Study On Pretreatment Of Coastal Seawater Desalination By Reverse Osmosis

Reverse osmosis (RO) has been widely used over many years for desalination of seawater due to the improvements of membrane technology. Enhanced pre-treatment and controlling membrane fouling are most critical factors for successful long-term performance of reverse osmosis seawater desalination plant. The seawater of Bohai bay near Tianjin was polluted seriously with the characteristic of higher organism, higher colloids, higher suspended solids and microorganism et al, which couldn't be treated to feed RO water requirement by conventional pre-treatment.Considering the seawater in Bohai sea,in order to make seawater up to the inlet grade of RO process , we perform the process of enhanced coagulation–sedimentation-sand filtration-Ultrafiltration(UF). The coagulation performance under different operation conditions is evaluated based on orthogonal test. It is indicated that the better removal efficiency would be under the operating conditions: FeCl3 15mg/L,KMnO4 2mg/L,oxidant contact time 45min,pH 6 and average velocity grads in flocculation pond G 69.8s-1.Than the outlet deposit in the inclined plate settler with countercurrent flow, which the plate range interval is small. The turbidity of the yielding water is 4.16NTU.Sand filtration can reduce the turbidity and iron content efficiently,the effluent is 0.62NTU and 0.075mg/L separately,this can alleviate the RO membrane pollution greatly, so it is necessarily. Hollow fiber UF reduces the turbidity of seawater effectively and the turbidity of UF permeate is always less than 0.20NTU, no matter what the inlet seawater quality is. UF can remove some of the UV254 and CODMn ,at the same time, the SDI15 figure constantly between 3.2-3.8. The UF need timing backwash, we use citric acid and NaOCl when washing with medicine cleaning the polluted membrane,after that the membrane flux can maintain better state.The results show that the permeate quality of ultrafiltration is stable. This could satisfy RO feed requirements basically. For the coastal seawater, this process can apply to the pre-treatment of SWRO.