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28 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT
substances (EPS). The phenomenon of accumulation of aquatic organisms and their metabolic products (EPS) on the membrane surface is known as biological fouling, biofouling, or microbial fouling.
Although bacteria constitute the majority of the membrane biofilm, other microorganisms such as fungi, algae, and protozoa can also attach to the membrane surface and contribute to biofilm formation. Usually, the most predominant bacteria causing biofouling are Pseudo- monas, Bacillus, Arthrobacter, Corynebacterium, Flavobacterium, and Aeromonas. Other microor- ganisms such as fungi (e.g., Penicillium, Trichoderma, Mucor, Fusarium, and Aspergillus) are typically present in the membrane biofilm in significantly lower levels than bacteria.
The biofilm formed on the membrane surface contributes additional resistance (pressure head losses) to the osmotic pressure that must be overcome in order to maintain steady production of freshwater by the membrane elements (Konishi et al., 2011).
In order to compensate for loss of productivity due to biofouling, the feed pressure of the RO-membrane system would need to be increased, which in turn would result in elevated energy use to produce the same volume of fresh desalinated water. Feed pressure increase beyond certain level (e.g., 83e85 bars for SWRO membranes) will cause irreversible damage of the membrane structure and ultimately will result in the need to replace all RO membrane elements. Similarly, the increase of DP above the threshold of 4.5 bars as a result of fouling would cause a permanent compaction and damage of the RO-membrane structure.
Biofouling is usually a significant operational challenge for saline waters of naturally elevated organic content and temperature, such as the seawater in the Persian Gulf and the Red Sea. Biofouling is also a challenge during intense algal blooms or periods when surface runoff from rain precipitation or nearby river water of high organic content enters the plant’s open intake.
2.6.2 The Membrane Biofouling Process
Bacteria that are the main contributor to the biofilm formed on RO membranes typically exist in two states in the source waterdmetabolically active and inactive.
In their active state, bacteria have high metabolic rate, grow fast, consume relatively large quantity of organic materials contained in the source water, and store some of the assimilated organics on the surface of their cells in the form of EPS, mainly polysaccharides, lipids, and proteins. As a result, when in active state, the individual bacterial cells are encapsulated in a layer of EPS, which they store as food reserves. Because the EPS are very adhesive, bacterial capsules easily stick to each other to form microcolonies and adhere to the surface of the RO membranes to form biofilm (see Fig. 2.3). Bacteria are typically in active state when biodegrad- able organics are abundant in the source water (TOC >2 mg/L) such as conditions when algal blooms occur. When the algal bloom event is over and the organic content in the saline source water drops significantly (<0.5 mg/L), bacteria use the accumulated EPS as a food source.
It should be pointed out that bacteria create EPS not only during times of abundant content of biodegradable organics in the source water to store food, but also encapsulate themselves with EPS during conditions when ambient water contains compounds that can destroy the bacterial cells, such as chlorine and other oxidants, strong acids and bases, and other biocides. In this case, bacteria maintain their EPS protection capsules only as long as the biocides are present in the ambient water. The protective EPS capsules are very resilient to strong