Urban Particle Capture in Bioretention Media. I: Laboratory and Field Studies

Bioretention is a novel stormwater best-management practice that uses a mixture of soil/sand/mulch as adsorptive filtration media that can capture both urban particulates and dissolved pollutants while promoting infiltration. This study conducted a series of laboratory column experiments and field observations, which showed that: (1) bioretention media stratification occurs with runoff percolation due to particulate deposition; (2) bioretention filter media are clogging limited, instead of breakthrough limited; and (3) both depth filtration and cake filtration significantly contribute to urban particle capture. Because of the fine size of bioretention media, incoming suspended solids cannot significantly penetrate below 5–10cm of the media in the column tests and approximately 20cm in the monitored field facility. Bioretention filters under intermittent flow conditions exhibited higher solids loading capacity (in kg∕m2 ) before clogging than under continuous flow conditions. The clay components in incoming total suspended solids assume critical responsibility for bioretention media clogging. The media resistance due to solids deposition was estimated through Darcy’s law. The hydraulic conductivity of two media types decreased from 54±23 and 72±46cm∕h to less than 10cm∕h due to particle capture. Experimental results suggest that a 20-cm media depth is sufficient for bioretention design and maintenance procedures (media replacement) for runoff particle capture.