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Removal of organic substances from water by ozone treatment followed by biological activated carbon treatment
~
Pergamon
Wa/. Sci. Teelt. Vol. 3S, No.7, pp. 171-178, 1997. tC> 19971AWQ. Published by Elsevier Science Ltd
PH: S0273- I 223(97)00 I 28-5
Pnnted in Great Bntain 0273-1223197 SINlO + 0'00
REMOVAL OF ORGANIC SUBSTANCES FROM WATER BY OZONE TREATMENT FOLLOWED BY BIOLOGICAL ACTIVATED CARBON TREATMENT Yasushi Takeuchi*, Kazuhiro Mochidzuki*, Noriyuki Matsunobu*, Ryozo Kojima**, Hiroshi Motohashi** and Syunichi Yoshimoto** • Department of Industrial Chemistry. Meiji University. Higashi-mita. Tama-ku. Kawasaki 214-71. Japan •• Municipal Water Plant Engineering Department. EBARA Corporation. 1-6-7 Konan. Minato-ku. Tokyo 108. Japan
ABSTRACf As an advanced water treatment, a combination of ozonation and biological activated carbon (BAC, hereafter) treatment is being applied to purify raw water for municipal use in some cases. The authors examined effects of ozonation on water quality in a batch system, using water samples containing organic substances fractionated to several molecular weight ranges. Also. a flow test of laboratory-scale was performed to study on the capability of the treatment in terms of removal efficiency of the dissolved organic substances, e.g., fumic substances, which preoxidized with ozone. As a result, the changes in equilibrium adsorption and in the biodegradability of organic substances dissolved in water before and after oxidation with ozone were made clear. © 1997 IA WQ. Published by Elsevier Science Ltd
KEYWORDS Adsorption; advanced treatment; biodegradation; biological activated carbon; drinking water purification; fumic substances; molecular weight fractionation; ozonation.
INfRODUCTION In these days, removal of dissolved organic pollutants in water, e.g., trihalomethanes and odorous substances, is attracting greater concerns to upgrade qualities of municipal water. Since these substances as above bring about unpleasantness other than negative effect on our health, municipal water treatment plants need to equip s(}ocalled advanced treatment processes to supply safe water. Activated carbon adsorption has been used to remove soluble organic contaminants from water, and applied as a final process in municipal water treatment. When granular activated carbon particles were used in a fixed bed, it was found that certain kinds of microorganisms grew on the surfaces of the carbon particles and their activities lengthened the life of carbon. Now, the activated carbon treatment with biodegradation is called "biological activated carbon (BAC) treatment" and is being used widely for various treatment processes to purify drinking water (e.g. Fujiki and Anraku, 1993) and to remove pollutants from industrial wastes (e.g. Takeuchi et. aI., 1996), 171
172
Y. TAKEUCHI etal.
because it brings about lower cost and higher performance than mere activated carbon adsorption treatment processes. It is said that BAC treatment combined with ozonation is especially effective to clean up raw water because of effects of preoxidation with ozone. Recently, it has been applied at several municipal water treatment plants, as one of advanced treatment processes to eliminate dosage of chlorine for disinfection, to remove odorous substances, to avoid formation of trihalomethanes and to make chromaticity lower (e.g. Amy et. aI., 1991; Moniwa, 1995; Okada, 1995). However, the mechanisms of removal in the treatment are more complicated than ordinary water treatment processes, since three different processes, i.e., ozonation, adsorption and biodegradation, occur simultaneously, and there remain some problems to be discussed in the treatment to make clear the removal mechanisms of organic substances and to obtain more detailed engineering data. This paper deals with results of ozone treatment of raw municipal water followed by BAC treatment using a batch tank reactor. Water samples used were a kind of leaf-mold extract diluted with tap water and a kind of raw water taken from an existing municipal water treatment plant, respectively. The behavior of equilibrium adsorption and biodegradation subjected to oxidation with ozone was examined, focusing on the effectiveness of ozonation for removal of organic substances fractionated to several average molecular weight ranges. Funhermore, a flow test was performed to investigate the effectiveness of ozonation on the BAC treatment placed after the ozone reactor. EXPERIMENTAL Physical properties of activated carbon samples A commercial coconut shell base granular activated carbon (LG-20; EBARA Co., Ltd. Japan) was used in this study, after following pretreatment. The panicles were sieved to 16/20 standard mesh fraction and washed with diluted aqueous solution of chloric acid. Then it was dried in nitrogen gas stream for 2 hours at 398 K. Physical propenies of the sample were determined to be as follows; true density was 2.00xl()3 kg/m 3,'BET specific surface area was 1.27xl06 m 2Jkg, total pore volume was 2.7Sxl0 4 m 3Jkg and apparent density was 7.Slx102 kg/m 3, respectively.
Qualities of water samples As a model of raw municipal water, a leaf-mold extract (hereafter, described as LME) prepared in the following manner was used, i.e., 2 kg of a kind of commercial leaf-mold was put into a tank filled with 0.02 m 3 of tap water, whose dissolved chlorine had been removed by keeping the water in air for several days, and components of leaf mold, e.g. fumic acids, were extracted for 24 hours with agitating several minutes in every 4 hours. A kind of actual raw water from a cenain municipal water treatment plant. which is originally one of small-scale secondary water filtration factories using underground water and has only three processes, i.e. ozonation, activated carbon adsorption and disinfection, was also examined. Some examples of their qualities are listed in Table 1. Table 1 Quality of Water Samples Used in the Study Item
LME
Actual raw water
[- I
6.8
[kg/m'l [kg/ml)
5.9xl0·2
8.1 3.lx 10.2
1.3x10·2
3.OcIO·2
[kg/mll NH4-N [kg/m 31
4.6xl0·2
1.3x 10"3
3.2x 10"2
8.Oc 10'"
1.1
3.7XI0"2
pH
TC IC TQC
E200
[-I
LME
Item
Actual raw water
Chromaticity [degree) Turbidity [degree) COD [kg/m 31
1.SxlO
4.5
25 J.IxlO'2
3.lxlO"'
[kg/ml)
4.3x 10"2
3.8xl0"3
0.25
1.2
BOD
BOD/COD [ - I
3.2
Removal of organic substances from water
173
Ozonation in a batch system To make clear changes of organic substances dissolved in water by ozone, batch ozone treatment of each water sample summarized in Table 2 was performed with a batch tank reactor shown in Figure I. Sample Number 1 was prepared from LME diluting with tap water to a certain concentration, and Number 2 was prepared from LME removing components of larger molecular weight (> I00(0) by coagulation with aluminum sulfate. Samples Number 3, 4 and 5 were fractions collected by dividing LME with a gel filtration chromatograph. For samples Number I and 2 and actual water, changes of total organic carbon (TOC), E2OO, chromaticity, turbidity, pH and ozone concentration with time by oxidation were measured. At the end of the experiments, chemical oxygen demand (COD), biological oxygen demand (BOD), molecular weight fractionation and adsorption eqUilibrium were also measured to examine their changes by oxidation with ozone. For samples 3, 4 and 5, changes of the indices as above by 2-hour ozone treatment were measured by the procedure described in Figure I(b). Table 2 The Initial Values of Each Water Sample Sample number
E260
TOe [kg!ml)
[
7.1
0.13
6.9
0.087
pH
-)
[
-)
1
LME
2
coagulated
8.6xlO·l 7.3xlo-l
3 4
fraction I
9.9x I0-3
7.0
0.34
fraction 2
l.lx10-2
6.8
S
fraction 3
6.9x I0-3
7.6
Qlromaticity [degree)
Turbidity [degree)
Molecular weight
18
6.2 4.8
-
( )
0.76
0.3S
33 29
> 10000-100 10000-200 10000- 1000
0.47
1000-200
O.IS
14
0.66
6.1
200>
t
Exhaust gas
Exhaust gas
..........
Sampling Cock
r=:::J
Ozone Monitor
o
00 0 00 0 o~o 0 0 0 0 0 0 00°0 0 o 000 0 00 00 0 0 00
o
Sparger
SUS Tank
00
'!,
Seal· less Pump
(a) Volume of reactor: 1.0 x 10.2 m3 Ozone concentration: 6.4 x 10.3 kg/m 3 Flow rate: 1.2 x 10. 1 m3/h
Glass Cylinder (b) Volume of reactor: 5.0 x 10.4 m3 Ozone concentration: 1.1 x 10·2 kg/m 3 Flow rate: 1.2 x 10.2 m3/h
Figure 1 Schematic Diagrams of Batch Tank Reactors Water qualities were measured by a TOC analyzer (TOC-5000; Shimadzu Co., Ltd. Japan), a pH meter (F13; Horiba Co., Ltd. Japan), a DO meter (D0-20A; TOA Electronics Ltd. Japan) and a dissolved ozone monitor (DOP-50; Ebara Jitsugyo Co., Ltd. Japan), respectively. Using a UV-VIS spectrometer (UV1200; Shimadzu Co., Ltd. Japan), E260, chromaticity and turbidity were determined by measuring
Y. TAKEUCHI
174
~tal.
absorbances of each water sample at wave length of 260, 390 and 660 nm, respectively. Molecular weight fractionation was performed as follows. After a certain amount of a water sample was fed to a pair of columns of 0.75 m in length and 2.2xlO-2 m in i. d., packed with two types of vinyl resin base gel (TOYOPEARL HW-55 and TOYOPEARL HW-40; Toso Co., Ltd. Japan), respectively, connected in series, and deionized water was fed to the system at a flow rate of 9.00xlo-s m3Jh through a constant feed rate pump to separate organic components dissolved in the sample, and the components eluted were detected by a refractive index detector (RID-6A; Shimadzu Co., Ltd. Japan). Flow tests for water samples A certain amount of the activated carbon particles was packed in a column, then the column was placed after an ozone reactor followed by an residence tank as illustrated in Figure 2. A water sample, which was prepared by diluting LME with tap water to about 5xlo-3 kg-TOC/m3, was fed to the ozone reactor, and a part of the water preoxidized with ozone was fed to the carbon column through a residence tank. Collection of effluent water from the ozone reactor and carbon bed was conducted at definite intervals, and several water quality indices as above were measured. Also, biological activated carbon treatment without ozone treatment was performed in the same manner as above, to compare with the case when organic substances in the water reacted with ozone. Exhaust gas Ozone gas
--·----.. . 1
ri----I--.....,
.......
00
2.4x10·3 m3/h
0
: ... ..
~
Water sample
over1low ....-----,1.45x10-S m=¥h
..... o
c:::::::J
Ozone Monitor
0
Ozone reactor
Residence tank
9.48x10" m31h
diameter of carbon bed: 5.0x10-2m weight 01 carbon: 0.04 kg volume 01 ozone reactor: 5.0x1()-4 mS volume of reSidence tank: 1.0x1()-4 mS flow rater of ozone gus: 2. 7x1 ()-4 m=¥h concentration of ozone: 1.3x1 O-S kg/mS
Figure 2 Outline of Experimental Apparatus for Continuous Treatment
RESULTS AND DISCUSSION
Ozonation in a batch system The changes of the indices with time, in the case when samples Number 1 and 2 were treated, are shown in Figure 3, and the removal ratios of TOC, E260 and chromaticity are illustrated in Figure 4. It was found that E260 and chromaticity were greatly lowered by ozone treatment, however, most ofTOC ingredients remained in the water. It is known that ozone react with organic substances dissolved in water to produce aldehydes or carbonic acids through primary ozonide and zwittertion (Somiya and Yamada, 1987). Thus, partial oxidation of organic substances can occur, e.g. breakage of double bonds and degradation to smaller molecular weight compounds, but ozone does not act to decompose most of organics up to carbon dioxides. Therefore, it was considered that ozonation was regarded as pretreatment for adsorption and biodegradation in the BAC bed placed after the ozone reactor if removal of dissolved organic carbon was aimed, though it was effective to decrease E260 and chromaticity. Regarding ozonation as a pretreatment process for BAC treatment, it is important to examine its influences to properties of organic substances in water, e.g.
Removal of organic substances from water
175
biodegradability and adsorbability. It was also found that biodegradability of raw water was improved by ozonation as shown in Figure 5. The panial oxidation of organic substances caused by ozone added increased the amount of hydrophilic group. Thus, it is thought that the affinity between organic substances and microorganisms can be improved greatly by ozonation.
1.0
e ._ IL-:.-to. .... ....
..
~_
~
0.8'.
~'-6.
0° 0.6
(3
.~
0.4 0.2 0.0
.-"--"---" .--.-...
..
".
....... -
1.0 ~:. __ .. _.• __ .__ ~
':0 0.8 r- ••••
'\:)--~~'O--Q
(3
'~ 0 .. 0.
".-'().
&--.-. --<>-"2-
0.4 0.2
a) sample 1
----..--.--
\.0.....
<.> 0.6
Y--o:~-il-:""-
...•. _
.--.-.....
b) sample 2.--...
·::-::t:t:-i"-~
0.0 .........""'---'---'--L---'---1---''--.L..-'''''---L..J 100 150 200 250 o 50 100 150 200 250 Time Elapsed [min) Time Elapsed [min) (----e---- TOC ----0---- E260 "--6---- ChromatlCny)
L.-"""-....L........._L..-....--'-___---II-.o....--L..J
o
50
Figure 3 Changes of Cleo for TOC, E260 and Chromaticty in Batch Ozone Treatment
o
TOC
•
E260
•
0.5,-:=-----------, Chromaticny
100
0.4
before olonation after olonation
-
o 0.3
:s
o
II:
~
•
o~ o CD
50
E CI>
II:
o
2
3 4 Sample No.
0.2 0.1
o 5
Actual Water
Figure 4 Removal Ratios of TOC. E260 and Chromaticity Obtained in Batch Ozone Treatment
1
2
Sample No.
Figure 5 Changes of BOD/COD Caused by Ozination
Next, the effects of ozonation on adsorption equilibria were studied and results are shown in Figure 6 with a list of Freundlich constants obtained for these cases. The adsorbability of the organic substances for samples I, 3 and 4 increased by ozone treatment but those for samples 2 and 5 were lowered. It was supposed, therefore, that ozonation caused the increase of the amount adsorbed of the substances of higher molecular weight. It was reponed that fumic substances with a molecular weight of more than several thousand were hard to adsorb on activated carbon (Lee et. aI., 1981; Takeuchi et. aI, 1988). However, the panial breakage of the structure of such organics by ozone made their molecular weight smaller, then they could leach adsorption sites through smaller pores. For example, mean molecular weight of sample 3 (> 500(0) was found to decrease to around 2000 - 3000, and it was thought that this size of substances could be adsorbed. On the other hand, it was suggested that hidrophilication by ozonation caused a decrease of adsorbability of organic substances (Nakano, 1996). The results obtained from samples 2 and 5, for substances with smaller molecular weight, agreed to his repon. Therefore, it was thought that the influence
176
Y. TAKEUCHI ~t al.
of hidrophilication to the adsorbability was much larger than that of decomposition by ozone treatment for organic substances with a molecular weight of less than several thousand. An example of the change of molecular weight distribution by reaction with ozone is shown in Figure 7.
No.
---2
" k
---~-
before 0.38
after 0.41
7.4xlct6 2.3xlo-' ---6.s-l--- -0:«--
. ______ ~ _. __ .2:3 ____ ~.4_X.IP!_
Sample 2
Sample 1
10"4 ~0-~3:--'-""""""'''''''''..L-a.......J
1
3 It 1.7 1.3 _______ k. __ ~.06I ___ . 1.6 4" 1.8 0.17 2.6 k 0.084 2.0xt017
10-2
-.------------------
~
1.2 1.1
It
A:
10-2
10~
1.3 0.19
C (kg-TOC,m3]
---.--(7...
Sample 3
~
Sample 4
10~~~~~~~~~ 10~ 10-3 10-210~
Before After
SampleS
10-3
10-2 10"
10-3
10-2
C (kg-TOC,m3]
Figure 6 Changes of Adsorption Equilibria by Ozonation
: l/Mt
",-B_ef_oTre_o_zo_n'Te_T_r...:e.:.a;;,tm_e;:...n...:t--lI~"·---r-"----·l .. i'II'\
I
100 ~ 1
• I
I
•
I
'E_
------r-.. ----T--------r--------1
1
' " • • I
1'"'-'
:
•
:
1 ' I
•
1 " I
_,
•
:
: •
:
:
i i
•
I
I
I
:
1
1 I, I
• I
:
:
:
:
•
r=A:::::=fte:::r~Oz::=o=n=e±':T:=re=a=tm:::::::::en~t-'I'T"-'----r'':;'''''!''-- --"1" •.•••• T' ••••• "r •
j
:
: : : r...: : N: : i :~:~:
~ CD
oq:
•
I
•
n
...... :
i:s i i i i : : : "l i ~ i i i: :: ~:-: :: :: : :
~' : - : ....
a:; II:
o
•
• •
:20
:40
::
I
I
: 6 0 : 8O·----!'i
•
•
•
lt40
lt60
"80
i : i : : ,
!200
•••••••• \ •••••••••••••• ••• J ............ ••••••• " •••••••• , •••••••• " ...................." •••••••• ,
RetentionTime[minl Figure 7 An Example of Molucular Weight Fractionation
Removal of organic substances from water
177
Flow tests for water sample Figure 8 shows the ratios of the concentrations of effluents to the initial concentration in terms of TOC in the case of two experimental runs, e.g., BAC treatment with and without ozone preoxidation. The removal ratio in the treatment without ozonation seemed to be higher than that with ozonation. The total amounts of TOC removed during this period with and without ozonation were found to be about 1.4xlO·3 kg and 1.6xlo-3 kg, respectively. Therefore, it can be concluded that reaction with ozone gives negative effects on adsorption behavior of organics. However, the outlet concentration started to decrease when 5 days passed, and the decrease was supposed to be caused by biological activities of microorganisms grown on the surface of the activated carbon particles. In fact, several species of microorganisms were found from both of carbon beds by observation under a microscope as listed in Table 3. There was remarkable difference between the species appeared under both conditions, that was, only Paramecium sp. existed under both conditions and other species appeared were different from each other. ROlaria sp. and Colurella sp. are known to appear when DO rich conditions were applied. It is thought, therefore, that the carbon bed was kept to be more aerobic by feeding preozonized water than the case of ozone free condition. Furthermore, there did not exist species which supposed to appear under bad conditions of biological treatment, i.e., UlonolUS sp. and Colpidium sp., when the carbon bed placed after the ozone reactor. Therefore, it was concluded that ozonation of raw water before feeding it to the activated carbon bed made biological processes better, while the ozonation gave a little bad effect to carbon adsorption in lowering of adsorbability.
--
1.2
..---c----
1.0 0
with ozonation without ozonation
0.8
~
(3
0.6 0.4 0.2 0.0 5
0
10 Time Elapsed [day]
15
20
Figure 8 Changes of CICo for TOC with Time
Table 3 Microorganisms Found in the Carbon Beds under the Different Conditions without ozonation
with ozonation
Paramecium sp. (wide BOD range) ROlaria sp. (high DO) Colurella sp. (low BOD, hidh DO) Actinophrys sp. (a lii1e1 high loding)
Paramecium sp. (wide BOD range) Arce1la sp. (low BOD,low pH) Vorticella sp. (good condition) Monostyla sp. (low BOD) Lepadella Sp. (very low BOD) Colpidium Sp. (bad condition) Utonotus sp. (bad-recovering condition)
(Japan water works association, 1993; Japan sewage warks association, 1984)
178
Y. TAKEUCHI "tal.
CONCLUSION Ozone reacted with organic substances to make their molecular weight a little smaller and values of E260 and chromaticity lower, but decomposition of them to carbon dioxide did not occur. This means that only partial breakage of C-C chains occurred but total amounts of dissolved organic substances did not decrease by ozone treatment. The resulting substances by ozone treatment were more easily decomposed by microorganisms, and that their adsorbability was also changed.
ACKNOWLEDGMENT The authors would like to express their deepest thanks to Mr. Mitsutoshi Taura and other senior students for their assistance in experimental works. UTERATURE CITED Amy, L. G., Tan, L. and Davis, M. K. (1991). The effects of ozonation and activated carbon adsorption on trihalomethane speciation. Wat, Res., 25,191-202. Fujiki, A. and Anraku, K. (1993). Treatment of raw water from the Sagami River by biological activated carbon fluidized bed. J. Water and Waste, 35, 716-722. Japan sewage works association. (1991). Earesyontankunaino-Biseibutu (Microorganisms in the aeration tank). Tokyo sewage works bureau (ed.), JSWA. Japan water works association. (1993). Josui-Shikenhou (Experimental methods in water), Public hygiene bureau, the ministry of health and welfare (ed.), JWW A. Lee, M. C., Snoeyink, V. L. and Crittenden, J. C. (1981). Activated carbon adsorption of humic substances. J. AWWA, 73, 440-446. Moniwa, T. (1995). Ozonation in water purification. J. JWWA, 64 (10), 2-6. Nakano, S. (1996). Water purification by biological activated carbon. Adsorption News, 10 (2), 4-9. Okada, M. (1995). Removal of odorous compounds by ozonation and effects of dissolved substances in water. J. JWWA, 64 (10), 18-20. Somiya, I. and Yamada, H. (1987). Products formed during ozonation of organic compounds in aqueous solution. EISEl KAGAKU (Japanese J. TOXicology and Environmental Health), 33, 365-384. Takeuchi, Y., Suzuki, Y., Koizumi, A. and Soeda, N. (1991). Removal of trihalomethane precursors from river and lake water by activated carbon adsorption. Wat. Sci. Thee., 23, 1687-1694. Takeuchi, Y., Suzuki, Y., Mochidzuki, K., Yagishita, Y., Fukuta, T., Amakusa, H. and Abe, H. (1996a). Biological activated carbon treatment of wastewater containing organics and heavy metal ions. Fundamentals of Adsorption, California, USA. LeVan, M. D. (ed.), Kluwer Academic Publishers, pp 937-944.
Pergamon
Wa/. Sci. Teelt. Vol. 3S, No.7, pp. 171-178, 1997. tC> 19971AWQ. Published by Elsevier Science Ltd
PH: S0273- I 223(97)00 I 28-5
Pnnted in Great Bntain 0273-1223197 SINlO + 0'00
REMOVAL OF ORGANIC SUBSTANCES FROM WATER BY OZONE TREATMENT FOLLOWED BY BIOLOGICAL ACTIVATED CARBON TREATMENT Yasushi Takeuchi*, Kazuhiro Mochidzuki*, Noriyuki Matsunobu*, Ryozo Kojima**, Hiroshi Motohashi** and Syunichi Yoshimoto** • Department of Industrial Chemistry. Meiji University. Higashi-mita. Tama-ku. Kawasaki 214-71. Japan •• Municipal Water Plant Engineering Department. EBARA Corporation. 1-6-7 Konan. Minato-ku. Tokyo 108. Japan
ABSTRACf As an advanced water treatment, a combination of ozonation and biological activated carbon (BAC, hereafter) treatment is being applied to purify raw water for municipal use in some cases. The authors examined effects of ozonation on water quality in a batch system, using water samples containing organic substances fractionated to several molecular weight ranges. Also. a flow test of laboratory-scale was performed to study on the capability of the treatment in terms of removal efficiency of the dissolved organic substances, e.g., fumic substances, which preoxidized with ozone. As a result, the changes in equilibrium adsorption and in the biodegradability of organic substances dissolved in water before and after oxidation with ozone were made clear. © 1997 IA WQ. Published by Elsevier Science Ltd
KEYWORDS Adsorption; advanced treatment; biodegradation; biological activated carbon; drinking water purification; fumic substances; molecular weight fractionation; ozonation.
INfRODUCTION In these days, removal of dissolved organic pollutants in water, e.g., trihalomethanes and odorous substances, is attracting greater concerns to upgrade qualities of municipal water. Since these substances as above bring about unpleasantness other than negative effect on our health, municipal water treatment plants need to equip s(}ocalled advanced treatment processes to supply safe water. Activated carbon adsorption has been used to remove soluble organic contaminants from water, and applied as a final process in municipal water treatment. When granular activated carbon particles were used in a fixed bed, it was found that certain kinds of microorganisms grew on the surfaces of the carbon particles and their activities lengthened the life of carbon. Now, the activated carbon treatment with biodegradation is called "biological activated carbon (BAC) treatment" and is being used widely for various treatment processes to purify drinking water (e.g. Fujiki and Anraku, 1993) and to remove pollutants from industrial wastes (e.g. Takeuchi et. aI., 1996), 171
172
Y. TAKEUCHI etal.
because it brings about lower cost and higher performance than mere activated carbon adsorption treatment processes. It is said that BAC treatment combined with ozonation is especially effective to clean up raw water because of effects of preoxidation with ozone. Recently, it has been applied at several municipal water treatment plants, as one of advanced treatment processes to eliminate dosage of chlorine for disinfection, to remove odorous substances, to avoid formation of trihalomethanes and to make chromaticity lower (e.g. Amy et. aI., 1991; Moniwa, 1995; Okada, 1995). However, the mechanisms of removal in the treatment are more complicated than ordinary water treatment processes, since three different processes, i.e., ozonation, adsorption and biodegradation, occur simultaneously, and there remain some problems to be discussed in the treatment to make clear the removal mechanisms of organic substances and to obtain more detailed engineering data. This paper deals with results of ozone treatment of raw municipal water followed by BAC treatment using a batch tank reactor. Water samples used were a kind of leaf-mold extract diluted with tap water and a kind of raw water taken from an existing municipal water treatment plant, respectively. The behavior of equilibrium adsorption and biodegradation subjected to oxidation with ozone was examined, focusing on the effectiveness of ozonation for removal of organic substances fractionated to several average molecular weight ranges. Funhermore, a flow test was performed to investigate the effectiveness of ozonation on the BAC treatment placed after the ozone reactor. EXPERIMENTAL Physical properties of activated carbon samples A commercial coconut shell base granular activated carbon (LG-20; EBARA Co., Ltd. Japan) was used in this study, after following pretreatment. The panicles were sieved to 16/20 standard mesh fraction and washed with diluted aqueous solution of chloric acid. Then it was dried in nitrogen gas stream for 2 hours at 398 K. Physical propenies of the sample were determined to be as follows; true density was 2.00xl()3 kg/m 3,'BET specific surface area was 1.27xl06 m 2Jkg, total pore volume was 2.7Sxl0 4 m 3Jkg and apparent density was 7.Slx102 kg/m 3, respectively.
Qualities of water samples As a model of raw municipal water, a leaf-mold extract (hereafter, described as LME) prepared in the following manner was used, i.e., 2 kg of a kind of commercial leaf-mold was put into a tank filled with 0.02 m 3 of tap water, whose dissolved chlorine had been removed by keeping the water in air for several days, and components of leaf mold, e.g. fumic acids, were extracted for 24 hours with agitating several minutes in every 4 hours. A kind of actual raw water from a cenain municipal water treatment plant. which is originally one of small-scale secondary water filtration factories using underground water and has only three processes, i.e. ozonation, activated carbon adsorption and disinfection, was also examined. Some examples of their qualities are listed in Table 1. Table 1 Quality of Water Samples Used in the Study Item
LME
Actual raw water
[- I
6.8
[kg/m'l [kg/ml)
5.9xl0·2
8.1 3.lx 10.2
1.3x10·2
3.OcIO·2
[kg/mll NH4-N [kg/m 31
4.6xl0·2
1.3x 10"3
3.2x 10"2
8.Oc 10'"
1.1
3.7XI0"2
pH
TC IC TQC
E200
[-I
LME
Item
Actual raw water
Chromaticity [degree) Turbidity [degree) COD [kg/m 31
1.SxlO
4.5
25 J.IxlO'2
3.lxlO"'
[kg/ml)
4.3x 10"2
3.8xl0"3
0.25
1.2
BOD
BOD/COD [ - I
3.2
Removal of organic substances from water
173
Ozonation in a batch system To make clear changes of organic substances dissolved in water by ozone, batch ozone treatment of each water sample summarized in Table 2 was performed with a batch tank reactor shown in Figure I. Sample Number 1 was prepared from LME diluting with tap water to a certain concentration, and Number 2 was prepared from LME removing components of larger molecular weight (> I00(0) by coagulation with aluminum sulfate. Samples Number 3, 4 and 5 were fractions collected by dividing LME with a gel filtration chromatograph. For samples Number I and 2 and actual water, changes of total organic carbon (TOC), E2OO, chromaticity, turbidity, pH and ozone concentration with time by oxidation were measured. At the end of the experiments, chemical oxygen demand (COD), biological oxygen demand (BOD), molecular weight fractionation and adsorption eqUilibrium were also measured to examine their changes by oxidation with ozone. For samples 3, 4 and 5, changes of the indices as above by 2-hour ozone treatment were measured by the procedure described in Figure I(b). Table 2 The Initial Values of Each Water Sample Sample number
E260
TOe [kg!ml)
[
7.1
0.13
6.9
0.087
pH
-)
[
-)
1
LME
2
coagulated
8.6xlO·l 7.3xlo-l
3 4
fraction I
9.9x I0-3
7.0
0.34
fraction 2
l.lx10-2
6.8
S
fraction 3
6.9x I0-3
7.6
Qlromaticity [degree)
Turbidity [degree)
Molecular weight
18
6.2 4.8
-
( )
0.76
0.3S
33 29
> 10000-100 10000-200 10000- 1000
0.47
1000-200
O.IS
14
0.66
6.1
200>
t
Exhaust gas
Exhaust gas
..........
Sampling Cock
r=:::J
Ozone Monitor
o
00 0 00 0 o~o 0 0 0 0 0 0 00°0 0 o 000 0 00 00 0 0 00
o
Sparger
SUS Tank
00
'!,
Seal· less Pump
(a) Volume of reactor: 1.0 x 10.2 m3 Ozone concentration: 6.4 x 10.3 kg/m 3 Flow rate: 1.2 x 10. 1 m3/h
Glass Cylinder (b) Volume of reactor: 5.0 x 10.4 m3 Ozone concentration: 1.1 x 10·2 kg/m 3 Flow rate: 1.2 x 10.2 m3/h
Figure 1 Schematic Diagrams of Batch Tank Reactors Water qualities were measured by a TOC analyzer (TOC-5000; Shimadzu Co., Ltd. Japan), a pH meter (F13; Horiba Co., Ltd. Japan), a DO meter (D0-20A; TOA Electronics Ltd. Japan) and a dissolved ozone monitor (DOP-50; Ebara Jitsugyo Co., Ltd. Japan), respectively. Using a UV-VIS spectrometer (UV1200; Shimadzu Co., Ltd. Japan), E260, chromaticity and turbidity were determined by measuring
Y. TAKEUCHI
174
~tal.
absorbances of each water sample at wave length of 260, 390 and 660 nm, respectively. Molecular weight fractionation was performed as follows. After a certain amount of a water sample was fed to a pair of columns of 0.75 m in length and 2.2xlO-2 m in i. d., packed with two types of vinyl resin base gel (TOYOPEARL HW-55 and TOYOPEARL HW-40; Toso Co., Ltd. Japan), respectively, connected in series, and deionized water was fed to the system at a flow rate of 9.00xlo-s m3Jh through a constant feed rate pump to separate organic components dissolved in the sample, and the components eluted were detected by a refractive index detector (RID-6A; Shimadzu Co., Ltd. Japan). Flow tests for water samples A certain amount of the activated carbon particles was packed in a column, then the column was placed after an ozone reactor followed by an residence tank as illustrated in Figure 2. A water sample, which was prepared by diluting LME with tap water to about 5xlo-3 kg-TOC/m3, was fed to the ozone reactor, and a part of the water preoxidized with ozone was fed to the carbon column through a residence tank. Collection of effluent water from the ozone reactor and carbon bed was conducted at definite intervals, and several water quality indices as above were measured. Also, biological activated carbon treatment without ozone treatment was performed in the same manner as above, to compare with the case when organic substances in the water reacted with ozone. Exhaust gas Ozone gas
--·----.. . 1
ri----I--.....,
.......
00
2.4x10·3 m3/h
0
: ... ..
~
Water sample
over1low ....-----,1.45x10-S m=¥h
..... o
c:::::::J
Ozone Monitor
0
Ozone reactor
Residence tank
9.48x10" m31h
diameter of carbon bed: 5.0x10-2m weight 01 carbon: 0.04 kg volume 01 ozone reactor: 5.0x1()-4 mS volume of reSidence tank: 1.0x1()-4 mS flow rater of ozone gus: 2. 7x1 ()-4 m=¥h concentration of ozone: 1.3x1 O-S kg/mS
Figure 2 Outline of Experimental Apparatus for Continuous Treatment
RESULTS AND DISCUSSION
Ozonation in a batch system The changes of the indices with time, in the case when samples Number 1 and 2 were treated, are shown in Figure 3, and the removal ratios of TOC, E260 and chromaticity are illustrated in Figure 4. It was found that E260 and chromaticity were greatly lowered by ozone treatment, however, most ofTOC ingredients remained in the water. It is known that ozone react with organic substances dissolved in water to produce aldehydes or carbonic acids through primary ozonide and zwittertion (Somiya and Yamada, 1987). Thus, partial oxidation of organic substances can occur, e.g. breakage of double bonds and degradation to smaller molecular weight compounds, but ozone does not act to decompose most of organics up to carbon dioxides. Therefore, it was considered that ozonation was regarded as pretreatment for adsorption and biodegradation in the BAC bed placed after the ozone reactor if removal of dissolved organic carbon was aimed, though it was effective to decrease E260 and chromaticity. Regarding ozonation as a pretreatment process for BAC treatment, it is important to examine its influences to properties of organic substances in water, e.g.
Removal of organic substances from water
175
biodegradability and adsorbability. It was also found that biodegradability of raw water was improved by ozonation as shown in Figure 5. The panial oxidation of organic substances caused by ozone added increased the amount of hydrophilic group. Thus, it is thought that the affinity between organic substances and microorganisms can be improved greatly by ozonation.
1.0
e ._ IL-:.-to. .... ....
..
~_
~
0.8'.
~'-6.
0° 0.6
(3
.~
0.4 0.2 0.0
.-"--"---" .--.-...
..
".
....... -
1.0 ~:. __ .. _.• __ .__ ~
':0 0.8 r- ••••
'\:)--~~'O--Q
(3
'~ 0 .. 0.
".-'().
&--.-. --<>-"2-
0.4 0.2
a) sample 1
----..--.--
\.0.....
<.> 0.6
Y--o:~-il-:""-
...•. _
.--.-.....
b) sample 2.--...
·::-::t:t:-i"-~
0.0 .........""'---'---'--L---'---1---''--.L..-'''''---L..J 100 150 200 250 o 50 100 150 200 250 Time Elapsed [min) Time Elapsed [min) (----e---- TOC ----0---- E260 "--6---- ChromatlCny)
L.-"""-....L........._L..-....--'-___---II-.o....--L..J
o
50
Figure 3 Changes of Cleo for TOC, E260 and Chromaticty in Batch Ozone Treatment
o
TOC
•
E260
•
0.5,-:=-----------, Chromaticny
100
0.4
before olonation after olonation
-
o 0.3
:s
o
II:
~
•
o~ o CD
50
E CI>
II:
o
2
3 4 Sample No.
0.2 0.1
o 5
Actual Water
Figure 4 Removal Ratios of TOC. E260 and Chromaticity Obtained in Batch Ozone Treatment
1
2
Sample No.
Figure 5 Changes of BOD/COD Caused by Ozination
Next, the effects of ozonation on adsorption equilibria were studied and results are shown in Figure 6 with a list of Freundlich constants obtained for these cases. The adsorbability of the organic substances for samples I, 3 and 4 increased by ozone treatment but those for samples 2 and 5 were lowered. It was supposed, therefore, that ozonation caused the increase of the amount adsorbed of the substances of higher molecular weight. It was reponed that fumic substances with a molecular weight of more than several thousand were hard to adsorb on activated carbon (Lee et. aI., 1981; Takeuchi et. aI, 1988). However, the panial breakage of the structure of such organics by ozone made their molecular weight smaller, then they could leach adsorption sites through smaller pores. For example, mean molecular weight of sample 3 (> 500(0) was found to decrease to around 2000 - 3000, and it was thought that this size of substances could be adsorbed. On the other hand, it was suggested that hidrophilication by ozonation caused a decrease of adsorbability of organic substances (Nakano, 1996). The results obtained from samples 2 and 5, for substances with smaller molecular weight, agreed to his repon. Therefore, it was thought that the influence
176
Y. TAKEUCHI ~t al.
of hidrophilication to the adsorbability was much larger than that of decomposition by ozone treatment for organic substances with a molecular weight of less than several thousand. An example of the change of molecular weight distribution by reaction with ozone is shown in Figure 7.
No.
---2
" k
---~-
before 0.38
after 0.41
7.4xlct6 2.3xlo-' ---6.s-l--- -0:«--
. ______ ~ _. __ .2:3 ____ ~.4_X.IP!_
Sample 2
Sample 1
10"4 ~0-~3:--'-""""""'''''''''..L-a.......J
1
3 It 1.7 1.3 _______ k. __ ~.06I ___ . 1.6 4" 1.8 0.17 2.6 k 0.084 2.0xt017
10-2
-.------------------
~
1.2 1.1
It
A:
10-2
10~
1.3 0.19
C (kg-TOC,m3]
---.--(7...
Sample 3
~
Sample 4
10~~~~~~~~~ 10~ 10-3 10-210~
Before After
SampleS
10-3
10-2 10"
10-3
10-2
C (kg-TOC,m3]
Figure 6 Changes of Adsorption Equilibria by Ozonation
: l/Mt
",-B_ef_oTre_o_zo_n'Te_T_r...:e.:.a;;,tm_e;:...n...:t--lI~"·---r-"----·l .. i'II'\
I
100 ~ 1
• I
I
•
I
'E_
------r-.. ----T--------r--------1
1
' " • • I
1'"'-'
:
•
:
1 ' I
•
1 " I
_,
•
:
: •
:
:
i i
•
I
I
I
:
1
1 I, I
• I
:
:
:
:
•
r=A:::::=fte:::r~Oz::=o=n=e±':T:=re=a=tm:::::::::en~t-'I'T"-'----r'':;'''''!''-- --"1" •.•••• T' ••••• "r •
j
:
: : : r...: : N: : i :~:~:
~ CD
oq:
•
I
•
n
...... :
i:s i i i i : : : "l i ~ i i i: :: ~:-: :: :: : :
~' : - : ....
a:; II:
o
•
• •
:20
:40
::
I
I
: 6 0 : 8O·----!'i
•
•
•
lt40
lt60
"80
i : i : : ,
!200
•••••••• \ •••••••••••••• ••• J ............ ••••••• " •••••••• , •••••••• " ...................." •••••••• ,
RetentionTime[minl Figure 7 An Example of Molucular Weight Fractionation
Removal of organic substances from water
177
Flow tests for water sample Figure 8 shows the ratios of the concentrations of effluents to the initial concentration in terms of TOC in the case of two experimental runs, e.g., BAC treatment with and without ozone preoxidation. The removal ratio in the treatment without ozonation seemed to be higher than that with ozonation. The total amounts of TOC removed during this period with and without ozonation were found to be about 1.4xlO·3 kg and 1.6xlo-3 kg, respectively. Therefore, it can be concluded that reaction with ozone gives negative effects on adsorption behavior of organics. However, the outlet concentration started to decrease when 5 days passed, and the decrease was supposed to be caused by biological activities of microorganisms grown on the surface of the activated carbon particles. In fact, several species of microorganisms were found from both of carbon beds by observation under a microscope as listed in Table 3. There was remarkable difference between the species appeared under both conditions, that was, only Paramecium sp. existed under both conditions and other species appeared were different from each other. ROlaria sp. and Colurella sp. are known to appear when DO rich conditions were applied. It is thought, therefore, that the carbon bed was kept to be more aerobic by feeding preozonized water than the case of ozone free condition. Furthermore, there did not exist species which supposed to appear under bad conditions of biological treatment, i.e., UlonolUS sp. and Colpidium sp., when the carbon bed placed after the ozone reactor. Therefore, it was concluded that ozonation of raw water before feeding it to the activated carbon bed made biological processes better, while the ozonation gave a little bad effect to carbon adsorption in lowering of adsorbability.
--
1.2
..---c----
1.0 0
with ozonation without ozonation
0.8
~
(3
0.6 0.4 0.2 0.0 5
0
10 Time Elapsed [day]
15
20
Figure 8 Changes of CICo for TOC with Time
Table 3 Microorganisms Found in the Carbon Beds under the Different Conditions without ozonation
with ozonation
Paramecium sp. (wide BOD range) ROlaria sp. (high DO) Colurella sp. (low BOD, hidh DO) Actinophrys sp. (a lii1e1 high loding)
Paramecium sp. (wide BOD range) Arce1la sp. (low BOD,low pH) Vorticella sp. (good condition) Monostyla sp. (low BOD) Lepadella Sp. (very low BOD) Colpidium Sp. (bad condition) Utonotus sp. (bad-recovering condition)
(Japan water works association, 1993; Japan sewage warks association, 1984)
178
Y. TAKEUCHI "tal.
CONCLUSION Ozone reacted with organic substances to make their molecular weight a little smaller and values of E260 and chromaticity lower, but decomposition of them to carbon dioxide did not occur. This means that only partial breakage of C-C chains occurred but total amounts of dissolved organic substances did not decrease by ozone treatment. The resulting substances by ozone treatment were more easily decomposed by microorganisms, and that their adsorbability was also changed.
ACKNOWLEDGMENT The authors would like to express their deepest thanks to Mr. Mitsutoshi Taura and other senior students for their assistance in experimental works. UTERATURE CITED Amy, L. G., Tan, L. and Davis, M. K. (1991). The effects of ozonation and activated carbon adsorption on trihalomethane speciation. Wat, Res., 25,191-202. Fujiki, A. and Anraku, K. (1993). Treatment of raw water from the Sagami River by biological activated carbon fluidized bed. J. Water and Waste, 35, 716-722. Japan sewage works association. (1991). Earesyontankunaino-Biseibutu (Microorganisms in the aeration tank). Tokyo sewage works bureau (ed.), JSWA. Japan water works association. (1993). Josui-Shikenhou (Experimental methods in water), Public hygiene bureau, the ministry of health and welfare (ed.), JWW A. Lee, M. C., Snoeyink, V. L. and Crittenden, J. C. (1981). Activated carbon adsorption of humic substances. J. AWWA, 73, 440-446. Moniwa, T. (1995). Ozonation in water purification. J. JWWA, 64 (10), 2-6. Nakano, S. (1996). Water purification by biological activated carbon. Adsorption News, 10 (2), 4-9. Okada, M. (1995). Removal of odorous compounds by ozonation and effects of dissolved substances in water. J. JWWA, 64 (10), 18-20. Somiya, I. and Yamada, H. (1987). Products formed during ozonation of organic compounds in aqueous solution. EISEl KAGAKU (Japanese J. TOXicology and Environmental Health), 33, 365-384. Takeuchi, Y., Suzuki, Y., Koizumi, A. and Soeda, N. (1991). Removal of trihalomethane precursors from river and lake water by activated carbon adsorption. Wat. Sci. Thee., 23, 1687-1694. Takeuchi, Y., Suzuki, Y., Mochidzuki, K., Yagishita, Y., Fukuta, T., Amakusa, H. and Abe, H. (1996a). Biological activated carbon treatment of wastewater containing organics and heavy metal ions. Fundamentals of Adsorption, California, USA. LeVan, M. D. (ed.), Kluwer Academic Publishers, pp 937-944.