An Innovative Approach to Nitrate Removal: Making a Dent in the Gulf of Mexico Hypoxia
This paper explores the feasibility of using existing biological nutrient removal technology to achieve a 30 percent reduction in nitrate loading from the Lower Illinois River Basin (LIRB), which is a highly agricultural watershed that drains to the Mississippi River. This level of nitrate reduction is in line with the overall goal of the Gulf of Mexico Hypoxia Action Plan. Agriculture in the LIRB accounts for 88 percent of the overall land area, and application rates of nitrogen in synthetic fertilizer and manure in the basin are among the highest in the Nation. Nitrates would be treated by diverting a portion of stream flow in multiple small, sub-watersheds to side-stream treatment facilities, which utilize Submerged Anoxic Packed Bed Reactors with methanol addition. It was estimated that approximately 70 treatment facilities with a capacity of 100 mgd would be required to achieve the targeted reduction in nitrate loading in the LIRB. This treatment approach, which could also be used to remove nitrates from highly agricultural areas in other Corn Belt states, is a potential alternative to nitrogen removal by Nutrient Farming with constructed wetlands. The paper includes a hydrological evaluation to identify feasible the numbers and locations of treatment facilities, a detailed description of the treatment technology, and planning level capital and operating costs.
Hypoxia Action Plan
The Hypoxia Action Plan is a national plan to reduce the hypoxic zone in the Gulf of Mexico by achieving a 30 percent reduction in nitrate discharges from the Mississippi River Basin over the next 15 years. The Hypoxia Action Plan indicated that 90 percent of the nitrate load comes from non-point sources, primarily agricultural runoff. (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, 2001) On-site control of fertilizers and off-site control of agricultural drainage by restoring riverine wetlands have been proposed as possible approaches to reducing nitrates from agricultural runoff. Figure 1 shows the Mississippi River Basin contributing to the Gulf of Mexico Hypoxia.
NAQWA Study of Lower Illinois River Basin
The USGS National Water Quality Assessment Program (NAWQA) of the highly-agricultural Lower Illinois River Basin (LIRB) concluded that nitrate concentrations in the basin were among the highest in the nation. Agriculture is the predominant land use—typically corn and soybean row crops. Agriculture accounts for 88 percent of the overall land area, whereas forests account for 7 percent and urban areas account for about 2 percent (Figure 2). The remaining land use, about 3 percent, is mostly grassland, wetlands, and water. (Groschen, G. E., Harris, M A., King, R. B., Terrio, P. J., Warner, K. L., 2000).
LIRB contributes about 58,000 tons of nitrogen per year from its 18,000 square mile watershed. It was estimated that the nitrogen contribution of the entire Illinois River Basin is about 15 to 20 percent of the total nitrate loading to the Gulf of Mexico. (Groschen, et al., 2000)