Bioretention is a widely utilized low impact development (LID) strategy to manage stormwater quantity and quality. Elevated underdrain outlets are incorporated to create a saturated internal water storage (IWS) layer, increase hydraulic residence time (HRT), and promote treatment processes like denitrification. Yet, it remains unclear how underdrain height within the IWS layer impacts HRT and consequently nitrate removal. Additionally, the implications of IWS geometry [on denitrification and mixing] are unexplored. We employed three laboratory columns with underdrains at the bottom, middle, and top of a 30 cm deep IWS layer comprised of gravel and woodchips. Laboratory tracer studies were coupled with VS2DRTI simulations considering seven different underdrain heights. Results revealed that hydraulic efficiency (HE = mean HRT / theoretical residence time) decreased as IWS underdrain height increased. Simulated column velocity profiles confirmed the presence of larger immobile fluid regions below higher underdrains, which inhibit solute transport and reduce HRT. These flow patterns impacted nitrate removal during laboratory column storms. The bottom underdrain always demonstrated superior performance in regulating nitrate effluent concentrations compared to the middle and top underdrains. However, the top underdrain performed best in regulating dissolved organic carbon. Simulated columns also considered the impact of IWS width-to-height ratios of 0.50, 0.76, and 1.56. HE was most sensitive to underdrain height for IWS width-to-height ratios &lt; 1. However, IWS width-to-height ratios greater than 1 contributed to the presence of immobile zones across all underdrain configurations. Results suggest that for geometries relevant to the field setting, immobile zones can be present even for bottom underdrains and theoretical residence time should not be used as a criterion for basin sizing. Flow and transport modeling can serve as a valuable tool to optimize IWS design to maximize water quality benefits. Optimal underdrain height will likely vary depending on site-specific constraints.

Raised versus inverted? The impact of internal water storage underdrain design on hydraulic residence time and nitrate removal

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.