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2.5 NATURAL ORGANIC FOULANTS 25
The largest natural organic foulants are the polysaccharides, organic colloids and proteins, followed in size by humic substances, organic acids, and low molecular weight organics of neutral charge. These compounds have different potential to cause membrane fouling. Polysaccharides along with living bacteria that excrete them and attach on the membrane surface have the highest potential to cause RO-membrane biofouling and therefore, they are classified as a separate group of foulantsdmicrobial foulants.
2.5.2 Measurement Parameters and Methods
The most frequently used parameters for quantification of natural organic fouling are total organic carbon (TOC) and UV254 absorbance.
2.5.2.1 Total Organic Carbon
Total organic carbon is one of the most widely used measures for organic content of saline source water. TOC concentration measures the content of both NOM and of easily biodegrad- able organics, such as polysaccharides, released during algal blooms.
This water quality parameter is widely used, because it is relatively easy to measure and it is indicative of the tendency of the source water to cause RO-membrane natural organic fouling and microbial fouling. TOC is measured by converting organic carbon to carbon di- oxide in high-temperature furnace in the presence of a catalyst.
Typically, open ocean seawaterdwhich is not influenced by surface freshwater influx (nearby river confluence); by man-made activities (i.e., wastewater or storm water discharges, or ship traffic); or by algal bloom event (i.e., red tide)dhas a very low TOC content ( 0.2 mg/L). When an algal bloom occurs, however, TOC concentration of the ocean water could increase by an order of magnitude (2e12 mg/L). Similar magnitude of TOC increase could be triggered by a storm water or river discharge during high-intensity rain event such as these occurring during rainy seasons in tropical and equatorial parts of the world.
Usually, an increase of TOC content in the source water above a certain threshold (2.0e2.5 mg/L) is observed to trigger accelerated biofouling of RO membranes.
Observations at the Carlsbad seawater desalination demonstration plant in California, USA (which is supplied by seawater collected using near-shore open ocean intake) indicate that when TOC concentration in the source water at that location exceeds 2.0 mg/L during algal bloom events, within one-to two-week period, the SWRO system experiences measur- able biofouling and associated increase in operating pressure.
Similar TOC level observations at the Tampa seawater desalination plant, in Florida, USA, (where the typical background TOC level of the seawater is less than 4 mg/L) indicate that accelerated biofouling occurs when TOC concentration exceeds 6e8 mg/L. Usually, acceler- ated biofouling at the Tampa facility is triggered by one of two eventsdheavy rains, which increase the content of alluvial organics in the source seawater, or algal blooms, which cause elevated organic concentration due to massive die-off of algae. The increase in alluvial organics during rain events is caused by the elevated flow and alluvial content of Alafia River, which discharges into Tampa Bay several kilometers upstream of the desalination plant intake. During high-intensity rains in the summer months, TOC level in the river water discharging into the bay may exceed 20 mg/L.