| 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
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
Within a day or two, heat generated by microbes begins to collect in
the core of the windrowknown 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 phasein which temperatures can reach
150 °Flasts 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.
The pathogen-reduction criteria include a temperature of at least 131°F
for 3 consecutive days in an aerated pile or 131 °F for 2 weeks
in the hot zones of a windrow pile with five turnings. This process
can kill nearly all pathogenic microbes and still maintain populations
of beneficial ones.
The demand for animal manure is projected to increase. As organic vegetables
and fruits gain popularity, more growers value its benefits to soil
quality and to the environment. Although in many states untreated manure
can be applied to farm fields, this practice can introduce pathogens
and parasites into soils and possible runoff or irrigation water. Various
state regulations must be considered before agricultural compost can
be marketed and sold.
"Agriculture is a farm-to-plate continuum, and we want to make
it as safe as possible," says Millner. "It is important to
make compost usable to agriculture and horticulture to prevent water
and food contamination." And this is where treatment of manure
comes into play.
Composting also results in stabilization of nitrogen in organic form
for use in soils. Compost may even be tailor-made to reduce phosphorus
availability and to remediate nutrient-deficient soils. Like pathogens,
excess unused nutrients make their way in runoff from fields into surface
water. "Nutrient stabilization in composted manure allows soil
microbes and plants to use nutrients in a slow-release and beneficial
manner," says Millner. "Compost may even help reduce demand
for nitrogen in certain crops."
Even though the basic technology may be ancient, composting still has
much to offer the world of modern agriculture.By Sharon
Durham, Agricultural Research Service Information Staff.
This research is part of Manure and Byproduct Utilization, an ARS
National Program (#206) described on the World Wide Web at www.nps.ars.usda.gov.
Patricia Millner is
with the USDA-ARS Environmental
Microbial Safety Laboratory, 10300 Baltimore Ave., Beltsville, MD
20705-2350; phone (301) 504-8163, fax (301) 504-8370.
"Composting: Improving On a Time-Tested Technique"
was published in the August
2003 issue of Agricultural Research magazine.