A COMPARISON OF ODOR AND HYDROGEN SULFIDE EMISSIONS FROM TWO METROPOLITAN WASTEWATER TREATMENT PLANTS

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1 A COMPARISON OF ODOR AND HYDROGEN SULFIDE EMISSIONS FROM TWO METROPOLITAN WASTEWATER TREATMENT PLANTS Chester M. Morton, P.E., BCEE, Phyllis Diosey QEP, Richard J. Pope, P.E., BCEE, Malcolm Pirnie, Inc. 14 Corporate Park Drive White Plains, NY 162 ABSTRACT Hydrogen sulfide is the most common odorous compound emitted from wastewater treatment plants (WWTPs) and is readily measured by numerous analytical devices. As a consequence this compound has historically been used as the primary parameter to measure and regulate wastewater odor emissions by States and local governmental agencies. Over the last 1 to 15 years, odor concentration, or dilution-to-threshold (D/T) has begun to be used to characterize odor emissions from WWTPs. The protocol for this parameter is presented in ASTM E In addition, a standard for D/T has been developed by the European Union, EN This standard is similar to ASTM E but specifies a greater presentation air flowrate to the odor panelists, provides quality assurance and control criteria, and provides for greater selectivity of panelists. A municipality had historically been using hydrogen sulfide as the primary means to measure odor emissions from its wastewater facilities. It was interested in evaluating other parameters that could be used to regulate and measure odor emissions from its facilities. A literature search was conducted that reviewed parameters and methods used by government agencies, municipalities, and the wastewater industry in measuring and projecting odor emissions. A sampling methodology was developed and field studies were conducted which measured odor emissions from two WWTPs, that are located in a metropolitan area. The treatment capacity of Plants A and B are 6 and 31 mgd, respectively. The plants unit processes are: headworks, influent distribution, primary clarification, activated sludge, final settling tanks and gravity thickeners. At the time of the sampling, Plant A s activated sludge plants were operated as step feed, with biological nutrient removal. Plant B s activated sludge tanks were operated as step-feed. Air and liquid phase measurements were made at the unit processes of both plants. Air phase measurements were made using EPA flux chambers from the influent distribution structures, primary clarifiers, aeration tanks, final settling tanks and gravity thickeners. Measurements were made on quiescent water surfaces and at weirs. At Plant A measurements were made at multiple zones within the activated sludge tanks (e.g. anoxic, oxic, etc). Air samples were analyzed for hydrogen sulfide, and dilution-to-threshold (D/T). The following will be presented in this paper: The methodology used to measure the plants emissions 21

2 The diurnal variation of hydrogen sulfide concentrations at the plants influent and a comparison to the plants influent flowrate Comparison of hydrogen sulfide and D/T concentrations and emission rates from the unit processes of each plant Dispersion modeling results for hydrogen sulfide and D/T showing offsite impacts in the form of isopleths from both plants. KEYWORDS hydrogen sulfide, dilution-to-threshold, D/T, flux chamber, odor emissions, dispersion modeling, ISCT3, isopleths INTRODUCTION The control of odor emissions from wastewater treatment plants is as important to some facilities as meeting their effluent limits because of the sensitivity of surrounding communities to a plant s odors, and to environmental issues in general. The heightened awareness of environmental issues is evidenced by the incorporation of environmental justice into the permitting process in some states. The regulation of odor emissions is often codified in terms of hydrogen sulfide because it is the most prevalent odorous compound associated with wastewater and is readily measured. In addition, many states and communities often have a general odor nuisance regulation or ordinance that prohibits the emission of odors that unreasonably interferes with the enjoyment of life and use of one s property. The limitation of the use of hydrogen sulfide to measure odor emissions has become recognized by the water pollution control industry and regulators because it is recognized that odorous compounds other than hydrogen sulfide are emitted from wastewater facilities. Quantifying odor emissions and developing control solutions based on this single compound may result in an inaccurate and misleading characterization of a facility s emissions, and putting in-place ineffective control technologies. In response, many countries, states, and other regulatory bodies have adopted odor regulations using the dilution-to-threshold (D/T) parameter. Malcolm Pirnie, Inc. conducted a study for a municipality located in a metropolitan area which is subject to a hydrogen sulfide regulation. Part of the study included characterizing odor emissions from its wastewater treatment plants in terms of hydrogen sulfide and D/T, conducting dispersion modeling to assess the impacts using these parameters and evaluating the predicted impact concentrations. This paper presents the characterization data and dispersion modeling results of that study. 22

3 MATERIALS AND METHODS Wastewater Treatment Plant (WWTPs) Descriptions Two WWTPs located in the northeastern U.S. were sampled and are referred to as Plant A and B. These plants are described as follows: Plant A The capacity of Plant A is 6 mgd. The unit processes of Plant A consist of a Headworks (screening and grit removal), primary sedimentation tanks, aeration tanks operated in step-feed and configured for biological nutrient removal (BNR), final settling and chlorine contact tanks. Each aeration tank has four passes. The first pass receives only the return activated sludge (RAS). Passes B, C, and D consist of an anoxic zone (nonaerated) for approximately one-third, and oxic (aerated) for two-thirds of each pass. Solids handling consists of co-gravity thickening of primary settled and waste activated sludge. Thickened sludge is anaerobically digested, dewatered by centrifuge and disposed off-site. Plant B The capacity for Plant A is 3 mgd. The unit processes for Plant B are the same as for Plant A except that the aeration tanks are traditional step-feed. Gas Phase Sample Collection and Analysis Gas phase samples were collected from quiescent water surfaces and weir troughs using surface isolation flux chambers (USEPA, 1986). Flux chambers used for weir troughs were custom designed to fit the weir troughs and included a sweep air header with an flowrate based on the standard USEPA flux chamber design. Sample locations in the primary settling tanks (PSTs) for Plants A and B were: the influent, mid-tank, just upstream of the effluent weir, and at the effluent weir/trough. At Plant A, sample locations in the aeration tanks were: Pass A, at the feed point of the return activated sludge (RAS); Pass A, at its mid-point; Pass B, at the feed point of the primary effluent; Pass B, at anoxic zone mid-point; Pass B, at the transition from the anoxic to the oxic zone; Pass B, at the oxic zone midpoint; and Pass D, at the effluent overflow weirs. At Plant B, the sample locations in the aeration tanks were: Pass A, at the feed point of the RAS; and Pass B, at the feed point of the primary effluent. Sample locations in the final settling tanks (FSTs) for Plants A and B were: the mid-tank, and at the effluent weir/trough. Samples that were to be analyzed for odor parameters were collected in 1-liter Tedlar bags. Each bag was pre-conditioned with air from the respective sample location before filling. Samples were analyzed for dilution-to-threshold (D/T) per ASTM E and EN Per EN13275, the presentation rate to the odor panelists was 2 liters per 23

4 minute. Hydrogen sulfide was measured using calibrated portable gold film, Arizona Instruments, 631X Jerome meters. Samples that were analyzed for hydrogen sulfide were collected in pre-conditioned 1-liter Tedlar bags. Sample Collection Dates, Frequency and Schedule Sample collection was conducted at the end of August and early September to ensure high wastewater temperatures and the presence of high odor conditions. Sampling at Plant A was conducted on eight separate days beginning at 4: pm. All locations were sampled simultaneously. Plant B was sampled during the same time period as Plant A, on two, 2-day sampling events, separated by 1 year. REGULATORY DISCUSSION Historically odor emissions from wastewater treatment plants have been regulated using hydrogen sulfide limits and general odor nuisance regulations. Ambient hydrogen sulfide limits range from.1 parts per billion by volume (ppb v ) to 5 ppb v (Mahin 21). The method of measurement can be either field measurements using portable analytical equipment, or by measuring source emissions and using dispersion modeling to predict ambient concentrations at offsite receptors. Over the past 1 to 15 years the use of the odor parameter dilution-to-threshold (D/T), or, odor concentration, has seen wider use. The D/T of an air sample is determined by an odor panel in a laboratory under controlled conditions, and is defined in ASTM E In addition, a standard for D/T has been developed by the European Union, in 23, EN D/T of ambient air has been determined in the field using hand-held devices, but this approach is being used less often due to the variability associated with measuring ambient air with a low odor concentration in favor of determinations made in odor laboratories. Where a D/T odor regulation is in use, it usually involves determining the odor concentration of an emission source using the methods noted above, determining the emission rate, and then using dispersion modeling to predict offsite D/T values. D/T limits being used in odor regulations range from 1 to 5, with the more commonly used values ranging from 5 to 7 (Mahin 21). DISCUSSION OF RESULTS Hydrogen Sulfide Time of day was a consideration in development of the sampling plan to ensure that peak emissions were measured. The intent was to sample the PSTs, aeration tanks and the FSTs simultaneously so that a peak odor emission could be modeled that is representative of elevated odor conditions that the surrounding community may experience. 24

5 In order to determine the time of day of elevated odor conditions, a hydrogen sulfide measuring/data logging device (OdaLog Unit) was installed in the influent/headworks areas of Plants A and B. At Plant A the unit was installed in the influent screenings channel. At Plant B the unit was installed at the PST distribution channel. The results of the monitoring is shown in Figures 1 and 2 and indicate that peak odor conditions, i.e., the time at which the most odorous wastewater is passing through these plants, occurs at Plant A and B beginning at approximately 2: pm and continues for about 2 to 3 hours. Hydrogen sulfide at the influent/headworks area at both plants was sampled for 5-days and the measured fluctuations shown in Figures 1 and 2 were repeated daily. It is noted that the peak odor conditions occur approximately 6 hours after the peak influent flowrate which occurs at approximately 8: am. The simultaneous sampling of the unit processes was scheduled for approximately 4: to 4:3 pm which accounted for the hydraulic retention time (HRT) through the PSTs and Pass B of the aeration tanks. The HRT for Pass A was not a factor because it receives only RAS. The HRT through the FSTs was approximately 12 hrs which precluded sampling these tanks coincident with the passage of the highly odorous wastewater. However, once the wastewater passes through the aeration tanks its odor potential is significantly reduced, therefore, the lack of coincident sampling at this location is not believed to have biased the sampling results. 15 Figure 1 - Plant A Influent Channel Hydrogen Sulfide Concentration and Influent Flowrate H2S (ppm) 6 28 Flow (mgd) H2S Flow : 2: 4: 6: 8: 1: 12: 14: 16: 18: 2: 22: : Time 25

6 Figure 2 - Plant B - Influent Channel Hydrogen Sulfide Concentration and Influent Flowrate H2S (ppm) Flow (mgd) H2S WW Flow : 2: 4: 6: 8: 1: 12: 14: 16: 18: 2: 22: : Time As would be expected, hydrogen sulfide and odor emissions were greatest at the upstream end of the plants, i.e., the distribution structure for the PSTs and the PSTs themselves, and were relatively lower in the aeration tanks and FSTs. The PST distribution structure at Plant A is non-aerated and at Plant B it is aerated. The average hydrogen sulfide emission concentration measured at the Plant A and B PST distribution structures was 18 and 1 parts per million by volume (ppm v ), respectively. At Plants A and B, the average hydrogen sulfide emission concentrations measured at the PSTs ranged from to.51 to 19 parts per million by volume (ppm v ), and.51 to.423 ppm v, respectively. The highest concentrations were measured at the effluent weirs. The lower hydrogen sulfide emissions measured at Plant B is believed to be due to the aeration of the influent wastewater in the Plant B PST distribution structure. The aeration stripped dissolved hydrogen sulfide from solution, added oxygen to the wastewater and reduced its odor producing potential. The retention time through the PSTs at both plants is approximately the same (2-hours), therefore a shorter HRT at Plant B is not the cause for the lower hydrogen sulfide emissions from these tanks. After the PSTs the wastewater enters aeration tanks where air is pumped into the wastewater, raising the dissolved oxygen concentration to support the growth of the microorganisms for the removal of biochemical oxygen demand (BOD). The average concentration of hydrogen sulfide in the aeration tank emissions from Plants A and B ranged from,.7 to.159 ppm v and.6 to.141 ppm v, respectively. At Plant A 26

7 where the aeration tanks are configured for BNR, there was an increase in the hydrogen sulfide emission concentration in the anoxic zone. The average hydrogen sulfide emission concentration from the water surface of the FSTs at Plant A and B was.5 and.11 ppm v, respectively; and from the effluent weirs, was.1 and.21 ppm v, respectively. Figures 3 to 5 give a profile of average hydrogen sulfide emission concentrations through Plant A. Figure 3 shows the concentrations through the entire plant, Figure 4 gives concentration in the PSTs and Figure 5 gives shows concentrations in the aeration tanks. Figure 6 gives a profile of hydrogen sulfide hydrogen sulfide emission concentrations through Plant B. Table 1 lists the average and maximum hydrogen sulfide emission concentrations through Plants A and B. Figure 3 - Plant A - Plantwide Hydrogen Sulfide Emission Concentration Profile 25 2 H2S (ppm) 15 1 Average 5 GT Weir GT Tank FST Weir FST Tank AT D - EF AT B - OMT AT B - AOMTT AT B - AMT AT B - PEF AT A - MT AT A - RASF Primary Weir Primary Bef Weir Primary Mid Primary In PIDB Headworks Location Figure 4 - Plant A - PST Hydrogen Sulfide Emission Concentration Profile 25 2 H2S (ppm) 15 1 Average 5 Primary Weir Primary Bef Weir Primary Mid Primary In PIDB Location 27

8 Figure 5 - Plant A - Aeration and FST Tanks Hydrogen Sulfide Emission Concentration Profile H2S (ppm).1.8 Average FST Weir FST Tank AT D - EF AT B - OMT AT B - AOMTT AT B - AMT AT B - PEF AT A - MT AT A - RASF Location Figure 6 - Plant B - Plantwide Hydrogen Sulfide Emission Concentration Profile H2S (ppm) Average Thickener Weir Thickener Mid Thickener In AT Pass B Mid AT Pass B In AT Pass A Mid AT Pass A In Primary Weir Primary Bef Weir Primary Mid Primary In Primary Influent Channel Location 28

9 Table 1 - Plants A and B Hydrogen Sulfide Emission Concentrations Location Average Values Maximum Values Plant A Plant B Plant A Plant B Headworks PIDB Primary In Primary Mid Primary Before Weir Primary Weir AT A RASF AT A MT AT B PEF AT B AMT AT B AOMTT AT B OMT AT D EF FST Tank.5 ND.11 ND FST Weir GT Tank GT Weir Dilution-To-Threshold The change in the relative D/T emission values measured through the plants were similar to the measured hydrogen sulfide values. That is, higher values were measured at the influent end of the plant, with lower values as one progressed downstream through the plant. The average D/T emission value at the PST distribution structures of Plant A and B, was 6,863 and 3,3, respectively. Average D/T values measured through the Plant A PSTs ranged from approximately 2, just upstream of the weirs to 8,15 at the effluent weirs. At Plant B the PST D/T values ranged from 21 just before the weirs to approximately 85 at the effluent weirs. At the aeration tanks of Plant A, the average D/T emission values ranged from approximately 26 in the oxic zone, to 1, in the anoxic zone. At Plant B the average D/T values ranged from 38 at the introduction of the RAS in Pass A to 2,75 at the feed point of the primary effluent. At the FSTs of Plant A the average D/T on the water surface and at the effluent weirs was 32 and approximately 7, respectively. At Plant B, the average D/T on the water surface and at the effluent weirs was 135 and 145, respectively. Figures 7 and 8 give a profile of average D/T emission values through Plants A and Plant B. Table 2 lists the average D/T emission concentrations through Plants A and B. 29

10 Figure 7 - Plant A - Plantwide D/T Emission Profile D/T 5 4 Average FST Weir FST Tank AT D - EF AT B - OMT AT B - AOMTT AT B - AMT AT B - PEF AT A - MT AT A - RASF Primary Weir Primary Bef Weir Primary Mid Primary In PIDB Headworks Location Figure 8 - Plant B - Plantwide D/T Emission Profile D/T 2 Average Thickener Weir Thickener Mid Thickener In AT Pass B Mid AT Pass B In AT Pass A Mid AT Pass A In Primary Weir Primary Bef Weir Primary Mid Primary In Primary Influent Channel Location 3

11 Table 2 - Plants A and B Dilution-to-Threshold Concentrations Location Average D/T Values Maximum D/T Values Plant A Plant B Plant A Plant B Headworks PIDB 6,863 3,3 8,8 4,7 Primary In 2, ,8 45 Primary Mid 2, ,2 54 Primary Before Weir 1, ,4 36 Primary Weir 8, , 1,2 AT A RASF AT A MT , AT B PEF 354 2,75 1, 2,9 AT B AMT 1, , AT B AOMTT , AT B OMT AT D EF FST Tank , 23 FST Weir ,5 18 GT Tank 1, ,6 26 GT Weir 6,775 1,16 11, 1,6 Hydrogen Sulfide and D/T Comparisons The average hydrogen sulfide and D/T concentration emission for Plants A and B were compared. In general, D/T values were found to correspond to the rise and fall of hydrogen sulfide values with some exceptions. Figures 9 and 1 show profiles of average hydrogen sulfide and D/T emission concentrations through Plants A and B. At Plant A, locations where D/T values increase greater than corresponding hydrogen sulfide concentrations are: within Pass A, and in the anoxic zone of Pass B. At Plant B, locations where D/T values increase greater than corresponding hydrogen sulfide concentrations are within Pass A and Pass B. These findings suggest that there are odorous compounds other than hydrogen sulfide being emitted at these locations. 31

12 Figure 9 - Plant A - Plantwide H 2 S - D/T Comparison D/T H 2S (ppm) D/T Average GT Weir GT Tank FST Weir FST Tank AT D - EF AT B - OMT AT B - AOMTT AT B - AMT AT B - PEF AT A - MT AT A - RASF Primary Weir Primary Bef Weir Primary Mid Primary In PIDB Headworks Location Figure 1 - Plant B - Plantwide H 2 S - D/T Comparison D/T H2S (ppm) D/T H2S. Thickener Weir Thickener Mid Thickener In AT Pass B Mid AT Pass B In AT Pass A Mid AT Pass A In Primary Weir Primary Bef Weir Primary Mid Primary In Primary Influent Channel Location Emissions and Dispersion Modeling Dispersion modeling was conducted of the emissions from Plant A in order to compare hydrogen sulfide and D/T offsite impacts. The latest version of the Industrial Source Complex Short-Term (ISCST3) model (Version 235, USEPA, 1995a, b) was used. ISCST3 is the USEPA preferred model for multi-source analyses because it can be used 32

13 to predict ambient air quality impacts from multiple area sources (e.g. open wastewater tanks) and point sources (e.g. odor scrubber exhaust stacks) in either urban or rural settings. The ISCST3 model reports 1-hr average impacts. However, the model s diffusion parameters are based on 3 to 15 minute averaging times and therefore the predicted impacts are reflective of this time interval. The ISCST3 model can also consider building downwash effects, stack-tip downwash, buoyancy-induced dispersion, block and running averages, and final/gradual plume rise. Inputs to the model were: Emission rates determined from the measured concentrations and the sweep air flowrate of the flux chambers. Emission rates used maximum measured hydrogen sulfide concentrations and 99% cumulative probability D/T values. One-year of meteorological data from a local meteorological station which included wind speed, direction, stability, temperature, and mixing heights. Discrete receptors were located along the Plant A fenceline at approximately 25- meter spacing. In addition, ground-level receptors were located at intervals of 25- meter spacing over a 1-kilometer by 1-kilometer Cartesian grid centered on the plant, and an additional set of Cartesian receptors at 1-meter spacing extended for and additional 1 kilometer, for a total grid size of 2-km by 2-km from the center of the plant. Terrain elevations of the receptor locations were obtained from 7.5 USGS Digital Elevation Model (DEM) files. Model control options used were: urban coefficient; the dimensions of nearby structures and the Building Profile Input Program were used to determine building wake effects. It was determined that there were no cavity impacts to be considered. One of the goals of the dispersion modeling was to produce conservative impacts because of the limited sampling providing only 8 data sets. Analysis of the data determined a greater variability of the D/T data as compared to the hydrogen sulfide data. While conservative results were sought, outliers from D/T were removed and 99% cumulative probability values were determined for each emission source. For the FST Tanks for instance, the maximum D/T value of 1, was replaced with the 99% cumulative probability value of 484. For the FST Weirs, the maximum D/T value of 3,5 was replaced with the 99% cumulative probability value of 72. This approach resulted in lower emission rates for sources with high variability; and slightly higher emission rates for sources with less variability. Another approach considered was the use of 95% cumulative probability values. Use of 99% probability values was chosen, with this issue to be addressed further before completion of the project. When removing outliers, consideration should be given to whether an outlying observation is an extreme manifestation of the random variable inherent in the data, is the result of nonrepresentative conditions of the system being sampled, or the result of error in sampling or analytical procedures. 33

14 Tables 3 and 4 present the emission flux and emission rates used in the dispersion modeling. Table 3 - Plant A Hydrogen Sulfide Emission Fluxes and Rates Emission Source Emission Flux g/m 2 -s Emissions g/s Headworks E-4 PIDB 2.45E-5 4.9E-4 Primary Tank 1.34E E-3 Primary In 1.56E-6 1.1E-3 Primary Mid 1.38E E-3 Primary Before Weir 1.7E E-4 Primary Weir - effluent 7.84E E-2 AT Pass A 2.98E-8 3.6E-5 AT Pass A Anoxic 2.51E E-6 AT Pass A Oxic 3.7E E-5 AT Pass B+C Anoxic 3.34E E-4 AT Pass B+C Oxic 1.1E E-4 AT Pass D 1.19E E-4 AT Weir 9.61E E-6 FST Tank 9.81E E-5 FST Weir 4.92E E-5 Table 4 - Plant A Dilution-to-Threshold Emission Fluxes and Rates Emission Source Emission Flux OU/m 2 -s Emissions OU/s Headworks E+3 PIDB 6.57E+ 1.31E+2 Primary Tank 2.65E+ 7.46E+3 Primary In 3.51E+ 2.48E+3 Primary Mid 1.87E+ 2.64E+3 Primary Before Weir 3.33E+ 2.35E+3 Primary Weir - effluent 2.63E+1 4.6E+3 AT Pass A 8.13E E+2 AT Pass A Anoxic 2.72E E+1 AT Pass A Oxic 9.16E-1 9.3E+2 AT Pass B+C Anoxic 1.15E+ 9.15E+2 AT Pass B+C Oxic 1.33E+ 2.16E+3 AT Pass D 1.7E+ 1.29E+3 AT Weir 1.29E+ 2.58E+1 FST Tank 3.1E E+3 FST Weir - effluent 1.2E+ 8.66E+2 Results of the dispersion modeling for Plant A are presented in the form of maps showing impacts as isopleths (lines of constant concentration) for hydrogen sulfide and D/T. Dispersion modeling results are presented in Figures 11 to 15 for the: PSTs, PST weirs, 34

15 aeration tanks, FSTs, and FST weirs. Isopleths are shown for 1 and 1 parts per billion by volume (ppb v ), and for 5 and 1 D/T values. 35

16 36

17 37

18 In addition to the concentration of a compound, the frequency of impact, i.e., hours per year, is a criterion that was considered as a measure for the need for odor control. As a reference, Germany limits odor impacts to residential and industrial areas to frequencies of 1% and 15%, respectively (Both and Koch, 24). Table 5 presents the hours of offsite impacts from the different sources at Plant A. Table 5 Frequency of Impacts Number of Hours (out of 876 hours) H 2 S D/T 1 ppb 5 Source Group Number of Hours % of total hours Number of Hours % of total hours PIDB.. PSTs PST Weirs Aeration Tanks FSTs. 1.1 FST Weirs The time of day at which impacts occur was considered. Figures 16 and 17 show the frequency at which off-site impacts above D/T values of 1 occur from the PSTs and aeration tanks from Plant A. These figures show most of the impacts to occur from 8 pm and 6 am. Figure 16 - PSTs - Frequency of D/T Impacts > Frequency of Occurrence am - 8pm 8pm - 6am 2 Jan Feb Mar Apr May June Month July Aug Sept Oct Nov Dec 8pm - 6am 6am - 8pm 38

19 Figure 17 - ATs - Frequency of D/T Impacts > Frequency of Occurrence am - 8pm 8pm - 6am 2 Jan Feb Mar Apr May June Month July Aug Sept Oct Nov Dec 8pm - 6am 6am - 8pm The results of the dispersion modeling is summarized as follows: PSTs. Offsite hydrogen sulfide concentrations range from 1 to 1 ppb v, with the 1 ppbv impacts extending south just beyond the site boundary. The 5 and the 1 D/T isopleths fall in between the 1 and 1 ppb v hydrogen sulfide isopleths. The 5 D/T isopleth extends offsite a maximum distance of 65 feet to the south. PST Weirs. The 1 ppb v hydrogen sulfide impacts are offsite and are coincident with the 5 D/T isopleth and extend a maximum distance offsite 47 feet. The 1 ppbv impacts extends to a wide area offsite. The 1 D/T isopleth extends offsite to the south, a maximum distance of approximately 35 feet. Aeration Tanks. The 1 ppb v hydrogen sulfide and the 1 D/T isopleths extends slightly off-site (1 ft.) at the south area of the plant. The 5 D/T isopleth extends offsite from 175 to 35 feet. FSTs. There are not offsite hydrogen sulfide or D/T impacts FST Weirs. There are no offsite hydrogen sulfide or D/T impacts. The number of hours of impacts per year can be considered as a criterion for the need for odor control. For Plant A, the source of the greatest number of hydrogen sulfide impacts was the PST weirs; and the source of the greatest number of D/T impacts was the aeration tanks. 39

20 Time of day can be considered as a criterion. At Plant A, the PSTs and aeration tanks had the greatest number of impacts which occurred between 8 pm and 6 am. CONCLUSIONS Following are conclusions from this study: Influent hydrogen sulfide concentrations to Plants A and B were found to follow a diurnal cycle. The peak concentrations were found to lag the peak influent flowrate by approximately 6 hours. Aerating the influent wastewater was observed to reduce hydrogen sulfide and D/T emissions from the PSTs. The PST weirs of Plant A were found to be the greatest hydrogen sulfide and odor mass emission source, accounting for 73% of the hydrogen sulfide and 22% of the odor mass emissions. The aeration tanks of Plant A caused offsite odor emissions, while accounting for 25% of the odor emission mass. The D/T data showed greater variability than the hydrogen sulfide data. Analyzing and removing outliers reduced the data variability. The use of D/T appears to provide a useful supplement to the use of hydrogen sulfide in determining odor emissions from WWTPs. REFERENCES USEPA, Measurement of Gaseous Emission Rates From Land Surfaces Using an Emission Isolation Flux Chamber, User s Guide, EPA Contract No , Work Assignment 18. Klenbusch, M.R., Radian Corporation, EPA/68-86/8, February ASTM E679-91: Standard Practice for Determination of Odor and Taste Thresholds By a Forced-Choice Ascending Concentration Series Method of Limits, American Society for Testing and Materials, Philadelphia, PA EN 13725:23 European Standard, Air Quality Determination of odour concentration by dynamic olfactometry Mahin, T.D., Comparison of Different Approaches Used to Regulate Odours Around the World, Proceedings of the First International Conference on Odour and VOCs: Measurement, Regulation and Control Techniques, March 21. USEPA, 1995a, b. User s Guide for the Industrial Source Complex (ISC3) Dispersion Models, Volumes 1 and II. EPA-454/B-95-3a, September Both, R., Koch, E., Odour Regulation in Germany an improved system including odour intensity, hedonic tone, and odour annoyance, In Environmental Odour Management, Intenaitonal Conference Proceedings, Cologne, November 24. 4