Bioretention System Enhancements for Nutrient Removal

Bioretention systems typically demonstrate limited nutrient treatment and often export nutrients just after installation. A variety of bioretention configurations were tested in duplicate field-scale mesocosms dosed with stormwater. A commercial bioretention soil media (BSM) was used by itself in some mesocosms and mixed with topsoil in others. Configurations tested included an activated alumina (AA) layer placed below the BSM, expanded gravel storage below the BSM, bottom outlets, and single and dual elevated outlets. Mesocosms with BSM/soil mix outperformed BSM-only mesocosms for both nitrogen and phosphorus removal. Mesocosms without AA exported considerable phosphorus. The AA layer substantially reduced phosphorus export and slightly improved nitrogen removal. Outlet types and elevations did not affect phosphorus removal. Compared to treatments with bottom drains, those with elevated outlets showed better nitrogen removal. The effectiveness of dual elevated outlets equaled or slightly exceeded single elevated outlets. Washing the media with tap water during construction reduced export only initially, with no differences noted after a few runs. The expanded storage mesocosms showed mixed results probably due to construction deficiencies.