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ARS researchers
have invented a bioreactor that removes nitrates from swine effluent, allowing
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Reducing Nitrate Discharge to Downstream
Ecosystems
By Ann
Perry January 27, 2009
Agricultural Research Service
(ARS) scientists are finding ways to stem the flow of nitrates that are washed
out of crop fields into regional surface and groundwater sources.
These nitrates come primarily from nitrogen fertilizers that are not
taken up by crops. After the nitrates are flushed out of the soil, they flow
into subsurface tile drains that channel excess water away from crop fields.
But these underground drains can facilitate the eventual passage of
nitrate-laden runoff into the Gulf of Mexico, the Chesapeake Bay and other
water bodies. When the runoff enters these areas, it can intensify the
development of oxygen-deficient "dead zones," a condition called hypoxia.
ARS research leader
Patrick
Hunt, agricultural engineer
Kenneth
Stone, and soil scientist
Matias
Vanotti developed a process for denitrifying nitrate-laden runoff in
subsurface drains before the runoff reaches sensitive aquatic ecosystems
downstream. They cultured and encapsulated denitrifying bacteria in polymer
gels and verified their denitrification rates. The resulting product was called
"immobilized denitrification sludge," or IDS.
They then devised a bioreactor by placing the IDS into a small reactor
cylinder. The team tested a bioreactor in the field, where nitrate
concentrations in runoff averaged 7.8 milligrams per liter.
Hunt and ARS environmental engineer
Kyoung
Ro determined that the hydraulic retention time (HRT)--how long the field
drainage water remained in the bioreactor--was crucial in the denitrification
process. With a one-hour HRT, 50 percent of the nitrogen was removed from the
runoff. When the HRT was increased to more than 8 hours, the nitrate removal
efficiency approached 100 percent.
The team concluded that the daily nitrate removal rate of a
one-cubic-meter bioreactor would be approximately 94 grams per square meter of
nitrate from field runoff. This is significantly higher than removal rates
reported for in-stream wetlands, treatment wetlands, or wood-based bioreactors.
Hunt, Ro, Stone, and Vanotti all work at the ARS
Coastal
Plains Soil, Water and Plant Research Center in Florence, S.C.
Read more
about this research in the January 2009 issue of Agricultural Research
magazine.
ARS is the principal intramural scientific research agency of the
U.S. Department of
Agriculture.