Denitrifying woodchip bioreactor leachate tannic acid and true color: Lab and field studies

Niranga M. Wickramarathne, Richard A. Cooke, Ruth Book, Laura E. Christianson

Research output: Contribution to journalArticlepeer-review

4 Scopus citations

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.

Original languageEnglish (US)
Pages (from-to)1747-1757
Number of pages11
JournalTransactions of the ASABE
Volume63
Issue number6
DOIs
StatePublished - 2020
Externally publishedYes

Bibliographical note

Funding Information:
The authors acknowledge funding and support from USDA Hatch Project ILLU-802-925 and project NR185A12XXXXC004 CESU under the Great Rivers Umbrella Agreement 68-3A75-18-504 (USDA-NRCS). Thanks to Dr. Shiyang Li, who designed and constructed the laboratory bioreactor cells. The Illinois Nutrient Research and Education Council provided funding for installation and nutrient monitoring at three of the field sites, with additional funding from the University of Illinois College of ACES at the Dudley Smith Farm. A debt of gratitude is also owed to the Illinois Farm Bureau and their innovative and inspirational Bioreactor Partnership, which funded installation and monitoring at the two other field sites. Within this partnership, the Illinois NRCS is acknowledged for bioreactor design, and the Illinois Land Improvement Contractors Association is acknowledged for bioreactor construction.

Publisher Copyright:
© 2020 American Society of Agricultural and Biological Engineers.

Keywords

  • Chemical oxygen demand
  • Oak
  • Tannin
  • Water quality
  • Wood leachate

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