Stack ‘em up: Field-scale performance of a stacked woodchip bioreactor and phosphorus removal structure

Authors: Penn, Chad; Williams, Mark; ASKAR, MANAL - The Ohio State University; Stinner, Jedediah; King, Kevin

Submitted to: Journal of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2024
Publication Date: 12/1/2024
Citation: Penn, C.J., Williams, M.R., Askar, M., Stinner, J.H., King, K.W. 2024. Stack ‘em up: Field-scale performance of a stacked woodchip bioreactor and phosphorus removal structure. Journal of the ASABE. https://doi.org/10.13031/aea.16145.
DOI: https://doi.org/10.13031/aea.16145

Interpretive Summary: Excess phosphorus (P) in surface waters leads to degradation of ecological systems and increases expense for drinking water treatment. However, the combination of nitrate and P loading to surface waters leads to harmful algal blooms. Soils that have excess P behave as long-term sources to surface waters, with traditional conservation practices ineffective. The P removal structure filters dissolved P from water through use of various filter media. The structures can be constructed in a variety of ways, including flow from the bottom-upward. Meanwhile, woodchip bioreactors act to filter nitrate in drainage water. While both have been shown to be effective, they have yet to be stacked together on a single field. We constructed and monitored a paired bioreactor-P removal structure for treating a tile drainage outlet from an 8.9 ha field. The woodchip bioreactor was a traditional design from the natural resource conservation service, and the P removal structure utilized activated alumina (AA) as the filter media. Discharge and nutrients were monitored going in and out of the structures. The P removal structure handled 100% of discharge while the bioreactor only handled 7%. Overall, the P removal structure removed 30% of the two-year dissolved P load, even though the input P concentrations were very low, which makes it more challenging to remove. A P spike test on the structure, using a higher input concentration in contrast removed 74% and 0.77 kg in only nine days, demonstrating the greater utility in choosing P "hot spots" for targeting P removal structures. While the bioreactor removed 93% of nitrate that flowed into it this equated to a small load removal due to the fact that it only treated 7% of the water. On the other hand, the P removal structure was able to remove nearly 10 times more nitrate than the bioreactor, mostly because it handled all water. The nitrate removal mechanism in the AA was unknown, but warrants further research. This research shows that edge-of-field conservation practices need to be designed to handle peak flow rates in order to be most effective, as well as target "hot spots" with the highest nutrient discharge concentrations.

Technical 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-shared by the Natural Resources Conservation Service, they have yet to be implemented in the same field as stacked practices. This study evaluated nitrate, 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 two-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 event discharge was spiked with dissolved P for nine consecutive days. Nearly 50% of the two-year DP and TP 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 and P; this emphasizes the need to design structural conservation practices for handling peak flow rates. The P removal structure removed 30% of the two-year load (not including spike test), despite inflow DP concentrations being much less than the minimum threshold 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 “hot spots” 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 than the bioreactor; 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.