Abstract
HighlightsWood media harvested from a nine-year-old denitrifying bioreactor were evaluated.Media physical changes had multiple causes and effects.Impacts of the physical changes may have been exacerbated by development of preferential flow.LCIs > 0.6 showed C quality declined but media still supported N removal.Abstract. There is a lack of information on denitrifying bioreactors treating subsurface drainage water at the end of their initial design life due to the relative newness of the technology and the relatively long estimated life. A denitrifying bioreactor (15 m L × 7.6 m W × 1.1 m D) installed in August 2008 in Greene County, Iowa, was recharged with new woodchips in November 2017 (age 9.25 years), providing the opportunity to evaluate the properties of the wood media at the end of design life. The objective was to pair a battery of physical, chemical, and nitrate-N removal tests on the wood media harvested from the bioreactor with field observations to assess likely reasons why denitrifying bioreactors treating tile drainage may need to be recharged. The two types of wood media harvested from the bioreactor (termed woodchips and mixed shreds) had median particle sizes (D50) of 12.1 and 7.7 mm, respectively, and saturated hydraulic conductivities of 4.2 ±3.0 and 3.1 ±1.0 cm s-1 (mean ± standard deviation), which were within the range of reported values for woodchips, albeit at the low end. The wood media carbon content and quality had degraded (e.g., lignocellulose indices of 0.63 to 0.74, nearing the range of decomposition stabilization), although batch tests suggested the robustness of wood as a carbon source to support nitrate removal (e.g., 65% nitrate concentration reduction in drainage water). Woodchip degradation along with sedimentation from the drainage system likely reduced conductivities over time. Development of preferential flow paths through the bioreactor was indicated by low bioreactor outflow rates (i.e., reduced permeability) and reduced hydraulic efficiency based on conservative tracer testing. These changes in media properties and linked impacts resulted in the need to recharge this bioreactor after nine years. Keywords: Denitrifying bioreactor, Hydraulic conductivity, Nitrate, Water quality.
Highlights
Denitrifying bioreactors are a nitrate treatment technology that has moved beyond proof-ofconcept, with demonstration and adoption promoted in agricultural areas around the world (Christianson and Schipper, 2016)
Changes in the physical and chemical properties of wood media over nine years of denitrifying bioreactor operation corroborated the reductions in performance assessed with field testing
Possible flow restriction may have led to the development of preferential flow paths through the bioreactor, which were indicated by the low bioreactor outflow rates and reduced hydraulic efficiency when the bioreactor was approximately eight years old
Summary
Denitrifying bioreactors are a nitrate treatment technology that has moved beyond proof-ofconcept, with demonstration and adoption promoted in agricultural areas around the world (Christianson and Schipper, 2016) Research on these systems has reached a critical mass, with an exponential increase in bioreactor studies over the past ten years (Addy et al, 2016). While other studies have reported changes in woodchip properties over relatively short times (e.g., 2 to 4 years; Ghane et al, 2018), this study provided the first opportunity to evaluate a fully operational field-scale bioreactor near the end of its initial design life. The objective was to pair a battery of physical, chemical, and nitrate-N removal tests on the wood media harvested from the bioreactor with field observations to assess likely reasons why denitrifying bioreactors treating tile drainage may need to be recharged
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