Modeling of Odors from a Wastewater Treatment Plant: A Comparative Study

There are a variety of processes at municipal wastewater treatment plants that have the potential to emit odors: grit tanks, influent channels, primary settling basins, gravity thickener tanks, digestion tanks, etc. The odors from these various processes are often very different in character and intensity. Odor regulations, on the other hand, are often compound-specific. For example, hydrogen sulfide is frequently used as the compound of concern with respect to odors in a number of state and local odor and quality of life ordinances. Some localities also use specific compounds like hydrogen sulfide as surrogate indicators of odors in general. This practice may have the potential of mis-identifying key odor sources if there are a number of different odors and odorous components. Without a full and accurate identification of odor sources, it is difficult to control odors from the facility.There is a need to reconcile these different odor approaches both in the analysis and in the compliance phases of a project. This paper is based upon an extensive study of odor-causing processes and odorous emissions from these processes at a municipal wastewater treatment facility. Odor regulations and guidance for the locality where the plant is situated are written in terms of hydrogen sulfide both as a compound that affects quality of life and as a surrogate for other odorous compounds. However, questions have arisen as to the appropriateness of the later assumption; namely, that requiring facilities to adhere to an extremely low level of hydrogen sulfide offsite will insure that other odorous compounds are also kept to below nuisance level. A large-scale field monitoring study was conducted to identify and quantify the emissions of odors from the various process steps at a municipal wastewater treatment plant. Measurements of both hydrogen sulfide (H2S) and dilution to threshold (D/T) were made using a number of specialized sampling devices. In this preliminary modeling study, the plant odor sources (as defined in the monitoring study) were set-up in the ISCST3 model. The model was run with the odor emissions expressed in terms of both H2S and D/T. The results of these alternative approaches were compared.