Corn Cobs as an Alternative Carbon Source to Enhance Bioreactor Performance for Improved Water Quality
The Iowa Nutrient Reduction Strategy identified woodchip bioreactors as a promising strategy for removing NO3-N from drainage water. While reported reductions are promising, bioreactor performance varies greatly and is influenced by temperature, carbon (C) availability, influent nitrate concentration, and residence. Besides temperature, bioavailability of C governs the N removal rate. Many lab scale studies have demonstrated increased N removal when exploring sustainable alternative C sources, especially agricultural residues, including corn cobs.
The overall goal is to compare nitrate and phosphorus fate in bioreactors using a combination of corn cobs and woodchips, plus just woodchip bioreactors, at the pilot scale. Objectives are to evaluate performance of bioreactors with three different ratios of corn cobs and wood chips by monitoring bioreactor influent and effluent for nitrate and phosphorus; conduct a tracer study to assess hydraulic properties; and conduct a technoeconomic analysis to determine the cost of N removed and identify opportunities to make bioreactor installation more cost effective.
Nine pilot scale woodchip bioreactors already in place will be used for this project. The woodchip fill material in six of the bioreactors will be modified in fall 2018. Corn cobs will fill one-third of three of the bioreactors, and two-thirds in another three. The remaining three will remain all woodchips. Experiments will begin in spring 2019 when flow is available, and run for about six months. Water samples will be collected weekly.
Converting the bioreactors from all woodchip bioreactors to a combination of woodchip and corncob fill materials in different proportions was completed during this quarter. The initial plan for excavating all bioreactors and mixing the existing woodchips for refill was reevaluated, and it was decided that the most practical (due to equipment capabilities) and cost-effective excavation plan would be to leave three bioreactors intact (not removing the existing woodchips), and partially excavating the other six bioreactors to fill with one-fourth or three-fourths corn cobs at the inlet ends. The sampling wells in the excavated portion of each bioreactor were replaced. During excavation, multiple samples of woodchips from the bioreactors and corn cobs from the stockpile were collected for particle size analysis and additional evaluation. The completed bioreactors were saturated with drainage or well water to keep rodents from burrowing into the bioreactors over winter. Supplemental work with collected corn cobs has been conducted in the laboratory, and analysis is ongoing.
Preparations have been made for the excavation and removal of woodchips from the pilot-scale bioreactors. A meeting was held in September to begin planning for this phase of the pilot-scale bioreactor study. It was determined that all bioreactors will be excavated and the woodchips will be mixed to ensure even distribution of chip sizes in the bioreactors. Internal plumbing repairs and modifications will be made while the bioreactors are empty, including anchoring the sampling wells to the flanges with screws, instead of relying on glue, to ensure the wells remain in place, and installing a diffusion pipe at the outlet end of each bioreactor. A contractor will be hired to complete the woodchip removal this fall.
Additional plumbing modifications discussed in September include reducing the inlet pipe and valve sizes to improve flow rate control and shortening the float control length in the supply tank to reduce the pressure variation impacting achieved flow rates.
Corn cobs were ordered from Green Products and delivered to the bioreactor site September 25th.