Location: Soil and Water Management ResearchTitle: Feasibility of recycling excess agricultural nitrate with electrodialysis Author
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/7/2017
Publication Date: N/A
Citation: N/A Interpretive Summary: Nitrate contamination of ground and surface waters is a serious problem in many agricultural areas. When concentrations are too high it can be a health hazard in drinking water, and it is a cause of hypoxia (oxygen depletion) in the Gulf of Mexico. Efforts to reduce nitrate loss from farm fields have not yet solved the problem, so there is a need for more tools to address the issue. One potential method that could be useful is electrodialysis, in which an electric current is passed through a series of membranes that are alternately permeable to positively and negatively charged ions. This causes disolved ions to be concentrated in one stream of water and diluted in the other, a technique that has been used widely to desalinate sea water. We tested the hypothesis that it could be used to remove nitrate from contaminated water and concentrate it in a form that could be reused as fertilizer. In a field test on a contaminated stream in southern MN we found that we could lower the concentration in stream water from 22 to 10 parts-per-million (ppm), while concentrating it to more than 500 ppm in a separate tank. An economic analysis indicated that the method would be somewhat more expensive than other remediation startegies, but has the advantage of recycling the nitrate rather than converting it to atmospheric N2 as other methods do. This approach will benefit producers by recovering lost nitrogen fertilizer and will benefit society by reducing nitrate contamination of streams.
Technical Abstract: 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. Thus it is desirable to develop methods to recycle nitrate, keeping it in a form that could be reapplied as fertilizer. One possibility is electrodialysis, in which a DC current is passed through alternating cation- and anion-permeable membranes, creating separate 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 was able to remove more than half of the nitrate in water passing through it, and concentrate it in a tank for subsequent application as fertilizer. The upper limit of concentration was approximately 500 mg l-1, above which precipitation of calcium carbonate limited operation. An economic analysis indicates that in comparison to denitrification methods such as bioreactors, electrodialysis is likely to be somewhat more expensive per unit of nitrate removed. The approach will be most feasible for situations where nitrate concentrations are well above environmental standards for extended periods, to maximize both operating time and nitrate removal rate.