Grantee Research Project Results
2000 Progress Report: Municipal Sewers as Sources of Hazardous Air Pollutants
EPA Grant Number: R827930Title: 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 Period Covered by this Report: January 2, 2000 through January 1,2001
Project Amount: $298,798
RFA: Urban Air Toxics (1999) RFA Text | Recipients Lists
Research Category: Air
Objective:
The U.S. Environmental Protection Agency (EPA) has developed National Emission Standards for Hazardous Air Pollutants (NESHAP) for Publicly Owned Treatment Works (POTWs). However, despite an acknowledgment that municipal sewers "have been identified as significant sources of HAP emissions from certain POTWs," sewers were omitted from the NESHAP because "little information is currently available to the EPA regarding these emissions." The primary objectives of the proposed research are to assess whether municipal sewers are significant area sources of hazardous air pollutants (HAPs), and whether such emissions can lead to localized "hot spots" that should be considered for future NESHAPs related to POTWs. 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.
Progress Summary:
Research to date has focused on a series of field experiments to determine the airborne release of a set of volatile tracers from operating sewers in Austin, Texas. A significant effort was made to select four sewer reaches in the City of Austin that span a wide range of conditions, particularly was related to channel slope, wastewater flow rate, and other relevant features (lengths of uniform channel, representative drop structures, etc.). Site visits were made to each sewer reach and assessments were made as to appropriate locations for tracer injection and sample collection. Sewer specifications were obtained from the City of Austin in order to plan the details of experiments on each reach and for post-experiment model evaluation. Target sewer reaches vary in length from approximately 4 to 8 km.
A significant effort also was made to select appropriate chemical tracers. Criteria associated with this process included the selection of tracers that span a wide range of physicochemical properties, particularly Henry's law constant, are easily resolved using purge and trap (liquid samples) and thermal desorption (gas samples) followed by GC/MS, are not present at detectable levels in City of Austin sewers, and that met the approval of the City of Austin, Texas Natural Resource Conservation Commission, and the EPA (Region VI). An "iterative" process led to the selection of the following volatile tracers: cyclohexane, dibromomethane, 1,2-dibromoethane, trans-1,3-dichloropropene, cis-1,3-dichloropropene, 1,4-dimethylbenzene (p-xylene), and 1,2,4-trimethylbenzene.
Following the selection of target sewer reaches and volatile tracers, a detailed quality assurance project plan (QAPP) was developed and submitted to the EPA. The QAPP contains information related to target chemicals, experimental protocols, sample analytical protocols, quality assurance procedures and quality assurance metrics. Experimental protocols include the injection of volatile tracers into the liquid-phase of each target sewer, injection of conservative tracers into the liquid (rhodamine dye) and gas (SF6) phases of each sewer in order to characterize fluid flows and dynamics, and sample collection in each phase at several downstream locations. Analytical methods include purge and trap followed by GC/MS for liquid samples containing volatile tracers, thermal desorption followed by GC/MS for gas samples containing tracers collected on adsorbent tubes, direct inject to a GC/ECD for gas samples to assess SF6 concentrations, and fluorometer analysis of liquid samples to quantify rhodamine concentrations.
A set of preliminary experiments was completed to refine experimental protocols, identify logistical difficulties (experiments are manpower intensive and require significant communication between researchers in the field), and to optimize analytical methods based on samples collected in actual wastewater and sewer headspace conditions.
Four actual experiments have been completed to date and data analysis has been completed on two of these experiments. Initial results verify previous modeling exercises that suggest that a significant fraction of volatile HAPS is removed from wastewater to air prior to reaching a downstream treatment plant. For the approximate 4 km sewer reach that has been completed to date, average mass removal efficiencies across the entire reach vary from complete removal (cyclohexane) to 54 percent removal (dibromomethane). Removal of the other five tracers was in a narrow band of between 72 to 78 percent removal. Approximately 2/3 of the removal occurred across a pimp station in which wastewater is pumped over a short distance to a gravity-flow sewer, a common occurrence in municipal collection systems.
It is anticipated that all field experiments will be completed by the end of April (2001). An effort is now underway to use existing and future field data to evaluate the performance of existing emission models previously developed at The University of Texas at Austin.
Future Activities:
Task 1 currently involves selective tracer releases in several operating sewers in Austin, Texas. Tracer migration is being monitored at distances ranging from 3 to 5 kilometers downstream of the discharge point, and tracer removal (stripping) efficiencies are being determined. It is expected that field experiments will be completed by April 31, 2001. The complete set of results will then be used to ascertain whether a significant fraction of volatile hazardous air pollutants (HAPs) is emitted to the ambient atmosphere prior to arriving at a POTW treatment facility, and to evaluate/calibrate two models (naUTilus and CMBA) that were recently developed at The University of Texas at Austin. Task 2 will involve the application of these models to estimate HAP emissions from a municipal sewer network in Houston, Texas. Locations that exceed prescribed emission thresholds (hot spots) will be identified. Task 3 will involve field monitoring to determine HAP emissions from those locations identified as hot spots during Task 2.
Journal Articles:
No journal articles submitted with this report: View all 7 publications for this projectSupplemental Keywords:
wastewater, VOCs, toxics, emissions, ambient air, Texas, TX, EPA Region VI., Air, Scientific Discipline, Geographic Area, Water, Toxics, EPA Region, State, Chemistry, HAPS, Environmental Chemistry, 33/50, Wastewater, Hydrology, air toxics, hazardous air pollutants, Benzene (including benzene from gasoline), Region 6, municipal sewer emissions, emissions, atmospheric chemistry, MTBE, benzene, Tetrachloroethylene, chemical composition, wastewater tracer studies, Toluene, effluents, air pollutants, Chloroform, Methyl tert butyl ether, Texas, municipal sewers, POTWs, Volatile Organic Compounds (VOCs), ambient air quality, Xylenes, Ethyl benzene, TX, acute toxicity, POTW, Methyl chloride (Chloromethane)Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.