EMISSIONS FROM COMBUSTION SOURCES AT WASTEWATER TREATMENT PLANTS

Orange County Sanitation District (OCSD) is the third largest wastewater management agency on the West Coast. It owns and operates two large wastewater treatment plants in Southern California. Plant 1 is located in Fountain Valley and currently treats an average 85–90 million gallons perday (MGD) of wastewater. Plant 2 is located in Huntington Beach and currently treats an average 150–160MGD of wastewater. Most of the digester gas produced by the anaerobic digestion of wastewater solids is burned in internal combustion engines that power generators producing the electricity to run the plants. It was shown that combustion processes (internal combustion engines, boilers, flares, emergency diesel generators and turbines) are major sources of criteria pollutants andair toxic emissions at both plants.Under California's Air Toxics “Hot Spots” Information and Assessment Act (AB 2588), the District is required to provide South Coast Air Quality Management District (SCAQMD) with an Air Toxics Inventory Report (ATIR) and Health Risk Assessment (HRA) to fulfill the mandate of the regulation. In 2003, the ATIR which the District submitted included significant changes to the previously reported emission levels. These changes were due to issues related to source sampling data and methods used for sampling and major upgrades occurring at both plants (Liang, Farrell, Kogan, Ahn, Torres, 2005). The Strategic Plan for reduction of the toxic emissions and the corresponding health risk iscurrently been developed by OCSD staff and their consultants — Malcolm Pirnie, Inc. from White Plains, New YorkAn updated air toxic inventory from combustion processes at both plants that utilized alternative emissions estimation techniques was developed. Formaldehyde was found to be the compound with the largest mass emissions. These revised emissions were then used to conduct a health risk assessment(HRA). Existing available emission factors and source sampling techniques related to developing the emission inventory were major issues. Formaldehyde, diesel particulate matter and acrolein werefound to be major contributors to health risk (Kogan Ahn, Torres, Liang, Farrell, 2005).Emissions from combustion sources are generally evaluated based on criteria pollutants standards and air toxics health effects and typically, not included in the evaluation of odor impacts associated with the wastewater treatment plant (WWTP). Some compounds in combustion source emissions, such as acetaldehyde, formaldehyde and sulfur dioxide are extremely odorous (for example, odor detection limit for acetaldehyde is several times lower than that of hydrogen sulfide). Typically, the odorous emissions from the combustion sources are not evaluated in WWTP odor control plan. In addition to providing the results of the air toxics emission inventory, this paper attempts to develop an approach to evaluating the odors associated with the combustion sources at the two WWTPs.