Emission of Odorous Gases from
Outdoor Hog Manure Basins

Charles J. Gantzer, Ph.D.
Gantzer Environmental Software and Services, Inc.

An odor episode downwind of an outdoor hog manure basin has three mechanistic components: emission, transport, and detection. As shown in Figure 1, the source of odorous gases is the emission from the water surface of the hog manure basin. Transport is determined by the wind speed, weather conditions, and terrain. The wind carries the odorous gases downwind and at the same time dilutes the odorous gases by atmospheric mixing in the vertical and lateral directions. Detection relates to the ability of people to smell the odorous gas if the atmospheric concentration is greater than the odor threshold concentration. The potential intensity of an odor episode is determined by the concentration of dissolved odorous gases in the hog manure basin, the potential of the odorous gases to leave the water-phase and enter the atmosphere, and the degree of dilution achieved by atmospheric mixing as the potentially odorous plume is transported downwind.

One means of objectively assessing if a specific hog manure basin can be a legitimate source of odor complaints is a two step process combining a chemical characterization of the stored hog manure with mathematical modeling. The chemical characterization of the manure can be determined by low-cost wet-chemistry methods (e.g., APHA, 1992). Based on the results of the chemical characterization, the surface area of the hog manure basin, and the expected weather conditions, the downwind atmospheric concentrations of selected odorous gases can be estimated using the HogOdor software package (GESS, 1997). The software also compares the estimated concentrations for each odorous gas to its reported odor threshold concentration to provide an index of potential odor problems. Because the chemistry/modeling assessment compares the estimated downwind atmospheric concentrations of individual odorous gases to their respective odor threshold concentrations, the approach does not model the release and downwind transport of odor per se. However, estimated atmospheric concentrations of individual odorous gases significantly above their respective odor threshold concentrations suggest a possible odor episode.

The HogOdor software package includes mathematical algorithms that account for the three mechanistic components of an odor episode. First, the software estimates the emission rate of odorous gases from an outdoor hog manure basin based on the same correlations used to predict gaseous emissions from wastewater treatment basins (WEF, 1995). Second, the atmospheric transport and dilution of the emitted odorous gases is described by same algorithms used in regulatory air quality models for area sources (EPA, 1995). Third, the estimated downwind concentrations of odorous gases are compared to the odor threshold concentrations reported by Nagy (1991) and Eaton (1996). The odor threshold concentration is defined as the gas-phase concentration at which 50 percent of the population can detect the gas' odor. The parameter obtained by dividing the estimated downwind concentration of an odorous gas by its odor threshold concentration is called an odor number. An odor number equal to 1 suggests that 50 percent of the population can detect the estimated atmospheric concentration for a specific gas. An odor number greater than 1 suggests that more than 50 percent of the population can detect the gas, while a value less than 1 indicates that less than 50 percent of the population can detect the gas. Typically, a odor number somewhere between 0.1 and 0.01 (and below) suggests that less than 1 percent of the population can detect the gas.

As an example of the types of information the chemistry/modeling assessment can provide, the emission of 8 odorous gases from a hypothetical outdoor hog manure basin will be presented. The assumed dimensions of the basin are 417 ft by 209 ft, which provides a surface area of 2 acres. The chemical characteristics of the stored manure are as follows: temperature of 20°C (68°F), pH 7.5, volatile fatty acids concentration of 1000 mg HOAc/L, phenol concentration of 50 mg/L, para-cresol of 50 mg/L, total dissolved sulfide concentration of 5.5 mg S/L, and an ammonia concentration of 1400 mg N/L. A worse-case scenario of weather conditions will be assumed, which are a clear night with a wind speed of 2.2 mph. These weather conditions correspond to the worse-case conditions that can be described by the dispersion-based air quality modeling algorithms.

The predicted odor numbers for 8 odorous gases downwind of the hypothetical 2-acre hog manure basin are provided in Figure 2. The modeled gases with atmospheric concentrations great enough to be detected by 1 percent of the population (assuming a odor number of 0.01 corresponds to detection by 1 percent of the population) at distance of 0.5 miles downwind are n-butyric acid, phenol, para-cresol, hydrogen sulfide, and ammonia. Only para-cresol and hydrogen sulfide have calculated atmospheric concentrations greater than their odor threshold concentrations and are detected by more than 50 percent of the population. Figure 3 provides the population response curves to the modeled atmospheric concentrations of hydrogen sulfide. At 0.5 miles downwind of the hypothetical basin, 78 percent of population is expected to detect the estimated atmospheric concentration and 42 percent of the population would find the odor intensity sufficiently high to complain, based on the population response curves of Nagy (1991). Thus, the modeling of the hypothetical outdoor hog manure basin suggests the potential for odor episodes downwind of the basin on clear nights with a 2.2 mph wind.

In addition to assessing the potential for odor episodes, the combination of chemical characterization and mathematical modeling can be used to evaluate compliance with existing environmental regulations. The Clean Air Act Amendments of 1990 classify a major source as a stationary source that emits more than 10 tons/year of listed individual hazardous air pollutants. One listed compound is hydrogen sulfide. Using the HogOdor software package, the relationship between surface area of a hog manure basin and the maximum allowable total dissolved sulfide concentration for specified pH values, temperatures, and weather conditions can be defined. Figure 4 provides one such relationship. Measuring the total dissolved sulfide concentration in a hog manure basin and comparing to a graph like Figure 4 can detemine if the basin is a major source.

In summary, the chemical/modeling assessment approach allows producers, consultants, and regulators to objectively address the following concerns: (1) does a specific hog manure basin have the potential to create downwind odor episodes; (2) will the proposed odor-control remedy address the type of odorous chemical compounds responsible for most of the potential downwind odors, and (3) is the proposed location of a hog manure basin far enough away from neighbors to sufficiently limit offensive odors.

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