Enhanced Biofilter Treatment of Urban Stormwater by Optimizing the Hydraulic Residence Time in the Media

Selecting the best media for a specific situation is critical when designing a biofilter or bioinfiltration stormwater control practice as the media affects the amount of runoff that is treated and the level of treatment that can be obtained. Appropriate hydraulic characteristics of the media, including treatment flow rate, clogging capacity, and water contact time, are needed to select the media and drainage system. This information, in combination with the media's ability to capture targeted pollutants with minimal clogging given the appropriate contact time, can be used to predict the performance of a biofilter device. This paper presents a series of tests being conducted to determine the hydraulic characteristics of sand-based filter media (having a variety of particles sizes representing a range of median particle sizes and uniformity coefficients) during pilot-scale trench tests. The drainage rate in biofiltration devices is usually controlled using an underdrain that is restricted with a small orifice or other flow-moderating component. These frequently fail as the orifices are usually very small (<10 mm) and are prone to clogging. A series of tests are conducted using a newly developed foundation drain material (SmartDrain^TM ) that offers promise as a low flow control device with minimal clogging potential. A pilot- scale biofilter using a fiberglass trough 3m long and 0.6 x 0.6m in cross section is used to test the variables affecting the drainage characteristics of the SmartDrains^TM (such as length, slope, hydraulic head, and type of sand media). The results indicated that slope of the SmartDrain^TM material had no significant effect on the stage-discharge relationship whereas the length had a small effect on the discharge rate. The information collected during this study will assist stormwater managers in the design of biofilters needing a slowly draining device.