Nutrient biofilters in the Virginia Coastal Plain: Nitrogen removal, cost, and potential adoption pathways

Abstract

Excess nonpoint source nutrient loss presents one of the most vexing water management challenges for water quality managers. Agriculture is the single largest contributor of nutrients to the Chesapeake Bay, and achieving nutrient reduction targets for agriculture will be costly. Biofilters offer new opportunities to reduce nutrient loads from artificially drained agricultural land. Nutrient biofilters consist of organic carbon (C) medium, typically woodchips, that when saturated with nitrogen (N)-enriched waters supports the activity of naturally occurring soil microorganisms that convert bioavailable N into N gases via denitrification. This study estimates the cost and N removal effectiveness for a biochar-amended woodchip biofilter draining a 10 ha (25 ac) field planted in a corn (Zea mays L.)–soy (Glycine max [L.] Merr.) rotation in eastern Virginia as well as for alternative design scenarios for biofilters ranging 25 to 150 m³ (883 to 5,297 ft³) with either woodchips alone or biochar-amended woodchip C substrates. Nitrogen removal effectiveness is estimated using modeled site-specific influent loads and N removal effectiveness estimates derived from experimental trials in a pilot scale system on the Delmarva Peninsula. This analysis estimates N removal costs as a function of biofilter size, which directly relates to residence time, per unit of N removed. Modeled N removal estimates over a five-year period (2009 to 2013) for five bed volumes range from 88 to 391 kg (194 to 862 lb) for the woodchip biofilters (21% to 95% removal) and 132 to 412 kg (291 to 908 lb) for the biochar-amended woodchip biofilters (32% to 100% removal) of the 412 kg total N load exported to the biofilters during the five-year period. The N removal costs range from US$15 to US$48 kg⁻¹ y⁻¹ (US$6.82 to US$21.82 lb⁻¹ yr⁻¹) for the woodchip biofilter and US$15 to US$33 kg⁻¹ y⁻¹ (US$6.82 to US$15 lb⁻¹ yr⁻¹) for biochar-amended woodchip filters. While biofilters are determined to be cost-effective for N removal compared to conventional agricultural best management practices, agricultural producers may be reluctant to voluntarily adopt biofilters because of the relatively high initial installation costs and the lack of direct incentive payments for pollutant removal effectiveness. Nutrient trading programs offer some potential to enhance adoption, but not in the immediate future.