Grantee Research Project Results
Final 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 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 acknowledgement that municipal sewers "have been identified as significant sources of hazardous air pollutant (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 this research project were to assess whether municipal sewers are significant area sources of HAPs, and whether such emissions can lead to localized “hot spots” that should be considered for future NESHAPs related to POTWs. Specific objectives included: (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.
Summary/Accomplishments (Outputs/Outcomes):
This study involved four major tasks: (1) completion of a series of field experiments to assess the removal of volatile tracers from operating municipal sewers; (2) use of tracer data to evaluate existing models for the prediction of volatile organic compound (VOC) removal from sewers; (3) development of a novel modeling approach to predict emissions of VOCs from the entire municipal sewer network; and (4) application of the modeling approach to several POTWs. Each of these tasks is described below, along with a summary of significant results.
Our team worked closely with the City of Austin to complete volatile tracer experiments in operating sewer reaches. Field experiments involved three sewer reaches in the City of Austin (North Austin Outfall [NAO = 3.6 km], Waller Creek Line [WCL = 2.9 km], and Lower Shoal Main [LSM = 2.7 km]). Five experiments were completed, two each in WCL and LSM and one in the NAO. The reaches spanned a wide range of conditions, particularly as related to channel slope, wastewater flow rate, and other relevant features (lengths of uniform channel, representative drop structures, etc.). Experimental tracers included (Henry's law constant at 25°C and units of (mg/L)gas/(mg/L)liq provided in parentheses) 1,2-dibromoethane (0.029), dibromomethane (0.036), 1,3,5-trimethylbenzene (0.14), 1,4-xylene (0.32), cis-1,3-dichloropropene (0.63), trans-1,3-dichloropropene (0.72), cyclohexane (7.9). Details of the experimental and analytical protocols are described in a detailed final report (volume 1) submitted to the U.S. EPA.
An example result from one field experiment is shown in Figure 1 below for the LSM. In this example, tracer removal efficiencies (fraction of mass removed from wastewater along the 2.7-km reach) ranged from 60 percent (1,2-dibrimoethane) to 100 percent (cyclohexane). A summary of average stripping efficiencies for each experimental reach is provided in Figure 2. Note that 1,3,5-trimethylbenzene (1,3,5-TMB) and p-xylene values are low for the WCL. It is believed that these chemicals were discharged to the experimental sewer reach from another connecting reach (i.e., increasing levels downstream of the tracer injection point). The lowest removal efficiencies were consistently observed for the WCL, which also was characterized by the mildest channel slopes. Even the VOCs, with relatively low Henry's law constants (1,2-dibromoethane and dibromomethane), were effectively stripped from wastewater along the NAO and LSM, both of which were characterized by short sections of steep channel slopes (> 10 percent).
Figure 1. Example Result (Lower Shoal Main).
Figure 2. Average Stripping Efficiencies for Each Experimental Reach and Tracer.
The results of field experiments were used to evaluate an existing mathematical model for estimating VOC emissions from sewer reaches. For individual reaches and compounds, model predictions were generally within a factor of two of experimental results. However, the model compared more favorably when compared with a series of experimental sections (entire reach), with relative differences generally less than 20 percent between predicted and experimental removal efficiencies.
We had originally intended to use the model described above to simulate VOC emissions from a specific sewer network to determine the significance of volatile HAP emissions from municipal sewers and to identify "hot spots" for HAP emissions. However, a major problem was encountered with respect to the unwillingness of municipalities to participate in this project; several large POTWs were asked to participate or provide data for this study, and each one declined. In response, we developed a novel approach for estimating systemwide volatile HAP emissions from municipal sewer systems, and used National Pollutant Discharge Elimination System (NPDES) and industrial pretreatment data submitted by municipalities to EPA Region 6 as a means of "back calculating" emissions from sewers. We also employed influent HAP data compiled by The Association of Metropolitan Sewerage Agencies (AMSA) based on a survey of their own members to derive "reasonable" estimates of systemwide HAP emissions normalized by total wastewater collected. These AMSA data are now a decade old and subject to some uncertainty. However, they were the best data set available and obtainable at the time of this study.
The modeling approach was based on Monte Carlo simulations of VOC discharges and transport through sewer systems with reasonable distributions of pipe sizes, channel slopes, and other operating conditions. The model described above was then used to develop probabilistic distributions for VOC removal efficiency for a wide range of VOCs, types of sewers, and drop structures. These distributions were further "sampled" 100,000 times to develop systemwide stripping efficiency distributions for individual VOCs. When coupled with reported VOC mass loadings into individual wastewater treatment plants, the calculated stripping distributions allowed for a back calculation of VOC mass emission distributions. Tenth, 50th, and 90th percentile values were then calculated for mass emissions of individual HAPs from several sewer systems from which data were obtained through the sources mentioned previously.
Predictions of sewerwide emissions are expected to be underestimated, as they do not account for all of the areas where significant turbulence can occur in sewers. Nevertheless, our estimates of volatile HAP emissions from the sewers of several POTWs in EPA Region 6 are significant relative to mass flow rates into the corresponding downstream treatment plant, even with respect to major atmospheric emitters listed in the Toxic Release Inventory (TRI) for some cities. For example, in one city with a population of approximately 500,000, we predict benzene, ethylbenzene, and naphthalene emissions from sewers to exceed those from the largest industrial emitters listed in TRI. Toluene emissions (50th percentile) were predicted to be 11 metric tons/year from the sewer system of that same city. Sewers serving 40 million gallons per day (MGD) plant in another city were predicted to emit between 55 (10th percentile) and 350 (90th percentile) metric tons of benzene each year, potentially alarming amounts that far exceed industries reporting to the TRI in that city. Sewers serving an 11 MGD plant in yet another city were predicted to emit between 55 and 310 metric tons/year of chlorobenzene.
Results of model applications based on plant liquid influent data for a large number of POTWs responding to the aforementioned AMSA survey are provided in Tables 1 and 2 below. For both sets of data, the mean, 10 percent, 50 percent, and 90 percent stripping efficiencies were applied, and the total annual emissions were calculated on the basis of 1 million gallons of treated wastewater. For the first data set (see Table 1: 1992-1993 data), total daily emissions (summed VOCs) of 186 grams per million gallons of wastewater treated were calculated, assuming the median stripping efficiency applied. Assuming that these influent concentrations are representative of a 20 MGD treatment plant, total HAP emissions from the municipal sewer system would be estimated at 1.5 ton/year. For the second set of data (see Table 2: 1994 data), total annual emissions were calculated as 0.5 ton/year for every million gallons of wastewater treated, based on the 10 percent value from the distributions. The median and 90 percent values resulted in annual emissions of 1.3 and 3.2 ton per million gallons of wastewater treated, respectively. Because these data represent large treatment plants, if a 200 MGD treatment plant was assumed to have similar influent concentrations, an estimated 256 tons of HAPs would be emitted by a municipal sewer system annually if the median stripping efficiency were applicable.
Because the average influent concentrations are presented by AMSA, it should not be assumed that every treatment plant experiences similar circumstances. Instead, the influent VOC concentrations are dependent on discharges to a specific POTW. It also is important to note that the AMSA data include concentrations for a much broader range of chemicals than found in most pretreatment reports. Municipalities are required to sample for many of these chemicals; however, at the average concentrations provided in the AMSA data, many of these compounds would fall below the required reporting limits. Thus, total HAP emissions based on pretreatment reports may be underestimated.
Table 1. Estimation of Emissions Based on AMSA 1992-1993 Liquid Influent Data (179 Respondents).
Chemical | Concentration | Stripping Efficiency | Emissions per million gallons | ||||||
Mean | 10% | 50% | 90% | Mean | 10% | 50% | 90% | ||
µg/L | % | % | % | % | ton/year | ton/year | ton/year | ton/year | |
1,1,1-Trichloroethane | 3.7 | 62 | 38 | 64 | 82 | 0.009 | 0.003 | 0.010 | 0.026 |
1,2-Dichloroethane | 0.6 | 23 | 10 | 20 | 38 | 0.000 | 0.000 | 0.000 | 0.001 |
Acrolein | 29.2 | 4 | <1 | 2 | 6 | 0.002 | - | 0.001 | 0.003 |
Acrylonitrile | 2.2 | 4 | <1 | 3 | 7 | 0.000 | - | 0.000 | 0.000 |
Benzene | 3.0 | 46 | 25 | 45 | 68 | 0.004 | 0.002 | 0.004 | 0.010 |
Carbon Tetrachloride | 0.1 | 69 | 45 | 71 | 87 | 0.000 | 0.000 | 0.000 | 0.001 |
Chlorobenzene | 3.2 | 39 | 20 | 38 | 58 | 0.003 | 0.001 | 0.003 | 0.007 |
Chloroform | 5.8 | 44 | 23 | 43 | 64 | 0.007 | 0.003 | 0.007 | 0.016 |
Dichloromethane | 10.3 | 32 | 16 | 32 | 53 | 0.007 | 0.003 | 0.007 | 0.018 |
Ethylbenzene | 2.4 | 50 | 28 | 50 | 71 | 0.004 | 0.001 | 0.004 | 0.009 |
Methanol | 137.7 | <1 | <1 | <1 | <1 | - | - | - | - |
Methyl Ethyl Ketone | 3.8 | 5 | 1 | 3 | 8 | 0.000 | 0.000 | 0.000 | 0.001 |
Naphthalene | 1.6 | 12 | 4 | 10 | 21 | 0.000 | 0.000 | 0.000 | 0.001 |
Styrene | 0.6 | 33 | 16 | 31 | 50 | 0.000 | 0.000 | 0.000 | 0.001 |
Tetrachloroethene | 4.6 | 62 | 38 | 63 | 82 | 0.011 | 0.004 | 0.012 | 0.032 |
Toluene | 14.1 | 48 | 26 | 47 | 68 | 0.020 | 0.008 | 0.019 | 0.046 |
Trichloroethene | 0.9 | 55 | 32 | 55 | 76 | 0.002 | 0.001 | 0.002 | 0.004 |
Xylenes (Mixed Isomers) | 12.8 | 45 | 24 | 44 | 65 | 0.016 | 0.006 | 0.015 | 0.036 |
Total Emissions Per Million Gallons of Wastewater Treated: | 0.0775 | 0.0290 | 0.0749 | 0.1836 |
Table 2. Estimation of Emissions Based on AMSA 1994 Liquid Influent Data (144 Larger POTWs).
Chemical | Concentration | Stripping Efficiency | Emissions per million gallons | ||||||
Mean | 10% | 50% | 90% | Mean | 10% | 50% | 90% | ||
µg/L | % | % | % | % | ton/year | ton/year | ton/year | ton/year | |
1,1,1-Trichloroethane | 3.8 | 62 | 38 | 64 | 82 | 0.009 | 0.004 | 0.010 | 0.026 |
1,1,2,2-Tetrachloroethane | 0.1 | 8 | 2 | 6 | 13 | 0.000 | 0.000 | 0.000 | 0.000 |
1,2,4-Trichlorobenzene | 0.4 | 28 | 13 | 26 | 45 | 0.000 | 0.000 | 0.000 | 0.000 |
1,2-Dichloroethane | 0.3 | 26 | 10 | 20 | 38 | 0.000 | 0.000 | 0.000 | 0.000 |
1,2-Dichloropropane | 0.1 | 35 | 17 | 33 | 53 | 0.000 | 0.000 | 0.000 | 0.000 |
1,4-Dichlorobenzene | 3.8 | 36 | 18 | 34 | 54 | 0.003 | 0.001 | 0.003 | 0.007 |
Acrylonitrile | 26.2 | 4 | <1 | 3 | 7 | 0.002 | - | 0.001 | 0.003 |
Benzene | 44.0 | 46 | 25 | 45 | 68 | 0.057 | 0.022 | 0.055 | 0.142 |
Carbon Disulfide | 1.7 | 64 | 40 | 65 | 83 | 0.005 | 0.002 | 0.005 | 0.013 |
Carbon Tetrachloride | 11.1 | 69 | 45 | 71 | 87 | 0.038 | 0.014 | 0.041 | 0.113 |
Chloroethane | 0.7 | 59 | 36 | 60 | 80 | 0.002 | 0.001 | 0.002 | 0.004 |
Chloroform | 49.4 | 44 | 23 | 43 | 64 | 0.059 | 0.022 | 0.057 | 0.134 |
Dichloromethane | 59.0 | 19 | 16 | 32 | 53 | 0.021 | 0.017 | 0.042 | 0.101 |
Ethylbenzene | 74.0 | 50 | 28 | 50 | 71 | 0.113 | 0.044 | 0.113 | 0.276 |
Methyl Chloride | 0.4 | 50 | 27 | 50 | 71 | 0.001 | 0.000 | 0.001 | 0.001 |
Methyl Ethyl Ketone | 93.8 | 5 | 1 | 3 | 8 | 0.008 | 0.001 | 0.004 | 0.012 |
Methyl Isobutyl Ketone | 42.2 | 11 | 3 | 9 | 18 | 0.008 | 0.002 | 0.006 | 0.014 |
Methyl Tert-Butyl Ether | 0.6 | 47 | 25 | 46 | 67 | 0.001 | 0.000 | 0.001 | 0.002 |
m-Xylene | 2.5 | 49 | 27 | 49 | 70 | 0.004 | 0.001 | 0.004 | 0.009 |
Naphthalene | 25.3 | 12 | 4 | 10 | 21 | 0.005 | 0.002 | 0.004 | 0.010 |
Nitrobenzene | 2.7 | 1 | <1 | <1 | 1 | 0.000 | - | - | 0.000 |
o-Xylene | 1.8 | 43 | 22 | 42 | 63 | 0.002 | 0.001 | 0.002 | 0.005 |
Styrene | 3.7 | 33 | 16 | 31 | 50 | 0.003 | 0.001 | 0.003 | 0.006 |
Tetrachloroethene | 18.9 | 62 | 38 | 63 | 82 | 0.047 | 0.018 | 0.049 | 0.131 |
Toluene | 49.7 | 48 | 26 | 47 | 68 | 0.070 | 0.027 | 0.067 | 0.161 |
Trichloroethene | 1.4 | 55 | 32 | 55 | 76 | 0.003 | 0.001 | 0.003 | 0.007 |
Vinyl acetate | 0.2 | 13 | 4 | 11 | 23 | 0.000 | 0.000 | 0.000 | 0.000 |
Xylenes (Mixed Isomers) | 707.0 | 45 | 24 | 44 | 65 | 0.881 | 0.340 | 0.846 | 2.000 |
Total Emissions Per Million Gallons of Wastewater Treated: | 1.3302 | 0.5174 | 1.3083 | 3.1518 | |||||
Note: 2,4-Dinitrotoluene, Cresol, and Methanol have stripping efficiencies <1% and are not included. |
The following conclusions are made based on the findings of novel field experiments and model applications associated with this STAR Grant:
· A large fraction of VOCs, including volatile HAPs, is likely emitted from municipal wastewater prior to reaching downstream wastewater treatment plants.
· Collective emissions of volatile HAPs are likely much greater from municipal wastewater collection systems than from municipal wastewater treatment plants to which they flow.
· In some cities, volatile HAP emissions from municipal sewers may exceed emissions from all regulated industrial sources in the city.
· Volatile HAP emissions are most likely emitted close to the point of discharge, where conditions for gas-liquid mass transfer and exhaust are greatest. These conditions include relatively shallow flows, steeper channel slopes, and relatively high degrees of air exchange between sewer and ambient atmospheres.
· Sewer ventilation is a key parameter affecting VOC emissions from municipal sewers, but is generally not well understood by either practitioners or academicians.
· The presence of drop structures can lead to significant VOC stripping efficiencies, often exceeding those from long sewer reaches.
· It is possible that "hot spots" for volatile HAP emissions can occur in municipal sewers. These locations would be characterized by significant HAP emissions at localized exhaust points in areas where significant exposure to the public may occur. However, this study did not lead to the identification of such "hot spots;" POTWs might identify such locations based on knowledge of dischargers, sewer design, sewer operating conditions, and selective field sampling and modeling.
· Modeling tools and field tracer protocols are now available for pro-active POTWs to better characterize volatile HAP emissions from municipal sewers.
Recommendations. As with many research efforts, the results of this study have generated numerous suggestions for practical applications and future research. The following recommendations stem from this study:
· Additional studies of sewer ventilation should be completed. Particular emphasis should be given to determining the extent of air exchange rates and flow patterns in municipal sewer systems in vicinities characterized by relatively significant mass discharges of VOCs.
· Given their potential importance, pilot and field experiments should be completed to better understand gas-liquid mass transfer across sewer drop structures and at hydraulic jumps. These studies should result in a more mechanistic model for estimation of stripping efficiency at these structures.
· POTWs should become pro-active in terms of identifying areas of potential volatile HAP emissions from their specific sewer systems. At best, this would involve selective tracer releases and system-specific characterization of HAP stripping potential, coupled with intermittent sampling of volatile HAPs at key locations in the sewer system. At least, POTWs should utilize the modeling tools stemming from this and other studies to investigate potential HAP hot spots in their sewer networks.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 7 publications | 4 publications in selected types | All 2 journal articles |
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Type | Citation | ||
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Koziel JA, Corsi RL, Lawler DF. Gas-liquid mass transfer along small sewer reaches. Journal of Environmental Engineering 2001;127(5):430-437. |
R827930 (Final) |
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Koziel JA, Corsi RL, Lawler DF. Closure to "Gas-liquid mass transfer along small sewer reaches" by Jacek A. Koziel, Richard L. Corsi, and Desmond F. Lawler. Journal of Environmental Engineering 2002;128(12):1190-1192. |
R827930 (Final) |
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Supplemental Keywords:
wastewater, volatile organic compounds, VOCs, hazardous air pollutants, HAPS, emissions, tracers, modeling, field experiments, EPA Region 6, air toxics, benzene (including benzene from gasoline), chloroform, ethyl benzene, methyl chloride (chloromethane), methyl tert butyl ether, MTBE, publicly owned treatment works, POTWs, tetrachloroethylene, toluene, xylenes, acute toxicity, ambient air quality, atmospheric chemistry, chemical composition, effluents, emissions, municipal sewer emissions, municipal sewers., Scientific Discipline, Air, Toxics, Geographic Area, Water, Hydrology, air toxics, Wastewater, Environmental Chemistry, Chemistry, HAPS, State, 33/50, EPA Region, ambient air quality, Methyl tert butyl ether, air pollutants, Toluene, Texas, municipal sewers, hazardous air pollutants, MTBE, Tetrachloroethylene, Xylenes, Ethyl benzene, Methyl chloride (Chloromethane), emissions, chemical composition, municipal sewer emissions, benzene, Chloroform, POTWs, POTW, Region 6, acute toxicity, wastewater tracer studies, effluents, Volatile Organic Compounds (VOCs), Benzene (including benzene from gasoline), Xylenes (isomers and mixture), TXProgress 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.