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 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 piping covering 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.
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 OP 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.
- Lake Simcoe Region Conservation Authority. Showcasing Water Innovation: Stormwater Performance Monitoring Report. 2013. http://www.lsrca.on.ca/Shared%20Documents/reports/swi_monitoring_2013.pdf. Accessed September 8, 2017.
- Credit Valley Conservation. George Richardson Stormwater Management Pond Retrofit. 2013. http://www.creditvalleyca.ca/wp-content/uploads/2013/08/CVC-Case-Study-George-Richardson_Aug_2013.pdf. Accessed September 8, 2017.
- Upper Thames River Conservation Authority. Red Sand helps Protect Water Quality. 2014. http://thamesriver.on.ca/2014/11/11/red-sand-helps-protect-water-quality-november-11-2014/. Accessed September 8, 2017.