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2.6 MICROBIAL FOULANTS 33
and algae, which naturally occur in the source water at any given time. The destroyed algal and bacterial cells release easily biodegradable organic compounds (such as polysaccharides) in the ambient water, which become food for the remaining aquatic bacteria that have survived chlorination by being in an inactive state. If the concentration of these organics reaches a certain threshold, it could trigger the conversion of these surviving bacteria from an inactive to an active state, followed by their attachment and excessive growth on the RO membrane surface, which in turn would manifest as membrane biofouling. Therefore, continuous chlorination or addition of other biocides often creates more membrane biofouling problems than it solves and usually does not provide sustainable long-term biofouling control solution. On the other hand, as indicated previously, intermittent chlorination has been found to provide effective control of microbial growth without generating a steady influx of easily biodegradable organics that can trigger a large-scale transfer of aquatic bacteria from their inactive to active state of existence.
Some pretreatment technologies such as granular media pressure filters and vacuum- and pressure-driven MF and UF membrane filters could potentially increase the biofouling poten- tial of the saline source water by breaking the algal cells contained in the water as a result of the vacuum or pressure applied for MF or UF separation, and releasing their easily biodegrad- able cytoplasm into the feed water to the RO system. Although pressure filters provide effec- tive removal of particulate and colloidal foulants, the filtration driving pressure applied by these systems could break some of the algal cells in the source water and cause the release of easily biodegradable organics, which in turn could result in accelerated RO-membrane biofouling. Most marine algae would break when the pressure applied to the UF/MF system feed water reaches 0.4e0.6 bars. Both pressure- and vacuum-driven filtration systems usually operate well above these thresholds. Similarly, pressure-driven granular media filters would have the same negative impact on algal breakage and ultimately would increase the biofouling potential of the saline source water.
From the point of view of minimizing biofouling associated with algal cell breakage, the most suitable pretreatment technologies are those that provide a gentle removal of the algal cells in the source water, such as down-flow gravity granular media filtration and clarifica- tion by dissolved air flotation and gravity sedimentation.
2.6.3.3 Concentration and Balance of Nutrients in Source Water
Results from research on desalination membrane fouling (Jiang and Voutchkov, 2013) indi- cate that the transformation of marine bacteria capable of producing EPS from passive to active state is triggered by imbalance of basic nutrients in the source water (i.e., organics, nitrogen, and phosphorus). Under normal non-algal bloom conditions, the ratios between the content of organics measured as TOC, nitrogen measured as total nitrogen (TN), and phosphorus, measured as total phosphorus (TP), are approximately equal (e.g., TOC:TN:TP1⁄41:1:1). Changes in ambient conditions which result in imbalance of the TOC:TN:TP ratio by intro- ducing uneven amounts of nutrients to the saline source water (e.g., algal blooms, surface runoff during rain events) trigger the transformation of bacteria from passive to active state and turns on their genes responsible for production of EPS.
For example, based on testing completed at the Carlsbad RO desalination plant in Califor- nia (USA) during non-algal bloom conditions, the saline water contains approximately 0.5 mg/L of TOC, TN, and TP and therefore, the ratio of these constituents is 1:1:1. During algal bloom events in 2010 and 2011, the TOC:TN:TP ratio has changed dramatically and