Role of sand size on bacterial retention in biochar-amended sand filters

Authors: Bolster, Carl

Submitted to: Biochar Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2019
Publication Date: 12/10/2019
Citation: Bolster, C.H. 2019. Role of sand size on bacterial retention in biochar-amended sand filters. Biochar Journal. 1:353-363. https://doi.org/10.1007/s42773-019-00027-0.
DOI: https://doi.org/10.1007/s42773-019-00027-0

Interpretive Summary: Animal manure is often applied to agricultural fields to add nutrients and organic matter to the soil to enhance soil quality and fertility. Manure can also, however, be a source of pathogenic microorganisms that pose a threat to humans, livestock, and wildlife. Of particular concern is runoff exiting tile-drained fields because tile drainage significantly alters field hydrology and can have a significant impact on the fate and transport of sediment, nutrients, pesticides, and bacteria. In particular, the transport of pathogens from agricultural fields to drinking water can be greatly enhanced in tile-drained fields because the distance from the soil surface to tile drains is relatively short and thus the ability of the soil matrix to filter out bacteria (through both physical and chemical processes) is reduced. One mitigation strategy for dealing with contaminated tile-drainage waters is the use of end-of-tile filters to treat the water prior to being discharged into the environment. In this study, the use of sand filters amended with biochar is investigated as a potential management practice for reducing microbial concentrations in tile-drainage waters. Specifically, this research focuses on the role of sand size on bacterial removal rates within biochar-amended sand filters. Results from this study further our understanding of the role of sand size on bacterial retention in biochar-amended porous media.

Technical Abstract: The addition of biochar to sand columns can enhance the retention of bacteria and thus may provide a management strategy for removing bacteria from tile-drainage waters. In this study, the role of sand size as a factor in controlling microbial retention in biochar-amended sand columns was investigated. Laboratory column experiments were conducted to quantify the removal of two bacterial isolates (E. coli and Salmonella) and polystyrene microspheres in 10-cm long columns packed with clean sand of three different sizes (0.25, 0.71, and 1.19 mm) at four biochar concentrations (0, 5, 10, and 15%). Sorption studies were also performed to help identify the relative roles of sorption and physical straining on the removal of bacteria and microspheres within the columns. For the large sand, the log10 removal values (LRV) for E. coli increased from 0.22 to 0.49 for the 0 and 15% biochar concentrations, respectively, with LRV for Salmonella increasing from 0.19 to 0.68. For the small sand, increasing biochar concentration from 0 to 15% increased LRV from 0.11 to 1.9 for E. coli and from 0.20 to 4.6 for Salmonella. In comparison, LRV for microspheres in the 15% biochar columns was only minimally higher than the unamended columns for all three sand sizes. Results from the sorption studies show that high sorption coefficients generally correlated with high LRV indicating that sorption rather than physical straining was the primary mechanism of retention in the columns. Results from this study further our understanding of bacterial retention in biochar-amended porous media.