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 priority Blend B: Water quality priority
Application Higher I/P ratioThe ratio of the catchment (impervious area) to the footprint area of the receiving BMP (pervious area).
Proportions

3 parts sand
1 part organic soil components and additives

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

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. Mixing of 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., topsoil and 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. should be done in a manner that preserves topsoil peds. The mixture should be free of stones, stumps, roots, or other debris larger than 50 mm diameter. 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 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 method
Texture <15 % finesSoil particles with a diameter less than 0.050 mm. Hygrometer
Organic matter (OM) 5 - 10 % ASTM D2974-14, Standard test methods for moisture, ash and organic matter of peat and other organic soils.
Phosphorus 12 - 40 ppm As measured by the 'Bray' method. Alternatives include 'Mehlich I or III', or 'Olsen'. Results from these are not directly translatable.[1]
Cationic exchange capacity(CEC) 10 meq/100 g ASTM D7503-10, Standard test method for measuring the exchange complex and cation exchange capacity of inorganic fine grained soils.
Hydraulic conductivity > 25 mm/hr
< 250 mm/hr
Falling head or constant head KSAT
Note that grain size distribution does not form part of the recommended acceptance criteria. Whilst this information may be useful in designing blends, the disconnect between hydraulic conductivityA parameter that describes the capability of a medium to transmit water. and the uniformity of gradation makes it far less important then measuring the 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 fro 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 LIDA 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 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.
  • Manufactured 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. from limestone or dolostone parent material is not acceptable.
  • 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. manufactured from crushed recycled glass is available locally, and is a viable alternative.
  • 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 a suitable soil mix for a vegetated stormwater practice include the following:

Specify that topsoil must be friable, neither heavy clay nor of a very light sandy nature.

An example of sandy loam topsoil is

  • 60 % 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.,
  • 25 % siltSoil or media particles smaller than sand and larger than clay (3 to 60 m),
  • 10 % clay,
  • organic matter 5 %, and
  • pH value of 6 - 7.5.

Topsoil must be capable of sustaining vigorous plant growth and to be free from subsoil, roots, vegetation, debris, toxic materials and stone over 50 mm diameter. Specify that topsoil sample must be provided to the consultant for testing and analysis including herbicide or atrazine content. All topsoil supplied must conform to a sample provided. Minimum topsoil depth is 150 mm for turf areas and ranging to 1.25 m for perennials, shrubs and trees.

Organic components

This is the first big opportunity to manage phosphorus export from a bioretention or stormwater planter system. Whilst 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 ingredient, designers working in sensitive watersheds 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 the nutrients required by the plants and the potential for leaching of excess nutrient.

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 must exclude 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 watersheds for which phosphorus pollution is a concern. There are alternatives which have undergone field study, each of which has a number of benefits and potential concerns:

Organic soil components
Material Benefits Concerns
Coconut coir[4] Doesn't leach phosphorus Requires importation
Wood chip Doesn't leach phosphorus
Promotes nitrogen removal from water
-
Peat Moss Doesn't leach phosphorus Sustainability controversial
Shredded paper [5] (see: Pittmoss)

Wood derivatives

https://www.unh.edu/unhsc/sites/default/files/media/unhsc_bsm_spec_2-28-17_0.pdf https://www.unh.edu/unhsc/sites/unh.edu.unhsc/files/STONE%20THESIS%20FINAL.pdf https://jbioleng.biomedcentral.com/articles/10.1186/s13036-017-0057-4 (focus on denitrification)

Additives

Typically these components would make up 5- 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 BMPsThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. 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.

Soil 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%[6] Proprietary
Fly ash[7]
Iron filings (ZVI) Proven phosphorus retention
Retained phosphorus is stable
May harm plants[8]
Removal efficiency declines with increased concentration of incoming phosphorus
Red 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, etc of WTR)

  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. Urban Drainage and Flood Control District. (2010). “Bioretention.” <http://udfcd.org/criteria-manual/volume-3/t-03-bioretention/> (Mar. 15, 2018).
  6. 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.
  7. Brown, G., Vogel, J., and Storm, D. (2016). “Using Fly Ash in Bioretention Cells to Remove Phosphorus from Stormwater.” <http://rgvstormwater.org/wp-content/uploads/2016/09/Fly-Ash-Brown.pdf>.
  8. Logsdon SD, Sauer PA. Iron Filings Cement Engineered Soil Mix. Agron J. 2016;108(4):1753. doi:10.2134/agronj2015.0427.