Hypochlorite Assessment Model

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American Water Works Association wishes to recognize and thank the following people and their associated organizations for their efforts in developing the Hypochlorite Assessment Model:

  • Benjamin D. Stanford, Ph.D. Director of Applied Research Hazen and Sawyer, P.C. 
  • Gilbert Gordon Distinguished Research Professor, Emeritus Department of Chemistry and Biochemistry Miami University 
  • Aleks Pisarenko, Ph.D. Post Doctoral Researcher Applied Research and Development Center Southern Nevada Water Authority
Hypochlorite Assessment Model

This predictive modeling tool, available exclusively to AWWA utility members, provides guidance on the expected levels of perchlorate and chlorate in stored bulk hypochlorite solutions. The calculations are based on predictive algorithms derived from earlier research on the subject and local, user-defined inputs (e.g., storage conditions, ionic strength, ambient temperature). The output is intended to help utilities determine the appropriate storage-time/life-cycle for the hypochlorite solution based on the expected rate of degradation and contaminant formation. Such a tool will better enable utilities to manage supply and/or take other countermeasures to mitigate further degradation that could cause an exceedence of a regulated contaminant standard.

Supporting resources

AWWA developed this resource based on the findings of the report, Hypochlorite - An Assessment of Factors that Influence the Formation of Perchlorate and Other Contaminants (WITAF # 712 / AwwaRF #4147).

This report found that perchlorate is present in hypochlorite solutions and continues to form at different rates depending on storage conditions. The perchlorate that is formed can be produced at levels that could cause regulatory compliance issues for utilities operating in states with existing perchlorate standards (California - 6 ppb and Massachusetts - 2 ppb) and under future regulatory limits that may be issued by the US Environmental Protection Agency or Canadian authorities.

Stanford et al. also addressed the foundations of this tool in the June 2011 issue of Journal - American Water Works Association. That article, titled "Perchlorate, Bromate, and Chlorate in Hypochlorite Solutions: Guidelines for Utilities" can be downloaded (free to AWWA members). This article has been selected as AWWA’s 2012 Water Quality & Technology Division Best Paper.

Hypochlorite Assessment Model FAQ and Glossary
1. Why can I not select or change the units for the output values on the model?
The model has been designed to provide the user with output units for hypochlorite concentration based on the input units selected. Thus, if weight percent sodium hypochlorite is selected, the output units will also weight percent sodium hypochlorite. In order to change the output units, select different input unit.
2. Can I change the units for the chlorate, perchlorate, and oxygen outputs?
Not within the program itself. Any conversion of units will need to be performed by the user after downloading the .csv file and importing those data into an appropriate program. The user takes full responsibility for any changes made to the data and proper conversion between units.
3. Why is the specific gravity needed for a weight percent solution?
Specific gravity of hypochlorite solutions may vary from manufacturer to manufacturer depending on multiple factors. Knowing the specific gravity of your solution will improve the accuracy of the model calculations. Note: Specific gravity is not the same as density as there is no temperature conversion. User is responsible for proper measurement of specific gravity. Consult the manufacturer or a local laboratory to determine the specific gravity of your solution.
4. How does the program calculate specific gravity when no value is provided?
Specific gravity is estimated based upon the following two equations in the normal user mode:
  • Specific Gravity = Wt% NaOCl x 0.016 + 0.985
  • Specific Gravity = Wt% FAC x 0.016 + 0.985
However, when the concentration of excess caustic is known, a more accurate calculation of specific gravity (SG) can be performed using one of the following equations:
  • Corrected SG = SG + (% excess NaOH X 0.017)
  • Corrected SG = SG + (g/L excess NaOH X 0.00135)
5. Why is knowledge of the excess caustic important?
Knowing the excess caustic can improve the model’s ability to estimate the specific gravity of the hypochlorite solution. However, if a measured value for specific gravity of the hypochlorite solution is used, then there is no benefit to providing the excess caustic concentration to the model.
6. What is the minimum user input required to run the model?
A given model calculation can be executed with only the strength (concentration) of the hypochlorite solution, temperature, and expected holding time. All other necessary values can be calculated by the program. However, knowledge of the other parameters will improve the accuracy of the model.
7. Will the model work for on-site generated hypochlorite solutions?
The model is designed to work with solutions with a pH range of 11 to 13, where hypochlorite decomposition is slower. Typical on-site generated (OSG) solutions are at a much lower pH (e.g. at pH 9), therefore would fall outside of the boundary conditions for this model. As a general practice, OSG bleach is generated for immediate use and is typically not recommended for long term storage.
8. Will the model work for calcium hypochlorite solutions?
Calcium hypochlorite behaves in a similar manner to liquid sodium hypochlorite. Data presented in the 1994 AWWA Research Foundation report (Gordon and Bubnis, 1994) clearly show a buildup of chlorate during storage, with predicted concentration in good agreement with the measured values (+/- 5%). Decomposition of calcium hypochlorite and formation of chlorate was modeled in Bleach 2001. Though perchlorate formation was found in calcium hypochlorite solutions based on the findings of the report ,“Hypochlorite--an Assessment of Factors That Influence the Formation of Perchlorate and Other Contaminants”, the present model was validated for sodium hypochlorite solutions, and thus, the current model does not include calculations for perchlorate formation in calcium hypochlorite solutions.
9. Why is the volume of oxygen evolved included in the model output?
The main decomposition pathways of sodium hypochlorite lead to production of of chlorate and chloride ions and oxygen gas. Under some conditions, (e.g. catalyzed decomposition) oxygen may be produced in significant quantities; however in most cases chlorate pathway is dominant. Calculations to predict oxygen gas formation are based on Bleach 2001 model to demonstrate the volume of oxygen produced during normal hypochlorite decomposition and the necessity of proper venting of hypochlorite storage tanks.
10. What is the basis for the default value for chlorate in the hypochlorite solution?
If a measured value for chlorate is not available, then the program calculates the chlorate concentration as 1/1000th of the molar hypochlorite concentration.
11. What is the basis for the default value for perchlorate in the hypochlorite solution?
If a measured value for perchlorate is not available, then the program calculates the perchlorate concentration as 1/1,000,000th of the molar hypochlorite concentration.
12. What is the basis for the default value for sodium chloride in the hypochlorite solution?
If a measured value for chloride is not available, then the program calculates the chloride concentration as the sum of the molar concentration of hypochlorite and 2.25 times the molar concentration of chlorate ion.
13. Why is the specific conductance (conductivity) of the solution requested?
The ionic strength of the hypochlorite solution plays a critical role in the rate of perchlorate formation. Thus, the program allows the user to measure the conductivity of the hypochlorite solution (making sure that appropriate dilutions are performed to stay within the calibration range of the conductivity probe) and converting that measurement to an empirical ionic strength using the following equation:
  • “Ionic Strength” (mol/L) = Conductivity x 0.000016
When a measured value for conductivity is unavailable, the program will estimate the ionic strength based on a sum of significant anions (molar basis) in solution from the following equation:
  • Calculated Ionic Strength = [OCl-] + [ClO3-] + [Cl-] + 10(-(14-[pH]
14. Where can I obtain more information on the basis of the Bleach 2001 model and the perchlorate model, including rate constants and kinetic equations used?
Several relevant sources of information are available including:
  • Stanford, B.D., A. Pisarenko, S. Snyder, and G. Gordon, Perchlorate, Bromate, and Chlorate in Hypochlorite Solutions: Guidelines for Utilities. Journal - American Water Works Association, 2011. 103(6): p. 71-83.
  • Stanford, B.D., A.N. Pisarenko, S.A. Snyder, and G. Gordon, Minimizing Perchlorate Formation in Hypochlorite Solutions, Opflow, 2009. 35(10): p. 10-13.
  • Snyder, S.A., B.D. Stanford, A.N. Pisarenko, G. Gordon, and M. Asami, Hypochlorite--an Assessment of Factors That Influence the Formation of Perchlorate and Other Contaminants. 2009, The AWWA and the Water Research Foundation: Alexandria, VA. p. 78.
  • Adam, L., Gordon, G. and Pierce, D. (2001) Bleach 2001 Predictive Model, Miami University, Oxford, Ohio. Copyright (c) 2001 AWWA Research Foundation and AWWA. Adam, L.C. and Gordon, G. (1999) Hypochlorite ion decomposition: Effects of temperature, ionic strength, and chloride ion. Inorganic Chemistry 38(6), 1299-1304.
  • Gordon, G.; Adam, L.; & Bubnis, B., 1994. Minimizing Chlorate Ion Formation in Drinking Water When Hypochlorite Ion Is the Chlorinating Agent. AWWA Research Foundation, Denver, CO.
15. Is the bromide ion concentration considered in the model?
No; bromide is not considered as bromide will rapidly react with hypochlorite to form hypobromite, which decomposes to bromate. Bromate has no effect on the rate of chlorate or perchlorate formation. Thus, it is assumed that any bromide will have already reacted to form bromate and will not have a long-term impact upon storage of the solution and subsequent chlorate and perchlorate ion formation.
16. Transition metal ions such as copper, nickel, cobalt, iron, and manganese will catalyze the decomposition of hypochlorite. Is there a way to account for this in the model?
No; there is no method to quantify the impacts of transition metals built into the existing model. It is assumed that the presence of such ions has been minimized by proper manufacturing, shipping, and storage practices. If transition metal ions are present, the rate of hypochlorite ion decomposition will be greatly increased and will invalidate any predictions from this model.
17. What is the “expected chlorine dose”?
The expected chlorine dose is the total amount of chlorine applied during treatment, NOT the residual leaving the facility. For example, there may be a pre-chlorination step where 0.5 ppm of chlorine is added to control for biogrowth in the transmission line, 0.5 mg/L added with ammonia for control of bromate formation during ozonation, 4 mg/L added to breakpoint chlorinate the finished water, and then a booster station in the distribution system adding another 0.5 mg/L chlorine, resulting in a total expected chlorine dose of 5.5 mg/L.
18. As of October, 2011 no chlorate or perchlorate maximum contaminant level (MCL) exists at the U.S. federal level. What value can I use here and what is the purpose of adding such information?
The chlorate and perchlorate MCL fields were added to allow the user to define (based on future or existing regulatory limits or specific goals of the facility) the maximum concentration of the specific contaminant within their finished drinking water. The program then uses the expected chlorine dose to calculate the amount of chlorate or perchlorate added to the drinking water during treatment. This calculation does NOT consider any background chlorate or perchlorate concentrations nor does it consider any other sources within the treatment plant. The “residual chlorate” and “residual perchlorate” output is therefore only the residual of each contaminant added to the finished water through application of the specific hypochlorite solution used within the model. The user must add any other chlorate or perchlorate sources to the model outputs post-processing.
19. Can I change the axis labels or other features on the graphical output?
No; the graphs are there only for quick reference. The data may be exported (via .csv file) to another data management program for user-defined graphing.
Term Operational Definition for this Model
Concentration A measure of the amount of dissolved substance contained per unit of volume.
Chlorine Refers to free available chlorine (FAC), "Cl2" is also used to describe FAC, a commonly used term in water treatment industry. Bleach is a more common term for chlorine. Cl2 has a molecular weight of 70.91 g/mol, where as NaOCl has molecular weight of 74.44g/mol and OCl- has a molecular weight of 51.5 g/mol The use of Cl2 for units of chlorine originates from using Cl2 gas in water treatment. When Cl2 gas is bubbled into water, 1 mol of Cl2 yields in 1 mol of NaOCl: Cl2 + 2 NaOH → NaCl + NaOCl + H2O As a chemical species chlorine exists as either hypochlorite ion or hypochlorous acid, depending on pH of the water. Commercial bulk sodium hypochlorite is usually defined as weight % Cl2 and has pH of 12-13.0 to minimize hypochlorite decomposition.
Hypochlorite Referring to the hypochlorite ion in "bleach" solutions. Formula: OCl-
Hypochlorous Acid Hypochlorous Acid has a pKa of 7.4 and free available chlorine will disproportinates into the form of hypochlorite ion if pH> 7.4, or hypochlourus acid if pH < 7.4. Formula: HOCl
Wt % NaOCl Concentration of Sodium Hypochlorite as Weight Percent.
Wt% FAC Concentration of Free Available Chlorine as Weight Percent.
Vol % NaOCl Concentration of Sodium Hypochlorite as Volume Percent.
Vol% FAC Concentration of Free Available Chlorine as Volume Percent.
NaOCl (g/L) Concentration of Sodium Hypochlorite as grams of NaOCl per liter (g/L).
FAC (g/L) Concentration of Free Available Chlorine as grams Cl2 per liter (g/L).
mol/L NaOCl Molar concentration of Sodium Hypochlorite. mol/L FAC Molar concentration of Free Available Chlorine. pH pH = -log [H+] This is an expression of the amount of acid or base in a liquid. Chlorate In the chlorate anion, the chlorine atom is in the +5 oxidation state. Formula: ClO3- MCL In this context, MCL may be defined by State or Federal regulations or may be defined by user criteria. Maximum Contaminant Level, the maximum permissible level of a contaminant in water delivered to any user of a public system. MCLs are enforceable standards. Perchlorate In the perchlorate ion, the chlorine atom is in the highest oxidation state of +7 Formula: ClO4- Specific Gravity (SG) The ratio of the density of a substance to the density of a standard, usually water for a liquid or solid, and air for a gas. When unknown, the program calculates S.G. by the following: SG = Trade Percent (% w/w FAC or NaOCl) X 0.016 + 0.985 Sodium Chloride A colorless crystalline compound occurring naturally in seawater and rock salt; common salt. Formula: NaCl Empirical Ionic Strength (E.I.S.) The measured ionic strength of a solution based on conductivity: E.I.S. = 0.000016 X [umho/cm] Specific Conductance A measure of the ability to conduct an electric current, in µmho/cm. Ionic strength (I.S.) Ionic strength, a measure of the concentration of ions in a solution. Enthalpy A thermodynamic quantity describing the heat gained or lost during a reaction. Here, Enthalpy of activation for the decompostion of hypochlorite ion = 102.2 kJ/mol Entropy A thermodynamic quantity describing the disorder or randomness. Here, Entropy of activation for the decomposition of hypochlorite ion = -55.2 J/(mol*K) kinf The second order rate constant for the decomposition of hypochlorite at infinite dilution (zero ionic strength)(mol/L/sec) k2 The rate constant for NaOCl decomposition. (mol/L/sec) kClO3 The rate constant for chlorate formation. (mol/L/day) kO2 The rate constant for oxygen formation. (mol/L/day) kClO4 The rate constant for perchlorate formation. (mol/L/sec) Half-life The time required for a chemical reactant/species to lose one-half of its original concentration. Equation: Half-life of hypochlorite in an ideal solution = 1/(k2*[Hypochlorite]initial) Normalization Converts concentration of each contaminant to the amount of contaminant added per unit measure of FAC Residual The amount of chlorate or perchlorate in solution (here, finished water) after adding hypochlorite at the user- defined dose. Equation: [Chlorate]Residual = [Chlorate]Norm*ChlorineDose Dose The amount of hypochlorite added during the drinking water treatment process in mg FAC/L