Study On RO Pretreatment And Silt Density Index Influence Factors

Reverse osmosis(RO) is one of the main processes for the water purification and desalination.It has remarkable merits of low energy consumption ,no pollution, strongly compatibility, convenient operation,low operating cost and so on. But the pretreatment technologies are the bottlenecks of the RO application Accordingly,it is very important to carry out the studies on water pretreatment. Silt density index(SDI) is the RO feedwater main requirement, which can reflect the effect of the colloid on the RO membrane. Researching the factor affecting the SDI test result has a positive significance to analyze the reason feed water SDI not to reach the requirment, and to take the effective pretreatment.The pretreatment methods was discussed in this paper by treating two kind of different water samples:tap water and the Bohai Sea sea water.It showed that:for tap water, 5μm cartridge filter+1μm cartridge filter or 1μm cartridge filter+1μm cartridge filter can be used as pretreatment step for small type RO system; Using the coagulation - precipitation– sand filtration craft could make tap water SDI to fall below 3, which was affected by sand particle size, bed depth, sand filtration rate, coagulant type, coagulant amount used, experiment temperature, pH value, salinity content . Selecting ALCl₃ 11mg/L, sand particle size 0.6 - 0.9mm, bed depth 60cm, sand filtration rate 10m/h ,SDI can achieve 2.31. In the coagulation - precipitation - MF craft, coagulant amount had no effect on the quality of water filtered by MF , but the appropriate coagulant amount can slow down the membrane flux depression. In the Bohai Sea sea water experiment, the beaker coagulation tests result indicated that: the FeCl₃ + PAM for coagulant had the best turbidity reducing effect. The SDI after coagulation -precipitation– sand filtration is still very big, which can be reduced to 2.48 by adding microfilters as further processes. MF flux depression is slow, and it can be basical restored by physical cleaning.The research of factors influencing SDI indicated that : with the survey time-gap decreasing and water sample volume reducing, the SDI result will be increased ; Very small amount SS can reflect as big SDI, in the different water sample pellet and the colloid are different, the influence is also different to SDI; Qualified SDI has the very low turbidity inevitably, but the low turbidity cannot guarantee SDI qualified; In the water natural organic matter like humic acid is influential to SDI, along with concentration enlargement, the pH value increasing, in the water salt content reducing, SDI increases. Existence of the iron colloid and aluminum colloid had effect on SDI, which will increase along with the concentration, pH value enlargement. When iron concentration is greater than 20 ppb, SDI will be higher than 3.When aluminum concentration is greater than 50 ppb, SDI could be still small .