Red sand

What is it?

Red sand, also known as a "Minnesota filter", is iron enhanced sand designed to capture soluble phosphorus that generally passes through a typical stormwater management facility. Iron has an affinity for dissolved phosphorus, which will serve to bind and therefore remove a portion of the dissolved phosphorus from the stormwater. The sand also filters the stormwater by removing a portion of the suspended solids and total phosphorus that may be attached to those particles.

How is it being used?

The first retrofit in Ontario to incorporate a red sand filter system was the George Richardson stormwater management pond in Newmarket. Red sand was installed as the last part of a treatment train to function as a final polishing unit. The underground system was first lined with a bentonite clay liner to minimize groundwater exchange, with various layers of clear stone and red sand sandwiched between nonwoven geotextiles. The water is distributed via a system of perforated pipes embedded in the top layer of the underground system just above the red sand filter media, with collector pipes located near the bottom. The system is estimated to have a reduction in phosphorus of 23 kg/year.

Another Ontario project was completed by the Upper Thames River Conservation Authority and Luckhart Transportation Limited, in which they retrofit an innovative wetland treatment system to test a red sand filter. This was the first of its kind for an agricultural application. In this project, a clear round stone was added to improve percolation and phosphorus retention..

Finally, in a two year STEP research study that compared standard bioretention media to red sand and Sorbtive™ amended media, the red sand plot was shown to have significantly lower total and dissolved phosphorus effluent concentrations. However the phosphorus reductions occurred only in year 2, after fine particulates in the red sand media had been washed out of the system (STEP, 2019).

Benefits

Early data from the George Richardson site indicate a reduction in Total suspended solids and phosphorus after going through the system. "Average TP concentrations into the filter were 0.12 mg/L with a maximum of 0.54 mg/L as compared to an average outlet concentration of 0.052 mg/L with a maximum of 0.13 mg/L. Average soluble phosphorus (orthophosphate) concentrations into the filter were 0.01mg/L with a maximum of 0.031 mg/L as compared with an average outlet concentration of 0.007 mg/L with a maximum of 0.044 mg/L." Poor orthophosphate removal was observed during hypoxic or anoxic conditions because the iron/phosphorus bond that facilitates the removal of orthophosphate can be broken under low oxygen conditions.

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.

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

A body of water smaller than a lake, often artificially formed.

Stormwater management following the hierarchical approach: Source Control measures, Conveyance Control measure and End of Pipe treatment to achieve the water quality and water balance target for lot level development of the preferred strategy.

A combination of lot level, conveyance, and end-of-pipe stormwater management practices.

1. 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).

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.

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

Sustainable Technologies Evaluation Program

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

Total phosphorus

A body of water smaller than a lake, often artificially formed.

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.