Virginia Law

A. Design standards for disinfection methods not specifically addressed in this chapter will be established upon evaluation of performance data.

B. Chlorine dioxide (ClO₂). Chlorine dioxide is characterized as a yellow-green to orange gas, its color changing toward red with increasing concentration. Upon cooling, it forms a red, highly unstable liquid which freezes at -59°C and boils at 11°C. Due to the sensitivity of ClO₂ gas to pressure and temperature, it shall be generated at the location where it will be used as a disinfectant. Chlorine dioxide is quite soluble in water, its solubility depending upon temperature and pressure. At temperatures less than 25°C and above 30 mm partial pressure, it is soluble to the extent of 10 grams per liter. Unlike chlorine, ClO₂ does not react with water; it is a true dissolved gas.

1. Chlorine dioxide gas is very toxic but, when dissolved, it is stable and safe to use in water solution. Since concentrated chlorine dioxide gas is unstable under pressure, chlorine dioxide shall be generated under controlled conditions.

2. The generation of chlorine dioxide involves the reaction between chlorine and sodium chlorite:

Cl₂ + 2 Na ClO₂——>2 NaCl + 2 ClO₂

Side reactions that also produce sodium chlorate (Na ClO₃) are also possible in dilute solutions, especially if the concentration of molecular chlorine, Cl₂, is low. Research has shown that high concentrations of sodium chlorite and molecular chlorine favor the formation of chlorine dioxide. Accordingly, chlorine dioxide generators should be designed and operated to provide these reaction conditions while minimizing the amount of chlorine gas that is mixed with the generated ClO₂.

3. As with chlorine, adequate disinfection with chlorine dioxide is achieved by maintaining a sufficient chlorine dioxide residual after a specific contact time in order to achieve the desired microbiological quality of the treated effluent. All the principles of good chlorination practice, proper pretreatment, rapid initial mixing, adequate residual, plug flow contacting, etc., are also applicable to disinfection with chlorine dioxide.

4. Thus, the required levels of residual ClO₂ shall be equivalent to the residual concentrations that would be required for chlorination of a specific effluent unless adequate information is submitted to the regulatory agencies verifying that acceptable disinfection can be achieved with a lower residual of ClO₂.

5. Design dosages of ClO₂ applied to treated effluent should be similar to the recommended levels for chlorination. The results of limited research to date indicate that for certain effluents, lower dosages of ClO₂, in comparison to Cl₂, may accomplish adequate disinfection. However, all proposals specifying design dosages of ClO₂ below the levels approved for chlorination, must provide supporting information based on field measurements or laboratory studies acceptable to the regulatory agencies.

6. The introduction of ClO₂ shall be in a manner to maximize mixing with the influent flow to the contact basin while minimizing vaporization. The same basic principles as for chlorine are to be adhered to in chlorine dioxide physical contacting with the wastewater. However, chlorine dioxide use should be optimized by appropriate selection of application points within the process scheme.

7. Contact periods approved for chlorination shall be directly applicable to chlorine dioxide contacting unless adequate supporting information is submitted verifying that the use of a particular design contact period can result in the acceptable level of disinfection.

8. Chlorine dioxide disinfection requires maintenance of a residual throughout the contact period. Conventional amperometric titration systems should be used to monitor chlorine dioxide residuals and, with some modifications, should be used to control the residual and generation of chlorine dioxide. Operator exposure to ClO₂ shall be minimized. Adequate ventilation shall be provided in areas where ClO₂ is generated and where concentrated mixtures of ClO₂ are sampled and tested. As ClO₂ to ambient air mixtures containing 10% or more ClO₂ are potentially explosive and highly corrosive, provisions shall be made to prevent this occurrence.

C. Electrolytic oxidants. Electrolytic processes produce a mixed group of oxidants consisting of ozone, hydrogen peroxide and chlorine constituents. This process is typically monitored and controlled by the chlorine residual level in the wastewater effluent. All electrolytic oxidant processes should be evaluated under the provisions for conventional disinfection of wastewater in accordance with this chapter. The department will evaluate the development of these methods of disinfection and the approval of this process will be handled on a case-by-case basis in accordance with the provisions of this chapter.