ESTABLISHING LIMITS AND DEFINING ODOR CONTROL NEEDS

How can we take the qualitative and quantitative data collected in the field during an odor study and combine it effectively with community involvement and outreach programs to solve wastewater treatment plant odor problems? Due to the unique features of odors, by combining these elements to develop source specific dose-response graphs, we can solve the question of how much odor control is needed and where. The presentation of a case history will demonstrate how this method was used effectively.Many states and local municipalities have settled on establishing fenceline (no odor detected beyond the property line) or receptor based (dilution to threshold, D/T, value not to exceed a specific level at any receptor-to be determined by air dispersion modeling) limits. Many of these limits are unduly restrictive because it does not distinguish between perception of an odor and the existence of a problem. We should also recognize that the activation of the sense of smell does not automatically/necessarily cause annoyance or unpleasantness to an individual. And if an odor is not a nuisance, then why would we be controlling it? Accordingly, odor control may be installed to achieve a regulatory limit, but not be necessary to satisfy neighborhood concerns over odors.As a regulator, owner, municipality or consultant, what other choice(s) do I have to limit odors? Establishing a D/T limit may not be effective, but it is a relatively safe and easy approach.Each WWTP odor source consists of a combination of many individual odorous compounds that when blended together provide a representative process unit characteristic odor. In the odor industry the fingerprint of that particular odor source is referred to as the odor's dose-response graph. The dose-response graph represents the change in odor intensity with dilution. Intensity (I) is related to concentration (C) by the following equation, where K is the constant and n is the exponent:I (perceived) = K(C)nThis relationship, known as Stevens Law, or the Power Law (Dravnieks, 1979) is an important concept related to the issue of reducing the odor intensity of a substance by air dilution. As a result, a complete dose-response graph provides more information on the sensory properties of a particular odor sample than the threshold value alone.Understanding the fundamentals of the dose-response graphs, we can use them to determine the requirements for odor control at a WWTP by following these steps:Collect odor samples at the key odor sources.Analyze the odor samples for D/T, and intensity at various dilution levels.Develop the dose-response graph for each source.Determine the D/T receptor impact value for each of the key odor sources by odor dispersion modeling.Read the intensity value from the dose-response graph corresponding to the receptor D/T value calculated by the model.Compare the intensity value to the accepted community odor annoyance index, which is the intensity at which the community feels odors are objectionable.If the intensity read off the dose-response graph is greater than the community accepted level, then odor control is necessary.A positive community outreach and/or public relations program helps to get the community involved. And since it is the community that is the driver behind providing odor control, it is important to get them involved in helping to solve the odor problems.A case history where this approach was used to assess the need for initial and/or additional odor control will be presented.