It is important to tailor a pretreatment practice to the specific site and type of receiving structural stormwater BMP. Many factors influence the choice of pretreatment practices, including but not limited to: (1) contributing area characteristics (drainage area, connected imperviousness, land uses, soils, slopes, dominant vegetation, source controls in place, and availability of public lands ); (2) existing infrastructure (above and below ground); and (3) type of structural stormwater BMP with infiltration/filtration BMPs having that have the greatest need for removal of solids prior to treatment areas. The following section describes the specific strengths, weaknesses, and applicability of pretreatment settling devices.
Pretreatment vegetated filter strips are designed to provide a specific set of processes to treat stormwater runoff prior to entering a structural stormwater BMP, including sedimentation, screening (by vegetation), and biological processing (transpiration, uptake, rhizospheric biodegradation). Pretreatment vegetated filter strips provide very limited (due to vegetated filter strip size) volume reduction, peak flow reduction, and infiltration. Pretreatment vegetated filter strips are especially effective at capturing excess sediment in stormwater runoff by settling solids. Stormwater management processes not provided in pretreatment vegetated filter strips include filtration and sorption.
The applicability of pretreatment vegetated filter strips with regard to the cold climate, retrofits, ultra-urban settings and other considerations is presented below.
During winter months, pretreatment vegetated filter strips will become frozen and covered to some extent by ice and snow. This may diminish the effectiveness of the practice if flows become channelized resulting in reduced trapping of sediments and reduced infiltration potential. Once the snow and ice melts, filter strips can provide a degree of snowmelt treatment. Pretreatment vegetated filter strips may also be less effective during cold climate conditions due to reduced biological activity (dormant vegetation), reduced settling velocities, reduced infiltration and snow management practices. In rare instances, invasive annual vegetation may reduce perennial covers and result in periods of unestablished vegetation during periods of runoff, particularly during winter thaws and spring snowmelt events, resulting in less pretreatment. Snow storage on pretreatment vegetated filter strips is not recommended.
Pretreatment vegetated filter strips tend to require more space than other pretreatment practices, which often limits retrofit suitability.
Pretreatment vegetated filter strips tend to require more space than other pretreatment practices, and thus they are often not suitable in ultra-urban settings where space is limited.
It is important that channelized inflows to the pretreatment vegetated filter strips be avoided as concentrated flows can reduce the effectiveness of the practice. Footprint area may be a constraint as pretreatment vegetated filter strips can require more surface area than other pretreatment practices. If a pretreatment vegetated filter strip is designed to infiltrate water, it shall be designed as a structural stormwater BMP
If designed properly, this practice meets the intent of the stormwater permit as identified in section III.D.1.D as pretreatment for an infiltration or filtration system. Section III.D.1.D in stormwater construction permit reads:
To prevent clogging of the infiltration or filtration system, the Permittee(s) must use a pretreatment device such as a vegetated filter strip, small sedimentation basin, or water quality inlet (e.g., grit chamber) to settle particulates before the stormwater discharges into the infiltration or filtration system.
The advantages and limitations vegetated filter strips with regard to the pollutant removal capabilities, cost, ease of construction and maintenance, space and other design considerations and compatibility are discussed below.
Pretreatment vegetated filter strips provide many advantages as a pretreatment practice. They effectively remove sediments from stormwater runoff and allow some degree of infiltration as a function of their smaller size to treat excess nutrients. The construction of this practice may also be relatively simple compared to other pretreatment practices. It may require a level spreader along with establishment of vegetative cover including periodic vegetation maintenance. Pretreatment vegetated filter strips are often easily accessible which makes maintenance more manageable and often have less non-routine maintenance costs (e.g. do not require the use of heavy equipment). Vegetated filter strips may offer aesthetic aspects enhancing stormwater designs desired in green infrastructure settings.
Pretreatment vegetated filter strips have limitations. This practice cannot treat runoff unless conveyed in shallow, distributed sheet flow. To maintain their effectiveness, pretreatment vegetated filter strips require regular routine inspection and maintenance to remove accumulated sediment and debris that can lead to bypassing or cause channelization of erosive flows through the vegetated filter strip. Sites requiring re-sloping/grading and imported specification soils may incur additional construction costs due to the use of heavy equipment and associated labor. High loadings of pollutants might damage the vegetation (e.g., vegetation adjacent to a roadway or parking lot with high application rates of deicers). Proper siting and design will improve long-term performance and reduce maintenance expenses. Although pretreatment vegetated filter strips are suitable for many receiving structural stormwater BMPs, space and site requirements may limit the applicability of this practice.
Pretreatment vegetated filter strips may provide limited volume reduction. Pretreatment vegetated filter strips are not to be confused with the structural stormwater BMP treatment vegetated filter strips, which are designed and used as a standalone structural stormwater BMP to reduce stormwater runoff volumes.
Properly designed pretreatment vegetated filter strips slow runoff velocities and allow sediment to settle. Pretreatment vegetated filter strips can also remove portions of other pollutants in runoff, including small particulates, hydrocarbons, heavy metals and nutrients such as phosphorus and nitrogen. Filter strips remove pollutant load through sedimentation, filtration, infiltration, biological uptake, and microbial activity.
The water quality benefit of a vegetated filter strip is dependent upon factors such as the length of the flow path and slope, stormwater flow rate through the vegetated filter strip, the type, density, and length of vegetation, the ability to maintain sheet flow through the vegetated filter strip and the soil properties that influence infiltration rate. The ability to maintain sheet flow is often cited as a crucial factor for the success of filter strips (lakesuperiorstreams.org; Virginia DEQ, 1999)
In general, pretreatment vegetated filter strips are very effective at removing solids from stormwater if there is adequate flow path length and if sheet flow is maintained through the BMP with low rate of flow (Goel et al., 2004; Abu-Zreig et al., 2003; Gharabaghi et al., 2000).
The following discussion provides a summary of information found in the literature. The information provides a general summary and indication of the range of pollutant removal and should not be used for design purposes.
Solids removal can vary between 50 and 98 percent depending on filter strip characteristics (NJ DEP, 2014; Goel et al., 2004; Abu-Zreig et al., 2003; Gharabaghi et al., 2000). Gharabaghi et al., 2000 found that 50 percent of sediments settled in the first 2.5 meters of a vegetated filter strip and an additional 25 to 45 percent of sediments settled in the next 2.5 meters. They also concluded that increased solids removal was minimal in flow path lengths greater than 10 meters. Some stormwater manuals require a minimum filter strip length of 10 feet with a slope of less than 2 percent (Inver Grove Heights, 2006; Wisconsin Department of Natural Resources, 2014). Other manuals specify that the minimum length for filter strips is 25 feet with a slope of less than 2 percent (Virginia DEQ, 1999), and requires four feet be added to the length for each 1 percent of slope greater than 2 percent. Virginia DEQ (1999) suggests that an ideal filter strip length is between 80 and 100 feet.
Assuming no infiltration, phosphorus removal is due to settling of solids and ranges from 32 to 79 percent based on the length of the flow path (Abu-Zreig et al., 2003). Some removal of dissolved nutrients occurs if the filter strip is designed to infiltrate water.