The following subsections describe many aspects of a stormwater BMP design. The design components were gathered from multiple sources of literature. This section focuses on other researchers’ BMP evaluations and design approaches.
2.1.4.1 Stormwater Biofiltration Systems
“Water biofiltration is the process of improving water quality by filtering water through biologically influenced media” (Facility for Advancing Water Biofiltration, 2009). The water quality that is improved is typically stormwater or wastewater, and the biofiltration occurs when the water flows through vegetation and a “porous filter media,” normally some form of soil, and exits through infiltration and/or a drainage pipe (Facility for Advancing Water
Biofiltration, 2009). Figure 5 shows the design of a typical biofiltration system.
Figure 5: Example Biofiltration System Design
This design includes vegetation such as grass and taller plants, filter media, a soil trench that allows infiltration into the surrounding soil, and a drain for the outflow. There are many
variations of design that can exist for biofiltration systems, and each variation has different consequences for the water treatment. If the outflow drain is removed from the design, the water will infiltrate into the surrounding soil. If the drain is included, but is placed above the bottom of the filtration system, more infiltration will occur during smaller wet-weather events and the design will still allow for water discharge to occur during larger storm events (Facility for Advancing Water Biofiltration, 2009). It is also possible to change the type of media and vegetation used for the design, which will be discussed further below.
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In addition to water treatment, one of the primary goals of a biofiltration unit is to reduce the volume of stormwater runoff entering a water body from roads and the contributing
watershed. Generally put, the purpose of a biofiltration system is to “maintain or restore runoff volumes and frequency to predevelopment levels” (Facility for Advancing Water Biofiltration, 2009). Since urban areas generate large volumes of stormwater runoff due to the amount of impervious area, biofiltration systems are an important method that helps to reduce the volume of runoff and improve the quality of the water. This goal is completed in three ways (Facility for Advancing Water Biofiltration, 2009):
1. Physical treatment occurs as water flows through the vegetation, which reduces the flow rate and helps with particle settling. Solids suspended in the water are also removed through filtration as the water infiltrates the soil.
2. Chemical treatment occurs when the particles in the water are attracted to the soil particles and are subsequently removed from the flow.
3. Biological treatment occurs when the vegetation utilizes the nutrients in the water for growth.
There are many aspects of the design that need to be considered before development of a biofiltration system. The individual elements of the design, such as the inflow pipe diameter, filter media, and vegetation type, are the most important aspects, but determining how the system will fit into the surrounding landscape and what impact it will have is also pertinent.
Each component of the overall biofiltration system design must meet certain standards or recommendations, often implemented on a state level. The design must be completed in a way that allows each element of the design to perform its duty in the most efficient manner. For example, the purpose of vegetation in a biofiltration system is to remove contaminants and promote evapotranspiration of the stormwater. Therefore, using vegetation types that are native to the area of the system is imperative, and it must also be able to withstand local weather conditions. Having a higher density of vegetation is also beneficial, as it will increase the nutrient uptake and evapotranspiration losses. An area at the inflow needs to be designed to slow down the flow entering the basin. This area can also be used as a small-scale settling device to remove suspended particles. An example of this a sediment forebay, where rocks slow down the flow rate and the small basin shape allows for settling to occur before the water travels into the main basin. Controls such as an outflow pipe and the amount of water allowed to infiltrate to groundwater are also important aspects to consider. The control method for infiltration is utilizing a liner and underdrain for the basin, which prevents the water from completely infiltrating. Not using a liner means all or most of the water that enters the basin will infiltrate to the ground.
The media contained within the basin itself is one of the most important aspects of a biofiltration system. The purpose of the media is to remove contaminants through filtration, support the vegetation, and retain the stormwater runoff to reduce flow rates (Facility for Advancing Water Biofiltration, 2009). The type of sand, silt, or clay that is used determines the infiltration rate of the overall basin. A soil type with particles packed closely together, like clay, will have a much slower infiltration rate than soil with loosely packed particles. This should be
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considered when determining what type of filter media will be used. The media must also be able to withstand nutrient leaching and compaction, so the infiltration rate does not change. According to the Facility for Advancing Water Biofiltration (FAWB) guidelines, the filter media should have these additional characteristics:
Table 3: FAWB Guidelines for Biofiltration System Filter Media
Parameter
Hydraulic Conductivity 100 – 400 mm/hr
Soil Breakdown < 3% silt and clay Minimum depth for plant growth
and heavy metal removal 300 mm Minimum depth for tree growth 800 mm
Another aspect of the biofiltration system design is a submerged zone. This area in the filter is not required, but has multiple benefits. It is a layer under the filer media that maintains a constant volume of water, which helps the vegetation in the system grow, increases nitrogen removal, and can be used when the weather is in a dry period (Facility for Advancing Water Biofiltration, 2009). FAWB recommends that this zone, if utilized, be at least 300 mm deep, with an optimal depth of 450 mm.
The drainage ability of the biofiltration system is an important consideration for design. This includes the specifications for the underdrain, liner, and drainage layer. The drainage layer is the section of the system that contains the treated water and transports it to the outflow pipe (Facility for Advancing Water Biofiltration, 2009). Typically, the drainage layer is made up of gravel at least 50 mm deep, and should be designed to be at least as large as the detention volume for the system so the detention capacity remains unsaturated, even if many rainfall events occur (Facility for Advancing Water Biofiltration, 2009). The underdrain should have be a pipe that is slotted for water entry to outflow under the filter media, and a vertical section that extends to the surface. Figure 6 depicts the shape of the drainage layer and location of the underdrain to accommodate different purposes of the biofiltration system.
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The design of furthest left image aims to collect as much water as possible. The middle and right images are designed to promote infiltration to groundwater. The raised location of the underdrain in the right image will have a greater infiltration success than the middle (Facility for Advancing Water Biofiltration, 2009). To determine the depth of the drainage layer for the furthest right design, the following equation can be used:
𝐷𝑟𝑎𝑖𝑛𝑎𝑔𝑒 𝑙𝑎𝑦𝑒𝑟 𝑑𝑒𝑝𝑡ℎ = 50 𝑚𝑚 + 𝑑𝑝+ 𝐷
Equation 1: Drainage Layer Depth Where:
50 mm = Pipe cover depth dp = Diameter of pipe
D = Depth from invert of pipe to bottom of drainage layer
If no infiltration to groundwater is desired, a liner is typically used to prevent it. Compacted clays or flexible membranes are two types of liners that can be utilized. The clay prevents infiltration because the water cannot easily flow through it, and the membrane provides complete infiltration prevention.
Once the biofiltration system is in place and begins treating the stormwater runoff, it
requires maintenance to keep it running in its most efficient state. This includes removing trash, built up sediment, and weeds from the system, and mowing the vegetation. Another important task after the system is in place is monitoring to see if it is performing as designed. A note the FAWB includes in this biofiltration system review is that “biofilters require an establishment period of approximately two years to allow the filter media to settle and the vegetation to reach its design conditions,” which means that the results of a monitoring plan may not be completely accurate if conducted before two years have passed (Facility for Advancing Water Biofiltration, 2009). The data that FAWB recommends gathering as part of the monitoring plan are as follows:
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Table 4: FAWB Recommendations for Monitoring Plan Data
General Specific
Catchment characteristics Catchment area Slope
Impervious area
Geological characteristics Land use
Biofiltration system
characteristics Layout – size, slope, elevation Design capacity
Material – filter media, vegetation, liner, submerged zone, underdrain Age Condition Maintenance practices Climate Rainfall Temperature Evapotranspiration
Constituents Total suspended solids
Total nitrogen Total phosphorus
Heavy metals – copper, cadmium, lead, zinc Physical parameters pH, conductivity, temperature, dissolved
oxygen
Flow Can use weir, flumes, pipes and water levels, area/velocity meters
Water quality Can use sensors for continuous collection, samples during rain events, base flow Ammonium
Oxidized nitrogen Organic nitrogen Orthophosphate
Metals – aluminum, chromium, iron, manganese,
nickel
2.1.4.2 Sand and Other Media Filters
Filters remove floating particulates from stormwater by only allowing particles that have a smaller diameter to pass through the outlets. This removes an extent of the suspended solids in the water, which increases the overall quality of the effluent. The extent of removal depends on the type and size of media. For a sand filter, every diameter media has varied water “flow- through rates” and remove different sized suspended solids (Urbonas). If a media has smaller diameter particles, such as fine sand or clay, there is less room for water to travel between
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them, so it takes longer for it to flow through the media. In this same circumstance, more particulate matter is removed because there is more media obstructing the pathway. Conversely, when the media has particles of larger diameter, such as coarse sand or gravel, there is more space between them, so water flows quickly and more suspended matter passes through the media. Other types of media provide additional suspended solid removal, such as ion exchange that occurs in peat-sand media (Urbonas). The ideal media filter factors in the flow-through rate, diameter of particles, size, typical suspended solids loading, and cost of the filter. It must also be able to handle most storms.
Installing a detention area upstream from the filter increases the efficiency of the system (Urbonas). It allows the filter to receive a steady flow of stormwater, instead of all of the water at once, and provides pretreatment to remove suspended solids that could clog the filter. Detention storage also removes some suspended particles through settling. When the detention area is located completely or partially above the media filter, the TSS removal is completed entirely by the filter, and no suspended solids are removed by settling (Urbonas).
The basic form of Darcy’s Law represents the characteristics of water flowing through a filter:
𝑞 = 𝑘𝐼
Equation 2: Darcy’s Law Where:
q = Flow velocity (inches/hour)
k = Hydraulic conductivity (inches/hour) I = Hydraulic gradient (feet/foot)
However, this equation is simple and is affected when the TSS builds up on the filter surface. When this occurs, the flow-through rate of the water decreases, and filtration takes longer to occur (Urbonas). When the filter clogs completely, the water will bypass the filter and become untreated effluent water. Therefore, the effluent water quality is a combination of the treated water leaving the filter and, if the filter malfunctions or a storm is larger than the filter is designed to handle, the quality of the water bypassing the filter (Urbonas).