Water quality parameters such as ATP, total direct cell counts, AOC, biofilm formation rate and destructive membrane studies are not suitable for biofouling monitoring and prediction. Therefore, a monitor named membrane fouling simulator was developed. In a comparison study, the same feed channel pressure drop development in time and the same fouling accumulation was observed in spiral wound membrane elements and membrane fouling simulators. Chemical dosing to the membrane fouling simulator feed water showed that a biofouling inhibitor was not inhibiting biofouling, but was even contributing to biofouling. It is shown that other chemicals such as acid and antiscalants may contribute to biofouling as well. It was found that the feed spacer presence strongly influences the feed spacer channel pressure drop increase caused by biofilm accumulation: in nanofiltration and reverse osmosis systems biofouling is a feed spacer problem. A new set of monitors for membrane fouling studies and methods for biofouling monitoring are described. A state of the art on global membrane fouling simulator use is given.

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