About Bioretention System (Bio II)
General Information Performance Data Water Quality Treatment Process Design




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System Performance

Water Quality Treatment
Bio II has proven effective at removing nearly all of the pollutants commonly associated with stormwater treatment performance assessment. It consistently exceeded EPA’s recommended level of removal for total suspended solids and meets regional ambient water quality criteria for petroleum hydrocarbons. However, it exhibits almost no discernable removal of total phosphorus. Like the other systems monitored at UNHSC, it does not provide chloride removal, although it does exhibit an ability to dampen chloride peaks.

The chart at top right reflects the bioretention performance in removing total suspended solids, total petroleum hydrocarbons, dissolved inorganic nitrogen, total phosphorus, and zinc. Values represent results from a half-year monitoring period.

Water Quantity Control
Like other infiltration and filtration systems, Bio II has a tremendous capacity to reduce peak flows and runoff volume in appropriate soils, i.e., those belonging to groups A and B. In the figure at bottom right, it demonstrates effective peak flow reduction and large lag times regardless of season. Vegetation contributes to stormwater volume reduction through the process of evapotranspiration.

Maintenance
Bioretention systems are designed to minimize maintenance. For the most part, the highest maintenance burden coincides with the initial stabilization of the system over the first three to four months. However, once vegetation is established and the system has been stabilized, the maintenance decreases and becomes similar to that required for standard landscaping, such as seasonal mowing and raking. With the exception of the stabilization period, no maintenance has been performed on Bio II since it was installed in fall 2005, and the system continues to function well.

Cold Climate
Bio II’s ability to treat water quality and control water quantity remained relatively consistent in all seasons. UNHSC researchers have observed that most LID stormwater systems, when properly designed and installed, are not negatively impacted by cold climate. In fact, these systems showed less seasonal variation than many conventional approaches that depend on sedimentation as primary removal mechanism. While some seasonal variation did occur in Bio II, significant design alterations do not appear to be necessary for cold weather applications of this system.