Road Salt Management

Salt management is an important component to the City’s winter road maintenance program. Effective salt management means that we use de-icing chemicals at the right time, in the right amounts, in the right place. The goal is to achieve safe road and sidewalk conditions while minimizing environmental impact.

Salt management is a priority for the City of Peterborough, with commitments including:

• Annual training for Municipal Operations staff
• Regular maintenance and calibration of application equipment
• Weather systems monitored closely to ensure the most efficient deployment of resources
• Research on best practices for operations, material and equipment.

The chemistry

Understanding the relationship between air and surface temperatures, current conditions, weather forecasts and chemical processes is key to maximizing the effectiveness of salt.

Salt needs time, temperature and moisture to work. It draws the heat required either from the air or the roadway surface.

Sodium Chloride is a chemical made up of sodium and chloride ions that require heat and moisture to become a solution. This solution depresses the point at which water will freeze. As it melts snow, it creates more moisture and forms brine across a roadway.

Pre-wet salt application

The City of Peterborough uses pre-wet salt applications in most weather conditions, apart from during freezing rain events, when the moisture is already present to break down the rock salt into a brine.

Pre-wet salt systems spray a 23 percent brine solution on dry salt as it is being discharged from the truck.

The City of Peterborough has 19 trucks equipped for pre-wet salt application. Each truck has a capacity of approximately 1175 litres for brine.

Use of salt as a pre-wet applications during snow clearing operations decreases the amount of sodium chloride applied in the municipality throughout the winter season, whereas sand treatments are required to be repeated numerous times due to it being a reactive treatment to provide traction and contains a sodium chloride content of 40 kilograms per 2-lane kilometre when applied.

Two advantages of pre-wet systems

1. Spraying brine onto the salt gives the salt a boost in moisture content that speeds up the formation of brine as it mixes with the snow. At lower temperatures, snow contains less moisture and it can take a longer time to start the chemical process.
2. Pre-wet salt sticks better to the roadway. This is especially important at the beginning of a weather event when the municipality salts the roadway ahead of the snowfall as part of the anti-icing strategy. Dry salt will very quickly be scattered to the curbs with traffic action and wind. Pre-wet salt stays in the roadway longer.

For example, when 100 percent of the material is applied to the center of the road, typically 15 percent of salt bounces off dry road surfaces onto the boulevard or other adjacent surfaces, compared to just 2 percent of of pre-wet road salt.

No one strategy covers all the scenarios in any given storm. Depending on the frost depth, weather forecast, current conditions, temperature trends and time of year, each snow event is approached a little differently. 

Sidewalk de-icing

While some of the principles of salt management apply to sidewalks, there are some significant differences.

• Because sidewalks don’t have the advantage of vehicle traffic, the salt applied doesn’t get moved around to create the necessary brine. An individual salt grain will melt some snow around it, but then stops working because there is insufficient moisture to continue the process. Therefore, more salt is required per square meter than a roadway to achieve the same result.
• Once there have been a few plowing events, the edges of the sidewalk form a dam from the plow curl, not allowing the excess water to drain off. This will re-freeze causing more ice. It should also be noted that there are no storm water drainage systems along sidewalks (only along the roadways where the curb meets the street) and this causes melting snow/ ice and rainfall to flood between snowbanks and potentially re-freeze.