Infiltration Systems Pretreatment Criteria Cont’d
Camp-Hazen equation may also be used to calculate pretreatment requirements
- Pretreatment area based on WQV
- Camp-Hazen equation: As = -(Qo/W)*Ln(1-E), where As is surface area of sedimentation basin
- As = 0.066 (WQV) ft2 for impervious cover ? 75%
- As = 0.0081 (WQV) ft2 for impervious cover > 75%
Exit velocities from pretreatment shall be non-erosive for 2-yr design storm
Notes:
The equation above is used to size pretreatment settling basin surface area. It was derived by the Washington State Department of Ecology from the Camp-Hazen.
As = Sedimentation basin surface area (ft2)
E = Trap efficiency; which is the target removal efficiency of suspended solids (set equal to 90%)
w = Particle settling velocity; for target particle size (silt) use settling velocity = 0.0004 ft/sec (0.0033 ft/sec for I > 75%, where I is percentage impervious area)
Qo = rate of outflow from the basin; which is equal to the water quality volume (WQv) divided by the detention time (td); use 24 hours.
The equations simplify to:
As = 0.066 ( (WQV) ft2 for I < 75%
As = 0.0081 (WQV) ft2 for I >= 75%