On-site generation of sodium hypochlorite as a means to deliver chlorine has been practiced for years on oil and gas production platforms worldwide. It provides a much safer disinfection mechanism than handling chlorine gas or biocides, and it is more proven than UV or ozone in use upstream of seawater filtration equipment.
Seawater can be used as the source of the chlorine ion needed to produce hypochlorite through electrolysis. Normal seawater has a chlorine ion concentration of 19000 - 20000 mg/l. DC current is passed through seawater in a cell to generate hypochlorite through a series of oxidation, hydrolysis and reduction reactions. The amount of hypochlorite produced is a function of the electricity passed directly through the cell. The overall net reaction is:
Sodium hypochlorite generation is generally done close to the seawater intake. It is a moderate pressure process at less than 15 barg (215 psig). Seawater is pumped into the generator. Sodium hypochlorite is generated and pumped back through a diffuser to ensure full contact with incoming seawater. Clean seawater should have a low chlorine demand. Generally, 1 mg/l of available chlorine is adequate for disinfection although most platforms retain the ability to deliver high hypochlorite doses intermittently as some organisms can become resistant to constant low dosages of chlorine.
UV sterilization is an effective method of disinfection. UV radiation is a form of physical disinfection. It penetrates the cell wall of microorganisms and destroys the organism's ability to reproduce. Its effectiveness depends on the amount of radiation and exposure time.
Ozone is also used for disinfection. It is a strong oxidizer, and is more effective at destroying bacteria and viruses than chlorine. However, an ozone generator is relatively expensive. Its effectiveness depends on ozone concentration, contact time and the susceptibility of target microorganisms.
