Nitrogen Farming for Pollution Control

Abstract

The use of free water surface treatment wetlands for nitrate reduction has an extensive basis in data from dozens of operating systems. Marshes are effective for denitrification, with first order areal annual rate constants centered on thirty-four meters/year. Performance improves at higher water temperatures, with a modified Arrhenius temperature factor of 1.090. Performance also increases with increasing hydraulic efficiency, created by prevention of short-circuiting, and reflected in values of the tanks-in-series parameter N > 5. Higher efficiencies are associated with submergent and emergent soft tissue vegetation, and lower efficiencies with unvegetated open water and forested wetlands. Hydraulic loadings of 2–7 cm/day can produce 30% nitrate load reductions, over the temperature range 6–20°C. Carbon availability limits denitrification at high nitrate loadings, however, wetlands produce carbon in sufficient quantities to support the loads anticipated in the upper midwest. The conversion of agricultural lands to treatment wetlands focused on nitrate reduction is termed nitrogen (N) farming. (D.H. Hey, Nitrogen farming: harvesting a different crop. Restoration Ecology, 2002, 10 (1), 1–11). A demonstration project is indicated to address local issues and scale-up considerations. Such a project would require thorough monitoring for the purpose of optimizing and refining design models. Significant ancillary benefits of ecological diversity and wildlife habitat are certain to accompany the project, but are of secondary importance until the water quality functions are demonstrated. Regulatory issues include permitting and wetland classification. Economic issues include proper pricing of services and methods of revenue generation. Resolution of these potential difficulties may require modification of existing policies and institutions.

Key Words:

• Wetlands
• Nitrate
• Nitrogen reduction
• Economics
• Agriculture.

Notes

^†Defined as discharge ammonia less than 2.5 mg/L.