ODOR MODELING AND CONTROL AT A BIOSOLIDS COMPOSTING FACILITY

The Snyderville Basin Sewer Improvement District (SBSID) operates two wastewater treatment plants (WWTP) at East Canyon and Silver Creek as well as a biosolids composting facility adjacent to the Silver Creek WWTP. In 1997, the composting facility handled an average of one-half of the biosolids produced at the two WWTPs, or approximately 1 dry metric ton per day (1.1 dry tons per day). The area served by the SBSID is currently undergoing rapid growth, due in part to the 2002 Winter Olympics. Because of the expected increase of up to four times the current production of biosolids, plus the occurrence of odor complaints at the composting facility, the SBSID decided to fund a study to evaluate alternative biosolids treatment options as well as model current and expected future odor impacts under a variety of compost management options.A total of 20 air samples from various odor sources was collected over a two-day period from the Silver Creek WWTP and composting facility. These samples were sent via next-day air for odor panel analysis of odor detectability using ASTM Method E679-91 and of odor intensity using ASTM Method E544-75. In addition to the odor samples, airflow data from active sources and area measurements from stationery sources were also taken and hours of operation for each unit function were determined. The odor analysis showed that the odors varied from a low of 14 dilutions-to-threshold (d/t) at the gravity thickener exhaust to a high of 6,834 d/t at the surface of a newly constructed pile. The analysis showed that 72.5 percent of the odors were produced by the WWTP, with the headworks and dewatering building being the main sources. Eleven percent of the odors came from sludge storage and mixing, 15 percent from the aerated composting phase, and only 2 percent from curing and storage.A total of 20 air samples from various odor sources was collected over a two-day period from the Silver Creek WWTP and composting facility. These samples were sent via next-day air for odor panel analysis of odor detectability using ASTM Method E679-91 and of odor intensity using ASTM Method E544-75. In addition to the odor samples, airflow data from active sources and area measurements from stationery sources were also taken and hours of operation for each unit function were determined. The odor analysis showed that the odors varied from a low of 14 dilutions-to-threshold (d/t) at the gravity thickener exhaust to a high of 6,834 d/t at the surface of a newly constructed pile. The analysis showed that 72.5 percent of the odors were produced by the WWTP, with the headworks and dewatering building being the main sources. Eleven percent of the odors came from sludge storage and mixing, 15 percent from the aerated composting phase, and only 2 percent from curing and storage.The odor sampling results as well as local topography and five years of weather data from the Salt Lake City airport were then incorporated into the Industrial Source Complex Short-Term (ISCST3) model recommended by the United States Environmental Protection Agency for the evaluation of short-term impacts of industrial sources on air quality. The model was run to obtain results in maximum d/t obtained in ten-minute peaks, and the results were plotted in isopleths. Additionally, the model was run to project the number of times the nuisance-level odor threshold would be exceeded under the five years of weather data utilized. For this study, a nuisance value was assumed to be 5 d/t, based on studies done elsewhere that showed that this level of composting odors could be found to be objectionable. A total of 15 odor receptors were included in the analysis.The model output for the current operation was found to closely correlate with odor complaints received and observations made by WWTP and compost facility operators. The model was also run under a number of odor treatment options, including:Treatment of WWTP odors, no treatment of composting facility odorsTreatment of mixing and biosolids storage odors, no treatment of WWTP or active composting odorsTreatment of WWTP and mixing odors, no treatment of active composting odorsEnclosing all composting operations and treatment of all off-gases or dispersion through roof ventsThe model showed that significant reductions in expected odor complaints could be achieved by treatment of the WWTP odors and containment and treatment of the biosolids storage and mixing odors and that no treatment of composting odors would be required.To verify these findings, the SBSID invited a number of neighbors who had complained of odors in the past to come to the WWTP and evaluate the various odor sources. This evaluation pointed to the compost piles, which were freshly built, as the odor of concern and that the WWTP odors were not a problem. Because of this citizen input, the SBSID is evaluating various alternatives for containing and treating or diluting this odor source. Interestingly, the model shows that the unique topography between the composting facility and the neighbors is a major problem. If the facility had been moved a little further from the WWTP, this odor problem might have been prevented.