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6.6 ADDITION OF BIOCIDES 129 6.6 ADDITION OF BIOCIDES
Oxidants such as sodium hypochlorite and chlorine dioxide are often used to suppress the growth of aquatic organisms (i.e., shellfish, barnacles) on the inner surface of intake pipes, equipment, tanks, distribution channels, and other structures in contact with the source seawater, as well as to minimize biofouling of RO membranes.
6.6.1 Sodium Hypochlorite
Sodium hypochlorite (NaOCl) is the most commonly used oxidant at present. When added to water, sodium hypochlorite generates hypochlorous acid (HOCl) and sodium hydroxide (NaOH):
NaOCl þ H2 O / HOCl (6.i) Hypochlorous acid in turn dissociates to hydrogen (Hþ) and hypochlorite (OCl) ions:
HOCl 4 Hþ þ OCl (6.ii)
The sum of sodium hypochlorite, hypochlorous acid, and hypochlorite ions is termed and measured as free residual chlorine. Chlorine in all of its forms is a toxicant that attacks all aquatic organisms and typically destroys them by the oxidation of their tissue and cells. It should be pointed out, however, that the use of chlorine has several drawbacks. Chlorination cannot destroy all forms of biofouling organisms and, therefore, it is not an absolute barrier to RO membrane biofouling. Chlorine or other oxidants added to the source water will need to be removed before they reach the RO membranes because these oxidants will cause a perma- nent damage of the RO membrane polymeric structure and ultimately the salt rejection of the membrane elements.
In addition, chorine and other oxidants break down otherwise nonbiodegradable NOM into biodegradable organic compounds and destroy the outer walls of bacterial cells and thereby cause the release of intracellular material into the source water. Because the intracel- lular material released from algal and bacterial cells as a result of oxidation is rich in easily biodegradable organics, it serves as a food to bacteria that have already colonized the RO membranes or survived the chlorination process.
Long-term exposure to chlorine triggers the production of extracellular polysaccharides or deoxyribonucleic acid by some of the microorganisms in the source water as a defense mech- anism, which in turn protects the biofilm-forming bacteria. As a result, although continuous use of chlorine may have a short-term benefit in controlling RO membrane biofouling, in the long term it usually does not solve this problem and, therefore, it is not recommended. Once microorganisms build a protective layer around their cells, the chlorine dosages that are needed to break this layer are very highdusually 200e500 mg/L or more. Typically, it takes 4e6 h for most aquatic organisms to build a protective layer that shields them from the harm- ful impact of chlorine at dosage of 3e15 mg/L usually added at desalination plant intake facilities. Therefore, after 6 h, further chlorination becomes ineffective and continuous addi- tion of chlorine becomes counterproductive.