Specialists in the Control of Odors and Corrosion in Sewer Systems and Wastewater Treatment Plants.

EVALUATING ODORS

The severity of an odorous condition is evaluated by conducting an odor emissions inventory that employs a number of methods to quantify and characterize the odor emissions. By knowing what chemical condition is causing the odor, the level of severity, and how the odor is experienced by people, the engineer can recommend the most appropriate method of controlling the odor. Standard methods used as part of an odor emissions inventory include:

Gas sampling for odor panel analysis
Gas sampling for chemical component analysis
Direct measuring of odorous contituents
Dispersion modeling
Odor Surveys

Gas sampling for odor panel analysis

Gas (air) samples are collected from each odor source in special Tedlar bags. For collecting air samples from liquid or solid surfaces, a floating "flux chamber" is used to isolate the surface. The samples are analyzed by an odor panel within 24 hours in accordance with ASTM standard E679. An odor panel is a team of six to eight trained people who participate in a series of scientifically controlled sensory tests. An instrument known as the Forced Choice Dynamic Triangle Olfactometer presents six different dilutions of the odorous sample for evaluation by a panelist. The panelist is presented with three air flows for each dilution. Two of the air flows are clean air "blanks" and the third is a diluted odorous sample. The air streams are emitted from identical funnel shaped sniffing ports at a controlled rate. The panelist is required to determine which sniffing port contains the odorous air. The individual panelist registers his/her selection by pressing a button corresponding to the funnel which he/she thinks contains the odor. After making their selection, the panelist moves to the next lower dilution level. All six dilutions are evaluated by each panelist.

The sensory properties that the odor panel test reveals are:

Odor Detectability - This test determines the number of times an odorous sample must be diluted with clean air before it is no longer detectable by 50 percent of the odor panel. The dilution ratio is known as the dilutions to threshold (D/T) ratio or ED50. A high D/T would indicate a strong odor requiring many dilutions to render it undetectable.

Odor Intensity - Odor intensity is a measure of the perceived strength of an odor. Odor panelists compare the odor intensity at various dilutions with standardized concentrations of 1-butanol (in accordance with ASTM E544). The intensity of an odor is not directly proportional to the concentrations of an odorous compound, but varies in proportion to a fractional exponent of the concentration. Thus, reducing the concentration of an odorant by one half will not result in an odor intensity reduction of one half.

Odor Character - Odor character is determined in response to the question "What does it smell like?" This is normally done at minimal dilution of the air sample. The results are analyzed to determine the most prevalent characteristics. The engineer can also use the tests to determine the relative distinctness and objectionability of the odor based on the response of the panel members. Common responses for wastewater treatment plant odors are rotten eggs, rotten cabbage, garbage, etc.

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Gas sampling for chemical component analysis

In some cases, particularly for odors from industrial sources, it is important to know the chemical make-up of the odor. Here, gas samples are collected in Tedlar bags or stainless steel canisters and analyzed via gas chromatography to identify and quantify the compounds present in the air. For many waste management applications, the principal odorants are reduced sulfur compounds such as hydrogen sulfide, mercaptans, dimethyl sulfide, and dimethyl disulfide. Industrial odors may contain a diverse array of volatile organic compounds.

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Direct measuring of odorous constituents

Concentrations of hydrogen sulfide (which results in that characteristic "rotten egg" smell) are measured by Bowker & Associates directly at the same point that gas samples are collected using an Interscan Model 4170 electrochemical H2S analyzer (0-200 ppm range).  Where low levels of H2S are anticipated, a Jerome 631X, sensitive to 1 ppb, is used to measure H2S.  Continuous monitoring of H2S is accomplished using an OdaLog instrument with datalogging capability.  Detector tubes may be used for screening-level quantification of ammonia, dimethyl sulfide, mercaptans, and other odorous compounds.

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Dispersion modeling

Bowker & Associates, Inc. uses Trinity Consultants, Inc. SCREEN3 dispersion modeling software, which is based on the U.S. EPA Industrial Source Complex - Short Term (ISCST) model and is designed to perform a screening level estimate of downwind pollutant concentrations. SCREEN3 predicts conservative or worst-case estimates of maximum short-term air quality impacts from specific pollutant sources. Using calculated odor emission rates, modeling of a facility's odor impact will produce a worst-case estimate of predicted odor concentrations at the nearest receptor (neighbor). Dispersion modeling can help to estimate the degree of odor control required to meet selected downwind odor levels.

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Odor Surveys

Bowker & Associates, Inc. conducts neighborhood odor surveys in the vicinity of suspected odor generators.  The purposes of such surveys include 1) assessing the frequency and intensity of downwind odor impacts, 2) identifying the various sources of odor in a community, and 3) verifying predictions from dispersion models.  Information collected at each odor "observance" include odor intensity and character, wind speed and direction, weather conditions, time of day, and air temperature and relative humidity.

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