Study On Hybrid Pretreatment Systems For Reverse Osmosis Desalination

In recent decades, reverse osmosis desalination has received widespread attention as the coping strategy for global water crisis. Ultrafiltration has been widely accepted as the preferred pretreatment technology for SWRO. However, problems including concentration polarization and severe membrane fouling seriously hindered the application of UF pretreatment technology. Thus, study the hybrid UF pretreatment technology system, and optimize the parameters and hybrid process during operation, is of far-reaching significant for the stability of RO desalination plant.The attenuation of flux was studied in direct UF system and coagulation-UF hybrid system. Six coagulants including ferric chloride, aluminum chloride, PAC and polyacrylamide were studied in the test. Results showed that ferric chloride provided better coagulation effect under low dosing quantity compare to other coagulants, and formed large and dense flocs. Aluminum chloride combined with polyacrylamide had better coagulation effect than being used alone. The turbidity removal effect of PAC was pretty close to ferric chloride, but the flocs was less denser and need longer settling time in PAC coagulation process. Ferric chloride coagulation conditions were studied in the experiment, results showed that ferric chloride coagulation can achieve better effect under the condition of neutral or weak alkaline environment. The coagulation effect will increase with the increase of stirring speed. Compared with direct ultrafiltration process, the coagulation-ultrafiltration was relatively slow and steady in terms of flux decline, proved that the coagulation process can effectively ease the pollution trend of UF membrane.The performance of coagulation-ultrafiltration pretreatment technology of seawater from Bohai Bay was investigated in this paper. The stability of the ultrafiltration system membrane at different membrane flux was assessed and the effects of FeCl3 coagulation on the trans-membrane pressure(TMP) and effluents quality were studied. Results showed that when the flux was below 60 LMH, long-term operation stability with low TMP(60-90kPa) can be reached with 6mg/L FeCl3 above 14 ?. During the experiment, UF provided water with consistently low turbidity(<0.15NTU) and stable silt density index(SDI15) around 2). The analysis of membrane resistance by Darcy model showed that coagulation can effectively lower the increase trend of membrane resistance.Hybrid pretreatment systems were studied in this pilot-scale experiment:(1) UF alone(2) coagulation/UF(3) sand filtration(coarse/fine/dual-stage)/UF(4) coagulation/ sand filtration(coarse/fine/dual-stage)/UF. The performance of these systems was estimated in the test by the stable of membrane specific flux(SF). Results of the research showed that the best treatment effect appear in coagulation/ dual-stage sand filtration/UF system, which can effectively reduce the attenuation ratio of SF to0.008L/(m5·h·m).