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Graduate student Alexis Martin prepares pots of trees for the next phase in the research study

Closing the loop: Biochar can create lasting environmental benefits across the landscape

There’s an adage in hiking: Leave a place better than you found it.

Brookhaven mayor John Ernst is taking that sentiment a step further, launching a plan to clean runoff water as it moves through the town.

The secret ingredient in the plan? Biochar.

“We want to see if we can take what was going to the landfill and repurpose it as great big water filters,” said Ernst in a Brookhaven news release. “Then the question becomes, can the used water filters then become a type of fertilizer?”

Biochar is a byproduct of burning wood in intense heat and limited oxygen supply. While a more traditional fireplace produces fine gray ash particles, biochar is black and has a larger surface area, said Warnell professor Daniel Markewitz. Because it is a byproduct of the manufacturing process, it often ends up in a landfill, although in recent years the agricultural sector has begun using it on crops as a soil amendment.

But another use for biochar is water purification.

“Biochar’s best quality for water purification is its large surface area and thus an ability to adsorb nutrients from the water,” said Markewitz. With Warnell researchers’ help, the plan is to test biochar’s ability to pull nutrients out of the water that flows through Brookhaven. Picture large tea bags floating in the tributaries that empty into Murphey Candler Lake at the north end of the town.

The concept isn’t too far from home water filters that use a charcoal-based filtration method to purify water. Just on a larger scale.

Through biochar’s adsorption process, researchers expect the material to draw excess nutrients and other contaminants out of the water and adhere to the biochar surfaces. As the filters are used and removed from the water, the biochar will then be applied as a soil amendment to planted areas around Brookhaven.

Already a good soil amendment, researchers say the additional nutrients pulled from the water will further increase biochar’s effects on soil fertility. “And because it’s so carbon heavy, when you apply biochar you apply carbon,” added Abney. “If you assume 50% carbon by weight—which is probably a little conservative—you’re just adding carbon, and it’s pretty stable in soil.”

The Brookhaven project faces several unknown, though, and that’s what the team is now trying to wade through.

First, there is the question of how much biochar is needed to clean a certain amount of water. Then, how much unwanted material will biochar adsorb from the water—and how do you know when you need to swap out your filtration bags? Then, once the biochar is removed from the water and placed on planted areas, how does it interact with the soil and act as an overall soil enhancer?

The Warnell research team is looking at all these aspects now, including collecting data from Brookhaven’s waterways to get a baseline of factors such as creek height, dissolved oxygen, turbidity and changes during large storm events. Associate professor Jay Shelton and research specialist Wesley Gerrin have installed sensors and began collecting data earlier this year.

In the meantime, Abney and Markewitz have been working to better understand how biochar works as a water filter. Abney is also working on a project to better understand how biochar works as an amendment for urban trees.

Because street trees are typically planted in small spaces, called “vaults,” and their environment is vastly different than those growing in a forest, there’s a possibility that biochar, used as an amendment for urban trees, may provide a carbon-friendly option to municipal arborists and planners.

“Street trees are often planted for stormwater mitigation—they are a place where stormwater can flow and slow down and come out cleaner,” said Abney. “The phase one of our study is, if you plant a tree with biochar, can you improve the quality of the water coming out of it and how slowly it comes out?”

In the second phase of the study, Abney will use biochar like a mulch around established street trees. This method will be applicable to arborists and city planners in Brookhaven as elsewhere, she added, as most street trees are already planted—but you can easily apply soil amendments to trees in the ground.

While wood ash is not a regulated soil amendment, Abney said there are many established benefits to adding biochar to soil. Along with increasing soil pH, it increases organic matter in the soil, which helps with water retention and drought tolerance. Depending on the structure of the biochar, it can also help with nutrient retention while also adsorbing contaminants.

“People look to it for adsorbing antibiotics and other pharmaceuticals and heavy metals—all the things you don’t want in your drinking water,” she said. “Agriculture has really adopted it, and it tends to improve yields because of all these soil improvements.”

The team is working on options that include wood ash sourced from an OSB manufacturer in Commerce, Georgia. The idea is, using materials that would otherwise go to a landfill could create a closed-loop solution that cleans a city’s water and provides a carbon-capturing soil amendment.

“I feel like if it goes well, there’s endless possibilities,” said Abney. “You can set up a floating biochar bag and pump water through it. And then having a place to put it is a nice, closed loop. We can truck it to Brookhaven, put it in the stream and then in the soil and it’s better than going in the landfill for sure.”


Assistant Professor of Forest & Disturbed Soils
Professor, Soil Site Productivity
Associate Professor, Fisheries

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