Runoff and tile drainage from agricultural activity is known to be a significant contributor of nitrogen pollution to surface waters. Denitrifying woodchip bioreactors, also known as Permeable Reactive Barriers (PRB), have been studied since 1994 as a possible edge-of-field technology for reducing nitrogen concentrations in agricultural runoff. The functional concept behind PRB is that they provide a readily available carbon source for denitrifying organisms (bacteria, archaea, fungi) to colonize, which then respond by consuming nitrogen compounds present in the drainage outflow for their metabolic processes. The denitrification process is a series of biotic enzyme catalyzed reactions that convert nitrogen in the form of nitrate (NO3- ) to N2 gas as follows: NO3-→NO2-→ NO(gas)→ N2O(gas) → N2(gas). If the denitrification process is incomplete, byproducts such as nitric and nitrous oxide, both powerful green-house gases, can be released. For denitrification to occur, conditions must be anaerobic at minimum. However, if redox conditions are too reducing, pollution swapping can also occur with the formation of undesirable compounds such as CH4 (methane), H2S (hydrogen sulfide gas) and methyl-mercury. Since denitrifying organisms are biotic in nature, conditions such as contact time with the nutrients as controlled by hydraulic retention time (HRT), water temperature, pH and oxidation state can all play a role in the functional success of a denitrification system.
Research on PRB has been performed mostly in the US Midwest and New Zealand, and primarily for irrigated crop systems. Little work has been done in alternative climate regimes such as those found in the Cascadia Region of Oregon and Washington State where agricultural runoff is most likely to occur during the winter rain season. A field-scale denitrifying woodchip bioreactor was installed at Oregon State University (OSU), designed to treat drain-tile runoff from about 40 acres of forage fields for the OSU Dairy Farm. Samples were collected daily over the 2019-2020 and 2020-21 winter seasons, and tested for nitrate, nitrite and ammonia/ammonium concentrations. Initial findings show significant nitrate reductions even during colder winter months. Influent concentrations ranged 87.7 to 2.0 mg NO3 L-1 and removal rates ranged from 57 to 870g N-NO3 d-1. Some periods of nitrite production were observed though on a much smaller scale (-0.039 – 0.058 gN-NO2 d-1) and some reductions of ammonium ion were also observed (0.021-5.864 gN-NH4 d-1). The ammonium reductions are somewhat unexpected as the conditions in the PRB are inherently anaerobic and reducing, not conducive to normal ammonium oxidation. Total N (TN) reductions ranged from 0.98 – 16.24 gN m-3d-1 with m-3 referring to active (saturated) chip-bed volume. Hydraulic Retention Time (HRT) plays a significant role as the best TN reductions occurred at HRT of >24 hours. This is to be expected because the HRT controls the “contact time” for the bacteria in the unit, however, 24 hours is longer than the 10 hour minimum currently recommended by the National Resource Conservation Service (NRCS).
Our initial conclusion is that denitrifying woodchip bioreactors can work well in a “Cascadia” climate regime, but unit design may need to be adjusted to accommodate longer HRT than currently recommended. Future research should be conducted to find the optimal HRT for our climate conditions: gaseous elements of the denitrification chain should be evaluated to see if complete denitrification is occurring and the potential for pollution swapping at the slower HRT should be explored.