Influence of Natural Organic Matter on Bromate Formation During

Downloaded by CORNELL UNIV on July 31, 2012 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0761.ch018

Chapter 18

Influence of Natural Organic Matter on Bromate Formation During Ozonation of Low-Bromide Drinking Waters: A Multi-Level Assessment of Bromate Christopher J . Douville and Gary L . A m y Department of Civil, Architectural, and Environmental Engineering, University of Colorado at Boulder, Boulder, C O 80309-0421 A multi-level approach is used to assess bromate formation. The size, structure and functionality of natural organic matter (NOM) and its role in bromate formation is being investigated via a nationwide survey of ozonation facilities, bench-scale ozonation and simultaneous NOM characterization of source waters, as well as scale-up comparison testing between bench-, pilot-, and full -scale ozone contactors. Initial results indicate that many utilities will be faced with the challenge of optimizing their ozonation process in order to achieve the desired Cryptosporidium inactivation, that may become a consequence of the proposed Stage 2 Disinfectant/Disinfection By-Product (D/DBP) Rule, along with compliance of the existing Stage 1 bromate standard of 10 μg/L. Ongoing work will continue to show that a solid understanding of the character of the NOM will enable utilities to predict how N O M will either inhibit or promote bromate formation.

Relevance of Research Cryptosporidium may become the target organism for disinfection of drinking water. Because this organism is much more resistant to disinfection than Giardia, water treatment plants may soon have to adjust their disinfection strategy to achieve an appropriate log-kill of Cryptosporidium (/). Many existing ozonation facilities will need to increase their ozone dosage i f Cryptosporidium disinfection is required. Many treatment plants have made the decision to implement ozone within their process, with many more upgrades and new constructions involving ozone anticipated. As the movement towards ozonation facilities is gaining momentum, a close eye must be kept on the balance between obtaining effective disinfection while minimizing the formation of ozone by-products. O f particular interest is the 282

© 2000 American Chemical Society

In Natural Organic Matter and Disinfection By-Products; Barrett, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

283 problematic compound bromate (Br0 ~). In Figure 1, a schematic portrays the tradeoff issue between the acute risk of Cryptosporidium and the chronic risk associated with bromate. In November of 1998, the U S E P A regulated bromate in drinking water with a maximum contaminant level (MCL) of 10 μg/L as part of Stage 1 of the Disinfectant/Disinfection By-Product (D/DBP) Rule (2,3). Due to the associated cancer risk of bromate, this standard may be lowered to 5 μg/L (or lower) in Stage 2 of the D/DBP Rule (2,4). 3


Players in the Bromate Formation Game As with any formed by-product, identification of the significant precursor(s) is vital to understanding it's formation and subsequently developing a minimization/control strategy. A large amount of research has been conducted on bromate in order to understand which disinfection treatment conditions and water quality constituents are factors in bromate formation. Important treatment conditions include ozone dose (measured as applied or transferred ozone dose), contact time, and water temperature. Increased levels of ozone dose and contact time as well as an increase in water temperature will ultimately result in higher levels of bromate (5,6). However, microorganism inactivation is also enhanced at higher temperatures. A n obvious precursor is the inorganic constituent bromide (Br") and many researchers have shown that with everything else being equal, increased levels of bromide contribute to elevated bromate formation (5-7). Research has shown that p H is a major factor affecting bromate formation, with higher bromate values resulting as pH increases (5,6). The parameter dissolved organic carbon (DOC) has been studied in previous research and is known to participate in reactions with ozone. D O C or natural organic matter (NOM) has been shown to exert an ozone demand (8). Some research has shown that high levels of D O C have resulted in increased levels of bromate (7). D O C can be dissected further into classes of ΝΟΜ. Bromate formation work has been conducted using fractions and isolates of N O M to expand the depth of study (9,10). Additionally, N O M has been shown to influence the formation of hydroxy 1 radicals ( Ο Η · ) and ultimately bromate formation (9). Since the Ο Η · pathway has been shown to be the dominant bromate formation pathway (10), promotion or inhibition of Ο Η · will influence bromate chemistry. Understanding N O M ' s composition and character and its associated role in bromate formation is the focus of this research.

Focus of Research There is a need to expand the understanding of how and to what degree certain components of N O M will react with ozone that may inhibit or promote bromate formation. B y utilizing a suite of N O M characterization techniques, the composition and functionality of the multi-faceted N O M compounds can be linked to bromate levels. With an increased understanding of the influence of N O M , disinfection practices can be more appropriately optimized. The U S E P A Surface Water



284 Treatment Rule (SWTR) established the " C T " system for disinfection credit (77), and provides tables that link C T to corresponding levels of Giardia inactivation (4). Inactivation requirements are currently being developed for Cryptosporidium inactivation as part of the Enhanced Surface Water Treatment Rule (ESWTR) (72). The challenge for treatment plants will be to realize the CTI Cryptosporidium inactivation level required, while at the same time acknowledging the associated C T versus bromate relationships. This is also one of the major challenges for the upcoming Stage 2 D/DBP and E S W T R regulation development. A nationwide survey of bromide in drinking water yielded an average value of approximately 80 μg/L (13). The research discussed here has focused on low- to moderate-bromide waters, designated as 10 to 100 μg/L levels. Up until recently, low-bromide waters were not targeted for study during bromate formation research. However, the likely onset of higher doses of ozone now creates a potential bromate issue with even low-bromide source waters. Low-bromide waters at treatment plants that ozonate are now a legitimate concern in terms of bromate.

Scope of Work A multi-level scope of work has been used to investigate bromate formation. Specifically, the components are: • • • •

A nationwide bromate survey of full-scale ozonation facilities N O M characterization of source waters Simultaneous bench-scale ozonation of the source waters to measure bromate formation A scale-up comparison study between full-, pilot- and bench-scale ozone contactors The details of each component are described below.

Bromate Survey The nationwide bromate survey, entitled the " A W W A R F Bromate Survey," was devised to understand bromate occurrence (formation) at the full-scale during realistic current treatment levels of ozone application. Twenty-four full-scale ozonation facilities are represented in the survey, including 21 from the U S and 3 from France. Two sampling campaigns (June 1998 and November 1998) were conducted to obtain sample pairs of "before" and "after" ozone application. Pre-, intermediate-, and dualozonation facilities (both pre- and intermediate-ozonation) are represented within the participants. Samples were analyzed for D O C , ultraviolet light absorbance at 254 nanometers (UVA254), alkalinity ( A L K ) , ammonia (NH -N), bromide ("before" ozone sample) and bromate ("after" ozone sample). Treatment conditions were also obtained from the utility at the time of sampling; information consisted of ozone dose 3


285 (transferred and/or applied), contact time estimates (hydraulic residence time (HRT) and/or ti ), pH and temperature. 0

N O M Characterization The goal of this component is to use a suite of characterization techniques to discern information about size, structure and functionality of ΝΟΜ. Raw water from each of the three participating utilities was subjected to analyses of D O C and U V A 5 4 to yield specific U V A (SUVA), U V spectrum (200 to 400 nm), high performance size exclusion chromatography (HPSEC) to give an estimate of apparent molecular weight (MW), and the XAD-8/-4 fractionation protocol which fractionates N O M into operationally defined classes of percent hydrophobic (XAD-8 adsorbable), transphilic (XAD-4 adsorbable), and hydrophilic D O C (neither X A D - 8 nor X A D - 4 adsorbable) (14). Additionally, differential U V spectra ( A U V A ) for the waters were developed. A differential U V spectrum, which provides insight to the relative degree of ozone reactivity, can be obtained by subtracting the U V spectrum of an ozonated water sample (after ozonation) from that of an un-ozonated sample (before ozonation). The analyses were performed on bulk water samples (no isolates or fractions). A t the same time of the N O M characterization analyses, the general water quality of the waters was assessed (i.e. alkalinity, N H N , pH, bromide).


2

r

Bench-Scale Ozonation True-batch ozonation (100% transfer efficiency) was used to determine bromate formation potentials (Br0 "-FPs) for the same three waters subjected to N O M characterization. A bench-scale reactor system (a modified graduated cylinder with a sample port) was used under standard conditions of temperature (20 degrees C), p H (7.0) and ozone to D O C dose ratio (2:1 mass based ratio). A 1.0 m M phosphate buffer was used to stabilize the experimental p H and bring the total volume of liquid in the reactor to 500 mL, thus incurring a dilution of bulk water. This dilution creates a new water quality matrix, and is cited as such. A 500 m L total volume was used because of the high volume necessary to obtain samples for kinetic data. For waters #2 and #3, dilution was also used to create equalization of D O C so that only the N O M properties would be influential in bromate formation. For these waters, experimentation was performed with a 1.0 mg/L D O C value in the reactor for each experiment. The reactor setup is shown in Figure 2. Samples of ozone residual were taken over time to generate an ozone decay curve as the ozone reacted with the ΝΟΜ. The area under the decay curve can be mathematically integrated to yield an estimate of C T called ozone exposure (OE). After completion of the reaction (i.e. an ozone residual of 0.0 mg/L), the Br0 *-FP was measured. With this data, the C T versus bromate relationship was evaluated. Each separate true-batch experiment resulted in one data point; multiple data points reflect replication efforts at different ozone doses. 3

3


286

IT'S ABOUT RISK CRYPTO


BROMATE

high

NOM Figure I. A risk schematic showing the Cryptosporidium versus bromate tradeoff.

Figure 2. The true-batch ozonation reactor used for bench-scale bromate formation experiments.


287 Scale-Up Comparison Study A scale study was performed to compare full-, pilot-, and bench-scale ozone contactors in terms of calculated C T versus B r 0 " formation. One of the aforementioned participating utilities also participated in the scale-up comparison study that involved on-site testing of full- and pilot-scale contactors and laboratory testing of a bench-scale ozone contactor. The bench-scale contactor was a novel continuous-flow system. In this reactor a bulk, undiluted water sample was continuously ozonated with dissolved ozone bubbles created by a frit as it flowed through a 380 m L glass column. The contactor has internal sample recirculation to create a completely mixed system as well as a water jacket for experimental temperature control. Full- and pilot-scale tests were performed at the facility to generate estimates of C T (as calculated per SWTR guidelines). Aliquots of ozonated water (quenched) were shipped to the university lab to determine bromate concentrations. Bench-scale experimentation took place in the university lab. To obtain different estimates of C T using the bench-scale system, separate experiments were run at a variety of liquid flow rates which resulted in a range of C T estimates, each with its respective Br0 *-FP. These C T estimates were calculated by multiplying the average ozone residual of the water times the H R T , and will be referred to as ozone exposure. The different scales were compared once each C T versus bromate relationship was derived and analyzed simultaneously.


3

3

Data Acquisition A l l of the analytical work mentioned in the paper was performed at the University of Colorado at Boulder (CU), with the exception of the low-level bromate analyses (< 2.0 μg/L) which were conducted at the University of Illinois at UrbanaChampaign (UI) (15). Final data was generated using calibration curves that were derived from appropriate, lab-grade standards. D O C was measured with a Sievers 800 T O C analyzer which utilizes the UV/persulfate oxidation method. U V analyses were conducted on a Shimadzu U V - V I S 160 spectrophotometer. Ammonia analysis was performed using a H A C H DR-2000 spectrophotometer and the Nessler method. Bromide and bromate values were obtained using a Dionex DX-300 ion chromatography system and an A S 9 - H C analytical column (high capacity). The detection limit at C U for both bromate and bromide was approximated at 2 μg/L by the lowest concentration of standard that was consistently detected. Dissolved ozone concentrations were obtained by the indigo method using H A C H Accu-Vac vials and a DR-2000 spectrophotometer. H P S E C work was performed using a Shimadzu highpressure liquid chromatography system and a Waters Protein Pak 125 column (16). At least 10% triplication was applied to the samples within each analytic tool.


288 Results and Discussion


Bromate Survey The intent of the survey was to assemble a representative cross-section of ozonation utilities as participants, with the hopes that data from the survey would yield insight to full-scale occurrence and formation of bromate. A geographic distribution of U.S. survey participants can be found in Figure 3. Although participation from California appears to be a geographic bias, the larger number of participants is somewhat justified due to the high amount of ozone usage within the state. Results from the survey are presented from two separate rounds of sampling. The first round was a late spring sampling session (June), while the second round was during the late fall (November). Minimum, mean, and maximum values of all the measured parameters for both rounds are presented in Table I. The data represents the water quality at the point of ozone application. There are several noteworthy results. First, the average formed bromate decreased from round 1 to round 2 (4.8 μg/L versus 3.6 μg/L). Potential reasons for this observed trend could be the decrease in average water temperature (June versus November sampling dates), or a significantly lower average level of bromide in the source water (66 μg/L versus 49 μg/L). Bromate levels did range significantly, from below the detection limit of

Table I. Summary Results of the A W W A R F Bromate Survey Parameter (Units) D O C (mg/L) UVA^icm- )

Min 1.3 0.012

1

-1

AUVA (cm ) S U V A (L/mg-m) N H - N (mg/L) 2 5 4

3

A L K (mg/L) Bffog/L) Br" conversion Br0 -^g/L) 3

r f

c

Round 1 Mean 3.1 0.053

a

Max 8.6 0.260

Min 1.0 0.011

Round 2 Mean 3.3 0.058

Max 8.9 0.247

-0.010

0.044

0.181

0.004

0.041

0.117

0.9 nd

2.1 0.17

3.4 0.70

0.9 0.01

2.2 0.16

3.2 1.11

8.7 6.4

111 66

270 180

6.8 3.7

98 49

280 150

0%

8%

39%

0%

7%

57%

Influence of Natural Organic Matter on Bromate Formation During

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