Modeling hydrogen sulfide emissions across the gas–liquid interface of an anaerobic swine waste treatment storage system
Introduction
Hydrogen sulfide (H2S) is a colorless, potentially harmful gas released from swine manure (US EPA, 2001a). It is produced as manure decomposes anaerobically, resulting from the mineralization of organic sulfur compounds as well as the reduction of oxidized inorganic sulfur compounds such as sulfate by sulfur-reducing bacteria (US EPA, 2001a). With a low odor threshold ranging from 0.0005 to 0.3 ppm (ATSDR, 2004), it is also one of the primary gases released from swine facilities that is associated with odor complaints due to its characteristic “rotten egg” smell.
Over the last few years, changes in livestock production methods in the US have led to the emergence of large-scale commercial livestock operations, substantially increasing the number of animals in geographically concentrated areas (Aneja et al., 2006). As emissions of trace gases (i.e., nitrogen and sulfur species) likely increase in parallel with the growth and consolidation of this industry, it is important to ensure that these operations do not exceed state regulatory levels for gases such as H2S.
To date, few studies have reported H2S emissions from waste storage treatment lagoons (Zahn et al., 2002; Lim et al., 2003; Blunden and Aneja, 2008). Arogo et al. (2000) studied the concentration and production of H2S from stored liquid hog waste in a laboratory experiment. Arogo et al. (1999) have investigated the effects of environmental parameters (wind speed and air temperature) and manure properties (solids content and liquid temperature) in the laboratory and developed an overall mass transfer coefficient for emission of H2S from liquid swine manure. The US Environmental Protection Agency (EPA) has developed a comprehensive model, WATER9, Version 2.0 (US EPA, 2001b) for estimating emissions of individual waste constituents in wastewater collection, storage, treatment, and disposal facilities.
In this study, a two-layer model of gas–liquid interchange for exchange between air and water is used to predict H2S flux across an air–water interface. The interface between the two layers is often considered a two-layer film system (Whitman, 1923; Danckwerts, 1970; Liss and Slater, 1974). The two-film layer consists of well-mixed gas and liquid films adjacent to the interface. The rate of transfer is controlled by molecular diffusion through the stagnant boundary layer.
Similar models have been developed to predict emissions of ammonia (Aneja et al., 2001), dimethyl sulfide (Aneja and Overton, 1990), sulfur dioxide, nitrogen oxide, methane, carbon monoxide (Liss and Slater, 1974), and carbon dioxide (Quinn and Otto, 1971). It is noted that other modeling approaches may be utilized to predict gas exchange at the air–liquid interface (e.g., Danckwerts, 1970).
For comparison, three process-based models have been developed in order to predict the rates of H2S flux from swine waste storage and treatment lagoons based on different conditions in the gas and liquid films. Two coupled Mass Transfer and Chemical Reactions Models based on the concept of simultaneous mass transfer and equilibrium chemical reaction were developed. One model considers flux based on the assumption of constant pH throughout the liquid film (MTCR Model I) and a second model considers a possible pH gradient in the liquid film due to diffusion processes (MTCR Model II). A third mass transfer model considers the hydrogen sulfide concentration in the bulk gas and liquid phases, neglecting chemical reactions in the gas and liquid films (MTNCR Model). Field experiments to measure H2S emissions from an anaerobic waste treatment lagoon were previously conducted at a commercial swine finishing operation in North Carolina over each of the four predominant seasons (Blunden and Aneja, 2008). These experimental results are used to evaluate the model's accuracy in calculating lagoon H2S emissions.
Section snippets
Experimental flux measurements
Hydrogen sulfide flux measurements were made at a commercial swine finishing operation in eastern North Carolina (Blunden and Aneja, 2008). Waste from the eight on-site animal confinement houses were flushed out with recycled lagoon effluent and discharged into the anaerobic lagoon from each house approximately once per week (varying days for each house).
Hydrogen sulfide flux was measured using a dynamic flow through chamber system (Aneja et al., 2000), consisting of a fluorinated ethylene
Sensitivity analysis
The three modeling approaches provide flux dependence for hydrogen sulfide emissions on lagoon temperature, lagoon pH, and aqueous sulfide content in the lagoon as well as atmospheric environmental factors such as ambient air temperature, wind speed, and the concentration of H2S in the ambient air. Practical ranges of these parameters have been considered for the sensitivity analysis. The effect of each parameter was examined by varying the values of the parameter throughout a given range while
Conclusions
A processed-based mass transport model has been developed in an effort to predict hydrogen sulfide flux from anaerobic waste treatment systems. Different conditions were considered, resulting in three variations of the model. The MTNCR Model considers mass transport and neglects chemical reactions. Two models consider mass transport coupled with chemical reaction in the gas and liquid phases (MTCR). The MTCR Model I assumes pH to be constant for mass transport though the liquid film, while the
Acknowledgments
Funding for this research project was provided by the US Department of Agriculture (USDA) as a part of the National Research Initiative (NRI) under Contract No. 2003-05360. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
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