About the author: Rip G. Rice, Ph.D., is the owner of RICE International Consulting Enterprises in Ashton, Md. He is the author of many papers regarding ozone and ultraviolet technologies, as well as a speaker on the subjects.
Rip G. Rice, Ph.D., RICE International Consulting Enterprises
undefinedOzone technology developments have opened new applications
for these established water treatment technologies. Driving these changes has
been the identification of new, more disinfection-resistant microorganisms such
as Giardia and Cryptosporidium cysts and governmental regulations designed to
protect the public health from the hazards of ingestion of these
microorganisms. Additionally, the desire to prevent or minimize the formation
of halogenated disinfection byproducts formed during chlorination has stimulated new interest in the use of ozone.
Combinations of ozone with hydrogen peroxide and/or ultraviolet (UV) radiation
can destroy many contaminants present in ground water.
Early on, ozone’s uses for water treatment began with
disinfection for potable water plants. Other (oxidative) applications for ozone
in drinking water have developed including the oxidation of iron, manganese,
sulfide, cyanide and nitrite in ground waters as well as man-made organic
compounds such as phenols and some pesticides, humics and organics discharged
in industrial waste waters.
In recent years, the use of small amounts of ozone for
coagulation assistance (microflocculation) and of slightly larger dosages for
partial oxidation of organic contaminants to increase their biodegradability
has developed. Following ozonation with filtration through media such as
granular activated carbon (GAC) allow development of a biomass on the GAC that,
in turn, converts significant amounts (up to 40–45 percent) of the
partially oxidized dissolved organic materials into carbon dioxide and water.
This process, known as “ozone-biofiltration,” only has developed
within the past 30 years.1,2,3 After ozone-biofiltration, smaller amounts of
chlorine or chloramine usually are added to provide a stable residual because
of the removal of chlorine-demanding organic materials.
Byproducts of Ozonation
Organic oxidation products. Most organic byproducts of ozonation contain more oxygen than did
their parent compounds. As such, they usually are less toxic and more easily
biodegradable than the starting materials. Generally, organic byproducts formed
by any strong oxidizing agent added to water (chlorine, chlorine dioxide,
ozone) include organic acids, aldehydes, ketones, alcohols, aldehyde-acids,
keto-acids, alcohol-acids, etc. Advantages of ozone for oxidation of organics
over chlorine are that ozone is a stronger oxidizing agent and, therefore,
forms higher quantities of the oxidized organics than does chlorine—and
these oxidized organics readily are “mineralized” (e.g., converted
to carbon dioxide and water during biofiltration). When residual chlorine is
present, the biomass cannot form effectively and, therefore, no mineralization
of chlorine-oxidized organics can take place.
Although the formation of organic oxidation byproducts from
natural humic-type precursors during ozonation might appear to be something of
a negative factor for ozone, it actually is an advantage. The oxidized organics
produced during ozonation of natural organics have simpler molecular structures
than do the parent humics. These simpler organics readily are mineralized
(converted to carbon dioxide and water) biochemically during the passage
through a biofilter. Once these oxidation-produced simple organic compounds
have been removed from solution, the chlorine demand of the biofiltered water
is lower than that of the water prior to ozonation and biofiltration.
Bromate anion. When
bromide ion is present in water to be ozonated, hypobromite ion (–OBr)
and hypobromous acid (HOBr) are formed (similarly with chlorine). HOBr is a
brominating agent capable of producing brominated organics of various types.
During chlorination of waters containing bromide ion, mixed bromo-chloro-organics
are produced (such as two of the trihalomethanes and several of the haloacetic
acids) as well as bromo-organics not containing chlorine (such as bromoform,
mono-, di- and tribromoacetic acids). During ozonation of waters containing bromide ion, the formed HOBr also can produce brominated organics such as bromoform and mono-, di- and possibly tribromoacetic acids. To date, however, only traces of bromoform have been identified in some ozonated water containing high levels of bromide ion.
In the presence of ozone, –OBr can be further oxidized
to bromate ion (BrO3–, which has been determined to be carcinogenic to
certain laboratory test animals. Consequently, BrO3– has been listed by
the U.S. Environmental Protection Agency (EPA) as a probable human carcinogen,
and a maximum contaminant level (MCL) of 10 mg/L has been established for
BrO3– in the Surface Water Treatment Rule (SWTR). Many source waters for
water treatment plants contain bromide ion, and the more bromide ion contained
therein, the more bromate ion can be produced during ozonation, particularly
when the pH is greater than 6.5. Consequently, it is important for water
treatment specialists to understand the chemistry of bromate formation and the
various chemical techniques for minimizing its formation.
BrO3– can be produced during ozonation when raw waters
contain bromide ion and under certain conditions of pH and ozone-demanding
materials. Ozone (or chlorine for that matter) quickly oxidizes bromide ion to
a mixture of HOBr and –OBr. However, ozonation is capable of slowly
oxidizing the –OBr (not HOBr) further to bromate ion provided that the pH
is above 6.5. At pH 6.5, no –OBr can exist, and ozone-formation of
bromate ion drastically is lowered or even eliminated. If bromate formation is
a potential problem in treating potable water supplies with ozone, one
technique for minimizing or eliminating bromate formation is to conduct the
ozonation at pH 6.5 or less, then adjust pH up at a later stage of treatment.4
Another technique to minimize bromate formation during
ozonation is to adjust the ozonation conditions to minimize the levels of
residual ozone. In this manner, other water contaminants tend to out-compete
the –OBr for the ozone. Still a third technique to minimize ozone production
of bromate ion is to add a trace of ammonia to the water prior to ozonation.
When HOBr is produced during ozonation, it will react immediately with the
added ammonia, producing monobromamine, which is much more slowly oxidized by
ozone to yield bromide ion again.5 Figure 1 summarizes the mechanisms of
formation of bromate ion and methods to minimize its formation during
ozonation.
Recent Developments in Potable Water Treatment
Until the passage of the Safe Drinking Water Act (SDWA)
Amendments of 1986, the use of ozone for drinking water treatment in the United
States was confined mainly to control tastes and odors. From the 1986 SDWA
Amendments came a requirement from the EPA in the SWTR to control
“new” microorganisms in raw water supplies (e.g., Giardia cysts and
enteric viruses). Although these organisms can be controlled by chlorination,
the increased quantities of chlorine generate increased amounts of halogenated
disinfection byproducts. UV disinfection was ignored in the SWTR because of a publication
indicating that UV was ineffective against Giardia cysts when excystation was
the end point.6 As a result, interest in the so-called “alternative
disinfectants” was stimulated, with particular attention focused on ozone
and chlorine dioxide.
The number of U.S. potable water treatment plants using
ozone rose starting in the late 1980s. Of interest is that of the 332 total
water systems using ozone, some 194 produce less than 1 mgd, and 120 of the 194
small plants serve fewer than 600 people. (See Figure 1.)
In the 1986 SDWA Amendments and the SWTR, the EPA also
introduced the “Ct” concept to U.S. drinking water utilities for
ensuring that any disinfectant used for inactivating Giardia cysts and enteric
viruses actually was doing its job. In this concept, the term “C”
refers to the concentration of disinfectant in aqueous solution (mg/L) and
“t” is the time (in minutes) the disinfectant is in contact with
the aqueous solution. By adopting the Ct concept, water treatment plants
operators can control disinfection online rather than wait for after-the-fact
microorganism counts.
With ozone, the Ct value for inactivation of three-logs of
Giardia cysts at 0.5° C is about 3 mg-min/L decreasing to about 0.5
mg-min/L at 25° C. Ct values for inactivating enteric viruses are less than
those for inactivating corresponding numbers of logs of Giardia cysts. Ten
years later, the 1996 SDWA Amendments required Cryptosporidium parvum oocysts
to be disinfected in addition to those microorganisms listed 10 years earlier.
However, at the time only ozone and chlorine dioxide were known to inactivate
Cryptosporidium. Since Cryptosporidium oocysts are considerably more resistant
to any chemical disinfectant than are Giardia cysts, considerably higher Ct
values are required. For example, the inactivation of two-logs of
Cryptosporidium at less than 5° C is about 20–30 mg-min/L, decreasing
to about 3–7 mg-min/L at 25° C. When inactivating Cryptosporidium
parvum oocysts, considerably higher concentrations of ozone and/or contact
times are required than for the inactivation of Giardia cysts or enteric
viruses. This means the generation of higher concentrations of organic (and
sometimes inorganic) oxidation byproducts is required.
On the negative side, the bromate issue continues to act as
a rein to the otherwise robust expansion of the installation of ozone. The
problem is two-fold—more ozone is required to inactivate Cryptosporidium
parvum than to inactivate Giardia cysts and viruses, yet more ozone usually
produces more bromate ion. If the amount of ozone added to control
Cryptosporidium produces sufficient bromate ion to exceed the current MCL of 10
mg/L, then the use of ozone becomes infeasible.
Advanced Oxidation
The term “advanced oxidation” was created to
describe several processes by which hydroxyl-free radicals are generated and
used for the oxidation of otherwise refractory organics in water.
The good news is that ozone is being installed for potable
water treatment in an ever-increasing number of plants. In the United States
alone (as of January 2000), ozone had been installed in approximately 194 small
systems (less than 1 mgd). In addition, some 363 ozone systems were known to
have been installed in residences and in small businesses as of January 2000.7
The ability to inactivate Cryptosporidium parvum oocysts also is good news for
ozone, since chlorine is ineffective for this purpose.
On the other hand, the bromate MCL of 10 mg/L discourages
the use of ozone for Cryptosporidium inactivation, particularly in waters
containing significant quantities of bromide ion. The Ct values for ozone
inactivation of Cryptosporidium are some 5–10 times higher than the Ct
for ozone inactivation of Giardia lamblia and enteric viruses.For ground water
systems contaminated with such refractory organics as TCE, PCE and probably
MTBE, the coupling of ozone and UV offers considerable promise to provide both
oxidation and disinfection.
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