Woodchip Bioreactors for Improved Water Quality
Woodchip bioreactors are a promising strategy for removing NO3-N from drainage water. However, field bioreactor performance varies greatly and is influenced by temperature, influent nitrate concentration, and hydraulic retention time (HRT). More research is needed on how to optimize the size of a bioreactor, while achieving adequate nitrate removal. Also, recent questions have emerged regarding ‘pollution swapping’ in bioreactors wherein nitrate is converted to alternate end products instead of being lost as N2 gas through complete denitrification.
The objective of this study is to evaluate NO3-N fate in woodchip bioreactors over a range of water retention rates, while gaining knowledge about improved bioreactor design for field implementation.
Previous funding provided for the installation of nine pilot scale woodchip bioreactors at the Iowa State University Agricultural Engineering Research Farm west of Ames. The reactors are designed to allow for differing hydraulic retention times (HRT), and influent nutrient concentrations. Experiments will begin as soon as flow is available in the spring in the county tile line to provide water to the pilot systems, and will run as long as flow is available. Water samples will be collected weekly, and experiments will be conducted in triplicate over a range of three HRTs - four hours, eight hours and 16 hours.
The pilot-scale bioreactor research site at the ISU ABE Research Farm is an invaluable tool for evaluating denitrification bioreactors, allowing for control of multiple factors under environmental conditions representative of changing field conditions. This research focused on building on the knowledge of kinetics and mechanisms for nitrate removal in woodchip bioreactors and identified flow conditions that may contribute to potential negative impacts such as incomplete denitrification resulting in potential greenhouse gas emissions. Analysis and evaluation of the results of this study are ongoing and will continue to add to the body of knowledge about woodchip bioreactors.
Equipment was purchased in 2017 for more accurate and real-time monitoring of flow rate at the outlet of each bioreactor. This quarter each of nine pressure transducers were paired with nine V-notch weirs to be installed in the nine bioreactors. The paired pressure transducers and weirs were calibrated in the lab for water depth level measurement, then installed in the pilot-scale bioreactors. Additional calibration was needed in the field due to “drift” of the calibration curve and inaccurate measurements. Tracer tests were done to evaluate the flow characteristics of the bioreactors for comparison with the initial tracer tests in 2015, and comparison of characteristics when the tests were conducted at various flow rates. Samples are being processed for analysis.
The pilot-scale bioreactors were winterized throughout this quarter, so work during this time included data analysis, two project presentations to the Iowa Legislature and planning for the 2018 season. Drainage nutrient removal, dissolved gases and greenhouse gas production at the three designated hydraulic retention times (HRTs) of 2, 8 and 16 hours were evaluated, as well as removal trends at the sample locations along the length of each bioreactor. This work supports considering load removal as a measure of bioreactor effectiveness as opposed to the current percent removal approach. The highest load reductions were measured at the 2-hour HRT, even though percent removals were low. However, consideration needs to be given to potential pollution swapping. At the 2-hour HRT, dissolved gases indicated incomplete denitrification and unwanted byproducts. Results of the first push-pull test at the pilot-scale bioreactors indicated extreme low plug recovery and the need to modify the initial procedure. Plug concentrations will be increased for the 2018 season planned tests, and multiple temporary wells will be installed to better evaluate the plug movement within the bioreactor.
Data processing and sample analysis from the 2017 testing season continued this quarter. Sample analysis included testing for water nitrate and ammonia concentrations, total organic carbon and gas samples for nitrous oxide and methane. Two papers on wood chip bioreactors and strategies for enhanced performance based on this research project are being written and prepared for publication.
The bioreactors were monitored throughout the early spring and summer until about mid-July when dry weather conditions led to a lack of flow in the main tile line. The remaining summer months were used to analyze data from 2016 and finalize the statistical methods. A push-pull study was run on one of the bioreactors during the months of May and June to better understand the kinetics of the system. After the one-hour rest period, the bromide decreased by 93.5% and the nitrate decreased by 95.4%. Based on these early results, this test will be conducted again in spring 2018 with a shorter rest period.