|Water Quality - Nutrient Recycling
One of the most serious environmental problems associated with agriculture is excessive nitrate-N in waters leaving fields. It is a local health hazard in drinking water and a primary cause of hypoxia in continental ocean waters receiving drainage from agricultural regions. Many of the recent mitigation efforts have focused on techniques that promote microbial denitrification – conversion of nitrate to N2. This seems inherently wasteful, since the industrial production of nitrate fertilizer from N2 requires a substantial input of energy and is a major source of greenhouse gas emissions. Consequently, we have been working on a method to recycle nitrate, keeping it in a form that could be reapplied as fertilizer.
We use electrodialysis, in which a DC current is passed through a stack of alternating cation- and anion-permeable membranes, which creates alternating streams of dilute and concentrated water. We tested the concept in a field setting on a contaminated trout stream with nitrate-N concentrations consistently above 20 mg l-1. The solar-powered system, suitable for off-grid locations, was able to remove more than half of the nitrate in water passing through it, while concentrating it in a tank for subsequent application as fertilizer. The upper limit of concentration was approximately 500 ppm, beyond which precipitation of calcium carbonate limited operation. The system and its performance are described in greater detail in Baker, J.M. and T.J. Griffis. 2017. Feasibility of Recycling Excess Agricultural Nitrate With Electrodialysis. Journal of Environmental Quality. 46:`528-1534. doi: 10.2134/jeq2017.05.0215.