Performance Evaluation of an Alternative Underdrain Material for Stormwater Biofiltration Systems

Underdrains are commonly used in biofilter stormwater treatment devices in areas having low soil infiltration rates. These usually are required to reduce the duration of standing water to less than 1-3 days to minimize nuisance conditions and reduce mosquito breeding. The drainage rates in biofilter underdrains can be controlled using a restricted orifice or other flow-moderating component in order to maximize contact time for improved water quality while still meeting the standing water criterion. Restricted orifices are of concern due to clogging potential, because they usually are very small in order to provide sufficient contact time with the treatment media for water quality benefits, or to detain the water for a significant time when used as part of green infrastructure components in areas that have combined sewers. If the orifices are large to minimize clogging, they are less effective for flow rate reductions or for maximizing water quality improvements. This study examines the flow capacity and clogging potential of an alternative belt drain underdrain device. The sediment-carrying capacity of the slowly flowing water entering a belt drain is very low and not capable of transporting particulates into the underdrain material due to its very low Reynolds numbers during normal operating conditions. The corresponding average Reynolds numbers based on the mean-flow velocities and the drain belt microchannel diameter range from 7 to 700. The results of these tests indicate that the drain belt provides an alternative option for biofilter underdrains, showing minimal clogging while providing very low discharge rates.