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130 6. CONDITIONING OF SALINE WATER
It is important to note that within 48 h of discontinuation of chlorination or addition of any other bactericide, most aquatic organisms retain their protective layer. However, after this period, the aquatic organisms begin to absorb the protective layer over their cells and again become vulnerable to destruction by chlorination. Therefore, if chlorination is reapplied again after a 48-h (or longer) period, the bactericidal effect of the disinfectant will be reinstated for at least 4e6 h, after which the bacteria will build another protective layer for their cells.
In the light of the practical observations presented earlier, intermittent chlorination for a period of 4 to 6 h with at least of 48 h between chlorination events has been found to be a more efficient method for RO membrane biofouling control than continuous chlorination. Typical chlorine dosage for periods of intermittent disinfection is between 2 and 5 mg/L and is determined based on a target amount of residual chlorine left in the pretreated water downstream of the cartridge filters (or membrane filters if cartridge filters are not used) of 0.1 mg/L or less.
Such intermittent addition of chlorine to the source water for biofouling control is some- times referred to as “shock” chlorination. Taking under consideration that for the shock chlo- rination to be efficient, the time between chlorination events has to be at least 48 h (e.g., 2 days), in most plants, such biofouling control approach is practiced two to three times per week.
Because aquatic organisms (shellfish and bacteria) are very adoptive to ambient condi- tions, usually a random schedule of intermittent chlorination works better than a preestab- lished chlorination schedule. The random nature of chlorination is of critical importance for controlling shellfish growth on the internal surface of the plant intake pipes because if chlorine is applied at a preset time schedule, within 1 week, shellfish will begin closing their shells a few minutes before the scheduled chlorination events occur, making the use of chlorine for shellfish growth control highly ineffective.
6.6.2 Chlorine Dioxide
This oxidant is weaker than chlorine but is fairly effective for most aquatic microorganisms and at the same time it is not as aggressive in terms of RO membrane oxidation. Therefore, if used intermittently and in low dosages (0.2e0.5 mg/L) and low pH, it could be applied without the need for dechlorinationdthat is, it is weak enough not to cause permanent damage to the RO membrane polymeric structure.
The feasibility of using chlorine dioxide for biofouling control is pH dependent and may not cause RO membrane degradation if the pH of the source water is below 8 (Erikson and Dimotsis, 2012). Ambient seawater has a pH in the range of 7.8e8.3 and the use of chlorine dioxide without dechlorination may not always be suitable, unless the source water is pH adjusted.
RO membrane manufacturers differ in their views regarding the use of chlorine dioxide without subsequent dechlorination and the RO membrane supplier for the specific desalina- tion plant has to be consulted regarding application dosages and the need for dechlorination before RO separation if chlorine dioxide is chosen as a biocide.
Because of its short useful life, chlorine dioxide has to be generated at the desalination plant site. However, most of the chlorine dioxide generators available on the market do also produce small amounts of chlorine in the form of HClO and ClO, which could oxidize