The Manual user will notice that many of the stormwater practices discussed in this Manual recommend pretreatment as an integral part of the best management practice (BMP) application. In fact, in many applications (e.g. infiltration, stormwater ponds), the BMP would not be properly used if pretreatment is ignored. Pretreatment techniques keep a BMP from being overloaded, primarily by sediment. Pretreatment is also required for post-construction practices covered under the Construction Stormwater General Permit.
Sediment, trash, debris, and organic materials found in stormwater runoff can clog and significantly affect the functionality of structural stormwater best management practices (BMPs). Reducing these burdens prior to entering structural stormwater BMP(s) will preserve their long-term functionality, particularly for filtration and infiltration BMPs. Pretreatment reduces maintenance and prolongs the lifespan of structural stormwater BMPs by removing trash, debris, organic materials, coarse sediments, and associated pollutants prior to entering structural stormwater BMPs. Implementing pretreatment devices also improves aesthetics by capturing debris in focused or hidden areas more amenable for removal.
Pretreatment practices serve an important role in the stormwater treatment network. They are installed immediately preceding one or more structural stormwater BMPs and are designed with consideration of the flow network and the downstream structural stormwater BMP characteristics. Pretreatment is highly recommended as an integral part of all post-construction structural stormwater BMPs and is required as part of Minnesota’s NPDES/SDS Construction Stormwater (CSW) General Permit for filtration/infiltration water quality BMPs (section III.D.1.d). Pretreatment practices are NOT stand-alone treatment practices and should only be installed in conjunction with a treatment practice immediately downstream.
Pretreatment practices include settling devices, screens, and pretreatment vegetated filter strips. Selecting the appropriate pretreatment device is critical and is primarily based on the downstream structural BMP, and the contributing drainage area (size, land use, underlying soils, trees/vegetation, etc.). The variety of pretreatment methods and flexibility of design allows for site-specific utilization of the most applicable pretreatment practice. It is recommended that pretreatment practices be designed to be easily maintained and capture a minimum of 25% of the sediment from runoff. Pollutants are captured primarily by physical screening or sedimentation/settling. The figure to the right shows the spectrum of particles sizes and classes and typical ranges of treatment for pretreatment, conventional structural stormwater BMPs, and structural stormwater BMPs that incorporate advanced treatment processes.
Pretreatment practices capture solids that are quickly settled or screened, including gross solids and most sand particles (roughly 100 microns (μm) and larger), although some pretreatment practices also capture floatables. In many watersheds, this material accounts for a large portion of the total pollutant load. Installing multiple pretreatment practices of the same type in series rarely increases performance because the pollutants will be captured by the first practice (if properly designed and maintained) and subsequent pretreatment practices will not be effective. Structural stormwater BMPs such as wet ponds and filtration practices generally capture large silts in addition to sands and gross solids, approximately down to 10 μm-sized particles. To capture fine silts, clays, and dissolved or colloidal pollutants, structural stormwater BMPs with advanced treatment such as infiltration, chemical reactions, or biodegradation must be used.
There are several recommended pretreatment practices that are compatible with structural stormwater BMPs. Differences in each pretreatment practice should be understood to ensure the selected practice is appropriate to the site, the downstream structural stormwater BMP, and maintenance capabilities of the individual or organization responsible for maintenance. The pretreatment descriptions presented below and in the following tables provide assistance in the selection of the proper pretreatment practice for the site-specific loading condition and proposed structural stormwater BMP. The descriptions highlight the types of pretreatment practices, their mechanism of pollutant removal, and their differences are described below.
Pretreatment settling devices are flow-through structures or devices, proprietary or non-proprietary, above or below ground, where settling is the primary mechanism of pollutant removal. Some of these devices also provide treatment in addition to settling by utilizing a variety of mechanisms to separate and capture pollutant-laden material. Aliases for these devices include water quality inlets, flow-through devices, hydrodynamic separators, grit chambers, forebays, sump manholes, and other descriptors.
Pretreatment screens are small catch basins or conveyance trenches in which screening is the primary mechanism of pollutant removal. These pretreatment devices use a perforated plate or mesh screen to separate and collect sediment, trash, debris and organic material as runoff passes over or through them. Screens often comprise thin sheets of metal, plastic, or fabric (e.g., geotextile) with holes or slots that allow water to pass through but limit particulate pollutant passage by deflection or sieving.
Pretreatment vegetated filter strips, also sometimes called buffer strips or buffers, are sloped surfaces that rely on shallow (i.e., water level less than the height of the vegetation), distributed flow through dense vegetation to reduce flow velocity, allow particles to settle, allow particle interception by the vegetation as their primary mechanism of pollutant removal. Pretreatment vegetated filter strips are not to be confused with treatment vegetated filter strips, which are designed and used as standalone structural stormwater BMPs. Concentrated or channelized flow is common to structural stormwater BMP treatment swales, but is not appropriate for pretreatment vegetated filter strips.
Pretreatment as defined above relies upon two primary treatment mechanisms (1) settling and/or (2) physical screening. Settling refers to the method of slowing flow velocities and temporarily detaining flow to allow solids denser than water to settle out of the flow. Screening refers to the method of removing solids by a porous material with a minimal thickness, such as a single sheet of metal with holes in it, as water flows through it, or forcing water to flow through submerged outlets to capture floatables.
The table below provides a general summary of the characteristics for the three types of pretreatment practice.
Pretreatment practice summary of characteristics
Link to this table
|Pretreatment practice||Mechanism of pollutant removal||Relative pollutant removal||Capital cost||Relative maintenance frequency||Relative maintenance effort||Relative space requirements|
|Pretreatment settling devices||Screening & settling||Medium||Medium to High||Medium1||Low to Medium||Low to Medium|
|Pretreatment vegetated filter strips||Screening & Settling||Medium||Low||Low||High||High|
1 Pretreatment settling devices are prone to plugging, which will drive maintenance costs up.
Although cost is variable depending on the specific site properties and the size of the pretreatment practice, the above table provides general guidance for selecting a pretreatment practice based on capital cost. In addition to capital costs, land value cost and lifecycle maintenance costs should be considered when selecting a pretreatment practice. The capital cost is the cost of purchasing the practice and/or the costs to implement the practice which will vary based on site conditions, type of practice, and size of the practice. Land value costs will be highly variable and largely dependent upon the potential of the site to generate revenue or other benefits within the pretreatment practice footprint. Maintenance costs are a long-term cost representing the combined frequency of maintenance and the effort required to carry out the maintenance, including acquisition of maintenance equipment, personnel training, labor, and disposal of accumulated sediment and debris. Maintenance costs should also consider how the pretreatment practice reduces maintenance costs in the structural stormwater BMP. To more fully understand the entire cost of installing one pretreatment practice from another, all three costs (capital, land value, and maintenance) should be considered on a site by site basis.
Some factors that may influence cost include the following.
Specific pretreatment practices may be more or less applicable under certain conditions. The table below provides a general summary of the applicability for the three types of pretreatment practice.
Pretreatment practice applicability
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|Pretreatment Practice||Cold Climate Suitability||Retrofit suitability||Suitability for ultra-urban settings|
|Pretreatment settling devices||Medium to High||Medium||Low to High|
|Pretreatment vegetated filter strips||Medium||Low to Medium||Low|
Pretreatment compatibility with structural stormwater BMPs is wide ranging and summarized in the following table. Site specific characteristics will help guide which practice is best suited in a given application. Factors such as size, topography, site use, pollutant type, and flow (overland/concentrated) will often distinguish which pretreatment practice is best suited for that BMP or site. Taking into consideration the most common type of pollutant (large debris, small debris, hydrocarbons, etc.), space limitations, topography, and receiving flow type will lead to the best-suited pretreatment practice at a given site. Assuming no other limitations, some examples include the following.
Pretreatment practice compatibility with downstream structural stormwater BMPs
Link to this table
|Structural treatment practices||Pretreatment settling devices||Pretreatment screens||Pretreatment vegetated filter strips|
|Bioinfilitration/bioretention with no underdrain (rain garden)||X||X||X|
|Permeable Pavement with no underdrain||(As elevations allow)||X|
|Tree Trench/Tree Box with no underdrain||X||X||X|
|Stormwater Re-use and rainwater harvesting||X||X||X|
|Biofiltration/Bioretention with an underdrain||X||X||X|
|Permeable Pavement with an underdrain||(As elevations allow)||X|
|Tree Trench/Tree Box with an underdrain||X||X||X|
|Iron Enhanced Sand Filter||X||X||X|
|Stormwater Re-use and rainwater harvesting||X||X||X|