UCLA

Stormwater biofilters, particularly woodchip biofilters, have been used to remove nitrate from stormwater, but their performance is expected to decrease under extreme weather conditions such as prolonged drying and high rainfall intensity, which are expected to be more frequent during climate change. The objective of this study is to examine the effect of biochar amendment on nitrate removal by woodchip biofilters subjected to increasing antecedent drying conditions and rainfall intensity. The experiments were designed to test the following hypothesis: the addition of biochar would increase the resiliency of woodchip-amended biofilter by enhancing the physical, chemical, and biological processes that support the removal of nitrate. Biochar-amended woodchip biofilters were packed with a homogeneous mixture of woodchips and biochar at 0, 5, 10, or 20% biochar volume in plastic columns (5.1 cm diameter, 61 cm height). Stormwater spiked with nitrate was injected through biofilters, and the effluent was collected by a raised outlet (30-cm submerged zone) to enhance denitrification. Antecedent drying duration was varied between 1 d to 8 d, and hydraulic residence time (HRT) was varied between 0 to 20 h to examine the effect of drying duration and high rainfall intensity on denitrification.

Results showed that biochar improved denitrification potential of woodchip biofilters. This improvement is attributed to changes in pore water chemistry, such as a decrease in dissolved oxygen (DO) concentration and an increase in dissolved organic carbon (DOC) trapped in pore water, and an increase in robust denitrifying biofilm supported on biochar particles, which has higher surface area than woodchips. Increase in antecedent drying duration had a net positive impact on denitrification, and the addition of biochar further increase nitrate removal during drying period. Antecedent drying periods helped replenish denitrification capacity of biofilters by increasing the dissolution of DOC and decreasing DO. Overall, the results suggest that addition of biochar could increase the resiliency of woodchip biofilters for denitrification during high intensity rainfall expected during climate change—the conditions at which the performance of woodchip biofilters typically deteriorates quickly.