ARS | Publication request: Denitrification of agricultural drainage line water via immobilized denitrification sludge

Submitted to: Journal of Environmental Science and Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 26, 2008
Publication Date: July 1, 2008
Citation: Hunt, P.G., Matheny, T.A., Ro, K.S., Stone, K.C., Vanotti, M.B. 2008. Denitrification of agricultural drainage line water via immobilized denitrification sludge. Journal of Environmental Science and Health Part A. 43:1077-1084.

Interpretive Summary: Nonpoint source nitrogen is recognized as a significant pollutant worldwide. It has been reported to dominate the riverine pollution of many coast regions of North America, Europe, and Africa to include the Chesapeake Bay and the Gulf of Mexico. This nitrogen pollution is often exacerbated by subsurface drainage lines that bypass the active riparian zones of agricultural streams. Constructed wetlands and bioreactors have been used to remove some of this drainage water nitrogen. However, removal of nitrogen from drainage waters in sensitive water locations and under land-limited conditions may require smaller reactors with even higher removal rates. In these conditions, reaction chambers with immobilized denitrifying sludge (IDS) may be very useful. In this study we used bioreactors containing IDS. In a field test with drainage water containing 7.8 mg NO3-N/L, 50% nitrogen removal was obtained with a 1 hr hydraulic retention time. Expressed as a cubically-shaped bioreactor, the nitrogen removal rate would be 94 g NO3-N per square meter per day, which is dramatically higher than treatment wetlands or passive carbonaceous bioreactors. IDS bioreactors offer potential for reducing nitrogen discharge from agricultural drainage lines. They also offer a direction for future research on emerging bioreactors technology as a component of improved water quality on both watershed and basin scales.

Technical Abstract: Nonpoint source nitrogen is recognized as a significant water pollutant worldwide. One of the major contributors is agricultural drainage line water. One potential method of reducing this nitrogen discharge to water bodies is the use of immobilized denitrifying sludge (IDS). Our objectives were to 1) produce an effective IDS, 2) determine the IDS reaction kinetics in laboratory column bioreactors, and 3) test a field bioreactor for nitrogen removal from agricultural drainage line water. We developed a mixed liquor suspended solid (MLSS) denitrifying sludge using inoculant from an overland flow treatment system. It had a specific denitrification rate of 11.4 mg NO3-N/gm MLSS/h. We used polyvinyl alcohol (PVA) to immobilize this sludge and form IDS pellets. When placed in a 3.8 L column bioreactor, the IDS had a maximum removal rate (KMAX) of 3.64 mg NO3-N/g pellet/d. In a field test with drainage water containing 7.8 mg NO3-N/L, 50% nitrogen removal was obtained with a 1 hr hydraulic retention time. Expressed as a 1m3 cubically-shaped bioreactor, the nitrogen removal rate would be 94 g NO3-N m-2/d which is dramatically higher than treatment wetlands or passive carbonaceous bioreactors. IDS bioreactors offer potential for reducing nitrogen discharge from agricultural drainage lines. More research is needed to develop the bioreactors for agricultural use and to devise effective strategies for their implementation with other emerging technologies for improved water quality on both watershed and basin scales.