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COMPARISON OF ENERGY REQUIREMENTS OF CONVENTIONAL OZONATION AND THE AOP O3/H2O2 FOR TRANSFORMATION OF TARGET MICROPOLLUTANTS IN DIVERSE WATER MATRICES

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Abstract

We used conventional ozonation and the AOP O3/H2O2 to estimate the required energy to achieve the desired pre-defined level of oxidation of target organic micropollutants by hydroxyl radicals. We used the probe compound para-chloro benzoic acid (pCBA) to assess the fraction ofOH available for oxidation of micropollutants. pCBA reacts slowly with O3 but reacts very fast withOH (kOH,pCBA = 5 x 10 9 M-1s-1). For our investigations we used three lake waters: Lake Zürich (ZH) and Lake Greifensee water (GF) from Switzerland and Lake Jonsvatnet water (NW) from Norway. The DOC concentrations were 1.4, 3.5 and 3 mg/L for ZH, GF and NW waters respectively and alkalinity was 2.7, 3.4 and 0.3 mM correspondingly. The results showed that for 90% pCBA transformation, the O3 consumption was roughly 2 mg/L (40 µM) for ZH and NW waters and approximately 4 mg/L (80 µM) for GF water. Bromate formation in ZH water was much higher than in GF and NW waters and exceeded the EU drinking water standard of 10 µg/L for all examined O3 concentrations, for initial bromide concentration 80 µg/L. The energy requirement to achieve 90% transformation was in the range 6.4 - 12.8 Wh/person/day for conventional ozonation of ZH-water and slightly higher for the use of AOP O3/H2O2. The use of O3/H2O2 however, reduces the required contact times and bromate formation.
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COMPARISON OF ENERGY REQUIREMENTS OF
CONVENTIONAL OZONATION AND THE AOP O3/H2O2 FOR
TRANSFORMATION OF TARGET MICROPOLLUTANTS IN
DIVERSE WATER MATRICES
Ioannis A. Katsoyiannis and Urs von Gunten
Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department of Water
Resources and Drinking Water, Ueberlandstrasse 133, 8600 Dübendorf, Switzerland. Tel:
+41 44 8235488, Email: ioannis.katsoyiannis@eawag.ch
ABSTRACT
We used conventional ozonation and the AOP O3/H2O2 to estimate the required energy to
achieve the desired pre-defined level of oxidation of target organic micropollutants by
hydroxyl radicals. We used the probe compound para-chloro benzoic acid (pCBA) to
assess the fraction of OH available for oxidation of micropollutants. pCBA reacts slowly
with O3 but reacts very fast with OH (kOH,pCBA = 5 x 109 M-1s-1). For our investigations
we used three lake waters: Lake Zürich (ZH) and Lake Greifensee water (GF) from
Switzerland and Lake Jonsvatnet water (NW) from Norway. The DOC concentrations
were 1.4, 3.5 and 3 mg/L for ZH, GF and NW waters respectively and alkalinity was 2.7,
3.4 and 0.3 mM correspondingly. The results showed that for 90% pCBA transformation,
the O3 consumption was roughly 2 mg/L (40 µM) for ZH and NW waters and
approximately 4 mg/L (80 µM) for GF water. Bromate formation in ZH water was much
higher than in GF and NW waters and exceeded the EU drinking water standard of 10
µg/L for all examined O3 concentrations, for initial bromide concentration 80 µg/L. The
energy requirement to achieve 90% transformation was in the range 6.4 – 12.8
Wh/person/day for conventional ozonation of ZH-water and slightly higher for the use of
AOP O3/H2O2. The use of O3/H2O2 however, reduces the required contact times and
bromate formation.
1. INTRODUCTION
Ozone (O3) is used widely in water treatment as disinfectant and oxidant.
Transformation of organic compounds with O3 occurs either via direct reaction with O3
or with hydroxyl radicals (OH), which result from ozone decay in water. O3 reacts
selectively with organic compounds and kinetic constants vary over 10 orders of
magnitude, whereas OH is an unselective oxidant and its reaction with the majority of
organic compounds is nearly diffusion controlled. The enhanced formation of OH, i.e.,
by addition of H2O2, comprises an Advanced Oxidation Process (AOP). Thus the
combined use of O3/H2O2 can accelerate ozone conversion to OH and can reduce the
reaction time required for micropollutant transformation [1].
In general, AOPs are more energy intensive than conventional ozonation.
However, because not all micropollutants are susceptible to direct ozone oxidation (in the
2
time scale of interest in water treatment) and moreover extensive ozonation of bromide
containing waters leads to bromate formation, AOPs can serve as a feasible alternative to
ozonation [2]. The efficiency of ozonation and AOPs is greatly dependent on the water
matrix composition and particularly on the concentrations of NOM and alkalinity, which
consume OH or influence the ozone decomposition [1]. There has been little research in
comparing the energy requirements of ozonation and AOPs to transform target
micropollutants. In a recent study by Rosenfeldt and co-workers, some of these issues
were addressed; however the issue of bromate formation was not taken into account [3].
Furthermore, in this study we used several water matrices, covering a broader range of
scavenging rates (the scavenging of OH occurring by the presence of NOM and
alkalinity), thus connected energy requirement not only to ozone dosage but to the
scavenging of hydroxyl radicals.
2. EXPERIMENTAL PART
2.1. Experimental setup
Ozonation experiments were performed in a 500 mL batch reactor. The solutions
were prepared as follows: firstly we filled the reactor with the desired water matrix,
adjusted the temperature at 20 oC and buffered with 5 mM borate and pH adjustment was
following, using 1M H2SO4 or NaOH. Next, pCBA was spiked to a final concentration of
1 µM and a sample was taken at time zero. Ozone was afterwards injected under vigorous
stirring from a stock solution of approximately 1.5 mM to achieve the desired initial O3
concentration. Samples were taken after 24 hours to measure pCBA transformation after
complete ozone consumption. The experiments with O3/H2O2 were performed exactly as
the ozonation experiments with the addition of H2O2 (1:2 molar basis H2O2:O3) prior to
the addition of ozone in the solution.
2.2 Analytical methods
pCBA concentrations were measured by HPLC with UV/vis detection. An eluent
consisting of 50%:50% methanol:H2O was used for pCBA measurement. Bromate was
measured with Ion Chromatography and UV detection after post column reaction [4].
3. RESULTS AND DISCUSSION
3.1. Calculation of scavenging rates of water matrices used in this study
Three lake waters were used in the present study to assess the effect of water
composition on the efficiency of pCBA transformation, thus on the fraction of hydroxyl
radicals which are available for transformation of target compounds. The basic
constituents of water affecting the hydroxyl radical availability are the concentration of
organic matter and the alkalinity. This can be estimated by calculating the scavenging
rate of the system from the concentration of DOC and alkalinity and the respective kOH
values (kOH, DOC=2.5 x 104 L mg-1 s-1, kOH, HCO3-=8.5 x 106 M-1 s-1, kOH, CO32- = 3.9 x 108
M-1s-1 ). Increasing the scavenging rate of the water source, reduces the fraction of
hydroxyl radicals, available for oxidation of target micropollutants. Table 1 shows the
3
concentration of DOC and alkalinity and the calculated scavenging rates of the examined
surface waters.
Table 1. Water composition and calculation of scavenging rates of the lake waters used
in this study
Lake Zürich
water (ZH)
Lake Greifensee
water (GF)
Lake Jonsvatnet
water (NW)
DOC (mg/L)
1.4
3.5
3
Alkalinity
(mM as HCO3)
2.7
3.4
0.3
Scavenging rate (s-1)
5.7 x 104
11.5 x 104
7.75 x 104
3.2. pCBA transformation by conventional ozonation as a function of ozone dosage
in different water matrices and comparison with the AOP O3/H2O2
The extent of pCBA transformation as a function of ozone dosage and the water
matrix is illustrated in Figure 1. It shows an increase of pCBA transformation with
increasing O3 concentrations. This is ascribed to the resulting higher OH-radical exposure.
For the same ozone dosages, the extent of pCBA transformation depends upon the
scavenging rate of the water matrix. Apparently, the higher the scavenging rate of the
water matrix, the higher fraction of hydroxyl radicals will be scavenged resulting in lower
pCBA transformation. Consequently, in waters with higher scavenging rates, like the GF
water, higher ozone dosages will be required to achieve a pre-defined micro-pollutant
transformation level, thus higher energy requirements for the same degree of oxidation.
The use of the AOP O3/H2O2 mostly affects the kinetics of pCBA transformation
and the kinetics of ozone transformation to hydroxyl radicals (Figure 2) and not the
extent of transformation. The additional use of H2O2 increases the energy requirements
and the treatment costs, but substantially reduces the required contact times in the reactor
and bromate formation, in bromide containing waters, as shown in Figure 3. For different
ozone dosages the use of AOP can reduce bromate formation up to 70%. However, the
use of AOP eliminates the disinfection capability of ozonation. Therefore, to ensure
disinfection and reduce bromate formation, H2O2 is usually spiked in the reactor after
ozone addition. In this case disinfection is ensured and bromate formation is reduced by
30% (data not shown), which in some cases might be sufficient to keep bromate
concentration below 10 µg/L. This depends strongly on the initial bromide concentration
and ozone dosages. Ozonation of GF and NW waters resulted in reduced bromate
formation (data not shown), most likely because of reduced ozone exposure, due to
increased rates of ozone decay.
4
0
20
40
60
80
100
120
20 40 80
Ozone dosage (µM )
Bromate (µg/L)
Bromate forma tion during Ozona tion
Bromate forma tion during AOP
0
20
40
60
80
100
120
20 40 80
Ozone dosage (µM)
% pCBA transformation
ZH w a te r
GF wa ter
NW w a ter
κ
Ο3
= 2.2 x 10
-3
s
-1
κ
Ο3
= 35 x 10
-3
s
-1
-3
-2.5
-2
-1.5
-1
-0.5
0
02468101214
Time (min)
ln[(O
3
/O
3
)ο]
O3 decay (conventional Ozonation)
O3 deca y (AOP)
Figure 1. pCBA
transformation by
conventional ozonation as a
function of ozone dosage in
waters with different
composition and scavenging
rates (pH = 8, T = 20oC)
Figure 2: Rate of O3
depletion using
conventional ozonation and
the AOP O3/H2O2 in Lake
Zürich water, 20oC, pH 8
Figure 3. Bromate
formation by conventional
ozonation and AOP
O3/H2O2 in lake Zürich
water (pH 8, T= 20 oC,
initial bromide = 80 µg/L)
5
3.3. Energy calculations and concluding remarks
Assuming that for the production of ozone, 10 – 20 kWh/kg O3 are consumed and
of H2O2 10 kWh/kg [3] and that for drinking water treatment a 90% removal of
contaminants would be desirable, we can calculate the amount of energy required for
complete elimination of pCBA and correlate it to bromate formation. For ZH water at pH
8, a rough estimate would be a 6.4 – 12.8 Wh/person/day with conventional ozonation
and 7.5 – 14 Wh/person/day with AOP (assuming 160 L consumption of drinking
water/person/day). However, the use of O3/H2O2 reduces bromate formation by 70% but
eliminates the disinfection capacity of ozone. Therefore, use of H2O2 after ozone addition
reduces bromate formation by 30% and ensures disinfection. The treatment of the
Norwegian water would require the same energy for oxidation as the ZH-water but
bromate formation would be much lower, because of lower ozone exposure. Greifensee
water would require more energy than ZH water to achieve the same oxidation efficiency.
Compounds which are susceptible to direct oxidation with ozone, such as olefins,
activated aromatics and deprotonated amines, will require 10 times less energy for 90%
transformation. The use of UV/H2O2 as an alternative to O3/H2O2 is expected to be
roughly 10 times more energy intensive than ozonation but circumvents bromate
formation and ensures disinfection. This would be a viable solution for waters with high
concentrations of bromide and scavengers and organic micropollutants, not susceptible to
direct ozone oxidation.
4. REFERENCES
1. von Gunten, U., 2003. Ozonation of drinking water: part I: Oxidation kinetics and
product formation. Water Research, 37, 1443-1467.
2. von Gunten, U., 2003. Ozonation of drinking water: Part II. Disinfection and by-
product formation in presence of bromide, iodide or chlorine. Water Research, 37, 1469-
1487.
3. Rosenfeldt, E.J., Linden, K., Canonica, S., von Gunten U., 2006. Comparison of the
efficiency of OH-radical formation and the advanced oxidation processes O3/H2O2 and
UV/H2O2. Water Research, 40, 3375-3384.
4. Sahli, E., von Gunten, U., 1999. Simultaneous determination of bromide, bromate and
nitrite in low µg l-1 levels by ion chromatography without sample pretreatment. Water
Research, 33, 3229-3244.
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