Denitrifying bioreactor woodchip sourcing guidance based on physical and hydraulic properties

Abstract

Denitrifying bioreactors using woodchips reduce nitrate loads in agricultural outflows, but there is a lack of practical guidance for woodchip selection from a hydraulic design perspective. The objectives of this study were to develop relationships between particle size, porosity, bulk density, and saturated hydraulic conductivity (Ksat) for a range of commonly available woodchip types to improve woodchip selection guidance and default design model values. Twenty woodchip types from across the US Midwest had their sizes quantified using both sieve analysis and manual axis measurements. Bulk and particle densities and porosity of the twenty types were measured using relatively easy “jar tests” and a selected eight types had bulk density determined across four compaction levels in larger permeameters. Saturated hydraulic conductivity was determined for six types each at three compaction levels. Mean conductivities generally ranged from 3.1 to 7.4 to 0.51–1.8 cm/s for uncompacted and highly compacted woodchips, respectively, and this work newly documented maximum limits of woodchip compaction occurred generally at bulk densities of 190 to 260 kg/m3. Woodchip types with D50 in the 13.5–16.8 mm range had predictable and relatively consistent drainable porosity and Ksat values across bulk densities, facilitating the development of predictive models (R2 ≥ 0.51; p < 0.0001). Even so, Ksat values were highly variable both within and between woodchip types, especially at the lowest compaction level, which creates challenges for precisely estimating this term for design models. Woodchip types with D50 > 12 mm and D90 > 26 mm with uniformity coefficients of 2–3.1 that were chip-shaped (length: width ratio < ∼3.5) were recommended for standardized denitrifying bioreactor designs.