SuDS have become a logical progression towards simple, low-cost treatment of diffuse non-point pollution. The need for SuDS has become increasingly important as the detrimental effects of urbanization become clear. Specifically, loss of greenspace, habitat and natural infiltration routes results in increased surface water runoff that eventually leads to higher peak flow, erosion and flooding (Brezonik and Stadelmann 2002). This, combined with a build-up of pollutants on impermeable surfaces being washed and accumulating untreated into watercourses, has led to development of the SuDS philosophy, with the overall aim to design systems that mimic natural drainage before development.
The premise of SuDS systems is three-fold: improve water quality, maximise amenity and biodiversity while providing attenuation capacity during high precipitation events (Woods-Ballard et al. 2007). While traditional drainage options may meet certain components of this philosophy, SuDS systems are designed to address all three functions as highlighted by the SuDS triangle (Fig.
1.1).
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Figure 1.1. The SuDS triangle
1.2.1 Types of SuDS
Many types of SuDS exist and their use is dependent on location, taking into account scenarios of hydrological capacity and pollutant load expected.
Comprehensive details of all types of SuDS can be found in the SuDS Manual (Woods-Ballard et al. 2007). The following is a list and description of typical SuDS in place throughout the UK.
• Filter strips – areas of grass or vegetation that treat runoff from adjacent impermeable surfaces.
• Swales – channels of grass or vegetation that allow for storage and conveyance of water and infiltration into the ground
• Infiltration basin – depression of land that stores runoff water and allows infiltration into the ground over time
• Ponds – basins that provide water quality treatment for a permanent source of water as well as providing temporary storage for excess runoff
• Detention basin – normally dry depression of land designed to provide water quality treatment for a for a specific volume of runoff water
• Constructed wetland – ponds with added wetland vegetation for enhanced pollutant removal and wildlife habitat
• Filter drains – trench filled with permeable material allowing for filtration, storage and conveyance of runoff from adjacent impermeable surfaces
• Infiltration device – designed to temporarily store runoff from a development and allow infiltration over time
Water Quality
Water Quantity Biodiversity
The SuDS Triangle
Chapter 1 Introduction ___________________________________________________________________________________
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• Porous pavement – surfaces that allow rainwater to infiltrate through to a storage layer for subsequent infiltration to the ground
• Sand filters – structure filled with sand that allows for treatment of surface water through filtration and temporary storage via surface ponding
• Bioretention – shallow landscaped areas with underdrainage and engineered soils and vegetation aimed towards enhancing pollutant removal and reducing runoff
• Green roofs – roofs with a cover of vegetation over a drainage layer
1.2.2. SuDS Performance
All types of SuDS benefit from a variety of pollutant removal mechanisms for improved water quality, though treatment capacity of the systems is not well defined. There are numerous reasons for this including limited field data available and over extended periods of time (Scholes et al. 2008), efficiency being highly dependent on design and location, and a lack of understanding of mechanisms at a fundamental level. Because of this, many removal efficiencies of target pollutants in SuDS systems are estimated and listed as simply high, medium or low (Claytor and Schuleler 1996). An example of the range of pollutant removal capacities of different types of SuDS design is shown in Table 1.1 as adapted from the U.S. EPA Handbook on Urban Runoff Pollution Prevention and Control Planning. This high level of uncertainty has led to the recommendation that several types of SuDS, or a ‘treatment train’, be utilized so that the level of redundancy in treatment assures removal over a series of SuDS (Pittner and Allerton 2009). While this philosophy may be effective, it is believed that a better understanding of removal mechanisms and thus removal capacities of SuDS systems can lead to better SuDS design.
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Table 1.1. Range of pollutant removal percentages for SuDS US EPA (1993)
For the sake of this research, focus will be narrowed to filtration based filter drains (Fig 1.2) in order to examine pollutant removal mechanisms typically associated with low-cost potable water treatment systems for SuDS applications.
Filter drains are trenches filled with gravel filter media intended to store and treat runoff from the adjacent roadway. Critical to road runoff is the drains potential to filter and treat vehicular pollutants including suspended solids, polycyclic aromatic hydrocarbons (PAHs), and an array of heavy metals (Ward 1990; Liu et al. 2001; Liu et al. 2005; Seelsaen et al. 2006; Genc-Fuhrman et al.
2007; Gan et al. 2008) at concentrations above regulatory limit. Thus, in the United Kingdom, treatment via SuDS is mandatory prior to discharge into nearby watercourses. It is therefore not surprising that filter drains are increasingly being fitted for urban drainage schemes, highlighting their widespread use even though an understanding of pollutant treatment mechanisms and performance is limited.
Figure 1.2. Schematic of a filter drain (Netregs.org.uk) and photo of a filter drain
Typical Pollutant Removal (percent)
SuDS Type Suspended Solids Nitrogen Phosphorus Pathogens Metals
Detention Basin 30 - 65 15 - 45 15 - 45 < 30 15 - 45
Pond 50 - 80 30 - 65 30 - 65 < 30 50 - 80
Constructed Wetland 50 - 80 < 30 15 - 45 < 30 50 - 80
Infiltration Basin 50 - 80 50 - 80 50 - 80 65 - 100 50 - 80
Filter Drain 50 - 80 50 - 80 15 - 45 65 - 100 50 - 80
Porous Pavement 65 - 100 65 - 100 30 - 65 65 - 100 65 - 100
Swales 30 - 65 15 - 45 15 - 45 < 30 15 - 45
Filter Strips 50 - 80 50 - 80 50 - 80 < 30 30 - 65
Sand Filter 50 - 80 < 30 50 - 80 < 30 50 - 80
Other Media Filter 65 - 100 15 - 45 < 30 < 30 50 - 80
Chapter 1 Introduction ___________________________________________________________________________________
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Pratt (2004) summarized the initial research into filter drain function which highlighted that Perry and McIntyre (1986) determined a working filter drain parallel to the M1 motorway significantly reduced effluent pollutant concentrations when compared to untreated runoff but efficiency varied between storm events and seasons. Subsequent research by Sansalone (1999) was carried out to compare performance of bench-scale experiments to a field partial exfiltration trench which combines porous pavement and porous media in a filter drain. While it was demonstrated that the trench could be used as an effective trap for suspended solids, breakthrough of particulate-bound heavy metals was found to be a controlling factor in design life. The SuDS Manual (Woods-Ballard et al., 2007) lists the pollution removal of filter drains as high for heavy metals and suspended solids and low to medium for nutrients. As with most published research on filter based SuDS, they are listed as a promising pollutant removal system, especially for particulate pollutants (Claytor and Schuleler 1996) though most research highlights a that a high clogging potential and poor maintenance are the main disadvantages of filter based SuDS systems (Jefferies 2004). While the clogging potential will influence the lifespan of filter drains, Hatt et al. (2007) demonstrated that the treatment capacity of gravel filter media for stormwater treatment remains high up until the point of clogging, and that a 0.5m depth can be effective for treating suspended solids and heavy metals, but not effective in treating nutrients, corroborating with the SuDS manual. While previous SuDS studies have demonstrated effective treatment of metals, the specific geochemical removal mechanisms and effect of lithology and biofilm growth has not yet been addressed.