Optimize Pollutant Capture of Construction Site Stormwater Runoff

Since the turn of the 21st century, Compost Filter Socks (CFS) have quietly guarded perimeters and surrounded water inlets at construction sites. As a highly-proven and now well-established best management practice, CFS treat sheet flow, rain and snowmelt to significantly reduce the amount of stormwater pollutants that leave a job site.

Also known as “silt socks,” CFS have replaced silt fences for a majority of construction projects because of their greater pollutant capture rates, cost savings and reduction in installation time over silt fences.

Compost filled socks are a combination of compost and other materials, encased in a tubular mesh skin. They work by slowing down stormwater runoff, giving sediment time to settle out while the stormwater filters through the sock.

Although many CFS look similar, what is under the skin can truly make a difference when it comes to capturing pollutants – especially pollutants other than sediment. Even the type of compost used in a sock affects flow rates, efficacy and longevity.

When CFS were first developed, the compost in the socks was carefully selected from mature stock and sanitized to help ensure that socks would have consistent results for capturing sediment in stormwater runoff. Although compost is abundant; mature, sanitized compost is a more finite resource. This meant that over time, socks began to be filled with any available compost, which then meant that even if you purchased all of your CFS from the same supplier, you could end up with widely varying results.

Variety in compost can also lead to CFS that have widely varying weights, strong odors, and socks that just don’t last very long in the field. Some socks have even been known to grow mushrooms or feedstock grains while in storage and in the field. Variable compost also affects sediment retention rates.

A new alternative to compost is chopped switchgrass. It is abundant, provides a consistent, stable organic stock and is lighter than compost. That means that up to 36% more footage can fit on pallets, and it’s easier for employees to install. It also helps to ensure that sediment removal rates have less variability.

Scrap metal recycling, manufacturing processes, fertilizers, mine tailings, sludges and pesticides can all contribute to heavy metal discharges in stormwater. Heavy metals create pollution problems because they are highly soluble in water and accumulate in water bodies where they are absorbed or digested by fish and aquatic organisms.

When CFS were first introduced, the principal focus was sediment retention. Over time, the socks became a great resource for capturing other pollutants as well. Blending carbon-based additives with compost can reduce heavy metal discharges by up to 71%.

BioChar is one of the common additives for heavy metal capture. However, like compost, there can be a lot of variety, from powders to large blocks. Carbon content – which is essential for capturing dissolved metals – also varies. Testing proves that large chunk, hardwood BioChar that has at least 80% carbon content provides longevity in the field with the ability to continue removing pollutants over time.

The addition of re-activated charcoal is another proven method for capturing heavy metals and other organic pollutants. Re-activated charcoal is recycled activated carbon that has been rebaked to bring it back to at least 95% effectiveness. This process is less labor and energy intensive than making virgin activated charcoal, and produces a viable product from what would otherwise be waste.

The addition of BioChar, activated charcoal and re-activated charcoal are all recognized methods for heavy metal and organic pollutant capture in stormwater. Continued research into pollutant capture has uncovered another additive: calcium silicate aggregate (CSA). This inert, fine-grain powder increases pH, which helps to precipitate heavy metals and enable their capture in carbon.

Capturing water pollutants on land – before they enter streams, rivers and other bodies of water can sometimes seem burdensome. But it is easier than trying to capture it once it has reached water. Products to control pollutants at perimeters and inlets continue to evolve to meet the ever-changing limits set by state and federal regulators.

Choosing high-quality CFS will help ensure more consistent pollutant removal, with easier installation and fewer changeouts. So you can focus on getting the job done.

Recent innovations in compost filter socks are addressing some of the biggest challenges in stormwater management—bringing improved pollutant capture, durability, and ease of use to the forefront. One such advancement is Stormtration™ Char21™ technology, which integrates sustainable materials like chopped switchgrass and additives such as BioChar, re-activated charcoal, and calcium silicate aggregate. These components provide consistent pollutant removal, lighter weight for easier handling, and longer-lasting performance in the field.

Stormtration™ Char21™ Filter Socks represent the next generation of stormwater management solutions, meeting the evolving needs of construction and industrial sites while maintaining compliance with stringent environmental regulations.

Perfect for post-construction stormwater filtration, industrial runoff control, erosion and sediment management, downspout filtration, catch basin surrounds, and perimeter protection.

Made in the USA, Stormtration™ Char21™ Filter Socks are the smarter choice for managing pollutants at your site. Upgrade today to cleaner water and hassle-free stormwater management.

Learn More: Full technical details and testing results are available in the Char21™ Overview PDF.