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 overall goal of this study is to evaluate NO3-N fate in woodchip bioreactors over a range of water retention times, while gaining knowledge about improved bioreactor design for field implementation.
Previous funding provided for the design and installation of nine experimental bioreactors at the Iowa State University Agricultural Engineering Research Farm west of Ames. The reactors are designed to allow for differing hydraulic retention times, influent nutrient concentrations, and fill materials. Sampling ports are located at two locations to provide access to water and fill materials, and at the effluent location. Experiments will begin in spring 2017 as soon as flow is available 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 - two hours, eight hours and 16 hours.
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 resource is being used to answer relevant questions regarding bioreactor design and performance, specifically related to nitrate removal and flow. In addition, greenhouse gasses above the bioreactors and in the water discharging from the bioreactors are being monitored. Researchers have found the nitrate mass removal was greatest from the bioreactors with a short retention time, but the concentration reduction was greatest from the bioreactors with the longest retention time. The pilot bioreactors are used frequently for outreach activities to state agencies, K-12 and college students, extension agents and farmers.
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.
The bioreactors were re-started in late April, and testing began in early May. Samples were taken for nitrate concentrations in the water as well as gas samples for methane and nitrous oxide. Testing continued once a week through the end of June. A push-pull study was run on one of the bioreactors during May and June to better understand the kinetics of the system. Data from 2016 was used to create a statistical model to produce results, and streamline future data processing from the bioreactors.
Project meetings were held in February to plan for spring start-up. It was determined during the 2016 season the manifold to and from the drainage tank trailer should be re-designed, separating flow from each tank to a block of three bioreactors, instead of all three tanks feeding the manifold simultaneously to route flow to the inlet of the bioreactors. The source drainage from the main pump/culvert also was routed directly to the base of the tanks, instead of being routed into the tanks through the manifold, in an effort to maintain more consistent influent flow rates to the bioreactors. A reconfiguration of the flow data collection was discussed, and plans made to implement the pilot-scale bioreactors with a more reliable flow rate measurement system to include pressure transducers with the potential for remote access.
The project team met in October to discuss the outcome of increased maintenance and monitoring at the site, the timeline for completion of the year’s sampling, and needs for winterizing the site. A full replacement set of meters was purchased and installed in October, and alternative pumps were purchased to better handle the clogging from excessive microbial growth. The site was winterized in November, and all external components were moved indoors for storage. Editing and response to reviewer comments were completed for a now published technical note, “Pilot-Scale Denitrification Bioreactors For Replicated Field Research” in Applied Engineering in Agriculture.
The project team met in September to discuss and refine the maintenance and sampling schedule. During the warm summer, there were significant challenges with the system including the buildup of black algae or bacteria, which interfered with set flows, and pump and flow meter failure due to the presence of particulates in bioreactor discharge. To overcome these challenging conditions, a schedule has been developed for flushing the system, re-setting flows, and cleaning sump pumps, filters, and flow meters three times per week. Flow data from 2016 has been analyzed. Water and gas analysis is ongoing. Woodchip samples were collected in the fall and analysis for denitrifying genes is underway.