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Bromate removal during transition from new granular activated carbon (GAC) to biological activated carbon (BAC)
PII: S0043-1354(98)00504-1
Wat. Res. Vol. 33, No. 12, pp. 2797±2804, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
www.elsevier.com/locate/watres
BROMATE REMOVAL DURING TRANSITION FROM NEW GRANULAR ACTIVATED CARBON (GAC) TO BIOLOGICAL ACTIVATED CARBON (BAC) MARI ASAMI1*, TAKAKO AIZAWA1, TAKAYUKI MORIOKA2, M M M WATARU NISHIJIMA3* , AKIHISA TABATA4* and YASUMOTO MAGARA4* Department of Water Supply Engineering, National Institute of Public Health, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8638, Japan; 2Water Treatment and Bioelectronic Laboratory, Fuji Electric Corporate R&D, Ltd., Kanagawa, Japan; 3Faculty of Engineering, Hiroshima University, Hiroshima, Japan and 4Department of Urban Environmental Engineering, Faculty of Engineering, Hokkaido University, Sapporo, Japan 1
(First received April 1998; accepted in revised form November 1998) AbstractÐBromate removal by activated carbon after ozonation is a subject of concern, since bromate is commonly found in the ozonation of bromide-containing water. Though new GAC (granular activated carbon) shows the capacity to reduce bromate to bromide, in the long-term use of GAC following ozonation, its bromate removal rate apparently decreases during transition from new GAC to BAC (biological activated carbon) after 3 months. Batch bromate reduction experiments using new GAC and BAC con®rmed new GAC's ability and BAC's inability to reduce bromate to bromide. Our experiment also indicated that ion exchangeable bromate adsorption on new GAC was very limited. Based on the results of our long-term experiment, the bromate removal rate during the transition from new GAC to BAC was calculated; 1.5 mg BrO3/g carbon when bromate concentration was 50 mg/l. BAC's inability to reduce bromate makes it necessary to optimize ozonation conditions to minimize the formation of bromate and other by-products while maintaining target levels of organic matter decomposition. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐdrinking water, bromate, chloride, GAC, biological activated carbon (BAC)
INTRODUCTION
Advanced water treatment in Japan has been applied to an increasing number of drinking water treatment plants in order to degrade odorous substances and trihalomethane precursors. Ozonation is mainly used as unit process to eliminate oensive odor and taste (Moniwa, 1995). Although ozonation does not yield chlorine-containing by-products, it yields larger amounts of aldehydes and other oxidized organic by-products than chlorination. It has been pointed out that the increase in amount of biodegradable organic substances due to ozonation reduces bacterial stability in drinking water distribution network. Ozonation also yields inorganic byproducts. Ozonation of water that contains bromide (ions) yields bromate, a possible carcinogen, though the bromate formation is partly restricted with coexisting organic matters (Asami et al., 1996). Bromate usually exists in its ionic form (BrOÿ 3 ) in water. The maximum allowable bromate level is still a subject of discussion: 25 mg/l according to provisional WHO guidelines and 10 mg/l according to *Author to whom all correspondence should be addressed. [Tel.: +81-3-3441-7111 ex. 273; fax: +81-3-3446-4314; e-mail: [email protected]].
proposed EU directives and U.S. regulations. The authors reported bromate concentrations in Japanese drinking water treatment processes between 7 mg/l and below QDL (quantitative detection limit) after ozonation and below QDL in all ®nished water. The QDL is 2 mg/l. A much higher concentration, approximately 50 mg/l, was detected in water from an experimental ozonation plant, which originally contained bromide at a 300 mg/l level (Asami et al., 1997). To eliminate ozonation by-products such as aldehydes and ketones, granular activated carbon (GAC) is applied as a posttreatment following ozonation. The GAC process results in the adsorption of organic matters, chemical reduction and mechanical ®ltration. GAC in actual water treatment processes naturally breeds a bacterial population on its surface, and eventually it works as biological activated carbon (BAC). GAC with a biomass that shows high biological activity is called BAC and it degrades biodegradable organic matter and ammonia (Nishijima et al., 1992). With the help of the bioactivity on BAC, the reduction of total organic carbon (TOC) and other organic substances is considered to maintain a longer service time before GAC regeneration (Baker et al., 1996). The combi-
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nation of ozonation and biological treatment with BAC enhances the degradation of organic substances, though it slightly degrades trihalomethane precursors and aldehydes (Cipparone et al., 1997). Many ozonation processes are followed by the GAC process which eventually works as the BAC process after some time. Therefore, the question remains whether BAC can reduce bromate or not. At present, the number of reports on activated carbon adsorption and removal of ionic substances, especially on BAC, is limited. There have been some reports of bromate reduction by GAC (Marhaba and Medlar, 1993) and new activated carbon (Yamada, 1993). The mechanism of bromate removal by activated carbon was analyzed by Siddiqui et al. (1996), in terms of surface conditions and properties of activated carbons, such as powdered activated carbon (PAC) and GAC. There are only few researches on the removal of ionic substances, especially on bromate removal by BAC in the water puri®cation process, though bromate behavior in new GAC and in BAC should be dierent. It is necessary to determine the mechanism and the duration for bromate reduction by GAC or BAC to satisfactory low levels in a treatment plant. This study deals with the behavior of bromate in new GAC and old GAC, i.e. BAC in an experimental ozonation-GAC/BAC water treatment plant. A batch experiment is also conducted in the laboratory to analyze bromate and bromide ion reduction and removal by GAC and BAC. MATERIALS AND METHODS
Bromate removal using GAC/BAC in an experimental treatment plant Bromate ion removal was observed at an experimental water treatment plant downstream of Minaga Lake, Hiroshima, starting in August 1995. Figure 1 shows the experimental setup that consists of an ozone contactor, a retention column and an activated carbon treatment tank,
all of which have a diameter of 10 cm and a height of 70 cm. Micro®ltred water (MF water) was introduced to the mixing tank. The ¯ow rate was basically set at 19.2 l/h (SV = 8). Activated carbon (Calgon F-400, 1024 g) was placed in a column with a diameter of 10 cm. The amount of dissolved organic carbon in MF water ranged from 2 to 3 mg/l. Ammonia was rarely detected during the experiments. The number of bacteria adhering on BAC was determined by direct counting, using an epi¯uorescence microscope after collecting approximately 0.2 g of activated carbon in the column, as described by Nishijima et al. (1997). No bromate was detected in raw water. In the long-term experiment, micro®ltered lake water with 50 mg BrO3/l of potassium bromate was continuously passed through the columns and sampled every 20 days. Ozonation was stopped during the sampling time of the long-term continuous experiment and during the shortterm experiment. In the short-term experiment, new GAC, two-month old GAC (BAC1) that had already been BAC, and one-year old GAC (BAC2) were examined. In each experiment, MF water containing 250 mg BrO3/l standard bromate was passed through the column for 6 h. Then MF water was passed through the column for more than 12 h after each experiment. Bromate reduction and adsorption in batch experiments A small amount of activated carbon was collected from the experimental treatment plant during the experiments, and batch experiments were conducted to clarify the chemical reaction of bromate with an activated carbon surface. Bromate removal by new GAC, one-year old GAC (BAC1) and two-year old GAC (BAC2) was examined in 100-ml ¯asks. Approximately 1 g of each activated carbon was added to a ¯ask that was ®lled with 100 ml of 250 mg/l bromate solution or 1 mg/l bromide solution and nitrogen gas. Samples were shaken for 1, 2, 3 or 7 days at 208C in a continuous shaker. Then they were ®ltered through a 0.45-mm membrane ®lter, and ®ltrates were analyzed by ion chromatography. The activated carbons were weighed after drying in a 1108C oven for 24 h. Because bromide was not chemically reduced by activated carbon, bromide ion adsorption was investigated using 0 to 10 gGAC/l for 1 mg/l of bromide for a 48-h contact time. Ion exchange capacity of activated carbon Each of the four samples of 0.0013 g GAC (washed and dried in a 1308C oven) were put in 100 ml of 10 mg/l bro-
Fig. 1. Schematic diagram of ozone-GAC/BAC pilot plant.
Bromate ion behavior on GAC and BAC mate solution. The solution was stirred for 2 h at 208C. Each activated carbon that adsorbed bromate was collected and then added to duplicate samples of pure water and 20 mg/l chloride ion (sodium chloride). After 2 h, samples were ®ltered through a 0.45-mm membrane ®lter and analyzed by ion chromatography. Bromate and bromide quanti®cation by ion chromatography Dionex ion exchange column, AS9SC/AG9SC and borate eluent (40 mM H3BO4+20 mM NaOH) were selected for better bromate separation. Ion chromatography (Yokogawa IC-7000) with an ultraviolet absorbance detector (210 nm) was employed for bromate analysis. A sample (250 ml) was injected after silver cartridge (Lida Co., Ltd.) ®ltration at a ¯ow rate of 1 ml/min to remove coexisting chlorides and bromides. The lowest QDL was 2 mg/l when coecient of variation was 10% (n = 5). For bromide analysis, Dionex DX-100 ion chromatograph with AS9SC/AG9SC column and carbonate eluent (1.6 mM Na2CO3+1.5 mM NaHCO3) were employed. A sample (50 ml) was injected without silver cartridge ®ltration then detected by electric conductivity detector. The detection methods of bromate and bromide were described elsewhere (Asami et al., 1997) in detail. RESULTS
Continuous bromate removal by GAC/BAC monitored at the treatment plant In the experimental treatment plant, new GAC was gradually transformed into BAC over a period of 2±10 months, which was determined from the bacterial count on the surface of the activated carbon. Bacteria concentration was 6.1 107 cells/g carbon at 3 months and 1.6 108 cells/g carbon after 1 year, both at the bottom part of the carbon column. TOC removal ratio was 100% at the beginning of the experiment and gradually decreased after 5 months. Monthly changes in the bromate removal were observed at the plant during the transition from new GAC into BAC. Bromate concentration after the GAC/BAC column is shown in Fig. 2. In the beginning, new GAC reduced over 60% of bromate, then the bromate removal ratio started to decrease after 50 days and there was reached almost
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no removal after 3 months. Total bromate removal ratio in the long-term experiment during the transition from new GAC to BAC was approximately 1.5 mg bromate/g activated carbon. From these results, the breakthrough of bromate was completed before the breakthrough of TOC in BAC, though Siddiqui et al. (1996) pointed out that the breakthrough of natural organic matter was completed before bromate breakthrough in GAC. Short-term experiment: bromate removal by new GAC/BAC To clarify bromate behavior in new GAC and BAC, short-term experiments on bromate removal were conducted at the treatment plant for new GAC, BAC1 and BAC2. Figure 3(a) shows the results of short-term tests of 250 mg/l bromate passing through the GAC/BAC column. No bromate was detected with new GAC treatment throughout the experiment. With BAC1, bromate concentration gradually increased during the 6-h experiment, and the apparent bromate removal ratio was approximately 70% after 6 h. With BAC2, bromate concentration showed a relatively rapid increase from the beginning of the experiment and reached the original bromate concentration in 150 min, while calculated empty bed contact time (EBCT) was 15 min. Bromate concentration after BAC2 reached a constant as the same concentration as the loaded bromate concentration after a few hours. The bromide concentration increased while bromate was passed through the GAC/BAC column in each short-term experiment, as shown in Fig. 3(b). The tendency obviously diers among GAC/BAC experiments, though bromide concentration showed some variance, since source water originally contains approximately 30 mg/l bromide on the average and since bromide QDL is 20 mg/l. In the case of new GAC, the bromate ion concentration decreased. The concentration of bromide detected in treated water was lower than that detected in MF water. Therefore new GAC is considered to adsorb bromide
Fig. 2. Continuous bromate removal by GAC/BAC. Initial bromate concentration 50 mg/l.
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Fig. 3. Bromate and bromide behavior in GAC/BAC in the short-term experiment. (a) Bromate concentration after GAC/BAC (initial bromate = 250 mg/l). (b) Bromide after GAC/BAC during bromate reduction experiment.
which was contained in MF water and bromide which originated from reduced bromate. After BAC1, bromide concentration increased due to chemical reduction of bromate. BAC1 seemingly showed little capacity of bromide adsorption. In the case of BAC2 used for more than 1 year, bromate was not reduced or adsorbed and was therefore detected in their original form and concentration. These results were parallel to the phenomenon observed by Siddiqui et al. (1996): bromate removal by activated carbon consists of two steps, adsorption and chemical reduction to bromide. Cumulative bromate removal and resolution in shortterm experiment To characterize bromate reduction by BAC, the accumulation of bromate and the resolution of bromate and bromide in the BAC column was calculated from the results of the bromate removal experiment and the following resolution experiment; 100 mg/l bromate solution was loaded for 6 h in the removal experiment and successively MF water was loaded for 2 h in the resolution experiment. The amount of accumulated bromate shown in Fig. 4 is obtained from the sum of the dierence between
the initial bromate concentration and the bromate concentration after the BAC treatment. The amount of accumulated bromide is obtained from the sum of the dierence between the initial bromide concentration in MF water and the bromide concentration after the BAC treatment. In the adsorption experiment, BAC showed very weak bromate adsorption. Approximately 0.28% of the total bromate ion loaded (5400 mmol) adsorbed onto BAC. In the resolution experiment (6 h after), loading bromate-free MF water, it was observed that bromate stored in the BAC column was suddenly redissolved into water, and approximately half of the bromate was redissolved into the resolution water. This portion is considered to be bromate that was diused in the BAC column and washed out. One-forth of the reduced bromate is chemically reduced and detected as bromide (ions). The remaining fraction is considered to be present in the form of hypobromite (OBrÿ) or hypobromous acid (HOBr), which is reduced from bromate or bromate adsorbed by activated carbon, as was described by Siddiqui et al. (1996). Hypobromous acid and hypobromite concentration were not moni-
Bromate ion behavior on GAC and BAC
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Fig. 4. Accumulated bromate adsorption, washout and redox reduction in BAC.
tored, since they are dicult to measure in low levels and are subject to chemical interference. Batch experiment: adsorption and reduction with GAC In the batch experiment with GAC, bromate reduction and bromide adsorption were observed. After 7 days of contact with GAC in the batch experiment, bromate decreased at a rate of 25.2 mg/g carbon. Bromate, at 250 mg/l, was reduced to half of the initial concentration by activated carbon (1 g/100 ml) in 1 day, and virtually 82% of bromate was detected as bromide after 7 days. The lost portion of bromate and bromide was considered to be hypobromide or adsorbed onto GAC (Fig. 5). Bromide has a very low adsorbability onto GAC. Bromide decreased at a rate of 381 mg/g carbon when 1000 mg/l of bromide was contacted with 1 g/l activated carbon for 48 h. Since bromide is dicult to reduce into another form, it is thus shown that
GAC weakly adsorbs ionic substances such as bromide. Batch experiment using GAC, BAC1, BAC2 and anthracite In order to distinguish new GAC and BAC capacity for bromate reduction, a batch experiment was conducted. Figure 6 shows the results of bromate reduction by new GAC, BAC (1 year) and BAC (used for 2 years in another experimental column) and anthracite after 1±7-day contact. As described before, new GAC reduced bromate to bromide. Compared to the results for new GAC, there was no signi®cant change in the amount of bromate in experiments using BAC. Our experiments clari®ed that BAC is incapable of bromate reduction under typical conditions, while new GAC has reduction and adsorption capacities. Bromate is least reduced by anthracite. Since the biomass on anthracite has been proved to be equal that of BAC (Nishijima et al., 1992), it should therefore be con-
Fig. 5. Bromate removal and reduction by new GAC (bromate = 2.0 mmol/l = 250 mg/l, GAC = 1 g/ 100 ml).
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Fig. 6. Bromate reduction by GAC, BAC and anthracite (bromate = 2.0 mmol/l = 250 mg/l, absorbants = 0.5 g/100 ml).
sidered that the biodegradation of bromate is very small by the biomass on anthracite and BAC. Ion exchange by GAC The question remains whether bromate adsorbs onto GAC via ionic bonds or diuses into GAC. No bromide, i.e. no bromate reduction was detected after 2-h's contact of 10 mg/l bromate solution contact with GAC. It was apparent that 5.5% of bromate adsorbed onto GAC. Then, after adding the GAC to NaCl-containing water and to pure water, 4.5% of the adsorbed bromate (0.25% in total) was eluted into NaCl water, but no bromate was eluted into pure water after 2 h contact. The elution was considered to be due to ion exchange between bromate and chloride ions. The dierence in bromate resolution between NaCl water and pure water is considered to be the ion-exchanged portion of bromate on GAC. DISCUSSION
New GAC showed bromate reduction capacity and a limited capacity for ion exchangeable adsorption; however, BAC exhibited a very small capacity for reduction and adsorption of bromate. Though the number of research on activated carbon adsorption of ionic substances is limited, ionic substances such as sulfate are known to adsorb to the surface
of activated carbon. This is considered to be due to the chemical adsorption between ionic substances and functional groups on the surface of activated carbon (Urano et al., 1976). The fact that the activated carbon surface washed with phosphate loses its chemical adsorption capacity for sulfate and other acids, indicates that the chemical reaction or chemisorption of ionic substances is aected by their chemical functional groups onto the activated carbon surface (Di Corcia et al., 1980). At the same time, activated carbon shows redox reduction capacity in the presence of oxidizing reagents, i.e. chlorine. The mechanism of bromate reduction by GAC is similar to that observed by Gonce and Voudrais (1994) who con®rmed GAC reduced chlorite (ClOÿ 2 ) to chloride. The batch experiments clari®ed the chemical capacity for bromate reduction by GAC and BAC. Bromate ion is adsorbed, reduced to hypobromite and ®nally reduced to bromide on GAC surface, as illustrated in Fig. 7. The term ``adsorption'' of bromate is considered to involve (1) chemical reduction and chemical bond to GAC surface, (2) diusion into GAC which is hard to elute in pure water, (3) diusion into GAC which is ion-exchangeable with chloride ions and (4) diusion into GAC which is easy to elute in pure water (weakest adsorption). In this classi®cation, bromate chemically bonded and that tightly diused into GAC are dicult to
Bromate ion behavior on GAC and BAC
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we have to consider oxidizing constituents rather than coexisting ionic substances. CONCLUSION
Fig. 7. Schematic bromate behavior on GAC and BAC. (a) Possible bromate reaction on new granular activated carbon (GAC). (b) Possible bromate reaction on biological activated carbon (BAC).
quantify separately. Chloride solution induced bromate resolution, which is then quanti®ed as a portion of ion-exchanged bromate in GAC. The mechanism of bromate removal by activated carbon was also analyzed by Siddiqui et al. (1996), especially in terms of surface conditions and properties of several activated carbons such as powdered activated carbon (PAC) and GAC. They postulated that bromate reduction by activated carbon occurs according to the following reactions: C BrOÿ ÿ4BrOÿ ÿ CO2 3ÿ , ÿ ÿ C 2BrO ÿ ÿ42Br ÿ CO2 where 0C represents the activated carbon surface and 0CO2 represents a surface oxide. According to their results, bromate removal was favored by a high pHzpc [isoelectric point (pH at zeta potential = 0)], and a high concentration of surface functional groups. As bromate is a strong oxidizing agent, bromate can be reasonably reduced by reaction with surface functional groups on activated carbon. Our result that BAC has a very low bromate removal capacity is attributed to the lack of reactive or reductive functional groups on its surface, that is, functional groups are consumed or oxidized by contact with oxidizing constituents during treatment. Though some bacteria showed bromate reduction capacity (Hijnen et al., 1995), in our long-term experiment and batch experiment, bromate reduction rate by biomass of BAC is considered very low. Considering bromate undergoes redox reduction,
In the long-term use of GAC following ozonation, its bromate removal rate apparently decreased during transition from new GAC to BAC after 3 months. According to our long-term experiment, bromate removal capacity of GAC/BAC was 1.5 mg BrO3/g carbon when bromate concentration was 50 mg/l. Batch chemical reduction capacity experiments on GAC and BAC con®rmed new GAC's capacity and BAC's inability to reduce bromate to bromide. Bromate removal ratio by GAC/BAC was found to depend upon the activated carbon service life in the water treatment plant experiment. The GAC/BAC bromate removal capacity is very limited, although GAC can reduce bromate to bromide. Regeneration of reduction capacity of GAC by a conversion process can be one alternative, though the frequency of GAC regeneration and exchange aects cost-eectiveness. Reducing bromate formation during the ozonation process will be an eective measure for reducing bromate in the treated water. As bromate formation is dependent on pH, pH adjustment is a reliable means of controlling bromate formation during ozonation (Siddiqui et al., 1995). Since bromate formation is dependent upon the ozone dose, minimization of the ozone dose in accordance with the ozonation objective is thought to be an eective means of restricting bromate formation. For instance, minimization of the ozone dose enough to degrade odorous substances is applicable to existing ozonation plants. Ozonation conditions should be optimized to minimize the quantity and toxicity of total ozonation by-products and maintain advantages such as odor reduction and other oxidation eects. In designing the overall treatment system, revision of the water source is a possible alternative when the bromide concentration is signi®cantly high. AcknowledgementsÐThis work was ®nancially supported by a Grant in Aid for Scienti®c Research (A) from the Japanese Ministry of Education, Science, Sports and Culture. The authors thank Dr. Motoyuki Suzuki and Mr. Takao Fujii for their helpful advice. REFERENCES
Asami M., Aizawa T. and Magara Y. (1996) Bromate ion formation inhibition by coexisting organic matters in ozonation process. J. Jpn. Soc. Water Environ. 19, 930± 936. Asami M., Hashimoto N., Aizawa T. and Magara Y. (1997) Determination of bromate ions in ozonated water by ion chromatography: analytical conditions and application. J. Jpn. Water Works Assoc. 66(4), 34±42. Baker T., Dyer-Smith P. and Boere J. (1996) Optimizing ozone and GAC in potable water treatment, Int. Water
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Supply Assoc., Asian Paci®c Conference, Hong Kong. pp. 42±52. Cipparone L. A., Diehl A. C. and Speitel G. E., Jr. (1997) Ozonation and BDOC removal: eect of water quality. J. Am. Water Works Assoc. 89(2), 84±97. Di Corcia A., Samperi R., Sebastiani E. and Severini C. (1980) Acid-washed graphitized carbon black for gas chromatography. Anal. Chem. 52, 1345±1350. Gonce N. and Voudrais E. A. (1994) Removal of chlorite and chlorate ions from water using granular activated carbon. Water Res. 28, 1059±1069. Hijnen W. A. M., Voogt R., Veenendaal H. R., van der Jagt H. and van der Kooij D. (1995) Bromate reduction by denitrifying bacteria. Appl. Environ. Microbiol. 61, 239±244. Marhaba T. F. and Medlar S. J. (1993) Treatment/removal of bromate and bromide ions using GAC and RO, AWWA Water Qual. Technol. Conf., Miami, FL. Moniwa T. (1995) Ozonation in water puri®cation. J. Jpn. Water Works Assoc. 64(10), 2±6. Nishijima W., Tojo M., Okada M. and Murakami A. (1992) Biodegradation of trace organic substances by
biological activated carbon. J. Water Environ. 15(10), 683±689. Nishijima W., Shoto E. and Okada M. (1997) Improvement of biodegradation of organic substance by addition of phosphorus in biological activated carbon. Water Sci. Tech. 36(12), 251±258. Siddiqui M., Amy G. and Rice R. G. (1995) Bromate ion formation: a critical review. J. Am. Water Works Assoc. 87(10), 58±70. Siddiqui M., Ahai W., Amy G. and Mysore C. (1996) Bromate removal by activated carbon. Water Res. 30, 1651±1660. Urano K., Sounai M., Nakayama T. and Kobayashi Y. (1976) Strong acid, strong alkali and their salt adsorption onto activated carbon and pH of activated carbon solution. Nihon Kagaku Kaishi 11, 1773±1778. Yamada H. (1993) By-products of ozonation of low bromide waters and reduction of the by-products by activated carbon. In Proc. 11th Int. Ozone Assoc. Congr., San Francisco, CA.
Wat. Res. Vol. 33, No. 12, pp. 2797±2804, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
www.elsevier.com/locate/watres
BROMATE REMOVAL DURING TRANSITION FROM NEW GRANULAR ACTIVATED CARBON (GAC) TO BIOLOGICAL ACTIVATED CARBON (BAC) MARI ASAMI1*, TAKAKO AIZAWA1, TAKAYUKI MORIOKA2, M M M WATARU NISHIJIMA3* , AKIHISA TABATA4* and YASUMOTO MAGARA4* Department of Water Supply Engineering, National Institute of Public Health, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8638, Japan; 2Water Treatment and Bioelectronic Laboratory, Fuji Electric Corporate R&D, Ltd., Kanagawa, Japan; 3Faculty of Engineering, Hiroshima University, Hiroshima, Japan and 4Department of Urban Environmental Engineering, Faculty of Engineering, Hokkaido University, Sapporo, Japan 1
(First received April 1998; accepted in revised form November 1998) AbstractÐBromate removal by activated carbon after ozonation is a subject of concern, since bromate is commonly found in the ozonation of bromide-containing water. Though new GAC (granular activated carbon) shows the capacity to reduce bromate to bromide, in the long-term use of GAC following ozonation, its bromate removal rate apparently decreases during transition from new GAC to BAC (biological activated carbon) after 3 months. Batch bromate reduction experiments using new GAC and BAC con®rmed new GAC's ability and BAC's inability to reduce bromate to bromide. Our experiment also indicated that ion exchangeable bromate adsorption on new GAC was very limited. Based on the results of our long-term experiment, the bromate removal rate during the transition from new GAC to BAC was calculated; 1.5 mg BrO3/g carbon when bromate concentration was 50 mg/l. BAC's inability to reduce bromate makes it necessary to optimize ozonation conditions to minimize the formation of bromate and other by-products while maintaining target levels of organic matter decomposition. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐdrinking water, bromate, chloride, GAC, biological activated carbon (BAC)
INTRODUCTION
Advanced water treatment in Japan has been applied to an increasing number of drinking water treatment plants in order to degrade odorous substances and trihalomethane precursors. Ozonation is mainly used as unit process to eliminate oensive odor and taste (Moniwa, 1995). Although ozonation does not yield chlorine-containing by-products, it yields larger amounts of aldehydes and other oxidized organic by-products than chlorination. It has been pointed out that the increase in amount of biodegradable organic substances due to ozonation reduces bacterial stability in drinking water distribution network. Ozonation also yields inorganic byproducts. Ozonation of water that contains bromide (ions) yields bromate, a possible carcinogen, though the bromate formation is partly restricted with coexisting organic matters (Asami et al., 1996). Bromate usually exists in its ionic form (BrOÿ 3 ) in water. The maximum allowable bromate level is still a subject of discussion: 25 mg/l according to provisional WHO guidelines and 10 mg/l according to *Author to whom all correspondence should be addressed. [Tel.: +81-3-3441-7111 ex. 273; fax: +81-3-3446-4314; e-mail: [email protected]].
proposed EU directives and U.S. regulations. The authors reported bromate concentrations in Japanese drinking water treatment processes between 7 mg/l and below QDL (quantitative detection limit) after ozonation and below QDL in all ®nished water. The QDL is 2 mg/l. A much higher concentration, approximately 50 mg/l, was detected in water from an experimental ozonation plant, which originally contained bromide at a 300 mg/l level (Asami et al., 1997). To eliminate ozonation by-products such as aldehydes and ketones, granular activated carbon (GAC) is applied as a posttreatment following ozonation. The GAC process results in the adsorption of organic matters, chemical reduction and mechanical ®ltration. GAC in actual water treatment processes naturally breeds a bacterial population on its surface, and eventually it works as biological activated carbon (BAC). GAC with a biomass that shows high biological activity is called BAC and it degrades biodegradable organic matter and ammonia (Nishijima et al., 1992). With the help of the bioactivity on BAC, the reduction of total organic carbon (TOC) and other organic substances is considered to maintain a longer service time before GAC regeneration (Baker et al., 1996). The combi-
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nation of ozonation and biological treatment with BAC enhances the degradation of organic substances, though it slightly degrades trihalomethane precursors and aldehydes (Cipparone et al., 1997). Many ozonation processes are followed by the GAC process which eventually works as the BAC process after some time. Therefore, the question remains whether BAC can reduce bromate or not. At present, the number of reports on activated carbon adsorption and removal of ionic substances, especially on BAC, is limited. There have been some reports of bromate reduction by GAC (Marhaba and Medlar, 1993) and new activated carbon (Yamada, 1993). The mechanism of bromate removal by activated carbon was analyzed by Siddiqui et al. (1996), in terms of surface conditions and properties of activated carbons, such as powdered activated carbon (PAC) and GAC. There are only few researches on the removal of ionic substances, especially on bromate removal by BAC in the water puri®cation process, though bromate behavior in new GAC and in BAC should be dierent. It is necessary to determine the mechanism and the duration for bromate reduction by GAC or BAC to satisfactory low levels in a treatment plant. This study deals with the behavior of bromate in new GAC and old GAC, i.e. BAC in an experimental ozonation-GAC/BAC water treatment plant. A batch experiment is also conducted in the laboratory to analyze bromate and bromide ion reduction and removal by GAC and BAC. MATERIALS AND METHODS
Bromate removal using GAC/BAC in an experimental treatment plant Bromate ion removal was observed at an experimental water treatment plant downstream of Minaga Lake, Hiroshima, starting in August 1995. Figure 1 shows the experimental setup that consists of an ozone contactor, a retention column and an activated carbon treatment tank,
all of which have a diameter of 10 cm and a height of 70 cm. Micro®ltred water (MF water) was introduced to the mixing tank. The ¯ow rate was basically set at 19.2 l/h (SV = 8). Activated carbon (Calgon F-400, 1024 g) was placed in a column with a diameter of 10 cm. The amount of dissolved organic carbon in MF water ranged from 2 to 3 mg/l. Ammonia was rarely detected during the experiments. The number of bacteria adhering on BAC was determined by direct counting, using an epi¯uorescence microscope after collecting approximately 0.2 g of activated carbon in the column, as described by Nishijima et al. (1997). No bromate was detected in raw water. In the long-term experiment, micro®ltered lake water with 50 mg BrO3/l of potassium bromate was continuously passed through the columns and sampled every 20 days. Ozonation was stopped during the sampling time of the long-term continuous experiment and during the shortterm experiment. In the short-term experiment, new GAC, two-month old GAC (BAC1) that had already been BAC, and one-year old GAC (BAC2) were examined. In each experiment, MF water containing 250 mg BrO3/l standard bromate was passed through the column for 6 h. Then MF water was passed through the column for more than 12 h after each experiment. Bromate reduction and adsorption in batch experiments A small amount of activated carbon was collected from the experimental treatment plant during the experiments, and batch experiments were conducted to clarify the chemical reaction of bromate with an activated carbon surface. Bromate removal by new GAC, one-year old GAC (BAC1) and two-year old GAC (BAC2) was examined in 100-ml ¯asks. Approximately 1 g of each activated carbon was added to a ¯ask that was ®lled with 100 ml of 250 mg/l bromate solution or 1 mg/l bromide solution and nitrogen gas. Samples were shaken for 1, 2, 3 or 7 days at 208C in a continuous shaker. Then they were ®ltered through a 0.45-mm membrane ®lter, and ®ltrates were analyzed by ion chromatography. The activated carbons were weighed after drying in a 1108C oven for 24 h. Because bromide was not chemically reduced by activated carbon, bromide ion adsorption was investigated using 0 to 10 gGAC/l for 1 mg/l of bromide for a 48-h contact time. Ion exchange capacity of activated carbon Each of the four samples of 0.0013 g GAC (washed and dried in a 1308C oven) were put in 100 ml of 10 mg/l bro-
Fig. 1. Schematic diagram of ozone-GAC/BAC pilot plant.
Bromate ion behavior on GAC and BAC mate solution. The solution was stirred for 2 h at 208C. Each activated carbon that adsorbed bromate was collected and then added to duplicate samples of pure water and 20 mg/l chloride ion (sodium chloride). After 2 h, samples were ®ltered through a 0.45-mm membrane ®lter and analyzed by ion chromatography. Bromate and bromide quanti®cation by ion chromatography Dionex ion exchange column, AS9SC/AG9SC and borate eluent (40 mM H3BO4+20 mM NaOH) were selected for better bromate separation. Ion chromatography (Yokogawa IC-7000) with an ultraviolet absorbance detector (210 nm) was employed for bromate analysis. A sample (250 ml) was injected after silver cartridge (Lida Co., Ltd.) ®ltration at a ¯ow rate of 1 ml/min to remove coexisting chlorides and bromides. The lowest QDL was 2 mg/l when coecient of variation was 10% (n = 5). For bromide analysis, Dionex DX-100 ion chromatograph with AS9SC/AG9SC column and carbonate eluent (1.6 mM Na2CO3+1.5 mM NaHCO3) were employed. A sample (50 ml) was injected without silver cartridge ®ltration then detected by electric conductivity detector. The detection methods of bromate and bromide were described elsewhere (Asami et al., 1997) in detail. RESULTS
Continuous bromate removal by GAC/BAC monitored at the treatment plant In the experimental treatment plant, new GAC was gradually transformed into BAC over a period of 2±10 months, which was determined from the bacterial count on the surface of the activated carbon. Bacteria concentration was 6.1 107 cells/g carbon at 3 months and 1.6 108 cells/g carbon after 1 year, both at the bottom part of the carbon column. TOC removal ratio was 100% at the beginning of the experiment and gradually decreased after 5 months. Monthly changes in the bromate removal were observed at the plant during the transition from new GAC into BAC. Bromate concentration after the GAC/BAC column is shown in Fig. 2. In the beginning, new GAC reduced over 60% of bromate, then the bromate removal ratio started to decrease after 50 days and there was reached almost
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no removal after 3 months. Total bromate removal ratio in the long-term experiment during the transition from new GAC to BAC was approximately 1.5 mg bromate/g activated carbon. From these results, the breakthrough of bromate was completed before the breakthrough of TOC in BAC, though Siddiqui et al. (1996) pointed out that the breakthrough of natural organic matter was completed before bromate breakthrough in GAC. Short-term experiment: bromate removal by new GAC/BAC To clarify bromate behavior in new GAC and BAC, short-term experiments on bromate removal were conducted at the treatment plant for new GAC, BAC1 and BAC2. Figure 3(a) shows the results of short-term tests of 250 mg/l bromate passing through the GAC/BAC column. No bromate was detected with new GAC treatment throughout the experiment. With BAC1, bromate concentration gradually increased during the 6-h experiment, and the apparent bromate removal ratio was approximately 70% after 6 h. With BAC2, bromate concentration showed a relatively rapid increase from the beginning of the experiment and reached the original bromate concentration in 150 min, while calculated empty bed contact time (EBCT) was 15 min. Bromate concentration after BAC2 reached a constant as the same concentration as the loaded bromate concentration after a few hours. The bromide concentration increased while bromate was passed through the GAC/BAC column in each short-term experiment, as shown in Fig. 3(b). The tendency obviously diers among GAC/BAC experiments, though bromide concentration showed some variance, since source water originally contains approximately 30 mg/l bromide on the average and since bromide QDL is 20 mg/l. In the case of new GAC, the bromate ion concentration decreased. The concentration of bromide detected in treated water was lower than that detected in MF water. Therefore new GAC is considered to adsorb bromide
Fig. 2. Continuous bromate removal by GAC/BAC. Initial bromate concentration 50 mg/l.
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Fig. 3. Bromate and bromide behavior in GAC/BAC in the short-term experiment. (a) Bromate concentration after GAC/BAC (initial bromate = 250 mg/l). (b) Bromide after GAC/BAC during bromate reduction experiment.
which was contained in MF water and bromide which originated from reduced bromate. After BAC1, bromide concentration increased due to chemical reduction of bromate. BAC1 seemingly showed little capacity of bromide adsorption. In the case of BAC2 used for more than 1 year, bromate was not reduced or adsorbed and was therefore detected in their original form and concentration. These results were parallel to the phenomenon observed by Siddiqui et al. (1996): bromate removal by activated carbon consists of two steps, adsorption and chemical reduction to bromide. Cumulative bromate removal and resolution in shortterm experiment To characterize bromate reduction by BAC, the accumulation of bromate and the resolution of bromate and bromide in the BAC column was calculated from the results of the bromate removal experiment and the following resolution experiment; 100 mg/l bromate solution was loaded for 6 h in the removal experiment and successively MF water was loaded for 2 h in the resolution experiment. The amount of accumulated bromate shown in Fig. 4 is obtained from the sum of the dierence between
the initial bromate concentration and the bromate concentration after the BAC treatment. The amount of accumulated bromide is obtained from the sum of the dierence between the initial bromide concentration in MF water and the bromide concentration after the BAC treatment. In the adsorption experiment, BAC showed very weak bromate adsorption. Approximately 0.28% of the total bromate ion loaded (5400 mmol) adsorbed onto BAC. In the resolution experiment (6 h after), loading bromate-free MF water, it was observed that bromate stored in the BAC column was suddenly redissolved into water, and approximately half of the bromate was redissolved into the resolution water. This portion is considered to be bromate that was diused in the BAC column and washed out. One-forth of the reduced bromate is chemically reduced and detected as bromide (ions). The remaining fraction is considered to be present in the form of hypobromite (OBrÿ) or hypobromous acid (HOBr), which is reduced from bromate or bromate adsorbed by activated carbon, as was described by Siddiqui et al. (1996). Hypobromous acid and hypobromite concentration were not moni-
Bromate ion behavior on GAC and BAC
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Fig. 4. Accumulated bromate adsorption, washout and redox reduction in BAC.
tored, since they are dicult to measure in low levels and are subject to chemical interference. Batch experiment: adsorption and reduction with GAC In the batch experiment with GAC, bromate reduction and bromide adsorption were observed. After 7 days of contact with GAC in the batch experiment, bromate decreased at a rate of 25.2 mg/g carbon. Bromate, at 250 mg/l, was reduced to half of the initial concentration by activated carbon (1 g/100 ml) in 1 day, and virtually 82% of bromate was detected as bromide after 7 days. The lost portion of bromate and bromide was considered to be hypobromide or adsorbed onto GAC (Fig. 5). Bromide has a very low adsorbability onto GAC. Bromide decreased at a rate of 381 mg/g carbon when 1000 mg/l of bromide was contacted with 1 g/l activated carbon for 48 h. Since bromide is dicult to reduce into another form, it is thus shown that
GAC weakly adsorbs ionic substances such as bromide. Batch experiment using GAC, BAC1, BAC2 and anthracite In order to distinguish new GAC and BAC capacity for bromate reduction, a batch experiment was conducted. Figure 6 shows the results of bromate reduction by new GAC, BAC (1 year) and BAC (used for 2 years in another experimental column) and anthracite after 1±7-day contact. As described before, new GAC reduced bromate to bromide. Compared to the results for new GAC, there was no signi®cant change in the amount of bromate in experiments using BAC. Our experiments clari®ed that BAC is incapable of bromate reduction under typical conditions, while new GAC has reduction and adsorption capacities. Bromate is least reduced by anthracite. Since the biomass on anthracite has been proved to be equal that of BAC (Nishijima et al., 1992), it should therefore be con-
Fig. 5. Bromate removal and reduction by new GAC (bromate = 2.0 mmol/l = 250 mg/l, GAC = 1 g/ 100 ml).
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Fig. 6. Bromate reduction by GAC, BAC and anthracite (bromate = 2.0 mmol/l = 250 mg/l, absorbants = 0.5 g/100 ml).
sidered that the biodegradation of bromate is very small by the biomass on anthracite and BAC. Ion exchange by GAC The question remains whether bromate adsorbs onto GAC via ionic bonds or diuses into GAC. No bromide, i.e. no bromate reduction was detected after 2-h's contact of 10 mg/l bromate solution contact with GAC. It was apparent that 5.5% of bromate adsorbed onto GAC. Then, after adding the GAC to NaCl-containing water and to pure water, 4.5% of the adsorbed bromate (0.25% in total) was eluted into NaCl water, but no bromate was eluted into pure water after 2 h contact. The elution was considered to be due to ion exchange between bromate and chloride ions. The dierence in bromate resolution between NaCl water and pure water is considered to be the ion-exchanged portion of bromate on GAC. DISCUSSION
New GAC showed bromate reduction capacity and a limited capacity for ion exchangeable adsorption; however, BAC exhibited a very small capacity for reduction and adsorption of bromate. Though the number of research on activated carbon adsorption of ionic substances is limited, ionic substances such as sulfate are known to adsorb to the surface
of activated carbon. This is considered to be due to the chemical adsorption between ionic substances and functional groups on the surface of activated carbon (Urano et al., 1976). The fact that the activated carbon surface washed with phosphate loses its chemical adsorption capacity for sulfate and other acids, indicates that the chemical reaction or chemisorption of ionic substances is aected by their chemical functional groups onto the activated carbon surface (Di Corcia et al., 1980). At the same time, activated carbon shows redox reduction capacity in the presence of oxidizing reagents, i.e. chlorine. The mechanism of bromate reduction by GAC is similar to that observed by Gonce and Voudrais (1994) who con®rmed GAC reduced chlorite (ClOÿ 2 ) to chloride. The batch experiments clari®ed the chemical capacity for bromate reduction by GAC and BAC. Bromate ion is adsorbed, reduced to hypobromite and ®nally reduced to bromide on GAC surface, as illustrated in Fig. 7. The term ``adsorption'' of bromate is considered to involve (1) chemical reduction and chemical bond to GAC surface, (2) diusion into GAC which is hard to elute in pure water, (3) diusion into GAC which is ion-exchangeable with chloride ions and (4) diusion into GAC which is easy to elute in pure water (weakest adsorption). In this classi®cation, bromate chemically bonded and that tightly diused into GAC are dicult to
Bromate ion behavior on GAC and BAC
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we have to consider oxidizing constituents rather than coexisting ionic substances. CONCLUSION
Fig. 7. Schematic bromate behavior on GAC and BAC. (a) Possible bromate reaction on new granular activated carbon (GAC). (b) Possible bromate reaction on biological activated carbon (BAC).
quantify separately. Chloride solution induced bromate resolution, which is then quanti®ed as a portion of ion-exchanged bromate in GAC. The mechanism of bromate removal by activated carbon was also analyzed by Siddiqui et al. (1996), especially in terms of surface conditions and properties of several activated carbons such as powdered activated carbon (PAC) and GAC. They postulated that bromate reduction by activated carbon occurs according to the following reactions: C BrOÿ ÿ4BrOÿ ÿ CO2 3ÿ , ÿ ÿ C 2BrO ÿ ÿ42Br ÿ CO2 where 0C represents the activated carbon surface and 0CO2 represents a surface oxide. According to their results, bromate removal was favored by a high pHzpc [isoelectric point (pH at zeta potential = 0)], and a high concentration of surface functional groups. As bromate is a strong oxidizing agent, bromate can be reasonably reduced by reaction with surface functional groups on activated carbon. Our result that BAC has a very low bromate removal capacity is attributed to the lack of reactive or reductive functional groups on its surface, that is, functional groups are consumed or oxidized by contact with oxidizing constituents during treatment. Though some bacteria showed bromate reduction capacity (Hijnen et al., 1995), in our long-term experiment and batch experiment, bromate reduction rate by biomass of BAC is considered very low. Considering bromate undergoes redox reduction,
In the long-term use of GAC following ozonation, its bromate removal rate apparently decreased during transition from new GAC to BAC after 3 months. According to our long-term experiment, bromate removal capacity of GAC/BAC was 1.5 mg BrO3/g carbon when bromate concentration was 50 mg/l. Batch chemical reduction capacity experiments on GAC and BAC con®rmed new GAC's capacity and BAC's inability to reduce bromate to bromide. Bromate removal ratio by GAC/BAC was found to depend upon the activated carbon service life in the water treatment plant experiment. The GAC/BAC bromate removal capacity is very limited, although GAC can reduce bromate to bromide. Regeneration of reduction capacity of GAC by a conversion process can be one alternative, though the frequency of GAC regeneration and exchange aects cost-eectiveness. Reducing bromate formation during the ozonation process will be an eective measure for reducing bromate in the treated water. As bromate formation is dependent on pH, pH adjustment is a reliable means of controlling bromate formation during ozonation (Siddiqui et al., 1995). Since bromate formation is dependent upon the ozone dose, minimization of the ozone dose in accordance with the ozonation objective is thought to be an eective means of restricting bromate formation. For instance, minimization of the ozone dose enough to degrade odorous substances is applicable to existing ozonation plants. Ozonation conditions should be optimized to minimize the quantity and toxicity of total ozonation by-products and maintain advantages such as odor reduction and other oxidation eects. In designing the overall treatment system, revision of the water source is a possible alternative when the bromide concentration is signi®cantly high. AcknowledgementsÐThis work was ®nancially supported by a Grant in Aid for Scienti®c Research (A) from the Japanese Ministry of Education, Science, Sports and Culture. The authors thank Dr. Motoyuki Suzuki and Mr. Takao Fujii for their helpful advice. REFERENCES
Asami M., Aizawa T. and Magara Y. (1996) Bromate ion formation inhibition by coexisting organic matters in ozonation process. J. Jpn. Soc. Water Environ. 19, 930± 936. Asami M., Hashimoto N., Aizawa T. and Magara Y. (1997) Determination of bromate ions in ozonated water by ion chromatography: analytical conditions and application. J. Jpn. Water Works Assoc. 66(4), 34±42. Baker T., Dyer-Smith P. and Boere J. (1996) Optimizing ozone and GAC in potable water treatment, Int. Water
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Supply Assoc., Asian Paci®c Conference, Hong Kong. pp. 42±52. Cipparone L. A., Diehl A. C. and Speitel G. E., Jr. (1997) Ozonation and BDOC removal: eect of water quality. J. Am. Water Works Assoc. 89(2), 84±97. Di Corcia A., Samperi R., Sebastiani E. and Severini C. (1980) Acid-washed graphitized carbon black for gas chromatography. Anal. Chem. 52, 1345±1350. Gonce N. and Voudrais E. A. (1994) Removal of chlorite and chlorate ions from water using granular activated carbon. Water Res. 28, 1059±1069. Hijnen W. A. M., Voogt R., Veenendaal H. R., van der Jagt H. and van der Kooij D. (1995) Bromate reduction by denitrifying bacteria. Appl. Environ. Microbiol. 61, 239±244. Marhaba T. F. and Medlar S. J. (1993) Treatment/removal of bromate and bromide ions using GAC and RO, AWWA Water Qual. Technol. Conf., Miami, FL. Moniwa T. (1995) Ozonation in water puri®cation. J. Jpn. Water Works Assoc. 64(10), 2±6. Nishijima W., Tojo M., Okada M. and Murakami A. (1992) Biodegradation of trace organic substances by
biological activated carbon. J. Water Environ. 15(10), 683±689. Nishijima W., Shoto E. and Okada M. (1997) Improvement of biodegradation of organic substance by addition of phosphorus in biological activated carbon. Water Sci. Tech. 36(12), 251±258. Siddiqui M., Amy G. and Rice R. G. (1995) Bromate ion formation: a critical review. J. Am. Water Works Assoc. 87(10), 58±70. Siddiqui M., Ahai W., Amy G. and Mysore C. (1996) Bromate removal by activated carbon. Water Res. 30, 1651±1660. Urano K., Sounai M., Nakayama T. and Kobayashi Y. (1976) Strong acid, strong alkali and their salt adsorption onto activated carbon and pH of activated carbon solution. Nihon Kagaku Kaishi 11, 1773±1778. Yamada H. (1993) By-products of ozonation of low bromide waters and reduction of the by-products by activated carbon. In Proc. 11th Int. Ozone Assoc. Congr., San Francisco, CA.