Study On High Efficient Pretreatment Of Seawater Desalination And Its Combination Process

Water shortage is a worldwide problem. In the past, due to unreasonable exploitation and utilization of water resources, it resulted in rivers and wetlands drying up, lake surface shrinking, ground sinking and water pollution. Domestic and foreign experiences have proved that seawater desalination was an important means to resolve the freshwater shortage of coastal areas, which was known as the sunrise industry of 21st century. Reverse osmosis desalination technology were widely used due to its low energy consumption. As the influent water of reverse osmosis membrane required higher quality, the pretreatment technology was very pivotal.This paper studied pretreatment effects of enhanced coagulation, preoxidation technology, enhanced filtration, coagulation/microfiltration, coagulation/ultrafiltration on low turbidity seawater. And it studied desalination efficiency of low turbidity seawater with different initial salinity by flocculation, manganese sand filtration, activated carbon filtration, security filtration, nanofiltration and reverse osmosis, which could provide a feasible desalination technology of seawater near Shanghai.While single coagulation pretreated low turbidity seawater by aluminum chloride, poly-aluminium chloride, ferric chloride and poly-ferric sulfate, most of turbidity and part of organic matters could be removed, and their desalination ability was poor. Toward the pretreatment of low turbidity seawater by enhanced coagulation, chlorine preoxidation had no improvement on removal of turbidity and salinity, and the removal efficiency of organic matters would decrease. Potassium permanganate preoxidation could increase the removal efficiency of organic matters, and its most optimized dosage was 1mg/L.Under the condition of ferric chloride and poly-aluminium chloride flocculation, while single medium filtration pretreated low turbidity seawater, the total effect of manganese sand filtration was the worst. The turbidity removal efficiency of all of them was good, and the removal efficiency of organic matters by activated carbon was the best. Desalination ability of all of them was very poor. The removal ability of organic matters by tandem filtration was markedly better than single medium filtration. Pretreating low turbidity seawater by coagulation-precipitation-tandem filtration, the effect of ferric chloride coagulation was better than poly-aluminium chloride coagulation under the condition of the same medium. If it used the same coagulant, coagulation-precipitation- manganese sand and granular activated carbon tandem filtration had the best effect.While coagulation/microfiltration or coagulation/ultrafiltration pretreated low turbidity seawater, and aluminum chloride, poly-aluminium chloride, ferric chloride or poly-ferric sulfate was used as coagulant, most of turbidity and part of organic matters could be removed. But all of their desalination efficiency was very low. The most optimized dosage of aluminium coagulants was 3mg/L, and that of ferric coagulants was 10mg/L.With ferric chloride flocculation and nanofiltration-reverse osmosis combination process pretreating low turbidity seawater, the removal effect became worse with the augment of initial salinity. While the initial salinity was 3.7～12.2g/L, nanofiltration-reverse osmosis combination process could remove almost all the turbidity and UV254, and the removal efficiency of COD_Mn and salinity reached more than 90%. While the initial salinity was 12.2～30.0g/L, with the augment of initial salinity, the effluent quality became worse.With poly-aluminium chloride flocculation and nanofiltration-reverse osmosis combination process pretreating low turbidity seawater, the removal efficiency of turbidity and UV₂₅₄ reached more than 98%. The removal efficiency of COD_Mn decreased with the augment of initial salinity. The desalination efficiency was better under the low initial salinity, and decreased while the initial salinity was higher than 19.2g/L.While nanofiltration-reverse osmosis combination process pretreated seawater near Shanghai, the effect of coagulation was better than that without coagulation. The treatment without coagulation would bring additional pressure to nanofiltration membrane and reverse osmosis membrane, which would shorten their useful life periods.