MODULE 5
OTHER CHEMICAL DISINFECTANTS
OZONE AND CHLORINE DIOXIDE
OZONE GAS (O3)
Physical properties
 Ozone is a toxic, bluish, unstable, potentially explosive gas which
is also an irritant to humans at low concentrations
 Its instability means it must be generated on site
 Ozone leak detectors should be installed to give audible/visible
warnings and shut down on site generator
 Highly corrosive in the presence of moisture
As a disinfectant
 More effective bactericide and virucide than chlorine
 Most effective chemical disinfectant against Cryptosporidium
 Ozone can only be used as a primary disinfectant due to its rapid
decay rate, which does not maintain a persistent residual
THE USES OF OZONE
IN WATER TREATMENT
 Oxidation of iron and manganese
 Enhancing flocculation
 Improving removal of algae
 The oxidation of colloidal organic compounds for colour
removal and the reduction in levels of organic carbon as
DBP precursors ahead of subsequent chlorination
 The oxidation of trace organic compounds, including other
micropollutants compounds that produce taste and odour,
phenolic compounds and some pesticides
 Biological stabilisation (in conjunction with GAC)
THE EFFECT OF WQPs ON REMOVAL
OF DBP PRECURSORS USING OZONE
 Oxidation of DBP precursors depends on ozone dose, pH,
alkalinity and the type of organic material in the water
 Below a pH of 7.5, DBP precursors is effectively removed by
ozone
 Higher alkalinities also help reduce THM formation potential
 At a pH of 7 and moderate alkalinity, reductions of 3-20% THM
formation potential at ozone doses ranging from 0.2-1.6 mg O3
/mg of TOC
THE DISINFECTION PERFORMANCE
OF OZONE
• Dissolved ozone can react directly or indirectly (with
formed hydroxyl radicals) with the water into which it is
dosed
• Ozone requires less contact time and lower dose rates than
NaOCl, ClO2, and chloramines
• Instability and reactivity means that ozone is unable to
provide an enduring disinfection residual in distribution
• Stability of ozone decreases with increasing pH and
temperature forming hydroxyl radicals which tend to have
higher reaction rates
THE DISINFECTION PERFORMANCE
OF OZONE – VIRUS INACTIVATION
Ct values (mg.min/l) for inactivation of viruses, pH 6-9
Log
Inactivation
Temperature, oC
≤1
5
10
15
20
2.0 (99%)
0.90
0.60
0.50
0.30
0.25
3.0 (99.9%)
1.40
0.90
0.80
0.50
0.40
4.0 (99.99%)
1.80
1.20
1.00
0.60
0.50
THE DISINFECTION PERFORMANCE
OF OZONE – CRYPTOSPORIDIUM
Ct values (mg.min/l) for inactivation of Cryptosporidium
Log
Inactivation
Temperature, oC
≤1
5
10
15
20
0.5
12
7.9
4.9
3.1
2.0
1.0
24
16
9.9
6.2
3.9
2.0
48
32
20
12
7.8
3.0
72
47
30
19
12
REDUCTION IN ORGANIC BY-PRODUCT
FORMATION FOLLOWING OZONATION
 Regulated halogenated organic by-products such as
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
trihalomethanes (THMs) are not formed by ozonation
Ozonation can have the additional benefit of reducing overall THM
formation arising from chlorine dosing as secondary disinfectant
Ozone is known to react with natural organic matter (NOM) and
may produce a range of by-products
The action of ozone on organic matter generally increases the
biodegradable fraction of TOC
 Assimilable organic carbon (AOC)
 Biodegradable dissolved organic carbon (BDOC)
AOC and BDOC removal is most often achieved by the use of
granular activated carbon (GAC) filtration downstream of
ozonation and/or the achievement of microbiological activity in
the downstream filter
BROMATE BY-PRODUCT FORMATION
• Ozone oxidises the bromide ion (Br-) to bromate (BrO3-)
• The current regulatory maximum concentration of 10mg/l
• The extent of bromate formation
–
–
–
–
–
–
–
increases with increasing bromide ion concentration
increases with increasing pH, up to pH 8.5
increases with increasing alkalinity
increases with increasing Ct
increases as the ratio of ozone dose to DOC increases
increases with increasing temperature
declines as ammonia concentration increases
THE RATIO OF OZONE DOSE TO DOC (MG/L) IS BEST KEPT BELOW 0.5
TYPICAL LAYOUT OF AIR FED
OZONATION EQUIPMENT
MONITORING AND VERIFICATION OF
OZONATION DISINFECTION
 Process verification of primary chemical disinfection systems
is always based on
 the measurement of Ct values for water entering the distribution
system to verify the achievement the required log inactivation of the
targeted pathogens,
 limiting the levels of inorganic by products in drinking water supplied
to consumers
 Determining the actual Ct achieved in a multiple chamber
ozone contactor is not straightforward
 Decay of C within both bubble diffuse and reaction contactors from
inlet to outlet is likely to be non-linear
 When used with bulk delivered hypochlorite there is potential
for bromate formation by both disinfection systems
ADVANTAGES OF OZONE
DISINFECTION
 Very effective disinfectant for bacteria, viruses and Giardia;
 More effective against Cryptosporidium than other chemical
disinfectants;
 Less sensitive to pH variation as a disinfectant than chlorine;
 Ozone does not directly produce THMs or HAAs;
 Other treatment benefits, such as oxidation of Fe and Mn,
pesticide removal, may occur in parallel
LIMITATIONS OF OZONE
DISINFECTION
 No lasting disinfectant residual into distribution;
 Ozone decay occurs particularly at high pH levels
 Capital cost of ozonation equipment is high compared to
other chemical disinfectants
 More expensive to operate compared to other disinfectants as
it requires on-site generation and high energy input;
 Complex plant for which a high skilled maintenance input is
required;
 Post process GAC filtration is usually required to remove the
consequent increased levels of AOC/BDOC formed by the
oxidation process – reactive THM precursors
 Verification of Ct is not straightforward
CHLORINE DIOXIDE PRODUCTION
Produced on-site as needed from:
 Acid/sodium chlorite
 Chlorine gas/sodium chlorite solution
 Hypochlorite/sodium chlorite solution
 Chlorine gas/solid sodium chlorite
More recently from:
 Sodium chlorate/hydrogen peroxide/sulphuric acid
(Chlorate cheaper than chlorite)
CT FOR 2 LOG (99%) INACTIVATION
(USEPA/WHO)
Temperature
1C
5C
10C
15C
20C
Cryptosporidium
1220
858
553
357
232
Giardia
17
15
13
Viruses
5.6
4.2
2.8
Bacterial inactivation needs lower Ct than viruses (e.g. < 1 mg.min/l)
Generally at least as good as chlorine, probably better at higher pH
ClO2 BY-PRODUCTS - CHLORITE
 Main by-product is usually chlorite, produced from
oxidation of organics/Fe/Mn:
ClO2 + e- → ClO2 Up to 70% of ClO2 can typically end up as chlorite,
particularly at higher pH (>9)
 Unreacted chlorite may also be present depending on
the production process
 Chlorite can be removed using GAC or Fe2+ but
practicality of introducing another process is
questionable
ClO2 BY-PRODUCTS - CHLORATE
 Chlorate (ClO3-) can arise from the production
process, depending on the conditions
 If subsequent oxidation (ozone) is used after ClO2
 Chlorate can also form from chlorite:
ClO2- + 2OH- → ClO3- + H2O + 2e-
ClO2 BY-PRODUCTS - OTHERS
 Normally no significant concerns regarding by-product
formation
 Organochlorines if excess chlorine from production
process
 Oxidation of bromide to bromine, and increased
brominated organochlorines?
ClO2 BY-PRODUCTS - REGULATION
 WHO guidelines of 0.7 mg/l for chlorate and chlorite
 No EU Directive standard
 UK limit (only if chlorine dioxide is used) of
0.5 mg/l for ClO2 + ClO2- + ClO3 USEPA MCL 1 mg/l chlorite (where ClO2 used) and
maximum ClO2 dose of 1.4 mg/l
ADVANTAGES AND LIMITATIONS
ADVANTAGES
 Generally an effective disinfectant – as good as chlorine
particularly at higher pH and better in some cases (Giardia)
 May reduce THM precursors
 Can provide a residual in distribution
LIMITATIONS
 Dose limitations from concern over chlorite
 Not effective for Cryptosporidium because of Ct
requirements and dose limitations
COPPER SILVER IONIZATION
 Proprietary systems use electrolytic ion generators with sacrificial

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electrodes contained within a chamber to control the concentrations of the
Cu and Ag dissolved ions
Scale build-up and cleanliness of the electrodes important
Copper Silver Ionisation systems do not result in halogenated organic byproducts such as trihalomethanes (THMs)
pH of the water should be less than 7.6
USEPA have a maximum concentration for silver of 0.1 mg/l while the WHO
states that available data is inadequate to permit derivation of a healthbased guideline value for silver – NOAEL limit for argyia
WHO - 3rd Edition of Guidelines for Drinking Water Quality (2008)
“Silver is
sometimes promoted as a disinfectant, but its efficacy is uncertain, and it requires
lengthy contact periods. It is not recommended for treating contaminated drinkingwater”.
Inadequate scientific data available to verify the process