Water Quality Chemistry: Generation of Sodium hypochlorite (NaOCl) disinfectant by electrolysis of NaCl Solution

SEAVURIASeattle + Kenya (2012)

Sodium hypochlorite (NaOCl)

• NaOCl is produced either when:– Chlorine gas is dissolved in sodium hydroxide

solution, or– A sodium chloride solution (brine) is treated with

electricity (electrolysis)• NaOCl solutions are called liquid bleach or

Javelle water• Onsite generation of NaOCl needed due to

decomposition over time(Disinfectant, 1999)

Image courtesy of Boal ( 2009)

Anode: + electrodeWhere the oxidation of chloride ions (Cl-), involving the lose of 1 electron (e-) per Cl-, results in the production of chlorine gas (Cl2).

Cathode: - electrodeWhere the reduction of water (H20), involving the gain of 2e-, results in the production of hydrogen gas (H2) and hydroxide ions (OH-).

Electrolysis: Production of NaOCl

NaCl + H20 → NaOCl + H2Sodium chloride

(table salt)

Water Sodium hypochlorite

Hydrogen gas

Oxidant

Oxidized Reduced

NaOCl + H20 → HOCl + Na+ + OH-

Electrolysis:

Water Treatment:

(Boal, 2009)

Hypochlorous acid (Disinfectant, 1999)

Typical Chlorine Dosages

(Disinfectant, 1999)

Chlorine: Pathogen Inactivation • In bacteria, chlorine found to adversely affect

cell respiration, transport and DNA activity.– Decreases oxygen utilization – Damages cell wall membrane– Decreases levels of DNA synthesis

(Disinfectant, 1999)

(Haas and Engelbrecht, 1980)

Chlorine: Environmental Effects

• Several environmental factors influence the inactivation efficiency of chlorine, including:– Temperature and pH,– Contact time and mixing,– Turbidity and interfering substances, and– Concentration of available chlorine

• Temperature and pH have the most impact on pathogen inactivation by chlorine.

(Disinfectant, 1999)

Chlorine: Effect of Temperature and pH

• Temperature:– Pathogen inactivation increases with temperature.– If temperature is lowered by 10C, contact time should be

increased 2-3X (Clarke et. Al, 1962)

• pH:– Most impact on pathogen inactivation by chlorine– Germicidal efficiency of hypochlorous acid (HOCl) much higher

than that of hypochlorite ion (OCl-)– Note: addition of OCl- to water increases pH– HOCl dominates at low pH = ↑ disinfection– At less effective pH contact time should be increased (Culp and Culp,

1974; Scarpino et al., 1972)

(Disinfectant, 1999)

Chlorine: Disinfection Efficacy• Bacteria Inactivation: Chlorine extremely effective as disinfectant for

inactivating bacteria– HOCl is 70-80X more effective than OCl- (Culp/Wesner/Culp, 1986)– If temperature is lowered by 10C, contact time should be increased 2-3X (Clarke et. Al, 1962)

• Virus Inactivation: Chlorine is highly effective viricide– In 1971 study (0.5 mg/L free chlorine; pH 7.8; 2⁰C):

• Reovirus (least resistant): 2.7 min. contact time for 99.99% inactivation (4 log removal)• Poliovirus (most resistant): 60 minute contact time for 99.99% inactivation• 99.99% inactivation for all 20 viruses studied was between 1.4 to >30 mg•min/L (CT values)

• Protozoa Inactivation: Chlorine has limited success inactivating protozoa, like Giardia (Hoff et al., 1984); see Figures 2-1, 2-2, and 2-3 for more details– Resistance of Giardia two orders of magnitude higher than some viruses– Resistance of Giardia >three orders of magnitude higher than some bacteria– Chlorine has little impact on viability of Cryptosporidium when used at the relatively low

doses encountered in water treatment (e.g., 5 mg/L)

(Disinfectant, 1999)

Figure 2-1. Free Chlorine Giardia and Virus CT Requirements

Shows that the CT values (mg•min/L) required to achieve recommended disinfection efficiency for conventional filtration systems (i.e., 0.5-log Giardia cyst and 2-log virus inactivation level) are 23 and 3 mg•min/L, respectively. (Disinfectant, 1999)

According to figure, does the inactivation efficacy of free chlorine increase or decrease with increased pH?

Figure 2-2. CT Values for Inactivation of Giardia Cysts by Free Chlorine a 10⁰C (at Cl2 dose of 3.0 mg/L)

(Disinfectant, 1999)

According to figure, does the inactivation efficacy of free chlorine increase or decrease with increased temperature?

Figure 2-3. CT Values for Inactivation of Giardia Cysts by Free Chlorine at pH 7.0 ( at Cl2 dose of 3.0 mg/L)

(Disinfectant, 1999)

Proposed Experiments

• Test the effect of pH on chlorine bacterial inactivation

• Test the effect of temperature on chlorine bacterial inactivation

• Test the effect of contact time on chlorine bacterial inactivation

• Test the effectiveness of coffee filters on removing yeast from solution before plating

Additional Information (Disinfectant, 1999)

• 2.7.6 Operational Considerations– 2.7.6.1 Application Methods (p. 2-41)– 2.7.6.2 Safety and Handling Considerations (p. 2-

42)• 2.8 Summary– 2.8.1 Advantages and Disadvantages of Chlorine

Use (p. 2-42 & 2-43)– 2.8.2 Summary Table (Table 2-22, p. 2-44)

References• Boal, Andrew K. "On-Site Generation of Disinfectants." National Environmental

Services Center. Spring 2009. Web. 19 Mar. 2012. <http://www.nesc.wvu.edu/pdf/dw/publications/ontap/2009_tb/onsite_generation_DWFSOM133.pdf>.

• Clark, N.A., et al. 1962. Human Enteric Viruses in Water, Source, Survival, and Removability, Internal Conference on Water Pollution Research. Landar.

• Culp, G.L., and R.L. Culp. 1974. New Concepts in Water Purification. Van Nostrand Reinhold Company, New York, NY.

• Culp/Wesner/Culp. 1986. Handbook of Public Water Systems. Van Nostrand Reinhold Company, New York, NY.

• "Disinfectant Use in Water Treatment: Chlorine." EPA Guidance Manual. 1999. Web. 19 Mar. 2012. <http://zenbackpacking.net/EPA/Chlorine.pdf>.

• Haas C.N. and R.S. Engelbrecht. 1980. “Physiological Alterations of Vegetative Microorganisms Resulting from Aqueous Chlorination.” J. Water Pollution Control Fed. 52(7): 1976.

• Hoff, J.C., E.W. Rice, ad F.W. Schaefer. 1984. “Disinfection and the Control of Waterborne Giardiasis.” Conference proceedings, ASCE Specialty Conference.

• Scarpino P.V., et al. 1972. “A Comparative Study of the Inactivation of Viruses in Water by Chlorine.” Water Research. 6:959.