4.2.1 Water Treatment

Where Am I: Food Processors Guidance Document

Commonly Used EPCRA Section 313 Chemicals - Chlorine, chlorine dioxide

Process Description - Chlorine is used to treat process water at food processing facilities. It is received as a compressed gas and added to process water as a disinfectant. The treated water uses include:

C                Component of a food product;

C                Wash or convey food products;

C                Control odor in fish meal processing; and

C                Cleaning purposes.

At a few plants, chlorine dioxide is manufactured by reacting liquid sodium chlorite, chlorine gas, and water for use in similar processes. During water treatment, the facility may coincidentally manufacture listed chemicals, such as chloroform, as byproducts.

Reporting Threshold - Depending upon the application, these chemicals may be manufactured, processed, or otherwise used, and therefore subject to different reporting thresholds. The most common situation is when purchased chlorine is used to treat water for washing or conveying fruits or vegetables or for cleaning equipment. In this case, the chlorine is otherwise used as a chemical processing aid, and subject to the 10,000-pound reporting threshold. Production of chlorine dioxide on site is considered manufacturing (produced for on-site use/processing), and should be applied to the 25,000-pound manufacturing threshold. If the chlorine dioxide is subsequently used as a disinfectant, the quantity used should also be applied to the 10,000-pound otherwise used threshold.

Estimating Release and Other Waste Management Quantities - The only release that would be expected in this application is small fugitive air releases of chlorine and chlorine dioxide. These releases occur from sources such as leaks in valves and fittings and losses during cylinder changeovers. For the quantities of chlorine typically used for water treatment purposes, engineering judgment can be used to estimate fugitive releases (e.g., based on the volume of the connecting hose and the number of changeovers). If significant quantities of chlorine are handled, these fugitive releases can be estimated using the SOCMI factors presented in Estimating Releases and Waste Treatment Efficiencies for the Toxic Chemical Release Inventory Form.

No releases are typically expected of either of these chemicals to water. Chlorine reacts very quickly with water to form HOC1, C1-, and H+. Although this is an equilibrium reaction, at a pH above 4 the equilibrium shifts almost completely toward formation of these products. Therefore, essentially zero releases of chlorine to water occur under normal circumstances. Releases of chlorine dioxide to water should be considered but are unlikely based on its strong oxidizing potential and the constant supply of organics in the waste stream.

Example - Chlorine Calculations

Example Release Estimate:

Once Cl2 is introduced into water or wastewater, it is transformed to HOCl or other compounds.

Cl2 + H2O ? HOCl + H+ + Cl-

The equilibrium constant, Keq, for the reaction is 4.5 x 10-4 at 25o C; therefore:

The brackets in Equation 1 represent the concentration of the chemical species in terms of gram moles/L. At a pH above 4, the reaction is essentially complete and little Cl2 remains. In addition, HOCl may further dissociate into OC1- and react with ammonia in water to form chloroamines. Cl2 also will react directly with other compounds in water.

Releases of chlorine as Cl2 to water or POTW will therefore be very small. By applying the Cl2/HOCl equilibrium relationship, Cl2 releases to water can be calculated. The following example presents Cl2 releases for a hypothetical chlorination process in which 100,000 pounds of Cl2 gas is used during the year, the applied dose is 10 mg/L (this is an extremely high value), the pH of the discharge is 8, and the facility chlorinates incoming well water and discharges the water to a stream.

[Molecular weight of C12 is 71 gms/mole] 

= 1.41 x 10-4 mol C12/L = [C12] applied

The goal of this calculation is to find the equilibrium concentration of C12 after its reaction with water (i.e., the [C12] equilibrium). From the stoichiometry of the C12/H2O reaction:

[C1-] = [HOC1] at equilibrium

Also from stoichiometry:

[C1-] = [C12] applied - [C12] equilibrium = 1.41 x 10-4 - [C12] equilibrium

The pH of the water is maintained at a level of 8; therefore:

[H+] = 1 x 10-8

Substituting into Equation 1:

Solving for [C12] equilibrium yields:

C12 equilibrium = 4.42 x 10-13 mol C12/L

Translating this into a mass-per-volume basis:

C12 equilibrium = (4.42 x 10-13 mol C12/L) * (71 g C12/mol) * (1000 mg/L)

= 3.14 x 10-8 mg C12/L

The applied C12 dose is 10 mg/L, and the remaining C12 after equilibrium is 3.14 x 10-8 mg/L. The fraction of the applied C12 dose remaining is therefore:

C12 releases to water = 100,000 lb x 3.14 x 10-9 = 0.0003 lb

Similar calculations can be performed for other pH levels at an applied C12 dose of 10 mg/L:

COMMON ERROR - Reporting

The most common reporting error for chlorine or chlorine dioxide treated water is reporting discharges or transfers when the chemicals have been treated on-site. As indicated above, if the discharge is maintained at a pH above 4, none of these chemicals are expected to be released to water streams. Although some facilities may monitor the residual chlorine concentration in their water, this parameter does not represent the C12 concentration. Residual chlorine is the sum of the C12, HOC1, and OC1- concentrations. Because the C12 concentration will be negligible under neutral pH conditions, direct discharges to water or transfers to a POTW may be reported as zero. Note that if your facility performs no other release and/or other waste management practices involving chlorine, you may be able to file a Form A rather than a Form R.