Denitrifying Bioreactor Woodchip Recharge: Media Properties after Nine Years

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.

Impact of flow on woodchip properties and subsidence in denitrifying bioreactors

“…Due to the controlled conditions at the Iowa State Research Farm, the observed subsidence is not likely attributable to damage by field equipment. Sedimentation is also unlikely in this system because of the use of a reservoir tank prior to the inlet into the bioreactors, although others have hypothesized sedimentation was occurring based on an increase in the proportion of media particles falling under 1.18 mm and an increase in observed ash content (Christianson et al., 2020; Feyereisen & Christianson, 2015; Ghane et al., 2018; Robertson et al., 2008).…”

“…Additionally, Christianson et al. (2020) reported a decrease in D50 over 9 yr. Both these studies also indicated a decrease in porosity, likely from sedimentation as subsidence was not reported.…”

“…Denitrifying bioreactor fill material and longevity has not previously been studied as a function of HRT. Recent studies evaluated the lifespan of the bioreactor fill material as these systems were initially designed to require little intervention by producers (Christianson et al., 2020; Ghane et al., 2018). Initial estimations of the timeframe for replenishing the fill material were 10‐yr intervals based on the rate of ligno‐cellulosic substrate depletion, although decades‐long intervals have since been estimated (Moorman et al., 2010; Robertson et al., 2000, 2008).…”

Performance of a Woodchip Bioreactor for the Treatment of Nitrate-Laden Agricultural Drainage Water in Northeastern Germany

“…For the evaluation of the hydraulic performance, the parameters, which purely target at preferential flow (e.g., S or λ), should be preferred compared to parameters, which also target at dispersion (e.g., MDI). In this case, the resulting S-value of 0.56 indicates moderate short-circuiting in the third drainage season; it is likely that some pore water regions in the woodchips are more mobile than others (caused for example by local compaction or bio-clogging) and this might also change further over time as the woodchips are being decomposed, which in turn leads to a change of the pore network and the saturated hydraulic conductivity [17]. In this particular case, the minimum residence time due to short-circuiting was still sufficiently long for the nitrate removal, but for bioreactors with shorter t n , the replacement of the woodchips after several years might be more necessary due to hydraulic malfunctions than due to carbon-supply limitations [17].…”

“…During the remaining months, the denitrification was complete and only N org was released from the woodchip bioreactor. However, compared to TOC, the release potential of the bioreactor for N org is low due to the high C/N ratio (>200 [17]) of the woodchips. In the drainage season 2019/2020, the N org -load in the outflow was only increased by 4% compared to the inflow, while for TOC an increase of 45.6% was observed.…”