Page 41 - ro membanes

24 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT
water. The combination of EOM and IOM of algae is referred to algogenic organic matter (AOM). AOM is easily biodegradable NOM and provides food source for biogrowth of bacteria on the RO-membrane surface.
Humic acids are polymeric (poly-hydroxyl aromatic) substances, which have the ability to form chelates with metal ions in the saline water, such as iron. This feature of humic acids is very important for seawater or surface brackish water pretreatment systems using iron coag- ulants, because they can form a gel-like layer of chelates on the surface of the membranes, which would cause fouling. Typically, such fouling layer can be dissolved at pH of 9 or more, at which condition both the membranes and the humic substances carry a negative charge. This feature is used for membrane cleaning.
Humic substances are hydrophobic and therefore, hydrophilic membranes are less prone to fouling by humic acids.
Most NOM in saline water consists of compounds of relatively large molecular weights (500e3000 Da). One Dalton (Da) is equal to 1.666,054  1024 g. A typical saline water reverse osmosis desalination membrane would reject over 90% of the compounds that have molecu- lar weight higher than 200 Da.
Humic and fulvic substances mainly differ by their ability to dissolve in strong acids. While humic substances are easily precipitated upon acidification of saline water, fulvic substances remain in solution at low pH.
Humic acids in their natural state are not a food source for most aquatic organisms. How- ever, when oxidized with chlorine or other oxidants, humic acids could become easily biode- gradable and serve as food source for aquatic bacteria growing on the RO-membrane surface. Therefore, continuous chlorination of source water containing large amount of humic acids often causes more membrane-biofouling problems than it solves.
Negatively charged NOM, which dominates in surface brackish water and seawater collected by open intakes, has the tendency to adhere to the surface of the RO membranes, which typically have slightly positive charge. Once adsorption occurs, the NOM begins to form cake/gel on the membranes and to affect their performance. It should be noted that depending on its properties and origin, NOM may also adhere to the surface of UF and MF pretreatment membranes and cause significant productivity loss by plugging membrane pores, adsorbing to the internal matrix of the membranes, and forming a cake of organic matter on the membrane surface.
Usually, saline water from surface saline water sources contains NOM that causes moder- ate fouling of UF and MF membranes, which could be removed by routine chemically enhanced backwash and periodic cleaning of the pretreatment membranes. This NOM can be removed with very little (typically less than 2 mg/L) or no coagulant addition to the saline source water. However, if the source water is influenced by surface runoff or large amount of alluvial organics, or if alluvial brackish aquifer is used for water supply, the NOM properties and ability to cause significant membrane productivity loss may increase dramatically. Under these circumstances, the efficient removal of NOM may require very high dosages of coagu- lant (usually over 20 mg/L).
A NOM fractionation study on surface waters completed by U.S. Bureau of Reclamation in 2002 demonstrates that this type of fouling depends on the type of membrane material and characteristics, the NOM polarity and molecular weight, and the chemistry of the source water.