| Summary |
Denitrifying bioreactors are commonly utilized as a best management practice in agricultural systems to reduce nitrate in drainage waters. The USDA recommends the utilization of woodchips as a carbonaceous substrate to enable denitrification. This study compared the nitrate reducing capabilities of pilot-scale in-stream bioreactors comprised of locally sourced woodchips, pine bark, and peanut hulls (a regional agricultural waste product) operating under hydraulic loading rates (HLRs) between 0.1-0.3 m/day. The phosphate adsorption capacity of expanded slate was also explored. This experiment took place in Greenville, North Carolina, from July-October 2021 (the warm season) and from January-March 2022 (the cold season). Samples were collected on a weekly basis, and the duration of flow and frequency of sample collection varied based on the influent flow rates being tested. The bioreactors were dosed with target concentrations of 20 mg nitrate-N/L and 1 mg phosphate-P/L. Overall, nitrate was reduced more effectively in pine bark (50.4% and 2.6 g N/m3/day median removal) than woodchips (31.4% and 1.1 g N/m3/day median removal) and peanut hulls (38.4% and 2.0 g N/m3/day median removal). Hydraulic loading rate (HLR) was found to significantly impact nitrate reduction. Woodchips and peanut hulls both exhibited negative correlations between nitrate-N percent reduction and HLR, while pine bark exhibited a positive correlation between nitrate-N percent reduction and HLR. Though these correlations were significant, they were not very strong ([rho] values between -0.30 - 0.34). This may be attributed to a poor representation of data across the flow regime, as data for this analysis was limited to HLRs ranging from 0.1 - 0.3 m/s. Temperature was also found to significantly impact nitrate reduction. As expected, pine bark exhibited a positive correlation between temperature and nitrate reduction. Contrary to what has been reported in the literature, woodchips and peanut hulls exhibited negative correlations between temperature and nitrate reduction. Although effective at reducing nitrate, peanut hulls released significant amounts of ammonium-N, organic N, organic P, and dissolved organic carbon (DOC). Expanded slate was found to be effective at reducing phosphate when paired with woodchips (64.2% and 0.90 g P/m3/day median removal), pine bark (46.5% and 0.11 g P/m3/day median removal), and peanut hulls (50.7% and 0.12 g P/m3/day median removal). These data suggest that in-stream denitrifying bioreactors paired with expanded slate as a phosphate adsorbent can be effective tools for reducing nitrate and phosphate. Given that most studies use woodchips as the carbonaceous substrate to promote denitrification, the increased denitrifying abilities exhibited by pine bark and peanut hulls in this study are of significance. |
