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2.5 NATURAL ORGANIC FOULANTS 23 TABLE 2.4 Threshold Levels of Common Scaling Compounds
Scalant
Maximum Increase in the Concentrate as Compared to Source Water Above Which RO Membrane Scaling Is Likely
Calcium carbonate, expressed as LSI, þ0.3 in source water without scale inhibitor
Calcium sulfate Strontium sulfate Barium sulfate Calcium fluoride
2.5.1 Description
230% 800% 6,000% 12,000%
2.5 NATURAL ORGANIC FOULANTS
Depending on its origin, saline waters can contain naturally occurring or man-made organic compounds and aquatic microorganisms. Since all microorganisms and most organic molecules are relatively large in size, they are well rejected by the RO membranes. However, some of the organic compounds and aquatic species may accumulate on the membrane sur- face and form a cake layer that may significantly hinder membranes’ main functiond rejection of dissolved solids. Depending on the main source of the fouling cake layer, these RO membrane foulants are typically divided into two separate groups: natural organic mat- ter (NOM) foulants and microbiological foulants (or biofoulants).
NOM is typically contained in surface saline waters (brackish water or open ocean seawater) and includes compounds that are produced by naturally decaying algal and other aquatic vegetation and fauna [i.e., proteins, carbohydrates, oils, pigments (i.e., tannins), and humic and fulvic substances (acids)]. High content of NOM in the source water used for pro- duction of drinking water is undesirable because it causes discoloration of the water, forms carcinogenic disinfection by-products (DBPs) when disinfected with chlorine, and results in complexation with heavy metals, which in turn causes an accelerated membrane fouling.
Under normal non-algal bloom conditions, typical seawater and brackish water collected using open intakes do not contain NOM concentration large enough to present significant challenge to desalination plant operations. High content of NOM is usually observed during algal blooms and/or when the desalination plant intake is located near the confluence with river or other freshwater source, or near wastewater-treatment plant discharge.
The easily biodegradable organic matter released by algae during their growth and respi- ration is referred to as extracellular organic matter or EOM (Edzwald and Haarhoff, 2011). When algae die off during the end phase of an algal bloom or their cells are broken by treat- ment or pumping processes, they also release intracellular organic matter (IOM) in the source