Bioretention: Performances

TSS Reduction

The performance results for Bioretention practices, located within TRCA's watershed originate from six primary sites:

• Kortright Centre
• Earth Rangers
• Seneca College
• Central Parkway (Mississauga)
• Wychwood Subdivision (Brampton)
• IMAX Corporation, head office

The mean performance value recorded at the outlet for Bioretention practices' ability to remove Total Suspended Sediments (TSS) was was calculated based on 301 separate recordings between 2005-2007, 2011-2019 and 2021 amongst the six sites previously mentioned.

As can be seen in the corresponding boxplot the mean performance removal efficiency of the bioretention practices monitored are well below the suggested guideline of 30 mg/L (Canadian Water Quality Guideline (CWQG), or (background (assumed at <5 mg/L)+ 25 mg/L for short term (<24 hour) exposure) (CCME, 2002; (TRCA, 2021).

The median value of the 301 samples taken was 9.50 mg/L whereas the mean was 18.63 mg/L, with a 15% guideline exceedance.

Phosphorus Reduction

The performance results for Bioretention practices, located within TRCA's watershed originate from seven primary sites:

• Kortright Centre
• Earth Rangers
• Seneca College
• Aurora Community Centre
• Central Parkway (Mississauga)
• Wychwood Subdivision (Brampton)
• IMAX Corporation, head office

The mean performance value recorded at the outlet for Bioretention practices' ability to remove Total Phosphorus (TP) was calculated based on 355 separate recordings between 2005-2007, 2011-2019 and 2021 amongst the seven sites previously mentioned.

As can be seen in the corresponding boxplot the mean performance removal efficiency of the bioretention practices monitored are not meeting the acceptable upper extent range of nutrients as of 0.03 mg/L (30 µg/L) (Environment Canada, 2004; OMOEE, 1994).

The median value of the 355 samples taken was 0.09 mg/L whereas the mean was 0.12 mg/L, with an 86% guideline exceedance. Given the age of most of these practices, more inspection, maintenance and necessary rehabilitation will be needed to ensure they are able to meet the federal and provincial governments' guideline requirement for stormwater quality.

Please refer to the Phosphorus page and the additives page for more information on how LIDs can reduce contaminant loading in stormwater

Recent Performance Research

Performance of bioretention with internal water storage

Location	Filter media composition	Media depth (cm)	Internal water storage depth (cm)	I/P ratio	Runoff volume reduction (%)	TSS reduction (%)	TN reduction (%)	TP reduction (%)
Montréal	88% sand, 8% fines, 4% OM	180	150	47	97	99	99	99
Virginia	88% sand, 8% fines, 4% OM	180	150	47	97	99	99	99
North Carolina	96% sand, 4% fines	110	88	12	89	58	58	-10
			58		93
		96	72	13	98
			42		100
North Carolina	loamy sand, 3% OM	120	60	20	99	-	-	-
North Carolina	98% sand, 2% fines	90	30	12	90	-	-	-
		90	60	12	98	-	-	-
North Carolina	15% sand, 80% fines, 5% OM	60	45	68	-	-	54	63
		90	75	68	-	-	54	58

• (USEPA, 2013) - Evaluation and Optimization of Bioretention Design for Nitrogen and Phosphorus Removal
  • USEPA conducted both field and laboratory testing on the performance of bioretention cells featuring filter media amended with drinking water treatment residuals (WTR) with low solids content (5-10% solids). Water treatment residuals were included at 10-15% of the total filter media mix by volume. Amended bioretention cells had median orthophosphate removal efficiencies of 90-99%. A second study found a bioretention design featuring WTR amended filter media and an internal water storage zone optimized to remove phosphorus and nitrogen had an orthophosphate removal efficiency of 20% and effluent concentrations below 0.02 mg/L.

Comparison of an Enhanced dephosphorization bioretention cell (EBC) (above) vs. a traditional bioretention cell (TBC) (below). The EBC includes evenly spaced apart soil mixture layers, which includes 70-80% native soil found on site mixed with 20-30% of charcoal, oregani matter and iron, along with permeable layers of gravel pumice and zeolite, all of which help adsorb phosphates out of stormwater entering the system. This differs from the TBC design which generally includes just a gravel bed to aid in the facility's drainage ability (Ho and Lin, 2022).

• (Ho and Lin, 2022) - Pollutant Removal Efficiency of a Bioretention Cell with Enhanced Dephosphorization
  • Authors Ho and Lin, 2022 note that bioretention practices perform poorly in reducing phosphorus from influent stormwater when compared to their ability to remove ammonia and COD pollutants. The authors tested a new type of enhanced dephosphorization bioretention cell (EBC) which improves phosphorus removal performance. The difference between EBC and a traditional bioretention cell is that the lowest level of an EBC feature is comprised of a mixed fill material layer (permeable layers - PLs and soil mixed layers - SMLs) instead of a traditional gravel bed layer. The SMLs include active charcoal powder, organic matter and iron, evenly spaced apart, while the PLs include aggregates of gravel, pumice and zeolite. Over the two years that the same sized EBC feature was monitored in comparison to a standard bioretention cell they found that the EBC outperformed the traditional bioretention cell by removing 92% of total phosphorus to 52%. The average inflow concentration for both features from May 2019 - April 2021 was 0.76 mg/L, whereas the outflow concentration averages were 0.36 mg/L for the traditional bioretention cell and 0.06 mg/L for the EBC, respectively (Ho and Lin, 2022).

• (STEP, 2019) - Improving nutrient retention in bioretention - Technical Brief
  • STEP researchers developed a study to examine the effectiveness of reactive media amendments as a means of enhancing phosphorus retention in a bioretention cell draining a 1150 m² parking lot in the City of Vaughan. For testing purposes, the bioretention was divided into three hydrologically distinct cells: (1) with a high sand, low phosphorus media mix (control); (2) with a proprietary reactive media (Sorbitve™) mixed into the sandy filter media, and (3) with a 170 cm layer of iron rich sand (aka red sand) below the sandy filter media. Outflow quantity and quality from each cell was measured directly, while inflows and runoff quality were estimated based on monitoring of an adjacent asphalt reference site over the same time period. The results found that the Sorbitve™ and the Iron rich (red) sand cells had lower concentrations of Total Phosphorus (among other contaminants) in its effluent outflow, and the TP measured was below the CCDME guideline of 0.03mg/L in both years monitored for Sorbitve™ (2016 & 2017) and 2017 for the cell with Iron rich (red) sand. Both cells had median concentrations lower than the control media cell used in the study by at least 68% for TP (STEP, 2019.

• (Ament, et al. 2022) - Phosphorus removal, metals dynamics, and hydraulics in stormwater bioretention systems amended with drinking water treatment residuals
  • Researchers from the University of Minnesota, the University of Vermont and the USEPA, conducted field experiment to test the effectiveness of Drinking water treatment residuals (DWTRs) as a filter media amendment additive for improve Total Phosphorus (TP) removal in roadside bioretention features. Influent phosphorus levels was relatively low when compared to normal influent stormwater P levels (dissolved = 0.002 mg/L, soluble reactive = 0.022, particulate = 0.036 mg/L) but the difference between the bioretention cell in the study with DWTR additives and the control bioretention cells were 95% (Large D.A) - 97% (small D.A) TP removal and 79 (large D.A)and 91% (small D.A) respectively. The outflows were well below the CCME guidelines of 0.3 mg/L coming in at 0.010 mg/L (large D.A) and 0.011mg/L (small D.A) (Ament, et al. 2022).)

• (Qiu, et al. 2019) - Enhanced Nutrients Removal in Bioretention Systems Modified with Water Treatment Residual and Internal Water Storage Zone
  • Researchers from Beijing University and Auburn University, conducted lab experiments with two bioretention columns (1) with Water treatment residuals (WTRs - i.e. polyaluminium chloride & dewatered sludge from a surface water treatment plant) (15% dried weight, the remaining 85% sandy loam) and the second (2) filled with traditional sandy loam for its filter bed material. Their pollutant rmeova lefficiency for TSS was virtually the same, treating between 100 - 400 mg/L over 10 separate test cycles in a 50-day period. The effluent TSS levels were bot hless than 20 mg/L (10 mg/L less than the CCME requirement in Ontario) with removal percentages above 90% on average to a maximum of 97%. Meanwhile, for Total Phosphorus removal (TP) the column with 15% WTRs added boated a mean TP removal of 99.6% with a maximum effluent of 0.08 mg/L after remoting an average influent concentration load of 4.0 – 7.0 mg/L) (Qiu, et al. 2019).

Total suspended solids

Toronto and Region Conservation Authority

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

Total phosphorus

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

The 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.

The ratio of the impervious catchment (drainage) area to the pervious (footprint) area of the receiving BMP.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Total nitrogen

Mineral 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.

Soil particles with a diameter less than 0.050 mm.

The 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.

The natural ground material characteristic of or existing by virtue of geographic origin.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

Sustainable Technologies Evaluation Program

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

A biological community, including humans and their natural environment.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

A perforated pipe used to assist the draining of soils.

The downward movement of water through the soil, the downward flow of runoff from the bottom of an infiltration BMP into the soil.

Loss of water from a drainage system as a result of percolation or absorption into the surrounding medium (e.g., the infiltration of water into the native soil through a perforated pipe wall as it is conveyed).

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

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.