An Overview of Chlorine
Dioxide Usage in Water
Dean Gregory
CDG Environmental, LLC
205 Webster Street Bethlehem, PA 18015 888-610-2562 www.cdgenvironmental.com
Agenda
Background
• Chlorine dioxide (ClO2 characteristics)
•
•
Regulatory concerns
•
ClO2 vs. free chlorine
ClO2 applications in water treatment
• Bench/full-scale data
• Analytical requirements (chlorite)
•
•
ClO2 generating systems
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Background
Chlorine dioxide (ClO2)
•Used
in drinking water treatment > 50 years
•First used in U.S. for T&O control (Niagara
Falls, NY)
•A
primary alternative to free chlorine for
oxidation and disinfection
•Generally
not used for secondary
disinfection
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Background
1st ClO2
application point
2nd ClO2
application point
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Background
General Characteristics
•
Highly soluble in water (10x more than Cl2)
•
Volatile & subject to photo-decomposition
•
Unstable at high concentrations (>15%)
and under pressure ~ must be generated
onsite
•
Oxidizes, does not chlorinate, using
selective one-electron transfer
•
Does not react with ammonia
•
Unaffected by pH over broad range (4-8)
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Background
Demand/Decay Characteristics
TOC = 5.1 mg/L
pH = 6.6
Mn2+< 10 μg/L
TOC = 1.2 mgl/L
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Background
Demand/Decay Characteristics
TOC = 5.6 mg/L
Mn2+ = 250 μg/L
TOC = 1.4 mgl/L
Mn2+ = 0 μg/L
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Regulations of Concern
Ct Disinfection Targets (20°C)
• 2-log Giardia lamblia: 10mg/L*min
• 2-log virus: 2.1mg/L*min
• 0.5-log Crypto (20°C): 58mg/L*min
• 0.5-log Crypto (10°C): 138mg/L*min
•
D/DBP Rule (MCL)
• TTHM: 0.08 ppm
• HAA: 0.06 ppm
• Chlorite: 1.0 ppm
•
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Regulations of Concern
Chlorite ion (ClO2)
• Must be measured daily at POE
• Monthly samples from distribution system
• Amp. titration ≈ 30 min. with experience
• LGB method
• Requires laboratory technician
• Colorimetric
• Provides ClO2 and ClO2 residuals
•
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Chlorine Dioxide vs. Chlorine
•
No THM/HAA formation
•
Giardia Ct requirements significantly lower
•
Superior manganese oxidation
•
Typical ClO2 dose ~ 0.6 - 1.7 ppm
•
Typical Cl2 dose ~ 2.0 - 5.0 ppm
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Chlorine Dioxide vs. Chlorine
Reactions with Natural Organic Matter
HOCl + NOM
TTHM, HAA
ClO2 + NOM
ClO2 + Low MW
organic
compounds
ClO2 reacts ONLY through a one-electron
oxidative pathway
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Water Treatment Applications
TTHM/HAA Precursor Destruction
• Greater impact on TTHM precursors
•
•
Manganese Oxidation
•
Taste & Odor Control (algae, diatoms)
Pre-ox.
Inactivates Giardia and viruses
• CT for 2-log Giardia inactivation: 10mg/L-min at
20°C
• CT for 2-log virus inactivation: 2.1mg/L-min
•
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Water Treatment Applications
Impact on TTHM/HAA-5 Formation
Raw TOC = 4.3 mg/L
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Water Treatment Applications
Impact on TTHM/HAA-5 Formation
Raw TOC = 5.1 mg/L
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Water Treatment Applications
Impact on TTHM/HAA-5 Formation
Raw TOC = 6.2 mg/L
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Water Treatment Applications
Impact on TTHM/HAA-5 Formation
Raw TOC = 7.8 mg/L
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Water Treatment Applications
Mn2+ Oxidation
•
Raw TOC = 6.3 mg/L
•
pH = 6.8 mg/L
•
Contact time = 5 min.
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Water Treatment Applications
Impact of ClO2 on Chlorophyll-a
Pilot Results (presented at ’05 WQTC)
Date
Raw Water Chlor-a
Post-ClO2 Chlor-a
Reduction
μg/L
μg/L
%
August 20
21
0
100
August 24
31
0
95
September 2
9
0
100
September 14
22
0
100
September 24
5
0
100
September 30
18
0
100
October 5
71
0
100
October 7
17
0
92
October 12
9
0
100
October 22
30
0
100
October 26
20
0
100
November 8
11
0
100
November 16
8
0
100
November 18
6
0
100
November 22
11
0
100
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Full-Scale Case Study
ClO2 for Pre-ox. and Disinfection
Raw water
Water Quality (TX Utility)
Raw TOC = 4.3 mgl/L (lower Rio Grande)
pH = 7.2, T = 23°
Pre-ox ClO2:
0.85 mg/L
Clearwell
Post-filt. ClO2:
0.85 mg/L
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Full-Scale Results
Texas Utility (6 mgd)
Total ClO2 dose = 1.6 - 1.8 mg/L
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Water Treatment Issues
Chemical sequencing
• Avoid NaOH/High-pH conditions
• pH > 9 = instant destruction
•
•
Avoid simultaneous application with NH2Cl
• 0.2 - 0.4 mg/L decrease in ClO2
residual
•
Interference from coagulant possible
•
Pre-ox. DT > 5 min. is optimum
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Analytical Methods
Amperometric titration
• Manual and semi-automatic systems
•
Ion chromatography
• Expensive, requires highly-trained
technician, turnaround time
•
•
Colorimetric (forming or removing a
color)
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Amperometric Titration
Hach™ AutoCat 9000 Amperometric Titrator
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ClO2 Generation Systems
Solution-based systems
• Chlorine Gas/25% Sodium Chlorite
(NaClO2) Solution
• Most common
•
•
Sulfuric Acid: Chlorate/Hydrogen Peroxide
Solution (Purate)
•
Electrochemical (small systems)
•
Acid-chlorite (less efficient)
•
Solid-based systems (CDG Gas: Solid™)
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Generation Systems
NaClO2(aq) : Cl2(gas)
2ClO2 + Cl2
2ClO2 + 2Cl
•
Efficient mixing of reagents improves purity
and yield (95% attainable)
•
High-concentration solution can be
batched and fed from day tank
•
Most efficient with chlorine overdose
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Generation Systems
NaClO2(aq) : Cl2(gas)
2ClO2 + Cl2
2ClO2 + 2Cl
Effluent must be analyzed regularly to
ensure efficiency
• i.e. addition amp. titration required
• frequent re-calibration possible
•
•
Disruption in Cl2(g) feed may cause ClO2
spike
•
Sodium chlorite storage/leaks are biggest
safety concern
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Generation Systems
Sulfuric Acid: NaClO3(aq)
2NaClO3 + H2O2 + H2SO4
2ClO2 + Na2SO4 + O2
Purate®
•
Effluent must be analyzed regularly to
ensure efficiency
•
Bulk storage of sodium chlorate and 73%
sulfuric acid required
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CDG Gas:Solid™ Process
Cl2
+
Chlorine Gas
2NaClO2
Solid Sodium Chlorite
SAF-T-CHLOR™
2ClO2
Chlorine Dioxide Gas
+
NaCl
Solid Sodium Chloride
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CDG Gas:Solid™ Plant-Scale
ClO2(g) Generators
SOLUTION
OUT
CHLORINE
EJECTOR
AIR
AIR
CHLORINE DIOXIDE + AIR
FILTER
RAW WATER
IN
SAF-T-CHLOR
SOLID
SODIUM
CHLORITE
SCALE
Cl2 + 2NaClO2
SAF-T-CHLOR
SOLID
SODIUM
CHLORITE
SCALE
CHLORINE
2ClO2 + 2NaCl
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CDG Gas:Solid™ Process
•
Produces pure ClO2 gas, minimizes
chlorite ion
•
Relatively simple operation: Cl2(g) feed
only
•
Flow-paced: Generator production
changes with plant capacity w/o need
for re-calibration
•
No need for additional titrations of
generator effluent
•
Safety
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CDG Gas:Solid™ System Safety
Considerations
Chemical handling
• Sodium chlorite shipped in sealed
cartridges w/ quick disconnects
• Maximum operator safety ~ no contact
with the chemical
• No liquid chemical storage & handling
•
Safety interlocks
• Low vacuum
• Low air flow
• Excessive chlorine gas flow
• Combustible gas presence
•
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CDG Gas:Solid™ System Safety
Considerations
Saf-T-Chlor™ Thermally Stable Sodium
Chloride
• Specially formulated
• Minimizes possibility of ignition source
• Absorbs and dissipates heat
• Does not propagate and ignite
• Sealed cartridges with ceramic
internal components
•
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New Option for ClO2 Applications
CDG Solution 3000®
•
Stabilized ClO2 solution - 3000 mg/L
•
Ideal for small systems, pilot plants
•
Available in 55g drums, 275g totes
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CDG Consultative Programs
Water Treatability Studies
• Protocols dependent on program
objectives
• Testing done on actual raw water
•
Trial programs
• Production unit installed at water plant
• Limited financial exposure
•
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Questions?
CDG Environmental, LLC.
205 Webster Street
Bethlehem, PA 18015
888-610-2562
www.cdgenvironmental.com
205 Webster Street Bethlehem, PA 18015 888-610-2562 www.cdgenvironmental.com
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