Phosphorus Removal in Woodchip Bioreactors

Research Published: Fall 2021

Denitrifying woodchip bioreactors are very effective at reducing Nitrate-Nitrogen concentrations in tile drainage water, but they have also been known to reduce Dissolve Reactive Phosphorus (DRP). However, there’s been limited studies on how and why the woodchips are reducing/absorbing this phosphorus. This study out of the University of Illinois looks at 7 different species of woodchips with various amounts of natural, P-absorbing metals to determine if and how we could potentially design better bioreactors capable of treating both Phosphorus and Nitrogen found on agricultural landscapes.

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The Takeaway

There weren’t many consistencies in the results that point to why some woodchips reduced P concentration compared to others, so we don’t need to start scattering woodchips along all grassed waterways or specifying a specific woodchip blend for dual purpose P and N reducing bioreactors. However, these results did back up previous research showing that it is possible to reduce DRP levels via denitrifying bioreactors. We just need to find out why.

The Site

  • Woodchips:
    • 7 types of wood: Cypress, white oak, hickory, poplar, cedar, maple, and woodchips taken from an actual bioreactor of unknown source (70/30 ratio hardwood/softwood)
    • Sources: plywood lumber, fallen branches, commercial woodchips
    • 2 particle sizes: small (1/8”-1/4”) and large (1/4”-1/2”)
  • Water:
    • Deionized (DI), “clean” water – Dosed with known amounts of P
    • River water – Similar to Ag runoff

The Testing

  • Woodchips investigated under microscope (for fungus & bacteria) & for initial metal concentrations (Figure 1)
    • These specific metals have been known to “capture” phosphorus
  • Water was added to woodchips, constantly shaken/mixed, and 11 samples were analyzed over 3 days
  • Multiple tests were done comparing P-reduction in woodchips based on factors:
    • Water Type: Deionized (clean) water vs. River water
    • Woodchip size: Small vs. Large
    • Initial P Concentrations: 1.0mg/L vs. 0.10mg/L

The Results

  • Bioreactor woodchips had much higher metal content for Al, Fe, and Mg (Figure 1)
  • Poplar, maple, hickory, and bioreactor chips showed reduction in P
    • Bioreactor (Figure 2) & Poplar wood chips reduced DRP the most
      • Over 80% reduction achieved for both (Table 1)
    • Poplar was the only wood with multiple trials that consistently reduced DRP
      • 11% to 87% reduction
    • Good removal with bioreactor chips (high metal content) & with poplar (low metal content)
      • Not able to prove hypothesis
    • Many other Chemistry-related factors are in play
      • Can’t determine exactly how the DRP is being reduced
  • a – A subset of batch solutions from one replicate of these tests were analyzed for metals and trace elements by inductively coupled plasma-atomic emission spectrometry.
  • * – Indicated the final test concentration mean at t = 72 h was significantly different (either lower or higher) from the initial test concentration (a = 0.05)


Sanchez Bustamante-Bailon, A.P., Margenot, A., Cooke, R.A.C. et al. Phosphorus removal in denitrifying woodchip bioreactors varies by wood type and water chemistry. Environ Sci Pollut Res (2021).