Woodchip Bioreactors for Improved Water Quality
An edge-of-field practice outlined in the Iowa Nutrient Reduction Strategy’s scientific assessment for nitrate removal is denitrification woodchip bioreactors. These bioreactors are widely reported to reduce 15 to 60 percent of the annual nitrate load from treated drainage water. Yet field bioreactor performance varies greatly, and is influenced by temperature, influent nitrate concentration and hydraulic retention time (HRT). Also, recent questions have emerged regarding ‘pollution swapping’ in bioreactors, where nitrate may be converted to something other than a harmless gas.
The goal of this project is to evaluate nitrate fate in woodchip bioreactors over a range of HRTs, which will lead to improved bioreactor design for field implementation.
Previous funding made it possible to install nine experimental bioreactors at the Iowa State University Agricultural Engineering/Agronomy Research Farm. The bioreactors are designed to allow for differing HRTs, influent nutrient concentrations and fill materials. Sampling ports are located at two locations within the reactors, to provide access to water and fill materials, plus at the effluent location. Experiments over a range of three HRTs, conducted in triplicate using these pilot-scale bioreactors, will begin in spring 2016 and run until mid-July when edge-of-field flow typically ceases.
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.
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.
Meetings were held in May and June to refine the experimental design and schedule. The pilot-scale bioreactors were blocked and randomly assigned a treatment hydraulic retention time (HRT). Woodchips were ground to fill 72 small mesh bags. Additional bags were filled with unground woodchips to evaluate carbon loss over multiple years of future studies. The bags were attached to a poly rope, weighted, and deployed in each of the sample wells. The portable water supply tanks were transported to the site, and all tubing attached. Flow rates were adjusted and set to achieve target HRTs of 2, 8 and 16 hours. The first season of monitoring began June 20, with weekly greenhouse gas sampling, dissolved gas sampling and nutrient sampling.
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 being conducted this winter.
The pilot-scale bioreactor outflow plumbing planning and installation continued during this quarter. The design was modified from existing sampling wells for a drainage project, with flow splitters installed to allow for flow-weighted sampling. One-inch river rock was ordered and delivered to line the base of the monitoring wells with 6 inches base. Specific placement of the discharge tile line sump was determined, and plans were made with the ISU Research Farm manager for excavation once the ground thawed.
The pilot-scale bioreactors remain winterized. Pre-planning consisted of estimating the expected nitrate concentrations in the tile drainage, a single sample collection from the main tile line to do a preliminary nitrate concentration analysis and a visual confirmation of continued flow in the tile line source culvert. Meetings with all principal investigators were held. The proposed sampling plan for water and greenhouse gas collection and analysis was presented. The required materials and equipment for the project, and the accompanying budget, were discussed. The overall experimental design also was discussed. The standard operating procedures for the greenhouse gas collection have been developed. The first step in completing the bioreactor outlet plumbing installation will be installing the tile line sump so the monitoring wells can be de-watered (the water table is currently above the rim of the sample collection wells). The large source tanks will be moved to the site by early April.