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12.5 ALTERNATIVES FOR CONTROL OF MICROBIAL FOULING 275 microorganisms from oxygen by applying strong reducing agents such as sodium bisulfite.
These methods are briefly described below.
12.5.2.1 Bacterial Growth Control by Microbial Oxidation and UV Inactivation
Several oxidants such as chlorine, chlorine dioxide, and chloramines can be used for sur- face source-water microbiological growth control. Microbial control by adding biocides to the source seawater is of controversy and focus of research at this time. This is because some membrane desalination plants have had significant microbial fouling problems after chlorina- tion or other means of microbial controldeven possibly worse than if no chemical biocides were used. It should be noted that continuous chlorination and dechlorination upstream of the SWRO membranes can increase bioactivity and associated membrane biofouling by increasing the content of assailable organic compounds in the source water. Some desalina- tion plants have experienced permanent damage of their SWRO membranes by exposure to the chemical oxidant when the dechlorination chemical system failed.
Chlorination is the most popular process for biofouling control practiced at present. This process was discussed in detail in Chapter 6. Ultraviolet (UV) inactivation of aquatic micro- organisms is an alternative method for membrane biofouling control. However, in some fa- cilities, microbial regrowth after UV treatment negated the benefits, so its use should be evaluated very carefully. UV irradiation method is power-intensive and, therefore, usually less cost-effective than chlorination/dechlorination. The cost-effectiveness of microbial UV inactivation is dependent on the source water quality and the design on the UV system. If the source water has high levels of turbidity, the UV dosage could be relatively high and biofouling control ineffective. For optimum performance, it is recommended that the turbidity of the saline source-water fed to the UV unit not to exceed 10 NTU. The best loca- tion of the UV system would be between the cartridge filter and the RO membranes. Because of space constraints, however, that is not usually possible, so as an alternate, installing the UV system just prior to the cartridge filters is acceptable.
12.5.2.2 Bacterial Growth Control by Reduction of Oxygen
Seawater collected by open intakes is usually rich in oxygen, which stimulates biological growth. Many marine bacteria use oxygen for bioassimilation of the dissolved organics in the ambient ocean water. Therefore, removal of oxygen by reducing chemicals such as so- dium bisulfite hinders their growth and ultimately suppresses the biofouling on the mem- branes (Wilf et al., 2007).
While inactivation of biofouling microorganisms by oxygen reduction is effective over short periods of time (several days to 1 week), in the long term, continuous addition of reducing chemical does not yield effective control. The reason behind this challenge is the fact that most marine bacteria are not strictly aerobic and they can also bioassimilate organics in the source water under anaerobic conditions.
While the bacterial growth in anaerobic environment is of slower rate than that under aer- obic conditions, such biogrowth would also cause accelerated biofouling of the membranes. Therefore, control of bacterial growth on the RO-membrane surface by continuous addition of reducing chemical (e.g., sodium bisulfite) is not sustainable long-term solution to biofouling challenges caused by algal bloomsdsuch blooms usually last for 4e6 weeks or more at a time.