Bioretention: Filter media
It is recommended that the mixture comprises:
|Blend A: Drainage rate priority||Blend B: Water quality treatment priority|
|Application||Impervious area to pervious area (I:P) ratio of 15:1 or greater|
|Porosity||This mixture may be assumed to have a porosity of 0.4 unless demonstrated otherwise||This mixture may be assumed to have a porosity of 0.35 unless demonstrated otherwise|
Filter media 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 media should be dried, ground and tested by a certified soil testing laboratory to ensure they meet the following specifications:
|Characteristic||Criterion||Recommended test method|
|Particle-size distribution (PSD)||< 25% silt- and clay-sized particles (smaller than 0.05 mm) combined;
3 to 12% clay-sized particles (0.002 mm or smaller)
|ASTM D6913/D6913M, Standard Test Methods for Particle-Size Distribution (Gradation) of Soils Using Sieve Analysis (pebble to sand fraction); and |
ASTM D7928, Standard Test Method for Particle-Size Distribution (Gradation) of Fine-Grained Soils Using the Sedimentation (Hydrometer) Analysis (silt and clay fraction).
|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.|
|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 conductivity, 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 Granular 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 media blend, but saturated hydraulic conductivity should be one. While information on particle-size distribution and soil texture are useful in selecting plants, the disconnect between hydraulic conductivity and uniformity of gradation makes it far less important than measuring the saturated hydraulic conductivity directly
- Coarse sand for LID construction shall be washed clean and free of toxic materials.
- The pH of the sand shall be ≤ 7.0.
- The coarse sand shall have a fineness modulus index between 2.8 and 3.1 according to ASTM C33/C33M, or otherwise meet the gradation below.
|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 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 bioretention filter media 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 Sand, Sandy Loam, Sandy Clay 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 bioretention filter media Blend B (water quality treatment priority), the topsoil must contain at least 9%, and not greater than 36% clay-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 media depth of 300 mm to support grasses, 600 mm for shrubs and perennials, and 1000 mm for trees.
This is the first big opportunity to manage phosphorus export from a bioretention or stormwater planter system. While compost is the most common choice, designers working in nutrient-sensitive receiving waters are encouraged to explore the alternatives listed below. Some of these materials may be combined 50:50 with compost to balance providing the nutrients required by the plants with limiting the potential for leaching of excess nutrients.
Compost is the most widely used organic component. It's use in bioretention facilities is well established and documented. In Ontario, compost should comply with mandatory Ontario Compost Quality Standards for Category 'AA'. See Compost page for a summary of Category 'AA" compost standards. Compost should also be certified to meet quality parameters recommended under the Compost Council of Canada Compost Quality Alliance (CQA) program. Low available phosphorus composts should always be sought for use in low impact development facilities, including bioretention. Low available phosphorus composts are typically created from feedstocks including yard, leaf, and wood waste, and excluding manures, biosolids, and food scraps.
Organic component alternatives
Even low-phosphorus composts are known to export phosphorus over many years. The use of compost is not recommended in nutrient-sensitive watersheds where phosphorus pollution is a concern, or an additive to enhance nutrient retention of the media should also be included in the blend, or a layer be included above the stone reservoir, or a reactive media vault be included in the treatment train downstream of the bioretention (see Additive below for available options). There are a number of alternative sources of soil organic matter which have undergone field studies which have benefits and potential concerns:
|Coconut coir||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 unsustainably extracted from natural wetlands|
|Shredded paper (e.g., Pittmoss)||Doesn't leach phosphorus
The 2017 guidance from New Hampshire specifically rules against the inclusion of compost in their bioretention media. 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. 
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>
Typically these components would make up 5 to 10% by volume of the filter media mixture.
A number of granular 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 precipitation and adsorption. 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.
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%||Proprietary|
|Iron filings or Zero valent iron (ZVI)||Proven phosphorus retention
Retained phosphorus is stable
|May harm plants |
Removal efficiency declines with increased concentration of incoming phosphorus
|Red sand or Iron-enriched sand||Proven phosphorus removal
Also removes TSS
|Poor orthophosphate removal in hypoxic or anoxic conditions|
|Smart SpongeTM||Removes phosphorus, as well as TSS, fecal coliform bacteria and heavy metals
|1-3 year lifespan, after which the product is removed as solid waste |
|Sorbtive MediaTM||High phosphorus removal efficiency||Proprietary|
|Water treatment residuals||Waste product reuse||Quality control (capabilities depend on source, treatment methods, storage time)|
- 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.
- CRC for Water Sensitive Cities. (2015). Adoption Guidelines for Stormwater Biofiltration Systems: Appendix C - Guidelines for filter media in stormwater biofiltration systems.
- Ontario Ministry of the Environment and Climate Change (OMOECC). 2012. Ontario Compost Quality Standards, July 25, 2012. PIBS 8412. Queen’s Printer of Ontario, Toronto, ON. https://www.ontario.ca/page/ontario-compost-quality-standards.
- A & L Canada Laboratories. 2004. Compost Management Program. London, ON. http://www.alcanada.com/index_htm_files/compost_handbook.pdf.
- 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.
- 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>
- UNHSC Bioretention Soil Specification. (2017). Retrieved from https://www.unh.edu/unhsc/sites/default/files/media/unhsc_bsm_spec_2-28-17_0.pdf
- 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
- 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.
- Logsdon SD, Sauer PA. Iron Filings Cement Engineered Soil Mix. Agron J. 2016;108(4):1753. doi:10.2134/agronj2015.0427.