Digging in to denitrifying bioreactor in situ bulk density

Woodchip drainable porosity and saturated hydraulic conductivity are important denitrifying bioreactor design parameters, but both strongly depend upon woodchip bulk density (or, how tightly woodchips are packed). These properties are relatively easy to estimate with packed columns in the lab, but woodchip bulk density in full-size bioreactors is relatively unknown. The overall objectives of this body of work were to (1) develop and test a proof-of-concept in situ woodchip bulk density estimation method and (2) use the new method to calculate bulk densities at two new and three old full-size bioreactors in Illinois, USA. Proof-of-concept testing involved excavating a pilot-scale bioreactor at the University of Illinois South Farms (Urbana, Illinois, USA) in three lifts and correcting the excavated woodchip masses by an appropriate moisture content. The volume of each of the three lifts was estimated using three methods: laser level surveying; lining the excavation with plastic and pumping the volume full through a flow meter; and capturing the excavation with iPhone LiDAR then using open-source volume software (SiteScape app, Cloud CompareTM). These three volume estimation methods are described in detail in this extended abstract proceedings paper. The survey method consistently resulted in the largest volume estimate for each lift, whereas the flow meter and LiDAR methods resulted in volumes within 3% of each other. Each method had limitations and advantages, but overall, cellphone-based LiDAR imaging plus open source volume software was an accurate and relatively easy volume estimation method for in situ bioreactor woodchip volumes.