About the Tree Box Filter
General Information Performance Data Water Quality Treatment Process Design




Back to List of Evaluated Treatments

System Performance

Water Quality Treatment
The tree box filter does a good job of removing many 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 products, nitrogen, and total zinc. However, UNHSC research demonstrates that water quality treatment effectiveness can be negatively influenced by an increased hydraulic loading rate, i.e., the filtration of a large surface area by a small filter area. The system does not remove chloride, but does exhibit an ability to dampen chloride peaks.

The chart at top right reflects system performance in removing total suspended solids, total petroleum hydrocarbons, dissolved inorganic nitrogen, total phosphorus, and zinc. Values represent results recorded over two years, with data further divided into summer and winter components.

Water Quantity Control
Unlike other filtration systems, the tree box filter does not reduce peak flows unless sited in appropriate soils, such as those in groups “A” (sand, loamy sand, or sandy loam with high infiltration rates) and “B” (silt loams or loams with moderate infiltration rates). In the figure at bottom right, the tree box filter displays no significant peak flow reduction or lag time for the range of seasons monitored.

Maintenance
No maintenance has been performed on the tree box filter since it was installed in fall 2005, and the system continues to function well. Generally speaking, these systems are designed to minimize maintenance. Aside from routine trash removal, the highest maintenance burden generally coincides with the establishment of vegetation over the first several months after installation. Once vegetation is established, the maintenance demand decreases. The tree may need to be replaced, depending on hardiness of the selected species. Adaptations to design can prevent root constriction in the planting vault.

Cold Climate
The tree box filter’s ability to treat water quality remained relatively stable in all seasons. This is consistent with UNHSC observations of most LID stormwater systems—when they are properly designed and installed, they are not dramatically impacted by seasonal fluctuations. While some seasonal variation in infiltration capacity and nitrogen removal does occur, cold conditions do not seem to warrant significant design alterations.