AIR EMISSIONS TESTING and ODOR MODELING at the REVITALIZED

Transcription

1 AIR EMISSIONS TESTING and ODOR MODELING at the REVITALIZED REGIONAL COMPOST FACILITY in HICKORY, NORTH CAROLINA Graham Gilley Sludge Consortium City of Hickory, North Carolina Todd Williams E&A Environmental Consultants, Inc. Cary, North Carolina Tim Muirhead Professional Services Group, Inc. Knoxville, Tennessee INTRODUCTION Full in-vessel capacity operation the of revitalized Regional Compost Facility (RCF) in Hickory, North Carolina was initiated on January 4, Professional Services Group Inc. (PSG) was contracted by the ownership of the RCF, to provide operations, maintenance, and management (OM&M) services via a multi-year agreement. The ownership is a Sludge Consortium, comprised of four local governments - Cities of Hickory, Newton, and Conover and Catawba County. The odor problems and revitalization program for this 2 DTPD in-vessel biosolids compost facility have been well documented in the past couple of years. In response to continued concerns by nearby residents and businesses of adverse health and environmental impacts fiom any odorous emissions being emitted the fiom RCF, the Consortium and PSG elected to perform a comprehensive air emissions testing and odor monitoring program during the first year of operations. E&A Environmental Consultants, Inc. was commissioned to perform the sampling and testing program to address the concerns of air emissions, odors, and any public health and environmental impacts attributable to the operations of the RCF. BACKGROU The $7.7 million RCF using the Ashbrook-Simon-Hartley (A-S-H) Tunnel Reactor technology was voluntarily shutdown by the Consortium in February, 1991 due to chronic odor problems and persistent complaints by nearby residents and businesses. In December, 1992 the Consortium selected PSG to implement a comprehensive odor mitigation and control program, which would Mly address all odorous emissions fiom the RCF. A major component to this program was the implementation of $1.5 million odor control facilities additions and modifications, substantially funded by a U.S. EPA Innovative/ Alternative (VA) Technology Replacement Grant. PSG was awarded in April, 1994 the notice to proceed as the first tier subcontractor to manage the construction of the additions and modifications to the odor control facilities. The project was completed by December, 1994 and the RCF was started-up on January 4,1995 and placed immediately into 111 facility operating capacity. Since start-up, more than 4,5 dry tons of biosolids have been successllly processed into "Exceptional Quality" compost with a high very reliability in facility operations, process performance, and odor control and reduction. Many the of odor control system improvements in Hickory were based upon PSG's comprehensive approach to control and eliminate odors kom point-source, hgitive and area emissions at the in-vessel biosolids composting facility in Schenectady, New York.

2 The technical highlights the of Odor Control Facilities Additions and Modifications Project included the foll-g: Rehrbished and relocated the 9,5 ch chemical scrubbing system fiom the dewatering and compost building to replace the 4, ch chemical scrubbing system at the biosolids receiving station and pretreatment plant. Retrofitted the existing 9,5 ch scrubbing system with high treatment efficiency packing media, upgraded spray piping and nozzles, installed W e plates for addition of mist elimination packing media in top cone sections and installed improved on-line instrumentation for process monitoring and chemical feed control. Optimized biosolids and composting unit processes to mitigate odors with an emphasis placed on biosolids quality and dewatering, amendment recipe type and metering, in-feed compost mixture and moisture, and in-vessel Tunnel Reactor aeration and retention time. Upgraded the 5,2 ch process air exhaust duct with a 72, ch fiesh air ventilation supply and foul air exhaust system. Installed new air handlinglodor exhausting system for the dewatering and composting building, which included high intensity capture hoods, fiesh air ventilation screens bird and push fans, foul air ducts with multipldtapered exhaust ports and dampers, and induced draft fans. Installed two parallel trains of 36, ch three-stage packed-bed wet chemical scrubbers with a combined treated discharge of 72, ch from a 5-foot tall exhaust stack. The packed-bed scrubbers were configured in a close-coupled arrangment to achieve the highest level of treatment and space efficiency. A new odor control building was constructed to house the scrubbing trains, as well as the MCC, PLC/annuciator panel, chemical metering pumps and laboratory bench areas. PSG s OM&M contract with the Consortium required that the odor control system be capable of capturing the odors produced fiom the various processes within the RCF and provide 99% concentration removal of the various odorous compounds. A daily and monthly odor control monitoring and testing program has been implemented to monitor and document contract compliance. As graphically illustrated in Figure 1, these levels of scrubbing treatment have been readily and continuously achieved for both high intensity nitrogen compounds (ammonia and amines), we11 as as very pervasive sulfur compounds (dimethyl sulfide, dimethyl disulfide, and mercaptans). Prior to start-up and during shakedown and acceptance testing of the odor control facilities additions and modifications, the Consortium and PSG initiated and implemented a public relations and outreach program the with various concerned residents and businesses which surround the RCF. Several informational meetings and progress reports were provided by the Consortium and PSG for the general public during the first few months of operation of the RCF. Based upon the feedback fiom the community, the majority the of previously offended nearby residents and businesses had not detected any odors fiom the RCF and were fully satisfied with the improvements made athe facility. However, the high level of odor control and treatment performance of the new facilities did not alleviate the concerns and complaints by a small group of individuals, who had historically been the most vocal about the RCF. The proactive and sincere communication concerning the OM&M activities and odor control system performance at the RCF became the basis for these individuals to elevate their concerns and complaints of the facility fiom an issue of odors to adverse health effects and environmental impacts. Accordingly, PSG and the Consortium elected to respond to the persistent and heightened concerns of these small group of residents by hiring E&A to perform a comprehensive air emissions study of odors and regulated toxic air pollutants to assess the potential impact of the RCF on nearby receptors and surrounding environment.

3 Figure 1 Odor Scrubbing System Performance CONC. (ppm) _, _ NH3 R-NH2 R-SH H2S DMS CL2 H2 OUTLET INLET 69 DESIGN EMISSIONS STUDY The Study focused on the remaining source of emissions fiom the RCF, which were the stack exhausts fiom the two odor scrubbing systems. These point-source emissions represented a combined atmospheric discharge of 81,5 ch. All other historical fugitive release of emissions fiom the processes and buildings athe RCF were eliminated by the new fiesh air ventilation and foul air capture system. Additionally, areas emissions sources at the RCF did not exist the since compost discharged fiom the Tunnel Reactors were being directly loaded into trucks and hauled to off-site agricultural sites for curing and application. The quantity of compost on the on-site curing pad was minimal or non-existent, and thus, the scrubber stacks represented the sources of emissions which could have an off-site impact. A comprehensive air sampling and testing program was developed to address the neighborhood residents expressed concerns of odors and health effects fiom the residual compounds in the scrubber stack exhausts. The program included a baseline analysis, followed by three scheduled quarterly tests, and random spot sampling (taken by the neighborhood residents) and testing. Eight different gas sample locations were included in the baseline testing and three of these locations were used for the quarterly and random testing programs. The samples were analyzed for Total Reduced Sulk (TRS) compounds, Volatile Organic Compounds (VOCs), sulfuric acid (H2S4), and chlorine (CL2 ). Limited odor analysis was included in the program since the TRS compounds are good indicator of odors, given their pervasive and low threshold characteristics. Table 1 summarizes the air emissions sampling and testing program. Table 1. Air Emissions Sampling and Testing Program. Biosolids Scruber Outlet m, VWS, a z, Hzs-4 Compost Scrubber Inlet TRS, vocs ComDost Scrubber Outlet m, VWS, CLz, Hzs.1 Combined Scrubbers Outlet vocs Property Line Upwind TRS, vocs Property Line Downwind TRS, vocs Neighborhood Downwind m, vocs TRS, vocs TRS, vocs TRS, vocs CL2, vocs cl2, vocs CL2, vocs

4 The purpose of the baseline program was to quantify the emissions in the scrubber exhausts at the property of the RCF and in the neighborhood community. Samples were collected during representative operating conditions with scrubber flow rates field verified during sampling. A combined sample was collected from both scrubber exhausts as a flow weighted composite in an effort to represent mixing of the scrubber exhaust stack emissions in the atmosphere prior to any off-site impact. A sample was also taken in the neighborhood to determine baseline information for comparison with hture sampling and testing events. Three quarterly sampling events were performed over the next nine months as follow-up to the baseline analysis, in an effort to gather ambient emissions data during varying weather and climatic conditions. As shown in Figure 2, two of the samples were taken the RCF property line and one sample was gathered at a downwind neighborhood location. The random testing usin grab sample cannisters allowed for either the plant personnel or one of the concerned residents in the neighborhood to obtain an ambient air sample for emissions testing. The Study allowed six for spot sampling events, with three of the cannisters given to neighborhood residents, after training was provided by W. These cannisters allowed for the immediate sampling by the residents when they perceived an odor harmll or emission from the RCF in their neighborhood. These random samples were analyzed for the regulated VOCs and chlorine gas measurements were also made by plant personnel using a pump and color detection tubes. Gas samples were collected in tedlar bags and shipped to a certified laboratory for performing detailed sulk compound analysis using gas chromotography/flame photometric detection (GCffPD). Twenty sui& compounds were analyzed to a detection limit 4. of parts per billion by volume (ppb/v). VOC samples were collected in summa cannisters and shipped to a certified laboratory for by gas analysis chromatography/mass spectrometry (GCMS). The analyses performed for these samples used the EPA Method TO-14 for 43 specific compounds which are common VOCs, 3 of which are regulated Clean Air Act compounds. In addition to these 43 compounds, 15 other tentatively top remaining compounds were also identified with the detection limits of these VOCs in the range of.2 to 2. ppb/v. Given the concerns of the neighborhood residents of the chemicals used in the scrubbing process being emitted in the exhaust stacks, the Study included gas detection tube analysis for sulkic acid and chlorine. Table 2 summarizes the analyses and testing approach performed the for Air Emissions Study. BASELINE TESTING RESULTS Air samples were collected on March 3,1995 during normal daytime facility operations with the weather during sampling being mild in temperature, mostly sunny and a gentle prevailing breeze. Very faint chemical-type odors were detectable in both scrubber system exhaust streams. These odors were not objectionable or pervasive. No facility odors were noticeable on the plant site and at the property line locations or in the neighborhood.

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6 The emissions samples collected the in tedlar bags and analyzed for the 2 Total Reduced Sulfur (TRS) compouids showed that only six compounds were detected in any of the samples, as listed below. Hydrogen sulfide was only detected the in headworks scrubber and both scrubber systems had very low inlet concentrations of TRS compounds. Both scrubber systems showed outlet concentrations below the detection thresholds for the sulfur based analytes, resulting in a removal efficiency >99% for these odorous and very pervasive compounds. This performance of the scrubber systems were also verified by snif3ng the outlet emissions from the exhaust stacks fiom inspection ports during gas sampling. Hydrogen Sulfide Carbonyl Sulfide Dimethyl Sulfide Methyl Mercaptan Carbon Disulfide Dimethyl Disulfide All ambient samples for TRS resulted in concentrations below detection limits and sulfur based odors were not detected athe upwind, downwind, or neighborhood locations athe time of sampling. The TRS concentrations measured in all samples were at least one order of magnitude below any regulatory limit. Based upon these results, the baseline testing showed that emissions from the RCF did not contain TRS concentration levels and thus, caused no off-site odors or health and environmental impacts. The baseline testing of summa cannister samples for VOCs showed that only 16 of the 43 possible compounds via the EPA TO-14 protocol were detected in any sample. Five of these 16 were detected in thqupwind or background location, and thus, indicative of their presence in ambient conditions. These five hazardous chemicals, four of which are listed as an air toxic by the North Carolina Department of Environmental Health and Natural Resources (NC DEHNR) were detected the at same concentration at both the downwind property line and neighborhood locations. These compound measurements, while not high enough in concentration to pose any health threat or adverse impact to the environment, do indicate they exist in the ambient air and that the RCF had no impact on their ubiqituous nature in the local area. The concentrations of the top 15 tentively identified VOC compounds were estimated for all samples. As shown in Figure 3, cumulative VOC emissions from the RCF were several orders of magnitude below the regulated limits by North Carolina OSHA, United States OSHA, and NOSH for worker and public exposure. Using 8-hour continuous exposure limits, the VOC emissions from the RCF included only 11 identifiable VOC compounds from the regulated lists, with the measured concentrations at the outlet of both scrubber system exhausts between two and five orders of magnitude less than the lowest allowable exposure limits and recommended concentration levels specified for any three of regulatory agencies. In addition to recommended exposure limits, all 11 compounds identified were compared against NC DEHNR s emission rate limits for each specific compound. This regulation governs the emissions control of all 15 air toxic compounds in North Carolina. Emissions rates for all 11 compounds identified in the scrubber exhausts athe RCF were an order of magnitude below the allowable emission rate limits. If any particular compound exceeded the regulatory limit would require an air toxic emission permit with NC DEHNR. The cumulative mass emission rates of air toxic compounds from the RCF was determined to be 18 1 lb/yr, as compared to the NC DEHNR emission rate limit of 6,795 lb/yr for the corresponding compounds. Thus, the results of the VOC analyses clearly showed that no air toxic or other regulated hazardous compound were present mcient in concentrations in either the RCF scrubber systems inlet or outlet exhausts to cause any health or environmental concerns.

7 ~ LOCATIONS Figure 3 VOCs Testing Program Results 1.E RCF NC OSHA US OSHA I HOdor Scrubbers I '_j NIOSH Sulfuric acid and chlorine were also analyzed in the exhaust gas of both scrubber systems as the last cpmponent of the baseline testing program. The gas detection tubes have a limits of 245 ppb/v for sulfuric acid and 2 ppb/v for chlorine. As expected, sulfuric acid was not detected in the scrubber exhausts, since this solubility of this chemical causes ito remain in the recirculating scrubbing solution and does not allow transfer into the vapor phase as a component of the exhaust emission. However, chlorine can be liberated fiom scrubbing solutions into the exhaust and be discharged as an offensive and harmll emission if the odor treatment devices are not properly operated and controlled. Only a trace (a ppb/v) of chlorine were measured in the scrubber exhausts athe RCF. At the levels measured, the chlorine emission rate was significantly below the NC DEHNR limit's for permissible exposure levels of toxic air pollutants, and thus, the scrubbing chemicals athe RCF did not contribute to adverse off-site health and enviromental impacts. Table 3 summarizes the air emissions results of the baseline testing program. Table 3. Air Emissions Baseline Testing Program Results. SAMPLE Biosolids Scrubbers Compost Scrubbers Combined Scrubbers Property Line Upwind Property Line Dowwind Neighborhood m. (ppbh) vocs. (ppbhr) : crr,. GO 4 8 H2W (ppb/v) (wpblv) QUARTERLY TESTING RESULTS Three follow-up sampling events were performed at the RCF to supplement the baseline air emissions sampling and testing. The samples were collected in approximate three month intervals to analyze for VOCs and TRS compounds during representative seasonal weather periods. Per the original sampling plan, 3 samples were collected at the property line upwind and downwind, as well as downwind in the neighborhood. Sampling was performed during normal day time operating conditions the at RCF and the exact sampling locations were adjusted fiom previous sampling events to account for the wind direction on the day of sampling. All samples were collected in duplicate to ensure a complete of set testing results.

8 For all three of the follow-up sampling events, none the of 2 Total Reduced Sulfur (TRS) compounds were detected in any of the 3 samples. These results were consistent with the baseline testing program and indicate that no odorous sulfiu compounds were being emitted off-site from the RCF. The quarterly emissions testings for VOCs showed consistent results amongst the sampling events and locations. Table 4 summarizes the number of VOCs identified and the cumulative concentration of these compounds. A detailed breakdown of the identified VOCs showed that 14 of the 17 compounds detected were present at similar concentrations in all three sample locations for all three quarterly emissions tests, indicating their ubiqituous nature during sampling. No VOCs were detected in downwind at the property line or in the neighborhood, which did not also exist at a comparable concentration the upwind sample at the RCF property line. Thus, the VOCs detected athe 3 sample locations for each quarterly test were reflective of their continued and consistent presence the in ambient background due to the environs near the RCF. The concentrations of all VOC s identified in any sample were compared against the three rigulatory limits for permissible worker exposure and public health levels. Once again, of all the detectable compounds were present in concentrations 4 to 6 orders of magnitude below the most stringent recommended exposure limit or specified air toxic level, and thus, the quarterly testing of RCF emissions showed no health threat or environmental impact for the levels of TRS compounds and VOCs measured. RAOM SPOT TESTING RESULTS The Air Emissions Study made provisions for up six to random spot sampling and testing by either PSG plant staff andor the concerned neighborhood residents. Several of the spot samples were conducted during early morning hours in attempts to gather samples during the worst case time periods, which occur at sunrise. This time of day would have the greatest possible accumulation of emissions fiom the RCF due to the early morning temperature inversion, which was often present in the Hickory area. Other spot samples were performed during normal night-time hours when typical operations the at RCF occurred, in order to evaluate emissions during typical evening, weekend and holiday periods when PSG personnel are not normally present at the plant. The neighborhood residents were concerned that the unit operations and especially the odor scrubber systems were loosing control and causing the odors during the unstaffed periods athe RCF. It was explained to them at the public meetings about the automated operations at the RCF with PLC control of the odor scrubber systems, all of which contain alarm annuciating and telemetry features to ensure that was not the case. However, the Emissions Study included the random spot sampling by the neighborhood residents to address their specific concerns. Given the simplicity of gas sample collection with the Summa cannisters, it was agreed that VOCs and the 15 of the most prominent and tentatively identified compounds would be analyzed so that any measurable compounds could be compared with federal and state regulatory exposure levels and air toxic emissions limits. When a sample was taken, the cannisters were picked up by PSG personnel, sampling event data recorded and then shipped fiom the RCF to the testing laboratory under a chain-of-custody. Chlorine gas was also analyzed by the PSG personnel using a hand held gas pump and colorimetric tube when contacted by the neighborhood residents concerning a random spot sampling event. Four the of available six cannisters were used by the neighborhood residents at times when they perceived odors fiom the RCF to be most noticeable and pervasive. The samples were collected fiom April through July of 1995, encompassing different weather conditions still of wind, temperature inversions, light haze, heavy fog, sunny and very humid conditions. Table 5 summarizes the four random spot testing results.

9 Table 5. Air Emissions Random Spot Testing Program Results. EMISSIONS NEIGHBORHOOD SAMPLES., REGULATORY LEYELs/Lm PARAMETERS (4!2/95) (4128/95) (616195) (7f7/%) NC OSHA US OSHA NIOSE vocs, ppbh ~1~ 5.6~1~ 3.4~1~ Other, ug/m ~1~ ~1~ EMISSIONS MODELING The Air Emissions Study comprehensively and definitively demonstrated the that RCF did not contribute to any adverse off-site health and environmental impacts based the upon testing analyses performed. Despite these very favorable results, the neighborhood residents shifted their continued concerns fiom health and environmental impact issues back to odors. Despite not identiwg any of the odorous and pervasive reduced sulfur compounds off-site in various gas samples collected during the Air Emissions Study, the Consortium elected to perform a comprehensive odor modeling evaluation to assess the level of off-site impact fiom the low, but quantifiable stack emissions fiom the two scrubber systems. The primary purpose of the emissions modeling work was to predict levels and/or periods of off-site odor detectability and the associated impact reductions for various compost system exhaust stack modifications.. Air samples were collected fiom both scrubber stack exhausts at the RCF on June 7, 1995 during normal operating conditions, as well as upwind and downwind areas during the early morning and/or evening periods when odor occurrences were perceived the by neighborhood residents as being most prevalent. Weather during the gas sampling effort was good, with partly cloudy conditions and a steady westerly breeze with an average temperature of 75 deg. F, which were very similar atmospheric conditions as the baseline emissions study sampling. Duplicate air samples were collected in conditioned tedlar bags and shipped overnight to another commercial laboratory using standard chain-of-custody procedures. The samples were analyzed for odor using a trained and screened odor panel by dynamic-dilution olfactometry. The odor results were quantified in both terms of ratio in accordance with ASTM Method E and intensity in accordance with ASTM Method E The emissions modeling utilized both aspects of odor analysis and perception; detectability and intensity. Odor detectability is quantified by the DIT ratio in which the detection threshold is expressed as the number of volumes of odor-pee air required to reduce one volume of the odorous gas to the median detection threshold. Typically the D/T ratio is defined at the ED5 detection threshold, where 5 percent of the trained panelists can sniff test different dilutions of sample air with non-odorous air and correctly distinguish the odorous air stream fiom the non-odorous sample. Odor intensity is typically quantified by a range of sample dilutions sniffed by the panel of trained odor technicians to standard dilutions of stable gas, n-butanol. For specified ranges of n-butanol concentrations, a corresponding scale, which is ranged fiom 1 to 8, defines the level of odor intensity. Both odor detectability and intensity measurements were performed in the Air Emission Study to model any off-site impacts at the nearest receptors. ODOR MONITORING RESULTS Table 6 summarizes the odor sampling results used in the emissions dispersion model to fiuther assess off-site intensity and detectability. As shown, both scrubber systems had equivalent inlet odor concentrations of 196 DIT with outlet concentrations which ranged fiom 23 to 81 D/T. The property line upwind and the neighborhood downwind had odor concentrations of and 6 5 DIT, respectively. The odor panelists reported significantly different sources of odors for these two samples as compared to both the scrubber inlet and outlet samples. While performing the on-site and off-site odor sampling, E&A staff confirmed that no RCF odors were detected, which supports the analysis of the odor panelists. Furthermore, the upwind and downwind odor sample results indicate that a detectable level of odor is prevalent in the local environment and neighborhood, independent of any odor impact fiom the RCF.

10 Table 6. Odor Detectability Results. SAMPLE BIOSOLIDS SYSTEM COMPOST SYSTEM LOCATIONS (am,) (p-m;) (am-) (p.nt) (avg.) (model) Scrubbers Inlet Scrubbers Outlet * 99* Property Line Upwind 6 1 Neighborhood Downwind 5 4 * flow-weighted average. The inlet odor concentrations were identified to be equivalent for the biosolids and compost scrubbing systems. Despite this coincidence, the compost scrubber system inlet was well below those reported values for other in-vessel biosolids composting facilities. The industry has reported odor inlet concentrations as high as 1,2 D/T. The lower inlet odor concentrations at the RCF is the result of the high level of success of in-vessel process control and odor mitigation of the composting process. The outlet concentrations of both scrubber systems are reflective of high odor treatment performance when compared to other chemical wet scrubbing levels reported at other in-vessel biosolids composting facilities. The results of the odor monitoring effort were used as inputs for the dispersion and off-site impact modeling. A conservancy was built in for the purpose of the dispersion model in that the outlet odor Concentrations for the compost scrubber system exhaust was assumed to be 1 D/T and for the biosolids scrubber exhaust was assumed to be 3 DIT. These model inputs provided a level of conservancy when the assessment was performed and represented worst case exhaust conditions for scrubber operations. Since the compost odor scrubber system represents 98% of the stacks emissions, the flow-weighted odor outlet concentration from the RCF during the odor sampling efforts as shown in Table 6 was 67 D/T. The equivalent flow-weighted outlet odor concentration for RCF stacks emissions in the model was 99 D/T. The dispersion model also used more rigorous property line upwind and neighborhood downwind odor concentrations. An odor concentration of 1.O D/T is a level in which odors can be detected but not specifically identified as to their source and type. Typically odor concentrations below 1 D/T are not detectable by most individuals. Similarly, odor concentrations above 4 D/T is often defined as a nuisance threshold, where they are distinctly detectable and even recognizable with an intensity which be may regarded as offensive by a receptor. Odor Concentration thresholds at compost facility property lines are being limited by state regulatory agencies at levels between 5 and 1 DIT to prevent off-site nuisances. Odor detectability using the value was related to odor intensity in the model to evaluate the sensitivity to nearby receptors using a dose-response formula and the n-butanol scale. While there is no nationally or North Carolina standards which define acceptable ambient odor levels, the research and experience in the composting industry has shown that n-butanol ratings 3 above will have an odor impact with an intensity that will be perceived as a nuisance. Thus, E&A determined fiom the odor sampling and testing results the calculated n-butanol equivalents which defined the specific odor in the Hickory area that may lead to a nuisance level at the nearest receptors. As shown in Table 7, the nearby off-site receptors (neighborhood downwind location) nuisance odor threshold was defined as less than 4 DIT. This off-site receptor nuisance threshold is lower than any existing regulatory odor standards for on-site levels at facility property lines, as well as lower than the levels measured in the ambient air in the Hickory area. Additionally, the Air Emissions Study defined the ambient air odor level of 1 Dm, which is lower than the measured upwind and downwind ambient odor concentrations Of 4 and 5 D/T. The contribution of these background odors, produced by other human activities and environmental sources the in area around the RCF, were not considered in the modeling analysis to evaluate nuisance conditions. Thus, a very rigorous odor impact level was used in the dispersion model to assess any off-site impacts contributed by the scrubber emissions fiom the RCF.

11 Table 7. Odor Intensity Results. n-butanol, MTENSITY SCALE 1 BIOSOLlDS COMPOST EQUIVALENT ODOR DISPERSION MODEL Several different emissions dispersion models were evaluated to identify the most applicable format and the Industrial Source Complex Short Term (ISCST2) model was selected. This is a refined model that employs historical weather data and can evaluate numerous emissions sources and receptors simultaneously. U.S. EPA recommends this model for evaluating simple and intermediate terrain using site specific source and receptor parameters and meteorological data. Discrete receptors can be evaluated, as well as odor concentrations in defined grid areas. The refined model estimates the odor concentration at each receptor for each hour of meteorological data and produces summaries of the highest concentrations recorded at each receptor and the fiequency of occurrence of concentrations higher than the model defined odor nuisance threshold. Frequency of occurrences can be narrowed to 3-secmd peak time periods. The model incorporated five years the of most recent meteorological data &om the Hickory Regional Airport and digitized terrain data around the RCF using topograhic maps of Catawba County. These maps provided the coordinates and elevations of all sources, terrain, and receptors in the area using the North Carolina coordinate system. A grid of 589 receptors points were identified within 5 a meter (1.1 mile) radius of the RCF and the 3 nearest and most sensitive receptors were discretely analyzed within the model. Figure 4 shows an area map around the RCF in Kickory, with identifications of the scrubber exhaust stacks and the 3 nearest and sensitive receptors. As seen in Figure 4, terrain features below 9 feet in elevation are not shown because they did not impact the modeling analysis. The primary odor inputs to the model were the RCF scrubber systems' exhaust stacks emissions, with a defined odor output threshold at the nearest receptors of less than 4 D/T. The dispersion model employed 5 scenarios, including existing conditions and 4 combinations of increased compost scrubber system exhaust stack height and emissions velocity, to consider commonly used source emission strategies to reduce any odor impacts at the near receptors. The compost scrubber system stack emissions were the focus of the modeling scenarios since was it determined that this point-source accounted for 93% of the discharged air flow and 98% of the discharged residual odors fiom the RCF. The 4 additional modeling scenarios included increased exhaust velocity to optimal dispersion conditions for existing stack height and two increased stack heights, as well as existing exhaust velocity an at increased stack height. The increased exhaust velocity scenarios used ambient air &om a dilution fan installed at the base of the stack. ODOR MODELING RESULTS The results of the ISCST2 dispersion model indicated that nuisance odor conditions (> 4 DD') would occur very infiequently for the existing hcilities. In addition, the model results showed that the most significant reduction in odor impacts could be achieved by increasing the stack exit velocity, with benefits also achieved &om an increase in stack height. In order to assess the degree of odor impacts on the nearest and most sensitive receptors for each model scenario, a tabulation was compiled of the highest odor concentrations projected during a thirty second (3-sec) peak occurrence, as summarized in Table 8.

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15 SUMMARY and CONCLUSIONS An air emissions study, which spanned 9 months in 1995, was performed during the fist year of operation of the revitalized Regional Compost Facility in Hickory, NC. A comprehensive air emissions sampling and testing program was implemented to evaluate the performance of the odor control systems and address public concerns of off-site health and environmental impacts. All emissions were compared to concentration thresholds and exposure levels of numerous compounds as regulated by 3 governmental (federal and state) agencies. Additional scrubber stack emissions and ambient air sampling and testing were performed for odors as inputs into a dispersion model to assess off-site nuisance impacts, both locally and at discrete sensitive receptors. Five different scenarios were evaluated by the model with use of conservative assumptions and the utilization of stringent impact criteria at receptors. Accordingly, the following conclusions were developed as a result of the Air Emissions Study: The Regional Compost Facility has minimal or no off-site health and environmental impact fiom its scrubber systems emissions as analyzed for total reduced sulfur (TRS) compounds, volatile organic compounds (VOC), sulfuric acid, and chlorine. TRS and VOC emissions in the scrubber systems exhausts were significantly below regulated concentration levels by U.S. and NC OHSA and NIOSH and air toxic emission limits by NC DEHNR. Both the biosolids and compost scrubber systems performed excellent for removing TRS compounds and emitted acceptably low levels of residual odor when compared to compost industry experience for odor treatment using wet chemical scrubbing. Only trace levels of total residual chlorine (TRC) were detected in the compost scrubber exhaust and well below regulatory limits. Chlorine odors were not detected off-site and had no impact on health or environmental concerns. Other chemicals used in the scrubbing process, i.e. sulfuric acid were not detectable in the stack exhausts and thus, did not contribute to any perceived or odors impacts. The existing facilities and operations the at RCF resulted in infrequent or nonexistent levels of odor detection, both on-site and off-site during emissions testing. The dispersion model predicts that for existing facilities and operating conditions, nuisance odor thresholds occur infrequently at the nearest three sensitive receptors. Peak nuisance odors athe nearest three sensitive receptors could be eliminated by increasing the compost scrubber system exhaust stack 25 by feet and increasing its emissions discharge velocity to the optimal point-source dispersion level. This scenario would also reduce detectable or perceived odor at nearest receptors by 5%. The implementation of this comprehensive air testing, emissions analysis, and odor modeling the at RCF in Hickory, NC demonstrated that the sensitivity of the nearby residents is such that no matter how favorable the results, their detection of nuisance odors and perceptions of adverse healttdenvironmental impacts may still exist. Thus, the potential still exists for concern by the Consortium and PSG of any periodic off-site detectability of odorous emissions which may be perceived as originating from the RCF. However, the Air Emissions Study results and E&A s independent assessment support the reality of the highly productive publidprivate partnership which exists between the Consortium and PSG. The very successful level of operations and odor control since start-up, a&ms the revitalization of the RCF. Its performance, reliability, and beneficial use for biosolids management the for Consortium communities, reflects a positive outlook on the hture of Composting in the Carolinas.