Filter Media Additives for Phosphorus Removal

Overview

Design innovations to improve water quality treatment performance of filter media mixtures involve the incorporation of additives to enhance retention of reactive or dissolved pollutants. 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 some cases to make significant improvements on the discharged water quality over several years. For instance, a two year STEP research study that compared standard bioretention media, to the same media amended with Sorbtive™ in one plot and iron enriched sand (aka red sand) in another showed statistically significant improvements in effluent phosphorus concentrations from the two media amended plots (STEP, 2019).

Determining when additive enhanced filter media needs replacing or maintenance represents a new challenge for stormwater asset managers, as there are no suitable visual indicators. Erickson et al. (2018) suggest effluent sampling and laboratory testing to identify when enhanced filter media pollutant retention is waning, or periodic sampling and batch (laboratory) testing of filter media to directly measure its capacity to retain the targeted pollutants. Periodic replacement of filter media at inlet locations should be considered as an operation and maintenance best practice to maintain treatment performance.

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Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
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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%	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 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)

Biochar
Photo credit: K.salo.85

What Is It?

Biochar is a carbon-rich material produced by pyrolysis of organic feedstocks such as municipal, agricultural, and forestry wastes. It has a high surface area, which enhances soil aggregation, water holding capacity, and organic carbon content. However, biochar properties and effectiveness for pollutant sorption depends on feedstock and pyrolysis conditions (Iqbal et al., 2015).

How is it being used?

• Biochar additions to green roof substrate were tested at the University of Toronto. Biochar-amended sedum green roofs presented the best integrated water quality, including reduced discharge concentrations of dissolved P (Liao et al., 2024).
• Ongoing biochar research at the British Columbia Institute of Technology is testing the response of native plants to various soil/biochar mixes to be used in rain gardens and the comparison of biochar with different physico-chemical characteristics in chemical contaminants removal efficacy (BCIT, 2025) .
• A bioretention system in China used biochar layered with or mixed into lateritic red soil, with some success in contaminant removal. The mixed biochar–soil design achieved the highest water retention, and both biochar-amended systems removed more contaminants (TN, NH₃-N, NO₃⁻, TP, PO₄³⁻, and Cu) than systems without biochar (Premarantha et al., 2023) .
• In Delaware, two roadside filter strips amended with biochar reduced peak flow and runoff volume, but showed no notable change in pollutant concentrations (Center for Watershed Protection, Inc., 2025) .
• In field experiments in Europe, biochar reduced nutrient leaching in green roofs, but did not reduce nutrient concentrations in effluent (Kuoppamäki et al., 2016) .

Benefits

One study states that the mixing of biochar with compost did not decrease the phosphorus leaching from the mixture (Iqbal et al., 2015). Based on this study, it would seem that biochar is ineffective for phosphorus removal. An Australian study found that phosphorus removal efficiency was inversely related to the biochar content of sand media when used to treat secondary sewage and septage in constructed wetland mesocosms (Rozario et al., 2016). Another study, located in Portland, found that biochar could reduce metal concentrations in stormwater when used as an amendment to bioretention systems but has a limited impact on nutrients .

However, some other papers indicate that biochar mixed with sand was able to retain some trace organic contaminants (TOrCs) (Ulrich et al., 2015), and that after six months of operation, biochar-amended biofilters improved removal of total dissolved phosphorus and other TOrCs by greater than 60% (Ulrich et al., 2017a). Amendment of LID systems with biochar has shown promise for improved removal of heavy metals, bacteria, nutrients, and TOrCs (Ulrich et al., 2017b).

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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.

Gravel, 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.

Any form of rain or snow.

The attachment of gas, vapour or dissolved matter onto the surface of solid materials.

Sustainable Technologies Evaluation Program

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

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.

Total suspended solids

The heating of an organic material in the absence of oxygen.

The process where one substance adheres to or is taken up by another.

Defined as those plants (including grasses, herbaceous species, shrubs, vines and trees) that have historically existed within a particular area. Native plants have co-evolved with the local ecosystems and natural processes.

Total nitrogen

Total phosphorus

the maximum flow rate occurring during a flood event measured at a given point in a river, street, or pipe system.

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.

The liquid waste from domestic, commercial and industrial establishments. City of Toronto Wet Weather Flow Management Guidelines November 2006 49

A vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year.

Low 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.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

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

Decayed 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.

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.

A bioretention BMP featuring an impermeable liner and underdrain that prevents infiltration of runoff into the underlying native soil; provides sedimentation and filtration of urban runoff as it passes through the mulch layer, engineered filter media and vegetation root zone.

A bioretention cell that features an impermeable liner that collects and treats stormwater through sedimentation and filtration only (i.e., no infiltration).

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

A thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens.

A bioretention BMP featuring an impermeable liner and underdrain that prevents infiltration of runoff into the underlying native soil; provides sedimentation and filtration of urban runoff as it passes through the mulch layer, engineered filter media and vegetation root zone.

A bioretention cell that features an impermeable liner that collects and treats stormwater through sedimentation and filtration only (i.e., no infiltration).

Reduction of peak flow and increase of the duration of the flow event.