Using the treatment train approach to BMP selection

The basic premise for selection of a Best Management Practice (BMP) or group of BMPs is to follow the treatment train approach. Under the treatment train approach, stormwater management begins with simple methods that minimize the amount of runoff that occurs from a site and methods that prevent pollution from accumulating on the land surface and becoming available for wash-off. Even though we know that we will never be able to fully accomplish either of these goals, we can make substantial progress using the Better Site Design, Low Impact Development (LID), pollution prevention, volume minimization, and temporary construction erosion and sediment control techniques.

After all of the efforts possible are made to minimize runoff and surface wash-off, we must recognize that some potential for runoff will occur. The next major BMP then becomes collection and treatment of runoff locally and regionally, either as stand-alone practices or in treatment train combinations. Some of the available BMPs are best used to reduce runoff volume, while others focus on water quality improvement. Some BMPs will be easy to implement, while others involve serious engineering and sophisticated design. Other BMPs used in a treatment train include bioretention devices, filtration practices such as swales, infiltration practices such as infiltration trenches and infiltration basins, stormwater ponds and stormwater wetlands.

Proper Treatment Accounting

When evaluating the benefits of various BMPs, it is essential to account for the amount of water that will enter the system versus the amount that will be by-passed or diverted. Water that does not fall within the design parameters of a BMP will be sent either to another down-gradient BMP or simply routed to a receiving water untreated (not recommended). Although some BMPs, such as ponds and wetlands, will minimally treat excess water because it is routed through the BMP, other such as filtration and infiltration systems, cannot operate properly if excess water flows into them. This is an important distinction that must be evaluated for each BMP installation.

The design recommendations and expected BMP performance contained within this Manual assume that only the amount of water contained within the design will actually be treated. It is not acceptable to assume that all water falling in any event and within the area draining to a BMP will, in fact, be treated by that BMP. An analysis of every BMP installation should include an identification of where by-passed water will flow and how it could be treated.

Pollutant Removal Mechanisms

The key to proper selection of a single or series of BMPs is to match the pollutant to be controlled against the pollutant removal mechanism of a specific BMP. For example, it is not appropriate to use a stormwater pond when temperature control is necessary; however it is very appropriate to use a pond for purposes of rate control. The definition of pollutant being utilized by the Minnesota Stormwater Manual includes both the traditional pollutants (nutrients, solids, etc.) plus the negative effects caused by thermal increases and excessive rate/speed of stormwater flows. Stormwater planners and designers will first need to understand the pollutant(s) of concern that may be generated at their sites. At the early stages of design, stormwater managers should be contacting local water management agencies (watershed districts, watershed management organizations, soil and water conservation districts, counties and/or cities) to learn which pollutants are necessary to control prior to discharge of new stormwater runoff to local waterbodies.

Pollutant removal mechanisms vary with each BMP. The key mechanisms for each group of structural BMPs can be used by stormwater managers as a preliminary screening tool.

Primary and secondary pollutant removal mechanisms

Water Quality Pollutant Removal Mechanisms

• Screening/Filtration: The capture of solid pollutants through screens and/or filters which use a media such as sand. Effective for removal of suspended solids.
• Infiltration/Ground Water Recharge: A technique to discharge stormwater runoff to ground water. Effective when runoff volume controls are required and surface water temperatures must be controlled.
• Settling: Deposition of solids in a water column, usually in a pond, wetland or hydrodynamic device. Typically a minimum of 12 hours of detention is needed to effectively settle solids in stormwater ponds and stormwater wetlands.
• Biological Uptake: Vegetative and microbial uptake of nutrients. Usually accomplished in biofiltration devices and stormwater wetlands.
• Temperature Control: Techniques to reduce the heating effects when runoff flows across hot pavements. Most effective technique is for groundwater to cool treated runoff.
• Soil Adsorption: The physical attachment of a particle, usually nutrients and heavy metals, to the soil.

Water Quantity Control Mechanisms

• Volume Control: Methods to limit the net increase in stormwater runoff volume caused by the creation of new impervious surfaces. Most common techniques include limitation of new surface areas, infiltration, and re-use by vegetation.
• Rate Control: Detention of stormwater runoff to slow the discharge of runoff to surface waters to rates comparable with pre-development conditions. Effective for peak rate control, but can significantly increase the time period of the peak flows.
• Velocity Control: Similar to rate control; intentional restriction of stormwater runoff such that velocity of discharged runoff through downstream channels does not cause channel erosion.
• Evapotranspiration: Specific volume control technique that utilizes evaporation from water surfaces and/or transpiration by vegetation.

BMP Organization

The following sections describe the BMPs that are recommended for Minnesota. The selection criteria are included in Chapter 7 and the specific information on each category is included in Chapter 12. Some additional support information occurs in Appendix D. Note that the order of the BMP presentation follows the treatment train sequence illustrated in Figure 6.1.

Non-Structural or Planning Level BMPs

The first level of BMP application occurs at the planning stage and is intended to minimize the impact of development. These practices are intended to prevent pollution and minimize the increase in stormwater volume and are considered prior to initiation of construction or land altering activity.

Pollution Prevention Practices

Residential, municipal and industrial/commercial practice categories (Water Quality Focus). Specific recommended practices include such things as:

• Housekeeping including landscaping, street sweeping, pavement maintenance, catch basin maintenance, yard waste reduction and litter control
• Atmospheric controls including wind erosion and dust, as well as regulatory emission regulations
• Chemical control of hazardous waste and salt, fertilizer/pesticides, spills (including prevention), swimming pool drainage
• Animal waste management
• Streambank stabilization
• Public works activities including chemical and sanitary wastes, and sewer maintenance

Better Site Design

(See Chapter 4 and Chapter 12)

Runoff Volume Minimization

(Water Quantity Focus; see also Table 7.1) Note that many typical runoff volume reduction techniques are included in the Chapter 4 discussion of better site design and Chapter 12 Fact Sheets, including:

• Compost amended soils
• Green roofs/rooftop gardens
• Pervious pavement/lattice blocks
• Rainwater harvesting (barrels/cisterns, evaporative and irrigation systems)

Temporary Construction Sediment Control

(Water Quality Focus; see Also Table 7.2; Reference MS4, NPDES, and Local References and Ordinances) These practices are described in terms of perimeter, slope, drainageway and “other” criteria, and include:

• Vegetated buffers
• Silt fence
• Access/egress and drainage inlet protection
• Soil and slope stabilization
• Exposed soil covers and reinforcement

Structural BMPs

These BMPs have design guidance describing the engineering details for the BMP category. This design guidance is used, for example, to determine storage volume and physical configuration that best meet the objectives of the BMP application. Also note that some of these BMPs, such as filtration, can be either a primary treatment technique or used for pre-treatment into another BMP.

Bioretention

This BMP suite includes vegetated systems that provide a combination of filtration and infiltration into a bio-system consisting of plants and soil, including:

• Rain gardens
• Depressed parking lot/traffic islands
• Road medians
• Tree pits/stormwater planters

Filtration

• Media (sand) filters (surface, underground, perimeter/Delaware filter)
• Surface (vegetative) flow (grass channels, dry or wet swales, filter strips)
• Combination media/vegetative filters

Infiltration

• Trenches
• Basins
• Dry wells
• Underground systems

Stormwater Ponds

Design based upon components needed to fulfill the desired function.

• Components include forebay/pre-treatment, various storage volumes, physical configuration
• Functions include water quality (including thermal impact) and flow control (rate and volume), which determine whether they are wet/dry or some combination

Constructed Wetlands

Selection criteria are similar to stormwater ponds.

• Components include pre-treatment, various storage volumes (detention needed), biologic character
• Functions include primarily water quality and flow control, but could also include ecological factors

Supplemental Pre- and Post-Treatment BMPs

The final category of BMPs present those that are generally, but not always, included in the stormwater treatment train as a supplement to the primary treatment device. Although this is not generally recommended, there is the possibility that these devices could be the only BMP used. These are described in less detail than the previous sections. The designer will be guided through a process of determining the function a generic device serves within the treatment train and evaluating the proposed device against the needed function and manufacturer claims. Proprietary devices are generically described rather than listed as individual companies to avoid risking some omissions and claims of certification in the Manual.

Supplemental Pre- and Post-Treatment

• Hydrodynamic
• Proprietary sediment and oil/grease removal devices
• Wet vaults
• Sorbents
• Skimmers

Filtration

• Catch basin inserts
• Sorbents
• Proprietary filtration devises

Chemical/biological treatment

• Chemical treatment (ferric chloride, alum, polyacrylamides). Note that these chemical treatments could be limited in the State of Minnesota because of the potential toxic effects associated with them; care will be taken to assess these impacts in the BMP discussion.
• Biological additives (ex. chitosan)