Bioretention: Filter media

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Sandy filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. mix.
Filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. being used for an online bioretention swale (also of previous, more sandy specification)

It is recommended that the mixture comprises:

One of these two blend options
Blend A: Drainage rate priority Blend B: Water quality treatment priority
Application ImperviousA hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil. area to pervious area (I:P) ratio of 15:1 or greater
Proportions

3 parts sand
1 part organic soil components and additives

3 parts sand
2 parts topsoil
1 part organic soil components and additives

PorosityThe porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e) This mixture may be assumed to have a porosityThe porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e) of 0.4 unless demonstrated otherwise This mixture may be assumed to have a porosityThe porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e) of 0.35 unless demonstrated otherwise

Filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. should be obtained premixed from a vendor and meet all municipal, provincial and federal environmental standards. Topsoil used to produce the mix should be passed through a 5 centimetre (2 inch) screen to remove large rocks, roots and other debris, while retaining soil peds. Samples of the filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. should be dried, ground and tested by a certified soil testing laboratory to ensure they meet the following specifications:

BioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.
Characteristic Criterion Recommended test method
Particle-size distribution (PSD) < 25% siltSoil or media particles smaller than sand and larger than clay (3 to 60 m)- and clay1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).-sized particles (smaller than 0.05 mm) combined;
3 to 12% clay1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).-sized particles (0.002 mm or smaller)
ASTM D7928, Standard Test Method for Particle-Size Distribution (Gradation) of Fine-Grained Soils Using the SedimentationDeposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.Settling-out or deposition of particulate matter suspended in runoff. (Hydrometer) Analysis.
Organic matter (OM) 3 to 10% by dry weight ASTM F1647, Standard Test Methods for Organic Matter Content of Athletic Field Rootzone Mixes.
Phosphorus, plant-available or extractable 12 to 40 ppm As measured by the 'Olsen' method for alkaline and calcareous soils (common in Ontario). Alternatives include 'Mehlich I or III', or 'Bray', which is better suited to acidic to slightly alkaline and non-calcareous soils. NB: Results from different test methods are not directly comparable.[1]
Cationic exchange capacity (CEC) > 10 meq/100 g ASTM D7503, Standard Test Methods for Measuring the Exchange Complex and Cation Exchange Capacity of Inorganic Fine-Grained Soils.
Hydraulic conductivityA parameter that describes the capability of a medium to transmit water., saturated (Kf) > 75 mm/h; Blend A
> 25 mm/h; Blend B
< 300 mm/h; Blend A and B
ASTM D2434, Standard Test Method for Permeability of GranularGravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices. Soils (Constant Head), when the sample is compacted to 85% of its maximum dry density in accordance with ASTM D698, Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort.
Note that you may choose not to use particle-size distribution as a criterion for acceptance of a filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. blend, but saturated hydraulic conductivityA parameter that describes the capability of a medium to transmit water. should be one. While information on particle-size distribution and soil texture are useful in selecting plants, the disconnect between hydraulic conductivityA parameter that describes the capability of a medium to transmit water. and uniformity of gradation makes it far less important than measuring the saturated hydraulic conductivityA parameter that describes the capability of a medium to transmit water. directly[2]

SandMineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

Particle size distribution graph for ASTM C33 sandMineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm., as described in table
  • Coarse sandMineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm. for LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. construction shall be washed clean and free of toxic materials.
  • The pH of the sandMineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm. shall be ≤ 7.0.
  • The coarse sandMineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm. shall have a fineness modulus index between 2.8 and 3.1 according to ASTM C33/C33M, or otherwise meet the gradation below.
Particle size distribution
Sieve Percent passing
9.5 mm 100
4.75 mm (No.4) 95 - 100
2.36 mm (No.8) 80 - 100
1.18 mm (No.16) 50 - 85
0.60 mm (No.30) 25 - 50
0.30 mm (No.50) 5 - 30
0.15 mm (No.100) 0 - 10
0.075 mm (No.200) ≤ 3

Topsoil

  • Topsoil may be material that was stripped from the project site and stored in stockpiles for re-use, or material imported to the site from a supplier provided the physical and chemical characteristics are within acceptable ranges.
  • Topsoil shall be in compliance with Ontario Regulation 153/04 Record of Site Condition standards for soil quality or as amended through Ontario Management of Excess Soil - A Guide for Best Management Practices.
  • Soil laboratory reports shall certify the material to be suitable for re-use on residential, parkland, institutional, industrial, commercial, or community landscapes for the germination of seeds and the support of vegetative growth.

The factors to consider in determining if a topsoil is suitable for use as planting soil for a vegetated stormwater practice, or use in producing a bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. mixture include the following:

  • Must be friable and capable of sustaining vigorous plant growth;
  • Must be free from toxic material and roots, stones or debris over 50 mm (2") in diameter;
  • Should not have been passed through sieves or screens smaller than 50 mm (2”) to avoid eliminating peds;
  • Should have a Loamy SandMineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm., Sandy Loam, Sandy Clay1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System). Loam, Loam or Silty Loam soil texture;
  • For use as planting soil for a vegetated stormwater practice , the topsoil must contain a minimum of 5% organic matter by dry weight or be amended so, through addition of an organic soil conditioner;
  • For use in producing bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. Blend B (water quality treatment priority), the topsoil must contain at least 9%, and not greater than 36% clay1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).-sized particles and at least 2% organic matter by dry weight.
  • Must have a pH of between 6.0 and 8.0;
  • Must have a sodium absorption ratio less than 15;
  • Must have a cationic exchange capacity greater than 10 milliequivalents per 100 grams (meq/100 g).

Specify that 4 litre samples of topsoil, from each source to be drawn upon, be provided to the consultant for visual inspection, along with topsoil quality test results from an accredited soil testing laboratory, or a quality assurance certificate from the supplier.

We recommend a planting soil or filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. depth of 300 mm to support grasses, 600 mm for shrubs and perennials, and 1000 mm for trees.

Organic component

This is the first big opportunity to manage phosphorus export from a bioretention or stormwater planter system. While compostDecayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil. is the most common choice, designers working in nutrient-sensitive receiving watersWatercourses and Lake Ontario, to which Stormwater and Combined Sewer Overflows discharge. are encouraged to explore the alternatives listed below. Some of these materials may be combined 50:50 with compostDecayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil. to balance providing the nutrients required by the plants with limiting the potential for leaching of excess nutrients.

CompostDecayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

CompostDecayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil. is the most widely used organic component. It's use in bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. facilities is well established and documented. Low-phosphorus composts should always be sought for use in low impact developmentA stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. facilities, including bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.. These are typically created from feedstocks including yard, leaf, and wood waste, and excluding manures, biosolids, and food scraps.[3]
Compost Specifications

Even low-phosphorus composts are known to export phosphorus over many years. The use of compostDecayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil. is not recommended in nutrient-sensitive watersheds where phosphorus pollution is a concern. There are a number of alternative sources of soil organic matter which have undergone field studies which have benefits and potential concerns:

Organic soil components
Material Benefits Concerns
Coconut coir[4] Doesn't leach phosphorus Must be imported
Wood chips Doesn't leach phosphorus
Promotes nitrogen removal from water
Peat moss Doesn't leach phosphorus Must be extracted from natural wetlands
Shredded paper (e.g., Pittmoss) Doesn't leach phosphorus
Promotes denitrification

Wood derivatives

The 2017 guidance from New Hampshire specifically rules against the inclusion of compostDecayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil. in their bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. media.[5] Instead they recommend "Shredded wood, wood chips, ground bark, or wood waste; of uniform texture and free of stones, sticks". The use of wood chip has been common in New Hampshire for some time, in this 2006 thesis 20% wood chips (not characterized) were incorporated into all of the test cases to match current practices at the time. [6]

Shredded paper has been tested as an additional source of carbon and as an electron-donor to promote denitrification in a number of successful laboratory and field studies. <ref>

Additives

Typically these components would make up 5 to 10% by volume of the filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. mixture.

A number of granularGravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices. amendments have been demonstrated to improve nutrient removal from discharge water in BMPs such as bioretention systems, stormwater planters, absorbent landscapes, sand filters or green roofs.

There are two primary processes involved, chemical precipitationAny form of rain or snow. and adsorptionThe attachment of gas, vapour or dissolved matter onto the surface of solid materials.. Both mechanisms are ultimately finite, but have been shown in come cases to make significant improvements on the discharged water quality over several years.

In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
Lists are ordered alphabetically; link updates are welcomed using the form below.

Filter Media Additives
Material Benefits Potential concerns
Biochar Renewable
Enhances soil aggregation, water holding capacity and organic carbon content
Currently expensive
Energy intensive to produce
Some sources say ineffective for phosphorus removal
Bold & GoldTM Documented total phosphorus removal of up to 71%[7] Proprietary
Iron filings or Zero valent iron (ZVI) Proven phosphorus retention
Retained phosphorus is stable
May harm plants[8]
Removal efficiency declines with increased concentration of incoming phosphorus
Red sand or Iron-enriched sand Proven phosphorus removal
Also removes TSSTotal suspended solids
Poor orthophosphate removal in hypoxic or anoxic conditions
Smart SpongeTM Removes phosphorus, as well as TSSTotal suspended solids, fecal coliform bacteria and heavy metals
Non-leaching
1-3 year lifespan, after which the product is removed as solid waste
Proprietary
Sorbtive MediaTM High phosphorus removal efficiency Proprietary
Water treatment residuals Waste product reuse Quality control (capabilities depend on source, treatment methods, storage time)

  1. Sawyer JE, Mallarino AP. Differentiating and Understanding the Mehlich 3, Bray, and Olsen Soil Phosphorus Tests 1. http://www.agronext.iastate.edu/soilfertility/info/mnconf11_22_99.pdf. Accessed August 1, 2017.
  2. CRC for Water Sensitive Cities. (2015). Adoption Guidelines for Stormwater Biofiltration Systems: Appendix C - Guidelines for filter media in stormwater biofiltration systems.
  3. Hurley S, Shrestha P, Cording A. Nutrient Leaching from Compost: Implications for Bioretention and Other Green Stormwater Infrastructure. J Sustain Water Built Environ. 2017;3(3):4017006. doi:10.1061/JSWBAY.0000821.
  4. Rheaume, A., Hinman, C., and Ahearn, D. (2015). “A Synthesis of Bioretention Research in Pacific Northwest.” Herrera, <http://www.modularwetlands.com/new/wp-content/uploads/2015/11/2-Bioretention-Synthesis-2015-DAhearn.pdf>
  5. UNHSC Bioretention Soil Specification. (2017). Retrieved from https://www.unh.edu/unhsc/sites/default/files/media/unhsc_bsm_spec_2-28-17_0.pdf
  6. Stone, R. M. (2013). Evaluation and Optimization of Bioretention Design for Nitrogen and Phosphorus Removal. University of New Hampshire. Retrieved from https://www.unh.edu/unhsc/sites/unh.edu.unhsc/files/STONE THESIS FINAL.pdf
  7. Hood A, Chopra M, Wanielista M. Assessment of Biosorption Activated Media Under Roadside Swales for the Removal of Phosphorus from Stormwater. Water. 2013;5(1):53-66. doi:10.3390/w5010053.
  8. Logsdon SD, Sauer PA. Iron Filings Cement Engineered Soil Mix. Agron J. 2016;108(4):1753. doi:10.2134/agronj2015.0427.