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

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 10 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 679 — 97. In addition, a standard for D/T has been developed by the European Union, EN 13275. This standard is similar to ASTM E 679 — 97 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 60 and 310 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' emissionsThe diurnal variation of hydrogen sulfide concentrations at the plants' influent and a comparison to the plants' influent flowrateComparison of hydrogen sulfide and D/T concentrations and emission rates from the unit processes of each plantDispersion modeling results for hydrogen sulfide and D/T showing offsite impacts in the form of isopleths from both plants.