwater defluoridation. The presence of silica in feed water was cause of decreasing in fluoride rejection rate. While, the results of silica fouling showed that membrane was fouled with polymerized silica fouled layer.