The membrane fouling index, widely used in the reverse osmosis (RO) membrane industry, is developed mainly to assess the fouling potential of particulate matter. However, the fouling potential by organic matter should also be accurately evaluated as the productivity loss by organic fouling is often more severe than particle fouling, despite its low concentration in feed water. This study was performed to investigate the influence of organic characteristics and feed water solution chemistry on the membrane fouling index, such as the silt density index (SDI). The results showed that the feed solution chemistry (i.e. pH, ionic strength and hardness) affected the SDI values of organic rich feed water to some degree. In addition, Aldrich humic acid (AHA) exhibited higher fouling potential by SDI than Suwannee river humic acid (SRHA), emphasising the importance of organic properties. Lastly, it is shown that although SDI values measured were the same, the degree of NOM fouling was significantly different for the membranes with different surface properties.

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