Chemical Oxidizers for Disinfection of Reverse Osmosis Systems

1 Chemical Oxidizers for Disinfection of Reverse Osmosis Systems 

Chlorine is commonly used to kill microbes in pretreatment prior to RO and to break up organics that may foul RO membranes. It is used rather than other halogens because of its higher ORP. Chlorine is available in many forms, such as chlorine gas, sodium hypochlorite (bleach), chloramines, and chlorine dioxide. Chlorine gas and sodium hypochlorite each react with water to form hypochlorous acid, as shown in Equations 8.5 and 8.6, respectively. 

NaOCl = sodium hypochlorite HOC1 = hypochlorous acid 

Hypochlorous acid then dissociates in water to form hydrogen ions and hypochlorite ions (Equation 8.7)  

Hydrochlorous acid has the highest disinfecting capability of the 3 species it is 100 times more active than hypochlorite ion.11 Chlorine gas contains 100% available chlorine (hydochlorous acid) while industrial-grade bleach has about 12% -15% available chlorine. 

NaOCl = sodium hypochlorite HOCl = hypochlorous acid 

Chemical oxidizers used to disinfect RO systems include hydrogen peroxide (peroxide), halogens, and ozone. Although halogens (and specifically chlorine) are the most popular oxidizers using in conjunction with RO pretreatment, they do not have the highest oxidization-reduction potential (ORP). Table 8.8 lists the ORP for several oxidizers. As the table shows, ozone and peroxide have nearly twice the ORP or oxidative power as chlorine. Despite the relatively low ORP, chlorine is the most commonly used disinfectant in brackish water RO pretreatment due to its ease of use and its ability to provide residual disinfection (for seawater desalination using RO, bromine (as HOBr) 

C12-E2H20 OHOCl+H -tCl NaOCl-tHjO OHOCl + NaOH 

Hypochlorous acid then dissociates in water to form hydrogen ions and 

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