Improving on a time-tested technique: microbiologist Patricia Millner and technician Michael Bzdil collect compose samples and gather data on temperature and moisture-content. The samples will be analyzed in the lab for pathogen content - Composting

Agricultural Research, August, 2003 by Sharon Durham

Composting, essentially a rapid, self-heating process by which organic material is decomposed and stabilized, was practiced by ancient Egyptians, Greeks, and Romans and is even mentioned in religious texts. During the past 20 years, this time-honored practice has developed into a robust waste-management technology that generates valuable organic soil amendments.

ARS microbiologist Patricia Millner, of the Environmental Microbial Safety Laboratory in Beltsville, Maryland, is working to make composting more feasible for animal producers, who face a daily challenge: manure management. Dairy cows produce about 80 pounds of manure every day, or 4 tons for every 100 cows. This can quickly become a large waste-management challenge in operations housing several hundred cows. When many operations are located in small areas, the materials-handling requirements are intensified.

Composting is one of several technologies used to treat animal manure, sewage sludge, and other organic residuals, which may contain pathogens or parasites of public health concern. In any manure slurry system, solids can be composted. Liquids can be further processed to stabilize nitrogen and phosphorus in soluble forms compatible with current nutrient-management requirements.

Biological treatment technologies may be either aerobic or anaerobic. Aerobic systems use oxygen, but anaerobic ones don't. Both may use heat to fuel the reactions that break down organic matter in manure. In composting, heat is generated by microbes that digest organic matter.

Millner has been conducting research on what she calls hybrid composting systems. These systems not only reduce numbers of pathogens like Escherichia coli O157:H7, Salmonellae, and Listeria, but they also reduce excess available phosphorus and keep the ratio of nitrogen to phosphorus within a range acceptable for use in areas that have nutrient-management plans. Her approach means that composting can address nutrient, pathogen, and odor concerns all at the same time.

Creating Quality Compost

Millner and her colleagues conduct their research on manure from the Beltsville facility's 200-head research dairy herd and various other research animals, greenhouse and landscape trimmings, old animal feed and bedding, other organic matter, and water. The compost mixture is formed into elongated piles, called windrows. Each windrow is 150 feet long, 8 feet wide, and 4.5 feet high. The initial carbon-to-nitrogen ratio of the mixture should be about 30 to 1, and the water content should be between 40 percent and 60 percent. A loose, porous texture must be achieved so that air can move in and out of the mixture. Millner's hybrid composting processes involve adding various inorganic byproducts containing aluminum, calcium, iron, and other minerals, as well as acid solutions.

Within a day or two, heat generated by microbes begins to collect in the core of the windrow--known as the hot zone. Temperature and oxygen content are monitored, and the mixture is turned to aerate it and stimulate the aerobic microbes that are rapidly eating the organic matter. This very active phase--in which temperatures can reach 150[degrees]F--lasts 3 to 4 weeks. Then, as the microbes gradually deplete the food sources, their metabolic activity declines and so does the temperature of the mix.

Other methods of treating manure include heat drying, air drying, pasteurization, or lagooning. Each method can accomplish the task, but all may require more time, external inputs, or money than composting. Composting seems to offer the most economically efficient method for animal-feeding operations to treat manure solids before land application.

The technique offers another advantage. Animal production facilities often smell bad because of manure stockpiles. But during the composting process, odorous compounds are contained in the pile, decomposed by the microbes, and ultimately replaced by more pleasant earthy ones. Chemist Laura McConnell of the Environmental Quality Laboratory, also in Beltsville, has been collaborating with Millner to determine which odor-producing compounds are present at the start, at what point they disappear, and how field personnel can monitor them.

Controlling Pathogens and Nutrients

Composting can effectively reduce pathogens and parasites commonly found in manure as well as those that have mutated into different strains with new abilities, like surviving in acidic environments. Millner estimates that once certain time-and-temperature criteria are achieved, E. coli and Salmonellae in the compost are nearly eliminated (99.9999 percent kill rate).

In a field that receives manure compost, "this reduces the numbers of pathogens that would find their way onto produce and into runoff after a rain," says Millner. It also stabilizes the nutrient content of the compost so that bacteria cannot regrow; the nutrients they require are depleted. Intense microbial competition further retards regrowth of the pathogenic bacteria in the final product.