Why Use ClO2
For Drinking Water Treatment?
Ron Hofmann
Department of Civil Engineering, University of Toronto
Osvaldo Conio, Fiorina De Novellis
Fondazione AMGA
ClO2 usage
North America
(1o)
Primary
disinfection and
oxidation
Secondary (2o)
disinfection
Surface
waters
3%
Ground
waters
0.2%
Italy
20%
< 1%
United States Environmental Protection Agency (2002)
Meucci, L., and O. Conio (1996). First European Symposium on chlorine dioxide and disinfection.
Why use ClO2?
DISINFECTION
Primary (1o) disinfection
… Secondary (2o) disinfection
…
TASTE AND ODOURS
DISINFECTION BY-PRODUCTS
IMPROVE DOWNSTREAM TREATMENT
IRON/MANGANESE
DESTRUCTION OF ORGANIC POLLUTANTS
Disinfection
1o and 2o disinfection
„
Primary: making the water safe before it leaves the
treatment plant (before the first person drinks the water)
„
Secondary: protecting the water as it travels to the home
Coag/flocc
Settling
Filter
Clearwell
Water Treatment Plant
ClO2 for 1o disinfection
„
ClO2 is a powerful disinfectant
(usually much stronger than chlorine, not as strong as
ozone)
„
Disinfectant doses in North America controlled by
CT requirements
„
CT requirements can be used to compare disinfectant
strengths (ClO2 vs. Cl2 vs. O3)
No
Note : log
= kCT
N
where N is number of surviving organisms
Number of
organisms
CT
CT requirements
If 20oC, and need 2-log Giardia reduction:
need CT = 10.0 mg⋅min/L
If 30 minutes available before the first customer → ClO2 =
10 mg ⋅ min/L
= 0.33 mg/L
30 min
CT For 2-log VIRUS Inactivation (10oC)
12
643
CT (mg min/L)
10
8
Cl2 and ClO2 are similar for
viruses
6
4
2
0
Cl2
ClO2
NH2Cl
O3
CT For 2-log GIARDIA Inactivation (10oC)
120
1230
100
CT (mg min/L)
ClO2 is about 5X stronger than Cl2 for
Giardia
80
60
40
20
0
Cl2
ClO2
NH2Cl
O3
CT For 2-log CRYPTO Inactivation (10oC)
1000
900
??
> 15,000?
CT (mg min/L)
800
700
ClO2 will get some Crypto kill
600
500
400
300
200
100
0
Cl2
ClO2
NH2Cl
O3
ClO2 for 1o disinfection
If Cryptosporidium disinfection not required:
„
ClO2 doses and/or times can be much lower than Cl2
for Giardia & virus control
…
„
ClO2 often a good replacement for Cl2 when
contact time (= volume) is limited
In practice in North America: ClO2 not a common
1o disinfectant.
…
UV, O3 normally used when Cl2 not possible
ClO2 for 2o disinfection
In Europe - fairly common. Reasons:
„
No regulation to maintain a residual at the tap
(often undesirable)
Note: ClO2 usually less stable than Cl2
„
Good multipurpose disinfectant/oxidant
In North America - almost never used. Reasons:
„
Less stable – can’t maintain residual at the tap
„
Difficulty with analytical method in the field
„
Taste and odour problems
Disinfection summary
In Europe
„
ClO2 can be used alone since no need for 2o disinfectant
…
attractive choice in some cases
In North America
„
ClO2 cannot be used alone → makes ClO2 less popular
Secondary
disinfectant
Preferred 1o
Less popular
Cl2
NH2Cl
Cl2, UV
O3, ClO2
Taste and odour control
„
Historically the most common ClO2 application
„
Experience: ClO2 destroyed T&O problems that Cl2 or
KMnO4 could not
„
Research: ClO2 not very effective for
…
geosmin, 2-methylisoborneol (MIB), common pyrazines
and anisoles
„
Research: ClO2 effective for sulfides, phenolics
„
Conclusion: ClO2 controls T&O in some waters,
but need research to identify the specific compounds
Disinfection By-Products (DBPs)
Trihalomethanes
Haloacetic
Acids
Chlorite
Chlorate
North
America
0.080 mg/L
0.060 mg/L
1 mg/L
None
Europe
0.025-0.150 mg/L
None
0.2-0.7
mg/L
0.7 mg/L
„
Pure ClO2 forms no THMs; very low HAAs
„
Pure ClO2 followed by NH2Cl (2o disinfectant) effective
„
Chlorite (ClO2-) and chlorate (ClO3-) are a concern
„
About 70% of applied ClO2 ends up as chlorite when
reacting with natural organic matter
5
Chlorite (mg/L)
Korn et al. 1998
8 waters, different pH,
temperatures
y = 0.6968x - 0.0487
2
R = 0.9518
4
3
2
y = 0.6815x
R2 = 0.951
1
0
Chlorite = 70% of reacted ClO2
0
1
2
3
4
5
6
Chlorine Dioxide consumed (mg/L)
Chlorate (mg/L)
0.6
Chlorate = 9% of reacted ClO2
y = 0.0906x - 0.0008
2
R = 0.8537
0.5
0.4
0.3
y = 0.0904x
2
R = 0.8537
0.2
0.1
0
0
1
2
3
4
5
Chlorine Dioxide consumed (mg/L)
6
ClO2 as a preoxidant:
sometimes lower Cl2 THMs/HAAs
ClO2
Cl2
Coag/flocc
Settling
Filter
Clearwell
160
Total THMs (μg/L)
140
No pretreatment
1 mg/L ClO2 pretreatment
„
Often an easy
change to make
„
Less effective for
HAAs than for
THMs
„
Does ClO2 affect
treatment?
120
100
80
60
40
20
0
Waterloo, ON
NY State #1
NY State #2
ClO2: impact on downstream treatment
„
Very little reported information; a lot of “stories”
…
ClO2 preoxidation make filters run longer
… ClO2 preoxidation improves coagulation
… Change Cl2 proxidation to ClO2, with Cl2 applied after filters
Result: turbidity formed after filters.
Why? Possible reducing conditions in filter releases Fe2+,
then Cl2 + Fe2+ → Fe3+ precipitate
„
AwwaRF 2005: pilot-scale tests of ClO2 preoxidation
…
settled water turbidity, total organic carbon, UV254nm
… filtered turbidity, particle counts, headloss, organic carbon
Example: settled water turbidity
3.00
Turbidity (NTU)
2.50
2.00
Raw Water
No ClO2
ClO2
1 mg/L ClO2
0.6
mg/L
ClO2
1.50
1.00
0.50
0.00
17-Aug
6-Sep
26-Sep
16-Oct
Sampling Dates
5-Nov
No statistically significant impact of ClO2 on settling tank or
filter performance for 1 water tested in California.
Fe/Mn control
„
Fe is easy to oxidize (air, Cl2, ClO2…)
„
Cl2 does not quickly oxidize Mn
(KMnO4 often used – but does not disinfect)
„
ClO2 oxidizes Mn within seconds
…
serves to disinfect at the same time
… reportedly unaffected by normal pH range
Fe/Mn control
Manganese stoichiometry:
2.45 mg ClO2 per mg Mn(II)
→ ClO2 reduced to ClO2- (not Cl-)
„
Natural organic matter interferes:
…
Competes for ClO2 reaction
… Forms Mn-complexes that are resistant to ClO2
„
Overcome by adding excess ClO2 (e.g. 200%-300%)
…
„
Limited effectiveness against some Mn-complexes
Must do tests to predict ClO2 effectiveness
Destruction of organic pollutants
Phenolics
…
Rapid oxidation
Pesticides
…
Very few studies on oxidation by ClO2: show poor results
Endocrine disrupting compounds and pharmaceuticals
…
Limited research; behaves similarly to chlorine
No strong reason to use ClO2 for destruction of specific
organic pollutants
Why use ClO2?
„
ClO2 is a very good disinfectant
…
European disinfection regulations make
ClO2 attractive
… North American disinfection regulations
make ClO2 less attractive
„
ClO2 may be best for multiple treatment
objectives
…
„
e.g. disinfection + taste and odour/Fe,Mn
Opportunity: learn more about taste and
odour control using ClO2
…
too many “stories”, too little science!