Physical Modeling of Particulate Matter Washout from a Hydrodynamic Separator
Publication: Journal of Environmental Engineering
Volume 139, Issue 1
Abstract
A hydrodynamic separator (HS) is commonly used as a preliminary unit operation for separation of coarser particulate matter (PM) and PM-associated constituents transported by urban rainfall-runoff. Although the small spatial footprint of an HS is advantageous for integration into sewer (storm or combined) or drainage systems, the small footprint also concentrates flow energy. In many HS units, where PM sludge is not isolated or in units that are not maintained (cleaned), washout of previously separated PM sludge can result in a net export of PM. This study analyzes PM washout from a baffled HS as a function of steady-flow rates and particle-size distributions (PSDs) guided by a physical modeling framework utilized for scour certification. The HS hydraulic signature is determined through velocity measurements and a residence time distribution (RTD) analysis. As a function of surface overflow rate (SOR), the predeposited PM washout rate ranges from 0.4 to for fine PM (sandy silt, SM ), from 0.3 to for moderately coarser PM (SM II,), and 0.2 to for coarser PM (SM III,) as SOR ranges from () to (). The washout rate decreases with time for each heterodisperse PM gradation, and the evolution of HS washout is modeled as a first-order exponential analogous to urban washoff models. Washout from the HS is modeled as a function of the densimetric Froude number and SOR. Results indicate that washout based on a Shields approach cannot be applied to HS units.
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Information & Authors
Information
Published In
Journal of Environmental Engineering
Volume 139 • Issue 1 • January 2013
Pages: 11 - 22
Copyright
© 2013 American Society of Civil Engineers.
History
Received: Sep 3, 2011
Accepted: Feb 21, 2012
Published online: Feb 23, 2012
Published in print: Jan 1, 2013
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