seavuria seattle + kenya (2012)
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Water Quality Chemistry : Generation of Sodium hypochlorite (NaOCl) disinfectant by electrolysis of NaCl Solution. SEAVURIA Seattle + Kenya (2012). Sodium hypochlorite (NaOCl). NaOCl is produced either when: Chlorine gas is dissolved in sodium hydroxide solution, or - PowerPoint PPT PresentationTRANSCRIPT
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Water Quality Chemistry: Generation of Sodium hypochlorite (NaOCl) disinfectant by electrolysis of NaCl Solution
SEAVURIASeattle + Kenya (2012)
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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)
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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
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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)
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Typical Chlorine Dosages
(Disinfectant, 1999)
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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)
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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)
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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)
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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)
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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)
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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)
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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)
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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
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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)
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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.