Denitrification of agricultural drainage line water via immobilized denitrification sludge

Abstract

Nonpoint source nitrogen is recognized as a significant water pollutant worldwide. One of the major contributors is agricultural drainage line water. A 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 g^{− 1} MLSS h^{− 1}. 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 (K _MAX) of 3.64 mg NO₃-N g^{− 1} pellet d^{− 1}. In a field test with drainage water containing 7.8 mg NO₃-N L^{− 1}, 50% nitrogen removal was obtained with a 1 hr hydraulic retention time. Expressed as a 1 m³ cubically-shaped bioreactor, the nitrogen removal rate would be 94 g NO₃-N m^{− 2}d^{− 1}, 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.

Keywords:

• Nitrogen
• nitrate
• denitrification
• bioreactor
• agricultural drainage
• immobilized denitrification sludge

Notes

^a 262 g IDS pellets, 1 reactor.

^b 430 g IDS pellets, 4 parallel reactors.

^c 877 g of pellets, 4 reactors.