Category: Water, Wastewater & Storm water
Treatment flow rates of bioretention media affect the design and performance of stormwater controls. High treatment flow rates allow smaller sized facilities, but also provide reduced contact time of the stormwater with the media, reducing the chemically active treatment in the media. Low treatment flow rates allow longer contact times with the media and usually better treatment, but require larger facilities. Small bioretention facilities with smaller amounts of chemically active media will require more frequent replacement. In addition, low treatment flow rates may result in extended standing water above the treatment media, leading to nuisance conditions. Rapid treatment flow rates with small facility surface areas, can lead to pre-mature failure due to clogging/silting. The media treatment flow rates can be moderated using outlet controls and underdrains.
This research summarizes a large number of laboratory column tests conducted to examine the treatment flow rates for various mixtures of stormwater bioretention media. Statistical analyses were conducted to assist in the selection of treatment media having targeted treatment flow rates, and to evaluate monitoring data from existing bioretention facilities. It was found that the bioretention media treatment flow rates were most affected by the median particle size (D50) and uniformity coefficient (D60/D10) of the media, and the amount of organic matter. As expected, larger particles with small uniformity coefficients had the largest treatment flow rates. Compaction had minor effects if the organic matter content was low, but had significant effects on the flow rates for high organic matter content.
Redahegn Sileshi– PhD, EIT, University of North Georgia, Oakwood, GA