2012 Annual Report
1a.Objectives (from AD-416):
1. To test the viability of a few available agricultural residues as denitrifying bioreactor media.
2. To identify a denitrifying bioreactor medium that enhances bioreactor performance, particularly at a temperature just above freezing, which represents soil temperature during spring snow melt.
3. To compare the economics of viable candidate bioreactor media with the currently used wood chip medium.
1b.Approach (from AD-416):
We propose a laboratory approach to investigate the flow and nitrate-N reducing properties of agricultural residue-based biofilter media. PVC pipe columns (6-in. dia. by 19 in. long) will be filled with candidate filter media and plumbed to treat water with a nitrate-N concentration of 50 mg/L at hydraulic residence times between 4 and 8 hours. The columns will be maintained in a controlled temperature chamber. We will have capacity to test up to 6 different materials, which will each be replicated 3 times. The performance of available materials from agricultural residue, including but not limited to corn cobs, corn stover briquettes, and wheat/barley straw/pellets, will be compared to wood chips, which are currently used in field-scale bioreactors in Minnesota. Variables that will be measured during the experiment include nitrate-N, total dissolved carbon, and dissolved organic carbon concentration of the effluent and temperature within each reactor. Hydraulic conductivity will be measured at the beginning and end of the experiment. We plan to execute 2 experimental runs, differentiated by the temperature in the chamber. The temperatures will represent field conditions during the spring snow melt (i.e. 36°F.) and during the summer (i.e. 60°F.). The nitrate-N concentrations of the effluent will be measured with an ion selective electrode (ISE). The dissolved carbon concentrations will be measured with a Dohrmann Total Organic Carbon analyzer. Temperatures within each reactor will be measured with thermocouples connected to data loggers.
Two major experiments are being conducted under this project: (i) laboratory denitrification of subsurface drainage water using agricultural residues packed in 15-cm diameter by 48-cm long PVC columns, and (ii) hydraulic conductivity of the residues packed in a one 30-cm diameter by 2.4-m long PVC pipe. The denitrification testing is in its fourth month under a controlled temperature of 17°C. The six treatments include wood chips, corn cobs, corn stover, barley straw, corn cobs + wood chips, and corn cobs + a plastic filter media. In addition to testing efficacy in removing nitrate-nitrogen, we are testing for dissolved organic carbon, total carbon, and dissolved gases (CO2, CH4, and N2O). The permeameter is being readied to test hydraulic conductivity of the wood chips, corn cobs, corn stover, and barley straw, and combinations of materials. These experiments are ongoing.
This progress contributes to meeting Objective 1b of our NP211 Project Plan which is to “Develop N management strategies for large dairy operations to reduce nutrient losses through artificial subsurface drainage.”