Volumetric Filtration of Rainfall Runoff. I: Event-Based Separation of Particulate Matter

As a unit operation, filtration generally requires flow equalization and primary clarification. This study examines the separation of runoff particulate matter (PM) in a volumetric clarifying filter (VCF). The VCF is a detention/retention vault integrating filtration after sedimentation. A paved source area $\left(\mathrm{1,088}{\text{m}}^2\right)$ directly loaded the vault $\left(4.2{\text{m}}^3\right)$ with five radial filters ( $4{\text{m}}^2$ of filtration surface area). PM separation was examined for 19 runoff events through monitoring of influent and effluent granulometric fractions. During the monitoring phase no maintenance was conducted and subsequent to the 19 events a measured material balance of the sedimentation vault and the radial cartridge filters generated a 94% recovery of PM. During 5 months of monitoring and PM mass, suspended sediment concentration (SSC) was reduced from 334 to 34 mg/L (90% reduction) with effluent sediment $\left(>75\mu \text{m}\right)$ of $<3\text{mg}/\text{L}$ , settleable of 14 mg/L, suspended PM $\left(<25\mu \text{m}\right)$ of 17 mg/L as event mean concentrations; with turbidity reduced from 96 to 23 NTU (76% reduction). Based on separate PM recovery from the vault and filters, 77% of the PM separation was sedimentation in the vault and 23% as filtration. Captured particle-size distributions are heterodisperse with a $d_{50\text{m}}$ of 300 μm in the vault and a $d_{50\text{m}}$ that ranged from 34 to 63 μm with filter depth. Filter forensics indicated PM capture was nonuniform, with the bottom and middle most heavily loaded by PM as compared to the upper third of the filter. While paired testing of automatic and manual sampling produced similar median effluent SSC, automatic sampling significantly misrepresented the median influent PM as SSC $\left(p⩽\alpha =0.05\right)$ .