Current through Register Vol. 41, No. 3, September 23, 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.
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.
