Virginia Administrative Code › Title 9 - ENVIRONMENT › Agency 25 - STATE WATER CONTROL BOARD › Chapter 790 - VIRGINIA POLLUTANT DISCHARGE ELIMINATION SYSTEM (VPDES) PERMIT REGULATION › Part III - Manual of Practice for Sewerage Systems and Treatment Works › Article 7 - Effluent Polishing and Disinfection Processes › Section 9VAC25-790-790 - Other disinfection methods

Current through Register Vol. 40, No. 24, July 15, 2024
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.