Municipal Sewers as Sources of Hazardous Air PollutantsEPA Grant Number: R827930
Title: Municipal Sewers as Sources of Hazardous Air Pollutants
Investigators: Corsi, Richard L.
Institution: The University of Texas at Austin
EPA Project Officer: Chung, Serena
Project Period: January 2, 2000 through December 31, 2002
Project Amount: $298,798
RFA: Urban Air Toxics (1999) RFA Text | Recipients Lists
Research Category: Air , Air Quality and Air Toxics
Description:There are greater than 15,000 publicly owned treatment works (POTWs) in the United States. These POTWs serve as pathways for the discharge of hazardous compounds in all urban areas and are potentially important with respect to urban air toxics for two reasons. First, the composition of hazardous air pollutants (HAPs) observed in wastewater provides a qualitative "snap shot" of the non-mobile use of such chemicals in urban areas. Second, sewers themselves may serve as important distributed (area) sources of volatile HAP emissions. Several HAPs that are commonly emitted by POTWs include toluene, ethylbenzene, xylenes, benzene, methylene chloride, chloroform, and tetrachloroethene. Additional compounds such as methyl-t-butyl ether (MTBE) are also observed at relatively high concentrations in some municipal wastewater. While the concentrations of most of these HAPs are typically observed at less than 10 to 20 mg/L in the influent streams of treatment plants, the large volumes of wastewater that are collected suggest that significant mass discharges and airborne emissions may occur from municipal sewers. Furthermore, the concentrations that are observed at the influent to treatment plants may be significantly lower than corresponding concentrations in sewers if significant emissions occur from the latter. The primary objectives of this study are to assess whether municipal sewers are significant area sources of HAPs, and whether such emissions can lead to localized "hot spots". Specific objectives include (1) development of a database that includes measured stripping efficiencies for a wide range of volatile chemicals in municipal sewers, (2) estimation of HAP emissions from a large urban sewer network, and (3) comparison of such emissions with other known sources of HAPs.
Approach:Task 1 will involve a series of experiments to track the migration and partitioning of several tracers over distances of 3 to 10 kilometers in operating sewers in Austin, Texas. At least four tracer chemicals will be used and will span a wide range of Henry's law constants. Four separate reaches (series of reaches) will be tested twice each for a total of eight experiments. Chemicals will be pre-dissolved in water and injected into a submerged diffuser system in the underlying wastewater. Rodamine dye will be injected from a second reservoir (drum), and will facilitate estimates of axial dispersion and wastewater flow rates. Wastewater and sewer headspace samples will be collected at several locations along the downstream flow path, including the head works of the treatment plant that serves each of the sewers to be tested, with subsequent analysis using gas chromatography. The experimental database will be used to ascertain the degree to which volatile HAPs are removed from wastewater prior to reaching a treatment plant. The data will also be used to evaluate two models (CMBA and naUTilus) that have been developed, but not evaluated based on field data, to estimate VOC emissions from sewer networks.
Task 2 will constitute the first rigorous modeling effort to predict volatile HAP emissions from a large and integrated municipal sewer system. The CMBA and naUTilus models will be used to predict volatile HAP emissions from an entire (or large section of) urban sewer network in Houston, Texas. This effort will involve extensive data collection regarding physical sewer system design, wastewater flow rates, locations and types of dischargers to the system, and mass discharge rates. Predicted emissions will be compared with other known sources of HAPs in the Houston Metropolitan area, including emissions from POTW treatment facilities. The modeling effort is expected to lead to the identification of localized emission "hot spots".
Task 3 will involve a field study to investigate emission " hot spots" that are identified during task 2. At selected sites, two headspace air samples will be collected as a pre-screening for the occurrence and concentrations of volatile HAPs. A set of three to six sites will be selected for study, depending on the relative magnitudes of predicted emissions and pre-screening concentrations at each site. Each event will occur over two or more days, and will involve the collection of wastewater samples in sewer(s) upstream and downstream of the predicted hot spot. Gas samples will be collected in the sewer headspace at one or more locations in the vicinity of each hot spot, depending on the sewer configuration, and will be multiplied by headspace air exchange rates to estimate mass emissions. Headspace exchange rates will be determined based on anemometry and selective addition and analysis of sulfur hexafluoride (SF6). One or more of the hot spots will be selected for additional study, and follow-up emission monitoring events will be completed as described above. However, parallel ambient monitoring will also be completed in the near vicinity of the hot spot. Ambient samples will be collected at three locations, e.g., 10 to 100 m, downwind and upwind of the emissions monitoring location.
Expected Results:The research proposed herein will be the most comprehensive study of volatile HAP emissions ever completed for municipal sewers.
Expected results include:
- A large database associated with task 1 tracer studies. The database will be
available in spreadsheet format, and will be beneficial for the evaluation of
existing and future models.
- A rigorous evaluation of the naUTilus and CMBA models. If the models are
proven to be acceptable emissions estimation tools, this effort will facilitate
their use in other urban areas.
- An answer as to whether a significant fraction of volatile HAPs that are
discharged to sewers are likely to be emitted prior to reaching a treatment
- A well-defined methodology for identifying air toxic "hot spots" in
- A state-of-the-art database associated with HAP emissions from sewers in a
large urban area.
- An assessment of whether sewers are major sources of volatile HAP emissions
in urban areas, and whether future regulations are warranted to assess or
control such emissions.
- A final report to the USEPA.
- Several papers in peer-reviewed journals and conference proceedings.
- Incorporation of findings into a short course that is offered on a regular
basis at The University of Texas at Austin.