Bioreactor Research and Assessment of Woodchip Tile Denitrification Bioreactors: Optimal Design/Performance and Experimental Bioreactor Installation and Study
An edge-of-field practice outlined in the Iowa Nutrient Reduction Strategy’s scientific assessment for nitrate removal is denitrification woodchip bioreactors. These are excavated pits filled with woodchips. Tile drainage water flowing through the woodchips converts nitrate into a harmless gas. Since bioreactors are relatively new, little research information from in and around Iowa regarding optimal management is available. This project will provide field-based research on the potential of bioreactors to reduce downstream transport of nutrients and other contaminants.
Bioreactors in Iowa typically are designed to receive 20% of the expected peak flow delivered by the incoming drainage tile. Flows exceeding this level are bypassed around the bioreactor and enter the receiving stream untreated. This project will assess the most efficient and effective water flow for water quality improvement.
One existing bioreactor will be used as a control, while four others will be manipulated in various ways to assess nitrate removal and carbon loss from the woodchips. Several years of past water quality data will provide insights into possible alternative management and design schemes. Past and future water quality and flow data will be used to evaluate nutrient reduction performance. Six pilot-scale bioreactors will be constructed to intersect an existing tile line and redistribute flow to the experimental reactors. A system bypass will allow for reduced or no flow through the pilot reactors. The design will allow collection of influent and effluent water for quality analysis, as well as access to bioreactor materials.
This project led to the design and installation of nine pilot scale bioreactors. This pilot scale system for testing bioreactors is unlike any other in the world. Retention time, fill material, and influent water quality parameters can be varied. This pilot scale system will be used to answer relevant questions regarding bioreactor performance, and the information from these studies will help improve the design of bioreactors for nutrient reduction.
Multiple meetings and a site visit were held to discuss sampling procedures and logistics of the project during the upcoming season. Materials were designed, purchased, and calibrated for greenhouse gas emissions. Static vented chambers will be used to measure greenhouse gas emissions from the surface of the bioreactors. Chambers were constructed from PVC pipe, then covered with reflective tape to minimize temperature change during deployment. Two chamber bases were installed at each of the nine bioreactors. Vials, syringes, and sampling kits were sorted and labeled in preparation for the sampling season.
Testing on nine pilot-scale bioreactors was completed this quarter, with some managed for a high retention time and others a lower retention time. Bioreactors did not perform more efficiently in terms of daily load removed, volumetric reaction rate and carbon use efficiency based on how the retention rate of each was managed. The low retention rate bioreactors did appear to have removed more nitrogen with a higher percent load reduction than the high retention rate bioreactors, but the difference was not statistically significant. Observing the flow characteristics of the bioreactors revealed there is variability among bioreactors and their internal flow dynamics, making it difficult to predict retention times and flow volumes. Due to this variability, bioreactors should have a basic level of monitoring to determine performance.
This project involving pilot-scale bioreactors is evaluating alternative flow management regimes to assess nitrate removal and carbon loss. One system involves high retention times and lower flow rates. Another is lower retention times and higher flow rates. Five bioreactors are being monitored in 2015. Two that were monitored in 2014 have been abandoned due to the lack of adequate flow from the tile system, so another bioreactor was added for the 2015 monitoring. Data from three additional bioreactors monitored by the Iowa Soybean Association are being used to supplement the data collected in this project. Cover crops are being surveyed and analyzed for nitrogen uptake this spring.
Carbon-based denitrification bioreactors are designed to intercept tile drainage and are a promising technology for reducing nitrate export to surface waters. While these systems have been tested in the lab, there has been limited evaluation of field performance. Nine pilot-scale bioreactors have been designed and installed for systemic field testing, allowing for variation in retention time, fill material and influent water quality parameters. The installation of the bioreactors was nearly finalized this quarter and a KBr-tracer test was conducted on three of those to characterize the flow through each of the reactors. Analysis of the remaining bioreactors is ongoing.
Weekly sampling of six pilot-scale bioreactors began in May. Three of the bioreactors were managed to have high retention times and lower flow rates. Another three were managed to have lower retention time and higher flow rates. One of the bioreactors set for a higher retention rate had limited data collected because heavy rainfall increased the stream depth to levels that were unsafe to cross. Two other bioreactors had limited data collected because the tile lines did not have adequate flow, as it appears those drainage systems underperformed. Data from additional bioreactors monitored by the Iowa Soybean Association were used to supplement the limited data collected. Results from the bioreactors with the low retention rate showed a 13% to 54% overall nitrate-N load reduction. Bioreactors with the high retention rate showed a 6-13% overall nitrate-N load reduction. The lower-than-expected reduction percentages were caused by the high amounts of bypass in late June and early July.
Weekly sampling of six pilot-scale bioreactors began with the onset of flow in May. Samples were taken at the inlet and outlet of the bioreactor and analyzed for nitrate-N, ortho-phosphorus and alkalinity. Flows were extremely low from April through mid-June. Heavy rains in late June and early July resulted in flows high enough to bypass all the reactors. Three of the bioreactors were managed to have high retention times, with the other three managed to have high flow volumes. Scientists are compiling and analyzing the flow data.
Weekly sampling of six pilot-scale bioreactors began with the onset of flow in May. Samples were taken at the inlet and outlet of the bioreactor and analyzed for nitrate-N, ortho-phosphorus and alkalinity. Five of the bioreactors were equipped with pressure transducers to continuously monitor flow from the field tile and the bioreactor. Flows were extremely low from April through mid-June. Heavy rains in late June and early July resulted in flows high enough to bypass all the reactors. Three of the bioreactors were managed to have high retention times, with the other three managed to have high flow volumes. The study will impact the management of the bioreactors by determining which flow regime removes the most nitrogen with the least amount of carbon expended.
The site and preliminary installation plans for six pilot-scale bioreactors have been finalized. Site preparation and installation is tentatively planned for August 2014. Three 2,500-gallon above-ground water storage containers will be purchased for collection of tile drainage, allowing differentiation of treatments (such as nutrient spikes) between the bioreactors. Use of these tanks will enable the bioreactors to be gravity fed, while also allowing for manipulation of nutrient/contaminant loads to the bioreactors. Consideration is being given to potential use of the site for outreach or educational purposes. Once construction is complete, water quality and bioreactor material analysis will be ongoing.
Site preparation is set to begin this spring as soon as weather permits for six pilot-scale bioreactors. Bioreactors are excavated pits filled with woodchips. Tile drainage water flowing through the woodchips converts nitrate into a harmless gas. The pilot-scale bioreactors will intersect an existing tile line and redistribute flow to the experimental reactors. Once construction is complete, water quality and bioreactor material analysis will be ongoing.