Predicting changes in saturated hydraulic conductivity of bioretention media amended with biochar

Date
2016
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
The addition of biochar to soil has been shown to improve soil quality for agronomic applications, demonstrating its usefulness in altering soil physical and chemical properties. While biochar has also been added to stormwater treatment media, little is known about its impact on hydraulic properties, especially the saturated hydraulic conductivity (Ksat). The objective of this research was to measure the Ksat of stormwater bioretention media with three dierent sizes of biochar amendment (4% w/w). Based on the Ksat results and the physical properties of the biochar and biochar-amended media (i.e. d 50, void ratio, etc.), existing models were evaluated for their ability to predict the changes in Ksat due to biochar addition. Soil reef biochar used in this study is a commerically available biochar produced through pyrolysis of softwood cuttings at 550°C, and it was divided into three groups with dierent particle sizes: small (≤ 0.841 mm), unsieved, and large (0.841–4.76 mm). The bioretention medium consisted of sand, clay, and sawdust with a percentage of 88%, 8% and 4% (w/w). Addition of these biochars universally decreased dry bulk density and increased the porosity compared to the unamended medium, regardless the biochar particle size. Ksat increased for all bioretention media: small biochar caused less increase (67%), and unsieved and large biochar amendment lead to greater increases (306% and 213%). Based on sediment-biochar particle size, porosity, etc., most of the existing models in the literature that predict Ksat based upon particle size were able to predict changes in Ksat to within an order of magnitude and in the correct direction. Biochar amendments were also tested with a uniform sand. In contrast to the bioretention medium, all biochars decreased Ksat. The Kozeny-Carmen model was best at predicting changes in Ksat, with the smallest average root mean square error, although this model (and all others) predicted Ksat to increase with large biochar amendment while experiments showed that it decreased. This perplexing result is hypothesized to be associated with the roughness of large biochar particles and the infuence of theses large particles on flow path tortuosity. Future work should seek to quantify this effect through X-ray computed tomography measurements.
Description
Keywords
Citation