Biological Control of Hog Waste Odor through Stimulated Microbial Fe(III) Reduction

Swine production is an important industry in the United States, with an annual value of more than 12 billion dollars. In 1997, the annual nationwide production was more than 100 million pigs, with a resultant production of 8.5 × 106 tons (dry weight) of manure per year requiring treatment and disposal (21, 28a). The low profit margins associated with swine production and the difficulties associated with poor public acceptance of hog facilities in many regions have resulted in a significant increase in the number of high-density-confinement rearing facilities in an attempt to improve efficiency based on economies of scale. Although hog wastes can be used as a good nutritional source of nitrogen and phosphorous for agronomic crops, the practice of land spreading has come under intensive scrutiny in the last decade because of runoff contamination of water resources and the odor of the untreated slurry. In addition, swine manures contain high concentrations of metals such as iron, copper, zinc, cobalt, and cadmium that are present as both soluble organic complexes and insoluble oxides and can accumulate in a soil environment as a result of land spreading (34). Thus, major air, soil, and water pollution problems are associated with storage and disposal of swine waste. Odor management has become a crucial issue for the swine industry, and the sustainability, productivity, and profitability of a producer depend on the extent to which odor emissions can be controlled (62). Malodorous components of swine waste can be divided into four classes: volatile fatty acids (VFAs), indoles and phenols, ammonia and volatile amines, and volatile sulfur compounds (63). Each of these components is primarily microbially formed through the activity of fermentative bacteria that degrade the complex organics present in the waste (56, 62). These compounds accumulate in storage facilities, where the mixtures of feces and urine collected from under-floor collection pits decompose under the prevailing anaerobic conditions. Although these pits are left exposed to the atmosphere, the high organic content of swine waste ensures that oxygen diffusing from the atmosphere is rapidly removed biologically (10). As such, although the malodorous components are readily biodegraded by many respiratory microbial species, the activity of these organisms is inhibited by the limiting availability of suitable electron acceptors or by the organisms' low rate of metabolism under methanogenic conditions (29, 48). Swine waste can be treated microbially in aerobic activated sludge systems; however, these systems are energy intensive, and there is a large production of microbial biomass (1.0 to 1.5 mol mol−1 waste treated) (33) that also requires treatment and disposal. Alternatively, anaerobic treatment processes, such as the use of methanogenic bioreactors, can be applied for the removal of odor and the generation of combustible biogases. This type of treatment has the advantages over aerobic systems of lower energy input and of much lower yield of biomass per liter of treated waste (0.032 mol mol−1 waste treated) (25). However, traditional methanogenic systems are slow due to the long doubling times of the fatty acid-degrading, syntrophic bacteria whose activity is central to the process (46). Alternative treatment systems based on sulfate- or nitrate-reducing bacteria could potentially be faster, due to the favorable thermodynamics of these metabolisms (46, 58). However, both of these systems can produce noxious and toxic products (e.g., sulfide, nitrite, and nitrogen oxides). Here, we describe an alternative approach for treatment of the malodorous compounds associated with hog waste by stimulating their removal through Fe(III) supplementation and bioaugmentation with a novel dissimilatory Fe(III)-reducing organism. This process can be used to treat hog waste directly in primary and secondary lagoons without the need to construct dedicated bioreactors. Microbial Fe(III) reduction is an energetically favorable process, and in the natural environment, Fe(III)-reducing bacteria (FeRB) can outcompete and inhibit both sulfate-reducing and methanogenic bacteria (10, 38, 44, 60). FeRB also have diverse metabolisms, and many pure-culture examples exist that can completely oxidize straight- and branched-chain fatty acids and aromatic organics without the need for the activity of the rate-limiting syntrophic bacteria (14, 37). The respiratory end product of microbial Fe(III) reduction, Fe(II), is nontoxic and can be recycled after abiotic reoxidation through its reaction with O2. In addition, added iron will abiotically react with malodorous HS− ions, forming non-odor-causing metal sulfide precipitates. This approach has the potential for the long-term and sustainable removal of the major odor-causing components of hog manure.