Stack’em Up: Field-Scale Performance of a Stacked Woodchip Bioreactor and Phosphorus Removal Structure
Authors: Penn Chad , Williams M. R. , Askar Manal , Stinner Jedediah , King Kevin W. 
Published: December 2024

Highlights

Bioreactor removed 5% and 2% of nitrate-nitrogen (N) and dissolved P (DP), respectively, from total drainage with 93% bypass.

P removal structure (PRS) handled all flow at 7 L s-1, thereby removing 30% and 29% of DP and nitrate-N from total drainage volume, respectively, despite low DP inflow concentrations.

PRS removed 74% of DP during 9-d P inflow spike test, 3x greater load removal than 2-yr natural loads.

Construct PRS at P critical source areas for max efficiency; design bioreactors and PRS for treating as much peak flow as possible.

Abstract. The combination of nitrate and dissolved phosphorus (P) has been identified as a cause of harmful algal blooms. Wood-chip bioreactors and P removal structures are structural edge-of-field practices that filter nitrate and dissolved P (DP) from drainage water. While both practices are cost-sharing by the Natural Resources Conservation Service, they have yet to be implemented in the same field as stacked practices. This study evaluated nitrate-nitrogen (N), DP, and total P (TP) removal from a stacked wood-chip bioreactor/P removal structure constructed on a 15 cm field tile drain outlet over a 2-yr period. A typical woodchip bioreactor was constructed at the tile outlet of an 8.9 ha field, up-pipe of a P removal structure that utilized activated Al (AA) contained in a buried tank with bottom-upward flow. All discharge was captured with automated flow measurements and sampling. Contributing soils were low in P and therefore not ideal for installation of a P removal structure, and attempts to substantially increase soil test P were not successful. Thus, an additional experiment was included in which discharge was spiked with dissolved P for nine consecutive days. Nearly 50% of the 2-yr DP load was lost within 40 days after a single fertilizer application event that only covered 10% of the field. Greater discharge rates corresponded to greater concentrations and loads of nitrate-N and P; this emphasizes the need to design structural conservation practices for handling peak flow rates. The P removal structure removed 30% of the 2-yr load (not including spike test), despite inflow DP concentrations being much less than the minimum threshold flow-weighted mean concentration (FWMC) of 0.2 mg L-1 for justification of a structure. Laboratory P removal experiments and the field P spike test demonstrated how greater inflow DP concentrations led to more efficient P removal (74% and 0.77 kg DP removal over 9d). This illustrated the importance of targeting critical source areas that optimize P load reductions. The P removal structure handled all discharge without restricting field drainage, resulting in a maximum overall event discharge rate through the structure of around 7 L s-1 while the bioreactor only treated 7% of the total discharge. As a result, the P removal structure removed nearly 10x more nitrate-N than the bioreactor; the nitrate removal mechanism is unknown, but adsorption to AA was not possible based on laboratory tests. Instead, it is possible that a redox-coupled Fe(II)–nitrate reaction was occurring on the surface of AA. Keywords: Conservation practices, Nitrate removal, Nutrient reduction, Nutrient transport, Phosphorus treatment, Stacked practices, Water quality.