Transcript Disinfectant, 1999

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
– 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)
Electrolysis: Production of NaOCl
Anode: + electrode
Where the oxidation of
chloride ions (Cl-),
involving the lose of 1
electron (e-) per Cl-,
results in the production
of chlorine gas (Cl2).
Cathode: - electrode
Where the reduction of
water (H20), involving
the gain of 2e-, results in
the production of
hydrogen gas (H2) and
hydroxide ions (OH-).
Image courtesy of Boal ( 2009)
Oxidant
Electrolysis:
NaCl + H20 → NaOCl + H2
Sodium chloride
(table salt)
Oxidized
Water
Sodium hypochlorite
Hydrogen gas
Reduced
(Boal, 2009)
Water Treatment:
NaOCl + H20 → HOCl +
Hypochlorous acid
+
Na
+
OH
(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
(Haas and Engelbrecht, 1980)
(Disinfectant, 1999)
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)
Figure 2-2. CT Values for Inactivation of Giardia Cysts by
Free Chlorine a 10⁰C (at Cl2 dose of 3.0 mg/L)
According to figure, does the inactivation efficacy of free chlorine
increase or decrease with increased pH?
(Disinfectant, 1999)
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)
According to figure, does the inactivation efficacy of free chlorine
increase or decrease with increased temperature?
(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. 242)
• 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
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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_generatio
n_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.