- Iron and Manganese Removal : - The standard
oxidation-reduction potential and reaction rate of ozone is such that it
can readily oxidize iron and manganese present in the raw water and in
water with low organic content.
While Iron and Manganese don't pose health problems, but water contaminated
by iron and manganese can stain water fixtures and clothing that is washed
with this water. Chlorine can also be used for oxidation of iron and
manganese, but significantly more chlorine is required versus ozone. This is
due to the fact that ozone has an oxidation potential 150 times greater than
chlorine. The use of chlorine can also result in the formation of THM's if
organic material is present in the water.
Ozone oxidizes Iron from Fe (II) to Fe (III). Fe (III) hydrolyzes to Fe
(OH3) which precipitates to a solid form which can be filtered. The
oxidation reaction requires 0.43 mg of ozone per mg of Fe (II). Excess ozone
can be used without negative effect. Fe oxidizes in the pH range of 6-9.
Ozone oxidizes Mn (II) to MnO2 (Mn IV) which is insoluble and can be
filtered out of water. The oxidation reaction requires 0.88 mg of ozone per
mg of Mn (II). Excess ozone beyond this ratio will form soluble Mn (VII),
permanganate. If oxidizable organic material is present in the water and
there is sufficient contact time, permanganate will be reduced back to MnO2
(Mn IV). Manganese oxidation is most effective around a pH of 8. In general,
when organic materials are present in water, more ozone will be required
than the amount shown above since ozone will also oxidize these materials.
- Color Removal : - Water appears colored when visible
radiation is absorbed by dissolved materials or when light is reflected
by suspended solids. Colored water is basically found at dye houses,
textile concerns, food and beverage processors, slaughter houses and
other industrial plants. Many industries have already started using
ozone, which is a more powerful and a safer substitute than chlorine for
color removal.
The best results are achieved when water has been treated to lower BOD, COD
and suspended solids (SS) values so that the ozone reaction is primarily for
color removal. Wastewater is Ozonated after it exists from chemical or/and
biological pretreatment at a dosage from 50 mg/lit to 150 mg/lit. At these
levels, color can be reduced by 90 to 98%. The dosage simultaneously reduces
chemical oxygen demand (COD) by about 45%. (Small increases of BOD, 5 to 9%,
may occur.)
Color removal efficiency depends on the ozone dosage, the feed's color
values, the wastewater type and temperature and water characteristics.
Temperature less than 30oC produce the optimum conditions for ozone
solubility. Ozone installations for this application represent a significant
capital cost, but offer lower operating expense than conventional treatment
using chemical coagulants. In addition to the cost of chemical, the
coagulant process generates sludge that requires disposal and further
expenditure. Generally, the investment for an ozone installation can be paid
back in 3 to 5 years, depending upon the size and other specifications.
- Taste and Odor Removal : - Odor and taste in drinking water
can be due to have several reasons. Odor and taste forming compounds can
be present in raw water, but they can also be formed during water
treatment. These compounds may derive from the decomposition of plant
matter, but normally they are a result of the activity of living
organisms present in the water. Inorganic compounds such as iron, copper
and zinc can also generate some taste. Another possibility is that the
chemical oxidation (chlorine treatment) leads to unpleasant taste and
odor.
Odor and taste forming compounds are often very resistant. This causes
elimination to be a very intensive process. For the elimination of taste and
odor, several processes can be appropriate such as oxidation, aeration,
granular active carbon (GAC) filtration or sand filtration. Usually, a
combination of these techniques is applied.
Ozone can oxidize compounds in a range of 20-90% (dependent on the type of
compound). Ozone is more effective for the oxidation of unsaturated
compounds. As was the case for the oxidation of pesticides, ozone combined
with hydrogen peroxide (AOP Process) is more effective than ozone alone.
Geosmin and 2-methyllisoborneol (MIB) are examples of resistant odorous
compounds, which are often present in the water. These are produced by algae
and have low odor and taste threshold. Nevertheless, ozone removes these
compounds very effectively.Generally, the most effective way to remove taste
and odor components appears to be a combination of pre-oxidation and
filtration, but ozone with sand filtration and GAC filtration is the most
efficient combination (upto 92% removal).
- Removal of Organic and Inorganic Matter : - All water
resources contain natural organic matters (NOM). Concentrations (usually
measured as dissolved organic carbon, DOC) differ from 0.2 to more than
10 mg. NOM creates direct problems, such as odor and taste in water, but
also indirect problems such as organic disinfection byproduct formation,
support of bacterial regrowth in the distribution system, etc. To
produce pure drinking water, the removal of NOM is a prior task in
modern water treatment.
Ozone, like any other oxidant, seldom achieves a complete mineralization of
NOM. Organic matter is partly oxidized and becoming more easily
biodegradable. This result in a higher amount of BDOC (Biodegradable DOC),
As a result ozone improves the removal process of NOM by a subsequent
filter, when it is used as a pre-oxidant. In a research, the effect of ozone
in combination with a biological filter is described. The combined treatment
resulted in a reduction of DOC by 40-60%. The removal is even greater when
ozone is used in combination with a coagulant. This is because ozone can
enhance the coagulation process. The combination coagulation-ozone-bio
filtration results in a DOC reduction by 64%. When only bio filtration was
applied, the reduction rate was only 13%. The optimal concentration to
remove organic matter by ozone was at an ozone dose of O3/DOC = 1 mg/mg.
Most inorganic matters can be eliminated by ozone quite fast. After
Ozonation, bio filtration is also required for inorganic matter. Namely,
oxidation forms unsoluble compounds that need to be removed during the next
water purification stage.
- Removal of Disinfection By-Products : -
- Disinfection byproducts (DBP) are mainly formed during the reaction
between organic material and a disinfectant. The reaction of chlorine
with matter can lead to the formation of chlorinated organic DBP's, such
as Trihalomethanes (THM). Ozone can also react with organic matter and
form DBP's. These are mainly organic disinfection byproducts, such as
aldehydes and ketones, which can be easily degraded in a bio filter
(90-100%). Generally, this organic ozone DBP's do not form risk of
violation of drinking water standards, when ozone is used as a
pre-oxidant.
To reduce the amount of DBP's at a conventional disinfection system
(disinfection by chlorine) it is important that potential to form DBP's
remains low. This is often expressed as DBP formation potential (DBPFP). The
potential to form DBP's can be reduced by the removal of (most of the) NOM,
for example by pre-oxidation with ozone (ozone-filtration). This combination
can lower the DBPFP by 70-80% when chlorine is used as a final disinfectant.
This concerns the DBPFP for THM's, HAA (haloacetic acids) and chloral
hydrate.
Ozone is a more effective disinfectant than chlorine, chloramines and even
chlorine dioxide. An ozone dose of 0.4 mg for 4 minutes is usually effective
for pre-tested water. Several studies proved that ozone, unlike chlorine
products, can deactivate resistant micro-organisms.
- Removal of Pesticides : - Micro-pollutants such as pesticides
may occur in surface water, but also increasingly in groundwater.
Drinking water standards for pesticides in the European Union are
strict.
Several surveys show that ozone can be very effective for the oxidation of
several pesticides. It is proved that three barriers
(storage-ozonation-granular active carbon filter) are effective and safe
enough for the removal of pesticides. From 23 tested pesticides, 90% were
degraded sufficiently (80% degradation). For highly resistant pesticides, a
higher dosage of ozone is advised, or ozone combined with hydrogen peroxide.
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