IDENTIFICATION AND MEASUREMENT OF PEAK ODORS

The following problems occur at wastewater treatment facilities because the peak odors are not accurately estimated or determined prior to the design of odor control programs:Peak odor data is not collected and average odors are instead used to design an odor control program. Consequently, the odor control program is designed to handle less than the actual peaks, odor complaints continue, and an unpleasant situation develops.Some odor sources with low average odor and high peak odors go undetected, odor control measures are not implemented, and odor complaints continue.Peak odor levels are sometimes over-estimated based on a limited odor sampling and testing program that may not collect appropriate information. In this case, the odor control program is over-designed and perhaps money is expended that was needed elsewhere.In this age of competitive utility business, odor control systems must be properly selected based on the characteristic source odors and developed for efficient utilization (both capital and operating costs). Yet these systems need to do the job, even under peak, or worst-case conditions.Defining peak conditions is quite important. It is obvious, from existing data, that odor emissions at different process units of a facility can vary by a large margin within any given day, or on the weekend, and even seasonally. Additionally, there may be planned or unplanned shutdowns that produce peak odor events that may be desirable to control. There is debate over what should constitute peak conditions for design criteria, and this paper presents information and suggestions based on several dozen odor control programs developed by the authors.Several of our odor control studies in recent years have been helpful in developing the following approach. Specific data are drawn from recent odor sampling programs at wastewater treatment facilities in San Francisco and Santa Cruz, California:Portable, continuous hydrogen sulfide (H2S) gas sampling and recording equipment is now available to place at critical locations, such as within sewer manholes, beneath process tankage covers and within existing foul air ductwork. Similar monitors are also available for other compounds such as ammonia (NH3) and chlorine (Cl2). To identify when peak odorant levels are occurring, these monitors can record gas-phase concentrations for days or even weeks at a time. Continuous monitoring allows for the collection of data from both planned and unplanned process changes. For example, downstream conditions can be monitored while upstream chemical addition is turned off or adjusted.Once the timing of peak odor emission occurrences is documented, subsequent sampling and testing work can concentrate on these times. This subsequent work would typically include more complete characterization of the odor, including speciation of odorous compounds, if warranted, coupled with total odor strength testing via bag sampling and dynamic dilution odor testing with an odor panel (ASTM Method E679).Even though continuous monitoring can identify high odor periods, subsequent testing cannot be conducted during the absolute peak times unless a sample team camps out at an odor source for days or even weeks. To avoid these logistics, odor strength measurements can be adjusted to represent peak odors. For example peak H2S concentrations from continuous gas-phase monitoring can be multiplied by a ratio of odor strength to H2S concentration measured from individual bag samples.