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The performance of an ozonation-biological activated carbon process under long term operation
~
Wat. Sci. T«h. Vol. 38, No.6, pp. 163-169, 1998. lAWQ
Pergamon
PII: S0273-1223(98)OO578-2
C 1998 Published by Ebevicr Science lAd. Prinled in Great Britain. All riahll reserved 0273-1223198 $19"00 +0-00
THE PERFORMANCE OF AN
OZONATION-BIOLOGICAL ACTIVATED CARBON PROCESS UNDER LONG TERM OPERATION Wataru Nishijima, Woo Hang Kim, Eiji Shoto and Mitsumasa Okada Faculty ofEngineering, Hiroshima University, J -4-J Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan
ABSTRACf 1be objective of this study is 10 evaluate removal of DOC and 1HMFP during long term operation of an
olonation-biological activated carbon (BAC) process. A pilol scale planl with raw waler in an eutrophic reservoir for drinking water supply was operated for 910 days. High DOC and 1HMFP removal were maintained al 36% and S7%, respectively, in the olonation-BAC process even after saturation of BAC by DOC. DOC and THMFP removal by ozanation were only 8% and 24%, respectively. High DOC and THMFP removal after saturalion was due 10 the increase in biodeeradable DOC by ozanation from 7% to 32% and the subsequenl biodegradation by bacteria at!ached on BAC. Although water temperature changed in the range from S to 30"C, seasonal changes in DOC lind 1HMFP removal were not observed in the ownation-BAC process. C 1998 Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Ozonation; biological activated carbon (BAC); dissolved organic carbon (DOC); trihalomethane (THM), biodegradable DOC. INTRODUCflON Synthetic organic chemicals and disinfection by-products (OBP) such as trihalomethanes (THM) in drinking water are of interest because of their potential mutagenicity, carcinogenicity, and toxicity (WHO. 1989). It is well known that these substances and precursors of DBP are difficult to remove by conventional drinking water treatment processes of coagulation, sedimentation followed by rapid sand filtration (Tsutsumi, 1989). Biological activated carbon (HAC) treatment is known to be one of the most promising processes for the removal of synthetic organic chemicals and precursors of DBP by simultaneous adsorption by activated carbon and biodegradation by bacteria attached on activated carbon. However, many synthetic organic chemicals such as pesticides and herbicides, and precursors of DBP such as humic substance in raw water for drinking purposes are refractory organic substances (Tsutsumi. 1989; Kim et aL, 1997a), This implies that significant biodegradation of organic substances on BAC can not be expected. 163
W. NISHUlMA tit al.
164
Preozonation followed by BAC is an effective process to improve removal of DOC and, subsequently, formation potential of DBP (Glaze et al., 1984; Maloney et al., 1985; Kaastrup et al., 1989; Bablon et aL, 1989; Suzuki et al., 1989; Dewaters et al., 1990). It is well known that ozonation can convert refractory organic substances into biodegradable organic substances (Malley et al., 1993; Murphy et al., 1993; Yolk et al., 1993). Using the classification scheme of DOC based on their adsorbability and biodegradability (Nishijima et al., 1996), the fate of biodegradable DOC produced by ozonation on BAC was analyzed with fulvic acid as influent in a laboratory scale experiment (Kim et al., 1997b). They also conducted similar studies on a pilot scale plant with raw water from a reservoir as influent (Kim et aL, 1997a). They reported that biodegradable DOC had possibilities to be removed both by adsorption and biodegradation. However, most of them were removed by biodegradation on BAC even after saturation of adsorption capacity for DOC, and biodegradation of biodegradable DOC produced by ozonation on BAC was responsible for the higher removal of DOC after saturation in an ozone-BAC process in comparison with simple BAC process. However, few studies have reported on changes in effluent quality during a long term use of the ozonation• BAC process and on the fate of biodegradable DOC on BAC. The BAC process with preozonation (ozonation-BAC process) is expected to show high DOC and THMFP removal for long term operation without replacement of GAC, because considerable DOC and THMFP can be removed by biodegradation on BAC. The objective of this study is to evaluate removal of DOC and THMFP in an ozonation-BAC process for long term operation. A pilot scale plant with raw water from an eutrophic reservoir was operated for 910 days. Raw water Simple SAC process
MF membrane
.. p Ozonation Retention SAC rector reactor column Figure I. Schematic diagram of the pilot scale plant
MATERIALS AND METHODS Plant confi~uration and operation parameters This study was carried out using a pilot scale plant with raw water coming from Minaga Reservoir, Hiroshima, Japan. Figure I shows a schematic diagram of the pilot scale plant. Parallel runs were conducted using an ozonation - BAC process and a simple BAC process. The raw water used was pre-treated by microfiltration. The membrane module used here is a hollow fiber type made from polypropylene (Japan 2 Memtech) with 0.2 mm of pore size and effective membrane surface area of the module is 1.0 m . The ozonation reactor has a working volume of 3.9 1. Contact time for ozonation was 24 min and ozone dosage
Ozonation-biological acivated carbon process
163
was 2.5 mg 0 3 mg C l . A retention column of the same size as the ozonation reactor was setup to remove residual ozone and was operated with 24 min of contact time. BAC reactors have working volume of 2.9 I packed with 2.5 I of GAC and were operated with an upflow mode and an empty bed contact time (BBCT) of 15 min. GAC used here was Calgon filtrasorb 400 with average particle size of 1 mm. Both GAC in the ozone-BAC and the simple BAC processes were not regenerated during the experimental period. The pilot plant was operated for 910 days (420 days in the simple BAC process). Analytjcal methods DOC was classified into ADOC, BDOC, A&BDOC and NRDOC based on biodegradability and adsorbability on activated carbon (Nishijima et al., 1996). ADOC is defined as the organic carbon which adsorbs on activated carbon and is not biodegradable. BDOC is the organic carbon which is biodegradable but does not adsorb on activated carbon. A&BDOC is the organic carbon which has both adsorbability and biodegradability, while NRDOC is the organic carbon which does not adsorb on activated carbon and is not biodegradable. Samples were concentrated at 40°C by rotary evaporator. The concentrate was sterilized by filtration through a 0.2 mm polycarbonate membrane filter (Costar). Adsorbable DOC was analyzed as follows: I g of powdered activated carbon (PAC) was placed in 100 ml of the sterilized sample. The mixture was shaken for 3 days on a rotary shaker at 125 rpm, 20°C. The removed DOC was defined as adsorbable DOC (ADOC + A&BDOC). Biodegradable DOC was determined following the procedure by Servais et aL (1987). Inoculum was taken from the Minaga Reservoir. The reservoir water was filtered through a 2 mm polycarbonate membrane filter (Costar) before use to remove phytoplankton, zooplankton, and other suspended solids. The volume of inoculum was 2 ml to 200 ml of the filtered sample. Incubation was conducted in the dark at 20°C for one week. The removed DOC was defined as biodegradable DOC (BDOC + A&BDOC). The remainder after the determination of biodegradable DOC consists of ADOC and NRDOC. The same procedure as the determination of adsorbable DOC was adopted. The removed DOC is ADOC and the residual DOC is NRDOC. A&BDOC is determined by the difference between (ADOC + A&BDOC) and ADOC. BDOC is determined by (BDOC + A&BDOC) - (A&BDOC). DOC, UV absorbance in 260 nm and THMFP were determined on filtered samples (Whatman OF/C). DOC was determined using a TOC analyzer (Shimadzu TOC-5000A). B260 was determined using a UV spectrophotometer (Shimadzu UV-180) with quartz cell of 1 em light path. THMFP was determined using the method outlined in the Standard Methods for the Analysis of Drinking Water (Japan Water Supply Association, 1993). Chlorination was performed by addition of sodium hypochlorite solution to 200 ml saIllple. The dose was determined to ensure a residual chlorine concentration between 1 to 2 mg I-I at the end of incubation period. The pH was adjusted to 7.0 with 0.2 M phosphate buffer solution. Samples were transferred to 160 ml glass vials, sealed without headspace and incubated in the dark at 20°C for 24 h. Residual chlorine was measured by the DPD colorimetric method. The excess chlorine in the samples was quenched with Na2S03 (0.5 WN%). THM analysis was carried out by the head space method using a gas chromatograph with an electron capture detector (Shimadzu GC-14B). RESULTS AND DISCUSSION ~ater Quality of raw
water
Long term variation of water temperature and pH in the Minaga reservoir (raw water) is shown in Fig. 2. Water temperature ranged from 5 to 30°C and pH was almost neutral (6.5-7.7) throughout the experimental period. Figure 3 shows DOC and THMFP in the influent. DOC was relatively stable for 910 days of operation (2.8-4.2 mg I-I) .except for days 6OO!0 660 when high DOC (4.4-5.7 mg I-I) was noted. Figure 4 shoWS fractions of OOC 10 raw water. DOC 10 raw water contained less biodegradable DOC (BDOC +
W. NISHIJIMA el al.
166
A&BDOC) which was only about 6% of total DOC. More than 90% of DOC wa~ adsorbable in activated carbon. Significant changes in DOC fractions were not noted during the operation period of 910 days. 9
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Time(d) Figure 2. Long tenn variation of water temperature and pH in the raw water.
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Time(d) Figure 3. Long lenn variation of DOC and THMFP in the raw water.
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Time (d) Figure 4. Long tenn variation of DOC fractions in the raw water.
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Ozonalion-biological acivatcd carbon process
167
THMFP in raw water ranged from 70 to 156 Ilg I-I. The ratio of THMFP to DOC fluctuated from 22 to 58 (Ilglm g). This suggests characteristc fluctuation of DOC in raw water during the operation period of 910 days.
W:ater QyaHty of effluent Figures 5 and 6 show DOC and THMFP in the membrane filtrate, the ozonated water and the BAC effluents, respectively. DOC removal by ozonation was small at about 9%, however, THMFP removal by ozonation was relatively large at about 22%. Effluent DOC and THMFP concentrations in the BAC, the ozonation• BAC, and the simple BAC processes, were almost the same at the start of operation. Effluent DOC increased rapidly after day 80. After day 80, effluent DOC and THMFP concentrations in the BAC and ozonation• BAC processes were lower than those in the simple BAC process.
5r------:-:=-----,...-----....., 4
2SO
500
7SO
1000
Time (d) Figure 5. DOC in the membrane filtrate (MF), the ozonated water (Oy and the BAC effluent in the ozonalion-BAC process (OJ-BAC) and BAC effluent in the simple BAC procell (SAC). 200
_ _ MF OJ
ISO
O).BAC BAC
-,; ::
w
100
~
Il.
~
~
so 0 0
250
soo
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Ttrne (d) Figure 6. THMFP in the membrane filtrate (MF). the ozonated water (Oy and the BAC effluent in the ozonalion• BAC process (03-BAC) and BAC effluent in the simple BAC process (BAC).
Figure 7 shows DOC fractions in the membrane filtrate, the ozonated water and the BAC effluents, respectively. Effluent DOC from membrane filtration had a small amount of biodegradable DOC (7%), but zonation increased this fraction from 7% to 32%. Biodegradable DOC was removed effectively on BAC ~th in the ozonation-BAC and simple BAC processes throughout 910 days of operation.
W. NISHIJIMA et al.
16M
Adsorption capacity of activated carhon for DOC was saturated at around day 300, because ADOC which could he removed only hy adsorption was hardly removed after day 300 (see Fig.7). High DOC and THMFP removal were maintained at 36% and 57%, respectively, in the ozonation-BAC process even after saturation of activated carhon for DOC. DOC and THMFP removal by ozonation was 8% and 24% for the same period. respectively. High DOC and THMFP removal after saturation was due to the increase of hiodegradahle DOC hy ozonation and following biodegradation of biodegradable DOC by bacteria auached on BAC. Non-hiodegradahle DOC (ADOC + NRDOC) was not removed after day 300. 4
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Time (d) Figure 7. DOC fractions in the membrane. ozonated water and BAe eflluents.
Production of hiodegradable DOC by ozonation was not significantly different in the experimental period. Seasonal changes in DOC and THMFP removal was not observed in the ozonation-BAC process. Bioactivity on BAC decreases generally in the winter season. However, DOC removal was 35% in the period with water temperature helow I We, which was almost the same as in other periods with high water temperature. Biodegradahle DOC was not detected in BAC rffluent even in the period when the water temperature was helow 10DC, These facts indicate that an EBCT f 15 min was enough for biodegradation of biodegradable DOC, if water temperatures ranged from 5 to 30 D C and concentration of biodegradable DOC ranged from 0.50 to o.\)\) mg I-I. CONCLUSIONS This study evaluated the removal of DOC and THMFP in a pilot scale ozonation-BAC process for 910 days. Raw water came from an eutrophic reservoir. The specific conclusions derived from this study are as follows. I. ()zollatioll increased hiodegradable DOC from 7% to 32%. Production of biodegradable DOC by o/.Ollatioll was not significantly changed throughout the experimental period. . 2. High DOC and THMFP removal was maintained in the ozonation-BAC process even after saturation WIth DOC, which was 36% and 57%, respectively. The high DOC and THMFP removal was due to the increase in hiodegradable DOC hy ozonation and the subsequent biodegradation by bacteria attached on BAC. 3. Although water temperature changed in the range from 5 to 30D C, seasonal changes in DOC and THMFP removal were not ohserved in the ozonation-BAC process.
Ozonation-biological acivated carbon process
169
REFERENCES Bablon, G. and Dagois, G. (1989). Recent development in the use of ozone and biological activated carbon reactors. Water Nagoya 89. ASPAC IWSA, 483-492. Dewaters, J. E. and DiGiano, F. A. (1990). The influence of ozonated natural organic matter on the biodegradation of a micropollutant in a GAC bed. Journal AWWA, 82, 69-75. Glaze W. H. and Wallace, J. L. (1984). Control of trihalomethane precursors in drinking water: granular activated carbon with , and without preozonation. Journal AWWA, 76, 68-75. J an Water Supply Association (1993). Standard methods/or the analysis 0/ drinJcing water, pp. 389-391. ~trup, E. and Halmo, T. M. (1989). Removal of aquatic humus by ozonation and activated-carbon adsorption. Advances in Chemistry 219, Suffel, 1. H. and MacCarty, P. (eds), American Chemical Society, Washington, DC, pp. 697-726. Kim, W. H., Nishijima, W., Shoto, E. and Okada, M. (1997a). Pilot plant study on ozonation and biological activated carbon process for drinking water treatment. Wat. Sci. Tech.• 35(8), 21-28. Kim, W. H., Nishijima, W., ~hoto: E. an~ Okada, M. (1997b). C::0mpetitive removal of dissolved organic carbon by adsorption and biodegradation on biological acttvated carbon. Wat. SCL Tech., 35(7), 147-153. Malley Jr., J. P., Eighmy, T. T., Collins, M. R., Royce, J. A. and Morgan, D. F. (1993). The performance and microbiology of ozone-enhanced biological filtration. Journal AWWA, 8S, 47-57. Maloney, S.W., Suffel, I.H., Bancroft, K. and Neukrug, H.M. (1985). Ozone-GAC following conventional drinking water treatment. Journal AWWA, 77, 66-73. Mu hy, B.. Amy, G. and Siddiqui, M. (1993). Ozone-induced conversion of DBP precursors (DOC) to biodegradable by-products rp (BDOC). AWWA /993 Annual Conference Proceedings (Water Quality), 389-416. Nishijima, W.• Kim, W. H., Shoto, E. ~d Oka~!d' (1996). Oassification of dissolved organic carbon based on adsorb ability against activated carbon and blodegradablhty. Journal 0/ Japan Water Worb Association, 65, 1-6. Servais. P., Billen, G. and Hascoe\, H.-C. (1987). Determination of the biodegradable fraction of dissolved organic matter in waters. Wat. Res., 21. 445-450. Suzuki, S., Magara, Y., Kaigo, N., Ishikawa, K. and Nishijima, M. (1989). Advanced water purification treatment by ozone and biological activated carbon. Water Nagoya 89, ~SPA.C IW~" ~03.511. Tsutsumi, Y. (1989), Advanced treatment on water punficatton factllttes. Japan Journal 0/ Water Pollution Research, 11, 146-
ISO. Volk, C., Renner, C., Roche, P., Paill,ard, H. and Jore\, J. C. (1993).. Effects of ozone on the production of biodegradable dissolved organic carbon (BDOC) dunng water treatment. Ozone:ScL& Eng., IS, 389-404. WHO (1989). Guidelines for drinking water quality.
Wat. Sci. T«h. Vol. 38, No.6, pp. 163-169, 1998. lAWQ
Pergamon
PII: S0273-1223(98)OO578-2
C 1998 Published by Ebevicr Science lAd. Prinled in Great Britain. All riahll reserved 0273-1223198 $19"00 +0-00
THE PERFORMANCE OF AN
OZONATION-BIOLOGICAL ACTIVATED CARBON PROCESS UNDER LONG TERM OPERATION Wataru Nishijima, Woo Hang Kim, Eiji Shoto and Mitsumasa Okada Faculty ofEngineering, Hiroshima University, J -4-J Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan
ABSTRACf 1be objective of this study is 10 evaluate removal of DOC and 1HMFP during long term operation of an
olonation-biological activated carbon (BAC) process. A pilol scale planl with raw waler in an eutrophic reservoir for drinking water supply was operated for 910 days. High DOC and 1HMFP removal were maintained al 36% and S7%, respectively, in the olonation-BAC process even after saturation of BAC by DOC. DOC and THMFP removal by ozanation were only 8% and 24%, respectively. High DOC and THMFP removal after saturalion was due 10 the increase in biodeeradable DOC by ozanation from 7% to 32% and the subsequenl biodegradation by bacteria at!ached on BAC. Although water temperature changed in the range from S to 30"C, seasonal changes in DOC lind 1HMFP removal were not observed in the ownation-BAC process. C 1998 Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Ozonation; biological activated carbon (BAC); dissolved organic carbon (DOC); trihalomethane (THM), biodegradable DOC. INTRODUCflON Synthetic organic chemicals and disinfection by-products (OBP) such as trihalomethanes (THM) in drinking water are of interest because of their potential mutagenicity, carcinogenicity, and toxicity (WHO. 1989). It is well known that these substances and precursors of DBP are difficult to remove by conventional drinking water treatment processes of coagulation, sedimentation followed by rapid sand filtration (Tsutsumi, 1989). Biological activated carbon (HAC) treatment is known to be one of the most promising processes for the removal of synthetic organic chemicals and precursors of DBP by simultaneous adsorption by activated carbon and biodegradation by bacteria attached on activated carbon. However, many synthetic organic chemicals such as pesticides and herbicides, and precursors of DBP such as humic substance in raw water for drinking purposes are refractory organic substances (Tsutsumi. 1989; Kim et aL, 1997a), This implies that significant biodegradation of organic substances on BAC can not be expected. 163
W. NISHUlMA tit al.
164
Preozonation followed by BAC is an effective process to improve removal of DOC and, subsequently, formation potential of DBP (Glaze et al., 1984; Maloney et al., 1985; Kaastrup et al., 1989; Bablon et aL, 1989; Suzuki et al., 1989; Dewaters et al., 1990). It is well known that ozonation can convert refractory organic substances into biodegradable organic substances (Malley et al., 1993; Murphy et al., 1993; Yolk et al., 1993). Using the classification scheme of DOC based on their adsorbability and biodegradability (Nishijima et al., 1996), the fate of biodegradable DOC produced by ozonation on BAC was analyzed with fulvic acid as influent in a laboratory scale experiment (Kim et al., 1997b). They also conducted similar studies on a pilot scale plant with raw water from a reservoir as influent (Kim et aL, 1997a). They reported that biodegradable DOC had possibilities to be removed both by adsorption and biodegradation. However, most of them were removed by biodegradation on BAC even after saturation of adsorption capacity for DOC, and biodegradation of biodegradable DOC produced by ozonation on BAC was responsible for the higher removal of DOC after saturation in an ozone-BAC process in comparison with simple BAC process. However, few studies have reported on changes in effluent quality during a long term use of the ozonation• BAC process and on the fate of biodegradable DOC on BAC. The BAC process with preozonation (ozonation-BAC process) is expected to show high DOC and THMFP removal for long term operation without replacement of GAC, because considerable DOC and THMFP can be removed by biodegradation on BAC. The objective of this study is to evaluate removal of DOC and THMFP in an ozonation-BAC process for long term operation. A pilot scale plant with raw water from an eutrophic reservoir was operated for 910 days. Raw water Simple SAC process
MF membrane
.. p Ozonation Retention SAC rector reactor column Figure I. Schematic diagram of the pilot scale plant
MATERIALS AND METHODS Plant confi~uration and operation parameters This study was carried out using a pilot scale plant with raw water coming from Minaga Reservoir, Hiroshima, Japan. Figure I shows a schematic diagram of the pilot scale plant. Parallel runs were conducted using an ozonation - BAC process and a simple BAC process. The raw water used was pre-treated by microfiltration. The membrane module used here is a hollow fiber type made from polypropylene (Japan 2 Memtech) with 0.2 mm of pore size and effective membrane surface area of the module is 1.0 m . The ozonation reactor has a working volume of 3.9 1. Contact time for ozonation was 24 min and ozone dosage
Ozonation-biological acivated carbon process
163
was 2.5 mg 0 3 mg C l . A retention column of the same size as the ozonation reactor was setup to remove residual ozone and was operated with 24 min of contact time. BAC reactors have working volume of 2.9 I packed with 2.5 I of GAC and were operated with an upflow mode and an empty bed contact time (BBCT) of 15 min. GAC used here was Calgon filtrasorb 400 with average particle size of 1 mm. Both GAC in the ozone-BAC and the simple BAC processes were not regenerated during the experimental period. The pilot plant was operated for 910 days (420 days in the simple BAC process). Analytjcal methods DOC was classified into ADOC, BDOC, A&BDOC and NRDOC based on biodegradability and adsorbability on activated carbon (Nishijima et al., 1996). ADOC is defined as the organic carbon which adsorbs on activated carbon and is not biodegradable. BDOC is the organic carbon which is biodegradable but does not adsorb on activated carbon. A&BDOC is the organic carbon which has both adsorbability and biodegradability, while NRDOC is the organic carbon which does not adsorb on activated carbon and is not biodegradable. Samples were concentrated at 40°C by rotary evaporator. The concentrate was sterilized by filtration through a 0.2 mm polycarbonate membrane filter (Costar). Adsorbable DOC was analyzed as follows: I g of powdered activated carbon (PAC) was placed in 100 ml of the sterilized sample. The mixture was shaken for 3 days on a rotary shaker at 125 rpm, 20°C. The removed DOC was defined as adsorbable DOC (ADOC + A&BDOC). Biodegradable DOC was determined following the procedure by Servais et aL (1987). Inoculum was taken from the Minaga Reservoir. The reservoir water was filtered through a 2 mm polycarbonate membrane filter (Costar) before use to remove phytoplankton, zooplankton, and other suspended solids. The volume of inoculum was 2 ml to 200 ml of the filtered sample. Incubation was conducted in the dark at 20°C for one week. The removed DOC was defined as biodegradable DOC (BDOC + A&BDOC). The remainder after the determination of biodegradable DOC consists of ADOC and NRDOC. The same procedure as the determination of adsorbable DOC was adopted. The removed DOC is ADOC and the residual DOC is NRDOC. A&BDOC is determined by the difference between (ADOC + A&BDOC) and ADOC. BDOC is determined by (BDOC + A&BDOC) - (A&BDOC). DOC, UV absorbance in 260 nm
water
Long term variation of water temperature and pH in the Minaga reservoir (raw water) is shown in Fig. 2. Water temperature ranged from 5 to 30°C and pH was almost neutral (6.5-7.7) throughout the experimental period. Figure 3 shows DOC and THMFP in the influent. DOC was relatively stable for 910 days of operation (2.8-4.2 mg I-I) .except for days 6OO!0 660 when high DOC (4.4-5.7 mg I-I) was noted. Figure 4 shoWS fractions of OOC 10 raw water. DOC 10 raw water contained less biodegradable DOC (BDOC +
W. NISHIJIMA el al.
166
A&BDOC) which was only about 6% of total DOC. More than 90% of DOC wa~ adsorbable in activated carbon. Significant changes in DOC fractions were not noted during the operation period of 910 days. 9
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Time(d) Figure 2. Long tenn variation of water temperature and pH in the raw water.
200
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150
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100
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Time(d) Figure 3. Long lenn variation of DOC and THMFP in the raw water.
5
.-
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o
A&BDOC
•
AD
•
BDO
cO
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1000
Time (d) Figure 4. Long tenn variation of DOC fractions in the raw water.
:I: f-
Ozonalion-biological acivatcd carbon process
167
THMFP in raw water ranged from 70 to 156 Ilg I-I. The ratio of THMFP to DOC fluctuated from 22 to 58 (Ilglm g). This suggests characteristc fluctuation of DOC in raw water during the operation period of 910 days.
W:ater QyaHty of effluent Figures 5 and 6 show DOC and THMFP in the membrane filtrate, the ozonated water and the BAC effluents, respectively. DOC removal by ozonation was small at about 9%, however, THMFP removal by ozonation was relatively large at about 22%. Effluent DOC and THMFP concentrations in the BAC, the ozonation• BAC, and the simple BAC processes, were almost the same at the start of operation. Effluent DOC increased rapidly after day 80. After day 80, effluent DOC and THMFP concentrations in the BAC and ozonation• BAC processes were lower than those in the simple BAC process.
5r------:-:=-----,...-----....., 4
2SO
500
7SO
1000
Time (d) Figure 5. DOC in the membrane filtrate (MF), the ozonated water (Oy and the BAC effluent in the ozonalion-BAC process (OJ-BAC) and BAC effluent in the simple BAC procell (SAC). 200
_ _ MF OJ
ISO
O).BAC BAC
-,; ::
w
100
~
Il.
~
~
so 0 0
250
soo
7SO
tOOO
Ttrne (d) Figure 6. THMFP in the membrane filtrate (MF). the ozonated water (Oy and the BAC effluent in the ozonalion• BAC process (03-BAC) and BAC effluent in the simple BAC process (BAC).
Figure 7 shows DOC fractions in the membrane filtrate, the ozonated water and the BAC effluents, respectively. Effluent DOC from membrane filtration had a small amount of biodegradable DOC (7%), but zonation increased this fraction from 7% to 32%. Biodegradable DOC was removed effectively on BAC ~th in the ozonation-BAC and simple BAC processes throughout 910 days of operation.
W. NISHIJIMA et al.
16M
Adsorption capacity of activated carhon for DOC was saturated at around day 300, because ADOC which could he removed only hy adsorption was hardly removed after day 300 (see Fig.7). High DOC and THMFP removal were maintained at 36% and 57%, respectively, in the ozonation-BAC process even after saturation of activated carhon for DOC. DOC and THMFP removal by ozonation was 8% and 24% for the same period. respectively. High DOC and THMFP removal after saturation was due to the increase of hiodegradahle DOC hy ozonation and following biodegradation of biodegradable DOC by bacteria auached on BAC. Non-hiodegradahle DOC (ADOC + NRDOC) was not removed after day 300. 4
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1000
Time (d) Figure 7. DOC fractions in the membrane. ozonated water and BAe eflluents.
Production of hiodegradable DOC by ozonation was not significantly different in the experimental period. Seasonal changes in DOC and THMFP removal was not observed in the ozonation-BAC process. Bioactivity on BAC decreases generally in the winter season. However, DOC removal was 35% in the period with water temperature helow I We, which was almost the same as in other periods with high water temperature. Biodegradahle DOC was not detected in BAC rffluent even in the period when the water temperature was helow 10DC, These facts indicate that an EBCT f 15 min was enough for biodegradation of biodegradable DOC, if water temperatures ranged from 5 to 30 D C and concentration of biodegradable DOC ranged from 0.50 to o.\)\) mg I-I. CONCLUSIONS This study evaluated the removal of DOC and THMFP in a pilot scale ozonation-BAC process for 910 days. Raw water came from an eutrophic reservoir. The specific conclusions derived from this study are as follows. I. ()zollatioll increased hiodegradable DOC from 7% to 32%. Production of biodegradable DOC by o/.Ollatioll was not significantly changed throughout the experimental period. . 2. High DOC and THMFP removal was maintained in the ozonation-BAC process even after saturation WIth DOC, which was 36% and 57%, respectively. The high DOC and THMFP removal was due to the increase in hiodegradable DOC hy ozonation and the subsequent biodegradation by bacteria attached on BAC. 3. Although water temperature changed in the range from 5 to 30D C, seasonal changes in DOC and THMFP removal were not ohserved in the ozonation-BAC process.
Ozonation-biological acivated carbon process
169
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