Up to 80% of the TOC, or dissolved organics, in natural waters is made up of humic and fulvic acids. Humic Acid compounds have a molecular weight range of 500 (fulvic) to 100,000 and are weakly acidic electrolytes containing carboxylic- and phenolic-OH groups (humic). As pH drops, they become more hydrophobic and clog hydrophobic membranes more. Calcium (Ca+), which forms a bridge between the membrane and the negatively charged membrane surface and/or between the negatively charged carboxyl groups of the Humic Acid, aggravates humic fouling in natural waters.
Polyhydroxy aromatics, polysaccharides, and microbial slime are other organic substances that damage membranes. To prevent organic molecule fouling as well as to reduce the possibility of microbial fouling since organics supply nutrients that enable microbial growth, feed water organic levels measured as TOC should be low.
