HighlightsOak leached more tannic acid, true color, and chemical oxygen demand (COD) than ash and mixed hardwood chips.The factors became similar (tannic acid, COD) or below stream levels (true color) after flushing.Eleven site-years of field bioreactor data showed decreasing tannic acid and true color over time.Post-startup tannic acid was lower in bioreactor outflow than in area streams.True color did not appear to be a reliable indicator of leachate tannic acid at low concentrations.Abstract. Woodchips have been a preferred denitrifying bioreactor medium to date, but concerns about potential harmful effects of tannins in the leachate have precluded the use of oak chips in many installations. A study was conducted to compare the suitability of oak (genus Quercus) woodchips as a denitrifying bioreactor medium relative to other types of woodchips, both in lab leachate tests and in the context of observed bioreactor leaching in the field. Assessment measures included the content of tannic acid and other compounds in the leachate, as well as leachate color, which can often be high during startup. An 84-day leaching test using rectangular bioreactor cells filled with either oak (Quercus rubra), ash (Fraxinus spp. L.), or a generic hardwood blend showed that oak initially leached higher concentrations of tannic acid, true color, and chemical oxygen demand (COD) than the other two media. The significant differences in leached concentrations among the three wood types were eliminated after a finite leaching period. Tannic acid and true color in 11 site-years of field bioreactor outflow data generally decreased over time, except following a dry period when one of the bioreactors received no drainage inflow for more than two months. The lab and field results indicated the capability of woodchip bioreactors to flush at least these two analytes to ambient stream levels. True color did not appear to be the best parameter for estimating the tannin content of woodchip leachate due to discrepancies at low concentrations. Mass normalized tannic acid leaching ranged from 0.03 to approximately 40 mg tannic acid g-1 woodchip across the lab and field assessments. Oak initially leached more tannic acid, color, and COD than the other wood types, but the eventual similarity among the wood types after flushing with a sufficient number of pore volumes meant that any potentially negative environmental impacts would likely be limited to the startup period or possibly after dry periods. Oak initially eluted higher mean total nitrogen (TN) concentrations than the other wood types, but the treatments were not significantly different by day 3, indicating that biological N removal was not significantly inhibited, even with high concentrations of tannic acid. Keywords: Chemical oxygen demand, Oak, Tannin, Water quality, Wood leachate.
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.
Treatment of nitrate loads by denitrifying bioreactors in centralized drainage ditches that receive subsurface tile drainage may offer a more effective alternative to end-of-pipe bioreactors. A paired denitrifying bioreactor design, consisting of an in-ditch bioreactor (18.3 × 2.1 × 0.2 m) treating ditch base flow and a diversion bioreactor (4.6 × 9.1 × 0.9 m) designed to treat high-flow events, was designed and constructed in an agricultural watershed (3.2 km2 drainage area) in Illinois, USA. Flow and water chemistry were monitored for three years and the woodchip and bioreactor-associated soil were analyzed for denitrification potential and chemical properties after 25 months. The in-ditch bioreactor did not significantly reduce nitrate concentrations in the ditch, likely due to low hydraulic connectivity with stream water and sedimentation. The diversion bioreactor significantly reduced nitrate concentrations (58% average reduction) but treated only ~2% of annual ditch flow. Denitrification potential was significantly higher in the in-ditch bioreactor woodchips versus the diversion bioreactor after 25 months (2950 ± 580 vs. 620 ± 310 ng N g−1 dry media h−1). The passive flow design was simple to construct and did not restrict flow in the drainage ditch but resulted in low hydraulic exchange, limiting nitrate removal.
Two common tree species, ash (Fraxinus sp.) and oak (Quercus sp.), could provide readily available media for denitrifying bioreactors that use wood-based carbon for biological nitrate treatment. However, it is not known if the wood from Emerald Ash Borer-killed (EAB-killed) ash trees is an effective carbon source for nitrate removal compared to other wood species or if the high-tannin nature of oak inhibits denitrification potential. This lab-scale study showed that EAB-killed ash woodchips did not significantly differ in nitrate removal or denitrification potential compared to a commercially available blend of hardwood chips. However, neither treatment performed as well as oak woodchips in these metrics. Use of high-tannin oak in bioreactors is currently restricted by a federal standard in the United States. Ash woodchips beneficially exhibited the lowest nitrous oxide production potential, and their dissolved phosphorus leaching fell within the range of other woodchip types. Emerald ash borer-killed ash wood could be an effective source for denitrifying bioreactors located within affected regions and oak woodchips merit additional investigation for the application of denitrifying bioreactors.
Highlights The bulk density of woodchips in denitrifying bioreactors in the field is unknown. In situ bulk density estimation methods were developed for use during construction or excavation. Dry bulk densities of aged woodchips at bioreactor bottoms were lower than previous literature values. Moisture and particle size and density explained some, but not all, of the variation in in situ bulk densities. Abstract. Woodchip bulk density in a denitrifying bioreactor governs system hydraulics, but this prime physical attribute has never been estimated in situ. The objectives were twofold: (1) to establish estimates of in situ woodchip bulk density at bioreactors in the field, and (2) evaluate causal factors for and resulting impacts of these estimates. Proof-of-concept bulk density methods were developed at a pilot-scale bioreactor using three ways to estimate volume: surveying the excavated area, pumping the excavation full through a flow meter, and using iPhone Light Detection and Ranging (LiDAR). These methods were then further tested at two new and three old full-size bioreactors. Additional ex situ (off-site) testing with the associated woodchips included analysis of bulk density along a moisture gradient and particle size, particle density, wood composition, and hydraulic property testing. In situ dry bulk densities based on the entire volume of the new bioreactors (206-224 kg/m3) were similar to values from previous lab-scale studies. In situ estimates for woodchips at the bottom of aged bioreactors (22-mo. to 6-y) were unexpectedly low (120-166 kg/m3), given that these woodchips would presumably be the most compacted. These low moisture-content corrected dry bulk densities were influenced by high moisture contents in situ (>70% wet basis). The impacts of particle size and particle density on bulk density were somewhat mixed across the dataset, but in general, smaller woodchips had higher dry bulk densities than larger, and several woodchips sourced from the bottom of bioreactors had low particle densities. Although dry bulk densities in the zone of flow in bioreactors in the field were shown to be relatively low, the resulting permeability coefficients under those packing conditions did not differ from those of the original woodchips. The LiDAR-based volume estimation method was the most practical for large-scale, full-size evaluations and allowed high precision with small features (e.g., vertical reactor edges, drainage fittings). Keywords: Compaction, Cone penetrometer, Drainable porosity, LiDAR, Moisture content, Survey.
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