The Western Lake Superior Sanitary District (WLSSD) operates a 40 MGD wastewater treatment plant in Duluth, MN which produces 27,000-29,000 wet tons of cake biosolids annually using a temperature phased anaerobic digestion (TPAD) process followed by dewatering with high solids centrifuges. These biosolids are spread on 1800-1900 acres of agricultural land (80-85%) and mine land reclamation sites (15-20%) annually in a year-round land application program.During the spring of 2003, WLSSD received several comments from farmers expressing that biosolids odors were upsetting neighbors surrounding land application sites. To gain a better understanding of biosolids odors in the field, WLSSD conducted a study during the summer of 2003 to document odor strength, extent and duration at 18 agricultural land application sites. The study was conducted using a Nasal Ranger field olfactometer developed by St. Croix Sensory.Biosolids odor strength at and around land application sites was influenced by a variety of factors, including distance, time, weather conditions, and application method. In order to minimize biosolids odor impacts to the people living around land application sites, WLSSD will develop an odor control and response plan based on the data collected and will continue to track the circumstances surrounding odor complaints. KEYWORDS: biosolids odor, olfactometer, land application odor INTRODUCTION BackgroundWLSSD operates a 40 MGD wastewater treatment plant receiving 50% industrial and 50% domestic wastewater. The primary industrial contributor is a kraft paper mill. The treatment process train consists of screening, grit removal, and pure oxygen activated sludge. After clarification, the clean water is filtered and discharged to the St. Louis River Bay. The settled activated sludge is thickened with dissolved air flotation, digested using a temperature phased anaerobic digestion (TPAD) process, and dewatered with high solids centrifuges.