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In order to clarify the fouling mechanism during silt density index (SDI) measurements of seawater in the seawater reverse osmosis (SWRO) desalination process, 11 runs were conducted under constant-pressure (207 kPa) dead-end filtration mode according to the standard protocol for SDI measurement, in which two kinds of 0.45 |im membranes of different material and seawater samples from the Mediterranean including raw seawater and seawater pre-treated by coagulation followed by sand filtration (CSF) and coagulation followed by microfiltration (CMF) technologies were tested. Fouling mechanisms based on the constant-pressure filtration equation were fully analyzed. For all runs, only t/(V/A) ~ t showed very good linearity (correlation coefficient R2>0.99) since the first moment of the filtration, indicating that standard blocking rather than cake filtration was the dominant fouling mechanism during the entire filtration process. The very low concentration of suspended solids rejected by MF of 0.45 urn in seawater was the main reason why a cake layer was not formed. High turbidity removal during filtration indicated that organic colloids retained on and/or adsorbed in membrane pores governed the filtration process (i.e., standard blocking) due to the important contribution of organic substances to seawater turbidity in this study. Therefore the standard blocking coefficient ks, i.e., the slope of t/(V/A) ~ t, could be used as a good fouling index for seawater because it showed good linearity with feed seawater turbidity. The correlation of SDI with ks and feed seawater quality indicated that SDI could be reliably used for seawater with low fouling potential (SDI15min<5) like pre-treated seawater in this study. From both ks and SDI, the order of fouling potential was raw seawater > seawater pre-treated by CSF > seawater pre-treated by CMF, indicating the better performance of CMF than CSF.