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2.6 MICROBIAL FOULANTS 29
FIGURE 2.3 Key steps of the membrane biofouling process.
oxidants. For example, chlorine dosage needed to inactivate EPS-protected bacteria is 150 mg/L or more. Therefore, the typical practice of chlorine addition at the intake of desa- lination plants at dosages of 5e15 mg/L is not very effective in suppressing bacterial growth once bacterial cells are EPS protected.
Usually, within 48 h of the discontinuation of biocide addition, bacteria assimilate their EPS capsules using it as food. If bactericide is added again 48 h after its first application, a large portion of the bacteria without protective EPS cover will be destroyed. If bactericide is added continuously or more frequently than once every 48 h, most bacteria will continue to maintain their protective EPS capsules, will survive chlorination and will continue to cause RO membrane fouling. Therefore, intermittent chlorination with time between two bacteri- cide applications of 48 h or more is significantly more efficient and cost-effective in suppress- ing bacterial growth than continuous chlorination.
It should be also pointed out that most bacteria that survive biocide addition will create protective EPS capsules within 4e6 h after the biocide feed to the source water is initiated. As a consequence, after these periods, all surviving bacteria will be protected against the damaging impact of the biocide. Therefore, continuation of biocide addition beyond 6 h at the dosages typically applied at desalination plant intakes is counterproductive and does not result in further control over the biofouling process. Quite the opposite, as indicated pre- viously, continuous addition of biocide would cause bacteria to maintain their protective EPS capsule, reducing significantly the opportunity for sustainable long-term biofouling control.
When bacteria are in an inactive state of existence, which occurs in saline source waters with low content of biodegradable organic substances (typically TOC <0.5 mg/L), they have very low metabolic and growth rates and exist as single cells or small cell clusters that behave as microparticles. The cells of these particle-like bacteria do not have capsules of EPS and, therefore, do not tend to stick to the surface of the RO membranes. Since in