Rapid urbanization and climate change are expected to increase eutrophication because of increases in stormwater runoff with nitrate as well as enhanced decomposition of leaves and increased growth rate of algae in a warmer climate. Stormwater biofilters, particularly wood chips biofilters, have been used to remove nitrate from stormwater, but their performance is expected to decrease under extreme weather conditions—prolong drying and high rainfall intensity—which are expected to be more frequent during climate change. We hypothesize that addition of biochar would increase the resiliency of wood chips biofilters by enhancing the physical, chemical, and biological processes that support the removal of nitrate at biochar solid-water interfaces. Biochar-amended wood chips biofilters were designed by packing a homogeneous mixture of wood chips and biochar at 0, 5, 10, or 20% biochar volume in plastic columns (5.1 cm diameter, 61 cm hight). The effective filter media height was 49 cm, of which 30 cm of the media at the bottom was submerged by a raised outlet to enhance denitrification. Stormwater spiked with nitrate (20 mg/L) was injected through biofilters. Antecedent drying duration was varied between 0.25 d to 8 d and hydraulic retention time (HRT) was varied between 0.5 to 8 h, to examine the effect of drying duration and high rainfall intensity on denitrification. Results showed that increases in HRT decreased nitrate removal due to exhaustion of denitrification capacity, but an addition of biochar decreased the exhaustion rate. Improve in denitrification by biochar addition is attributed to an increase in HRT, dissolved organic carbon (DOC) trapped in immobile water, and robust denitrifying biofilm. Increase in antecedent drying duration generally had a net positive impact on denitrification, and the addition of biochar further increase nitrate removal. Characterizing change in pore water chemistry before and after drying, we showed that antecedent drying periods help replenish denitrification capacity of biofilters by increasing the dissolution of DOC and decreasing the dissolved oxygen. Overall, our results suggest that addition of biochar could increase the resiliency of wood chips biofilter for denitrification during extreme weather conditions expected during climate change.

Resiliency of biochar-amended woodchips-biofilter to remove nitrate from urban stormwater during climate change

256th National Meeting (2018)

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