Denitrifying Woodchip Bioreactor Leachate Tannic Acid and True Color: Lab and Field Studies

Abstract

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.

Biological Nitrate Removal With Emerald Ash Borer-Killed Ash and High-Tannin Oak Woodchips

“…The bioreactors were packed with either EAB-killed ash woodchips, oak woodchips (Quercus rubra), or a "mixed hardwood" blend of commercially supplied woodchips. The previous study evaluated tannic acid leaching and leachate true color to assess that aspect of high-tannin oak wood in bioreactors (Wickramarathne et al, 2020). A major procedural difference between the previous study, where only nine bioreactors were evaluated, and the present study was that nine additional bioreactors containing an upstream rock filter were included here.…”

“…Eighteen rectangular Plexiglas R denitrifying bioreactors (15 × 15 × 20 cm) were constructed and operated as previously described by Wickramarathne et al (2020). The bioreactors were packed with either EAB-killed ash woodchips, oak woodchips (Quercus rubra), or a "mixed hardwood" blend of commercially supplied woodchips.…”

“…), caused concern due to the potential for these tannins to negatively impact the denitrifying community within the bioreactor or the aquatic organisms downstream. Wickramarathne et al (2020) showed that while oak woodchips do initially leach more tannic acid than other woods in a bioreactor application, the significant differences between wood types were eliminated given a sufficient flushing period. That woodchip flushing study reported that any negative impacts due to the high-tannin nature of the oak would likely be a bioreactor start-up effect or have limited occurrence after periods of woodchip drying/re-wetting.…”

Effectiveness of Denitrifying Bioreactors on Water Pollutant Reduction from Agricultural Areas

“…Many small-scale bioreactor fill media comparisons have reported these start-up nutrient flushing issues (e.g., Gibert et al, 2008;Cameron and Schipper, 2010;Healy et al, 2012), and colored bioreactor outflow water has been observed during start-up at the field-scale (e.g., the Belgium section above). Additional woodchip leaching concerns, such as phenols and tannic acid, are starting to be investigated (Wickramarathne et al, 2020;Lepine et al, 2020b). These issues subside given sufficient flushing, which has ranged from 15 to roughly 300 cumulative pore volumes, or from a week to approximately six months, depending on the bioreactor size, fill media, and initial flow rate (Healy et al, 2012;von Ahnen et al, 2016;Wickramarathne et al, 2020).…”

“…Additional woodchip leaching concerns, such as phenols and tannic acid, are starting to be investigated (Wickramarathne et al, 2020;Lepine et al, 2020b). These issues subside given sufficient flushing, which has ranged from 15 to roughly 300 cumulative pore volumes, or from a week to approximately six months, depending on the bioreactor size, fill media, and initial flow rate (Healy et al, 2012;von Ahnen et al, 2016;Wickramarathne et al, 2020).…”