Sand Filter bed Sizing Criteria
Darcy’s Law
- Af =WQV*(df)/[k*(hf + df)(tf)] where:
- Af = surface area of filter (ft2)
- WQV = treatment volume (ft3)
- df = filter bed depth (ft) - can vary depending on the site conditions but should not be more 24" (18" is the standard)
- k = coefficient of permeability (ft/day)
- hf = average head above filter bed (ft) - varies depending on the site conditions, but should not exceed 6 feet
- tf = time to filter through bed (days) - A value of 40 hours is recommended
Notes:
The principles of Darcy’s Law are used for sizing the sand filter bed area as derived by the City of Austin, TX, Environmental and Conservation Services Department. The primary design parameter for filtration basins is surface area. The necessary surface area is a function of the water quality volume, permeability of the filter medium, the bed depth, the hydraulic head (height of water above the bed), and the time for water to filter through the bed. The equation in this slide can be used to size all types of filter media.