Understanding Tree Box Filters and Their Importance

Tree box filters are a crucial component in urban drainage systems, designed to capture and filter stormwater runoff before it enters our waterways. They're essentially large boxes or tanks that use a combination of physical and biological processes to remove pollutants and debris from the water.

These filters can be made from a variety of materials, including concrete, steel, or plastic, and come in different shapes and sizes to suit specific needs. In some cases, tree box filters may be installed underground, while others are above ground.

Tree box filters are designed to accommodate a low volume of rainfall, with a filter surface area of 48 feet covering up to 0.25 acres of impervious or nonporous surface. Strategically positioning multiple tree boxes around the area of coverage is vital to reduce costs and work.

The design of a tree box filter consists of four main parts: the tree, open-bottom concrete box, porous soil mix, and underdrain. A soil layer of at least two feet deep is required to sustain a tree with a canopy of thirty square feet, and the soil mixture should include construction sand, unscreened topsoil, and compost.

The underdrain consists of a layer of crushed stone, at least two feet deep, surrounding a perforated drainage pipe that connects to the municipality's existing storm drain system. This allows excess water to flow out, preventing overflow.

Design is a crucial aspect of tree box filters, and it's essential to consider several factors to maximize their effectiveness. A tree box filter's design should take into account the area available, area of coverage, types of contaminants, level of rainfall, aesthetic appeal, maintenance, and budget.

The location, design, and type of material of the box filter may be altered to accommodate these considerations. Tree box filters are designed to accommodate a low volume of rainfall, with a filter surface area of 48 feet (15 m) covering up to 0.25 acres (0.10 ha) of impervious or nonporous surface.

A tree box filter consists of four main parts: the tree, open-bottom concrete box, porous soil mix, and underdrain. The tree is planted in a soil mixture of construction sand, unscreened topsoil, and compost, with a recommended 2 cubic feet (0.057 m) of soil for every 1 cubic foot (0.028 m) of tree canopy.

Here's a breakdown of the required soil depth for different tree canopy sizes:

This ensures the tree has enough space and nutrients to thrive. The underdrain consists of a layer of crushed stone, at least two feet (0.6 meters) deep, surrounding a perforated drainage pipe that connects to the municipality's existing storm drain system.

Tree box filters work by slowing down water with an energy dissipator, such as stones, to catch sediment before it filters through the mulch and engineered soil media.

The runoff then passes through the plant's root system, where the plant absorbs water and nutrients. This environment is well-suited for water quality treatment, with biological and chemical reactions taking place in the engineered soil media, mulch, and root zone.

The underdrain, a perforated PVC pipe, conveys treated water to the storm drainage system. This is where the treated water is released into the environment.

Tree box filters can significantly reduce the amount of pollutants in the stormwater that it infiltrates. In fact, they can remove up to 90% of total suspended solids, 38-65% of nitrogen, and 50-80% of phosphorus.

Here's a breakdown of the pollutants that tree box filters can remove:

Maintenance is crucial for extending the life and efficiency of a tree box filter. Regular inspections are recommended to catch any potential issues before they become major problems.

The cost of care can range from $100 to $500 per year for each tree box filter, depending on the specific needs of the filter. This cost can add up, but it's worth it to keep your tree box filter running smoothly.

Inspecting the facility twice per year is a good starting point for regular maintenance. This can help identify any issues before they become major problems.

Removing sediment, leaves, and trash as needed is another important part of maintenance. This can help prevent clogs and keep the filter functioning properly.

Inspecting the overflow pipe for damage and function is also essential. This can help prevent backups and keep the filter running smoothly.

Here is a list of maintenance tasks to consider:

If excessive volumes of stormwater are bypassing the tree box filter or if the box holds water beyond two days, corrective measures should be taken to restore performance.

Tree box filters are a game-changer for reducing pollutants in stormwater. They can remove up to 90% of total suspended solids from the water.

These filters show impressive load ratios, with a reduction of 0.09 in total suspended solids, 0.1 to 0.3 in soluble metals, and 0.35 to 0.6 in organics and nutrients.

By implementing tree box filters, you can expect a significant reduction in pollutants like nitrogen (38-65%), phosphorus (50-80%), and zinc (54%).

Here are some specific pollutant reductions you can expect from tree box filters:

Tree box filters are a key part of stormwater management, and their benefits extend to environmental engineering, hydrology, and urban planning.

The area I'm talking about was a very old neighborhood with a lot of missing infrastructure. It had gas, water, and sanitary sewer services, but no stormwater drainage system.

Flooding was a big problem every time it rained, making it unsafe to drive due to hydroplaning. The road surfaces were in terrible condition.

The city proposed a Low-Impact Development (LID) green solution to improve the area with a sustainable stormwater and irrigation system. This solution was two closely related projects that enabled the city to make better use of stormwater runoff.

The first phase of the project collected runoff from the streets and piped it to two large new sedimentation ponds. This reduced the city's current expenses for bringing water in from another county.

Building regional stormwater infrastructure creates an incentive for businesses to build in the area. Companies can save $250,000 to $350,000 by tying into the regional system instead of building an individual detention system.

The city assumed that the area would eventually be 10% green space, with the rest either hardscape or rooftop surfaces. This informed the design of the stormwater system, which had to be able to handle the entire future build-out of the neighborhood.

The project was approved at a budget of $5 million, making it the largest stormwater management project of its type in the southeast.

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Tree box filters are a type of bioretention practice that uses a combination of mulch, engineered soil media, and plant roots to treat stormwater runoff.

They feature an underground precast or cast-in-place concrete structure and an above-ground planting, typically including small trees and shrubs.

Tree box filters are best suited for areas that don't receive high levels of sediment, such as parking lots, courtyards, and privately maintained roads.

A common feature of tree box filters is an inlet where runoff enters a concrete box filled with engineered soil media.

The concrete box is often topped with a grate similar to those found on urban street trees, which helps to prevent debris from entering the system.

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