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Biochar for Enhanced Trace Organic Contaminant Retention in Stormwater Biofilters

Bridget Ulrich
Colorado School of Mines
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PDF icon 4.2.4 Ulrich, Bridget.pdf6.86 MB
Abstract: 

Biofilters are popular low impact development technologies because they can improve stormwater quality while maintaining landscape permeability. However, conventional biofilters may fall short for removal of polar trace organic contaminants (TOrCs), such as pesticides. Biochar is a promising geomedia to enhance the sorptive retention of TOrCs. Biochar-amended biofilters are of particular interest, as biological activity could degrade contaminants and delay the need for media replacement. However, biological fouling could potentially diminish the sorptive capacity of biochar and limit system lifetime. The objective of this work was to evaluate the effectiveness of biochar-amended biofilters for long term removal of TOrCs from stormwater. The approach was threefold: (1) verification and application of a forward transport model to predict sorption-controlled breakthrough times in a neighborhood-scale biofilter, (2) evaluation of the effects of biological activity in laboratory microcosm and column experiments, and (3) assessment of long term performance in intermittently saturated, vegetated columns. Forward modelling predictions indicated that a biofilter composed of 30 vol% of a gasification biochar in sand could prevent TOrC breakthrough (removal > 95% maintained) for approximately five years. Experiments evaluating biological effects revealed that biological attenuation processes can enhance overall TOrC removal in the presence of biochar (i.e. reduce TOrC mass recovery in the effluent), while fouling reduced breakthrough times relative to clean (biologically inactive) systems. Results from vegetated columns have shown that biochar-amended media (33 vol% biochar in sand) maintained at least 99% removal of several TOrCs after treating the equivalent of half a year of runoff volume, while non-amended columns only maintained approximately 50% removal. Efforts to further evaluate long-term performance are ongoing: columns will be dosed with three years of runoff volume by the conclusion of the experiment. The results from this study motivate future efforts to evaluate TOrC removal effectiveness at the pilot and field scale.

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