UF study shows sewage plant by-product is good for seagrass

by Kirsten Romaguera, UF/IFAS

Seagrasses are vital to the marine ecosystem, where they provide food, habitat, shelter and other services to humans and aquatic organisms. But while the success of seagrass beds is threatened by a few factors, including human activities like boating, a crystallized version of human waste could be the answer to revitalizing these marine habitats.

Urine, it turns out, includes two key ingredients in plant fertilizers: phosphorus and nitrogen. Even better, wastewater treatment facilities are already treating this abundant resource and creating byproducts that would otherwise be sent to landfill.
In its crystallized form, this byproduct is called struvite, the material applied to seagrass beds in a recent study by scientists at the University of Florida.

“Struvite occurs during the wastewater treatment process because magnesium, ammonia and phosphate are all readily available to form the crystalline byproduct,” said Conor MacDonnell, who conducted the study as PhD student. student in the Department of Soil, Water and Ecosystem Sciences at UF/IFAS. “The result is a relatively insoluble and durable compound that is found in wastewater treatment plants.”

This relative insolubility led MacDonnell – who graduated in 2021 and now works as a UF postdoctoral associate studying seagrass restoration – to team up with Gdańsk University of Technology student Franciszek Bydalek and the UF/IFAS faculty members Patrick Inglett and Todd Osborne to investigate whether struvite could be used to fertilize seagrass beds.
“Coastal ecosystems depend on seagrass beds,” Inglett said. “As they decline, it leads to problems such as declining water quality and the disappearance or migration of marine life to other areas.”

To add to the challenges, current seagrass restoration methods are relatively expensive and unsuccessful compared to other coastal ecosystems. The study indicates that nutrient problems and competition with algae explain these difficulties.
In the study, scientists grew three types of seagrass plots in a simulated setting at the UF Whitney Laboratory for Marine Biosciences. One type received an application of struvite, another received a regular controlled-release fertilizer, and the last received no fertilizer. Two experiments were conducted, testing different fertilizer dosages.

“From both experiments, we found that struvite worked better than controlled-release fertilizer in seagrass growth,” MacDonnell said. “Struvite appears to provide a slower, more consistent release of nutrients into the seagrass.”
According to the researchers, the benefits of using struvite in these efforts extend to environmental sustainability.
“Struvite is potentially a win-win solution for the environment,” Inglett said. “It is removed from wastewater, which reduces impact on downstream ecosystems and does not over-fertilize when used for restoration.”

These downstream effects were investigated in a previous study by another then PhD student. student in the same UF department. John Hallas worked with faculty members from the UF/IFAS North Florida Research and Education Center, Cheryl Mackowiak and Ann Wilkie, to research results from water treatment facilities near the Quincy-based center.
“Recovering struvite from wastewater treatment plants is an effective diversion of these useful nutrients for plant growth, rather than allowing them to enter the landfill,” Mackowiak said.

“It also results in a more useful biosolid product, making the wastewater treatment process more sustainable,” Wilkie added.
MacDonnell explained that sourcing struvite also adds to its sustainability credentials compared to more traditional fertilizers. Phosphorus mining, for example, depletes this finite natural resource and degrades the land.

Potential struvite research is just beginning, predicts MacDonnell. He continued to study herbal restoration and his current role is to work with Osborne as a postdoctoral associate. This work, he says, could include further struvite studies in the future.
“Although the use of struvite in aquatic systems seems very promising, there are not many studies on it in marine restoration projects,” he said, “especially in combination with other restoration techniques”.

Measurements of seagrass growth after struvite application.
Photo credit: Conor MacDonnell.

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