Development of a Bench-Scale Test to Predict the Formation of Nitrosamines [Project #4180]

Transcription

1 Development of a Bench-Scale Test to Predict the Formation of Nitrosamines [Project #4180] ORDER NUMBER: 4180 DATE AVAILABLE: May 2012 PRINCIPAL INVESTIGATORS: Stuart W. Krasner, Chih Fen Tiffany Lee, William A. Mitch, and Urs von Gunten BACKGROUND: Nitrosamines are emerging chemicals of health and regulatory concern. They have 10-6 excess cancer risks (i.e., 1 chance in 1,000,000) at low ng/l levels. The U.S. Environmental Protection Agency has included five nitrosamines on the contaminant candidate list 3 (CCL3) and may regulate this class of compounds in the next few years. The predominant nitrosamine observed in drinking water is N-nitrosodimethylamine (NDMA). NDMA may be formed via a nitrosation mechanism when sufficient nitrite is present and chlorine is primarily in the form of hypochlorous acid (HOCl). However, NDMA is typically not formed in the presence of free chlorine. NDMA may also be formed via a non-nitrosation mechanism in the presence of chloramines. Moreover, research has shown that dichloramine is more reactive in forming NDMA than monochloramine. The order of addition of chlorine and ammonia can impact NDMA formation, where the addition of chlorine to ammonia-containing water can result in a localized region with dichloramine and, thus, more NDMA formation. In other research on reclaimed wastewater, NDMA formation peaked at the breakpoint. Sources of NDMA precursors include treated wastewater, soluble microbial products (SMPs) from activated sludge, certain pharmaceuticals and personal care products (PPCPs) with tertiary amine functionalities (e.g., ranitidine), and particular quarternary amines (e.g., the polymer polydadmac, certain anion exchange resins, shampoo constituents). Strong pre-oxidants (chlorine, chlorine dioxide, ozone) can destroy or transform NDMA precursors. HOCl reacts with deprotonated amines, where the maximum rate constant for amine oxidation might be at a ph level of ~8.6. OBJECTIVES: Formation potential (FP) tests have been used to determine levels of nitrosamine precursors, but they do not provide an indication of how much might form in full-scale applications. Simulated distribution system (SDS) (bench-scale) tests have been used to estimate trihalomethane (THM) formation in the presence of free chlorine. Thus, the objective of this project was to develop an SDS test for nitrosamines. In addition, bench-scale tests with different pre-oxidants were conducted to evaluate the impact on NDMA formation and control. Moreover, tradeoffs in the formation of other disinfection by-products (DBPs) were evaluated. APPROACH: The project was divided into the following tasks in order to assess nitrosamine formation and control while balancing the formation of regulated and other emerging DBPs and achieving adequate microbial inactivation:

2 Evaluate the impact of ph, temperature, and time Assess the effect of the ratio of chlorine to ammonia-nitrogen (Cl 2 /N) Evaluate the ability of pre-oxidation to control nitrosamine formation and determine tradeoffs from the formation of other DBPs Examine the effect of bromide or nitrite on nitrosamine formation Assess the impact of order of addition and the mixing effect at pilot scale Mimic full-scale nitrosamine and halogenated DBP formation in site-specific SDS tests As part of this project, a standard SDS test with uniform formation conditions (UFC) was used as a baseline. Filtered water was chlorinated for 3 min, with a dose sufficient to result in a residual of ~2.5 mg/l as Cl 2. Ammonia was then added to be at a Cl 2 /N weight ratio of 4.75:1 mg/mg (just to the left of the maximum on the breakpoint curve). The chloraminated water was held for 3 days at ph ~8 at 25 C. In other tests, SDS conditions were varied from UFC, with one parameter varied at a time. The range of conditions evaluated included ph levels of 7, 8, and 9; temperatures of 5, 15, 25, and 35 C; post-chloramine contact times of 1 hr, 1 day, 3 days, and 7 days; and Cl 2 /N weight ratios of 3.6:1, 5.1:1, 6.3:1, 7.6:1, and 8.6:1 mg/mg; as well as site-specific conditions. Pre-oxidants evaluated included chlorine, chlorine dioxide, ozone, and low- and mediumpressure ultraviolet (UV) irradiation. CT levels included ~ mg*min/l for free chlorine, ~0.2-2 mg*min/l for ozone, ~2-20 mg*min/l for chlorine dioxide, and mj/cm 2 for UV, which is germicidal fluence. Additional lower or higher CT or fluence levels were evaluated to further characterize the tradeoffs in DBP formation. In addition, higher fluence would be associated with an advanced oxidation process (AOP). Pre-oxidant experiments were followed with post-chloramination using a UFC test. RESULTS/CONCLUSIONS: Impact of ph, Temperature, and Time For polydadmac-impacted waters, typically there was more formation at ph 8, where temperature often did not significantly affect NDMA formation at ph 8 or 9. Alternatively, at ph 7, there was more formation in warmer water than in colder. At all ph levels, NDMA formation occurred slowly and typically plateaued after several days. For wastewater-impacted waters, laboratory-generated SMPs or at plants with nitrified biofilters (e.g., at plants using the ammonia-chlorine process to control bromate formation), NDMA formation usually appeared to be higher at colder water temperatures at ph 8 and/or 9. At ph 8 or 9, chlorine was more effective at destroying/transforming NDMA precursors in the warmer water, such that there were fewer precursors available to form NDMA during postchloramination. At ph 7, there were less deprotonated amines, such that pre-chlorination temperature was often less of an effect, whereas post-chloramination temperature was more of an impact on NDMA formation. In the chloramine SDS tests, THMs and haloacetic acids (HAAs) typically had higher formation at ph 7 than at ph 8 or 9. At ph 8, there was more formation in warmer water. Thus, factors

3 that affect DBP formation often had a different impact on halogenated DBPs than that of NDMA. Effect of Cl 2 /N When the Cl 2 /N ratio was varied while keeping other UFC parameters constant (e.g., chlorine added first, ph ~8), peak NDMA formation for polydadmac-impacted waters occurred at Cl 2 /N weight ratios of 5.1:1 mg/mg or less and there was little to no formation at the breakpoint (Cl 2 /N weight ratio of 7.6:1 mg/mg); whereas for wastewater-impacted waters, peak formation sometimes occurred at Cl 2 /N weight ratios to the right of the maximum on the breakpoint curve (e.g., 6.3:1 mg/mg) and there was usually substantial formation at the breakpoint. When the SDS tests were changed to more closely match that of previous research (i.e., ammonia added first, plus ph ~7 in selected tests), peak NDMA formation for polydadmac-impacted waters was still at Cl 2 /N weight ratios of 5.1:1 mg/mg or less; whereas for wastewater-impacted waters, peak NDMA formation was at Cl 2 /N ratios at or approaching the breakpoint. THM and HAA formation increased with increasing Cl 2 /N ratio, especially after breakpoint where there was free chlorine. In tests with a blend of treated wastewater and river water, THM yields (normalized to total organic carbon [TOC]) were higher for the waters with more river water. Although the wastewater was an important source of NDMA precursors, it was low in reactivity to form THMs. Pre-Oxidation Control and Tradeoffs In general, ozone and chlorine were found to be the most effective pre-oxidants in controlling the formation of NDMA. Ozone worked the best at low CT and can be considered broad screen. At higher CT, chlorine often achieved comparable control of NDMA formation. In some wastewater-impacted waters, there was more NDMA formation at low CT compared to a control sample without pre-chlorination. In one case, this was found to be due to a nitrosation mechanism. In other cases, chlorine may have transformed precursors to a form more nitrosatable. Nonetheless, at higher CT, pre-chlorination reduced NDMA formation. Furthermore, sequential use of ozone and chlorine was quite effective in minimizing NDMA formation. In general, chlorine dioxide was the least effective and appeared to be a selective oxidant. Increasing CT with chlorine dioxide generally did not improve NDMA control. Mediumpressure UV did reduce NDMA formation in some waters, but usually at high fluence that would not be used for disinfection. In each case, the pre-oxidants formed other DBPs of concern (THMs and HAAs with chlorine, chlorite with chlorine dioxide, bromate with ozone). Moreover, pre-oxidation with ozone increased the formation of chloropicrin during post-disinfection. Likewise, use of mediumpressure UV increased the formation of chloropicrin, as well as that of chloral hydrate. Nonetheless, for most waters studied, there was a CT level (especially for ozone or chlorine) that could achieve some degree of NDMA control while balancing the formation of other regulated or emerging DBPs.

4 Limited testing did show that high levels of bromide may increase NDMA formation. Temperature also impacted pre-oxidation to various extents. In cases where a low chlorine CT had limited control of NDMA formation at cold water temperatures, an increase in free chlorine CT resulted in more substantial NDMA control. The Mixing Effect The order of addition of chlorine and ammonia at pilot scale often had no impact with polydadmac-impacted waters. However, when there was an effect, chlorine added first resulted in less NDMA formation. Mimicking Full-Scale Formation Site-specific free chlorine contact times, chloramination conditions, and water quality were used in conducting SDS tests at a number of plants. Typically, the bench-scale tests produced similar levels of NDMA, THMs, and HAAs as the full-scale plant and distribution system. Because a plug of water was not followed, water collected at a maximum detention time sometimes had somewhat different levels of DBPs. In addition, SDS tests in glass bottles cannot simulate all of the phenomena that can occur in a distribution system. Nonetheless, the tests show that this can be a good predictive tool. Moreover, for plants that did not use chloramines, SDS tests were used to provide information on likely levels of NDMA that would form and what reduction in THM and HAA formation was possible. RECOMMENDATIONS: Researchers that are studying the formation and control of nitrosamines with FP tests (a measure of precursor levels) should also evaluate the use of SDS and/or UFC tests to predict likely formation under actual operating conditions. Utilities that produce high levels of NDMA can use SDS tests to evaluate treatment, pre-oxidation, or chloramination conditions that may be able to lower nitrosamine formation. Chloramine plants that plan to make changes in source water (e.g., to a wastewater-impacted watershed), treatment (e.g., polymer usage), disinfection (e.g., preoxidation), or are considering implementation of the ammonia-chlorine process should conduct SDS tests to examine the impact of these changes on nitrosamine formation. Plants that currently do not use chloramines but are considering a switch to chloramines should use SDS and/or UFC tests to predict likely levels of nitrosamine formation. Lowering the polydadmac dose or a switch to an alternative polymer may result in lower levels of NDMA formation, but changes in polymer usage must consider turbidity and filtration goals. Pilot-scale experiments with SDS testing could be used to evaluate optimizing polymer usage. Many chloramine plants have minimized or eliminated free chlorine contact time to substantially reduce THM or HAA formation. Pre-chlorination tests should be conducted to determine if an increase in free chlorine CT can be used to decrease NDMA formation while not producing excessive amounts of halogenated DBPs, presuming that the infrastructure and operations will allow for changes in free chlorine contact time. In some cases, a seasonal approach may be warranted, where more chlorine CT could be used in the winter to better control NDMA formation while producing less THMs or HAAs in the colder water. Moreover, such an

5 approach might satisfactorily control nitrosamine and halogenated DBP formation on a running annual average basis. If pre-chlorination alone cannot be used to meet an NDMA goal or future regulatory standard, ozonation should be evaluated. Moreover, sequential use of ozone and chlorine may be quite effective. Note, if ozone is added at the plant influent, it will be able to destroy/transform wastewater-derived precursors, whereas if ozone is added to the settled water, it will also be able to control precursors from polymer usage. In cases where ozone can only be added to the raw water, polymer-derived precursors may be controlled in the filter effluent with sufficient free chlorine contact time before ammonia addition. If a plant uses chlorine dioxide, tests can be conducted to determine the extent to which it can be effective in NDMA control in particular waters. If UV is used or is being considered, tests can be run to determine the impact on NDMA at the range of fluences being considered. FUTURE RESEARCH: More research is needed on studying other nitrosamines that may be present in drinking water. Currently, research on the reactivity of certain PPCPs, agricultural chemicals, and other anthropogenic chemicals with chloramines to form nitrosamines is taking place. In addition, research is needed to determine the relative contribution of these chemicals in treated wastewater or in watersheds as sources of nitrosamine precursors. More research is needed on SMPs as a source of precursors. Moreover, studies on minimizing SMP issues while still getting the benefits of biofiltration would be ideal. Because polydadmac is such a widely used polymer, it is clear that more research will be conducted on optimizing its use, as well as exploring alternative polymers. Because all chloramine plants have chlorine feed capabilities, more research is needed on evaluating how pre-chlorination conditions can be optimized for balancing the formation and control of NDMA and that of halogenated DBPs. For example, some plants with little or no free chlorine contact time have significant NDMA formation, which may be controlled with some level of preoxidation. However, additional research is needed to determine optimum conditions that control NDMA formation without forming too much THMs or HAAs. Although this project studied the use of pre-ozonation and UV, additional research is needed to understand the mechanisms (molecular ozone versus hydroxyl radicals, direct or indirect photolysis or oxidation). In addition, work with ozone- or UV-based AOPs should be explored. Research is also needed to evaluate the efficacy of powdered or granular activated carbon for nitrosamine precursor removal. Plants have been making treatment and disinfection modifications for decades to meet a range of regulatory and operational goals. More research is needed to find cost-effective and technically feasible strategies that will allow plants to control nitrosamines while meeting other standards and water quality objectives.