Wastewater sludge byproduct shows promise and risks for sustainable agriculture

A liquid byproduct generated during the treatment of wastewater sludge could help recycle nutrients for agriculture, but new research shows it may also subtly disrupt key ecological processes at the soil water interface.

In a study published in Agricultural Ecology and Environment, researchers investigated how hydrothermal carbonization aqueous phase, or HAP, affects periphyton, a thin, living biofilm made up of algae, bacteria, and fungi that plays a crucial role in nutrient cycling in agroecosystems such as rice paddies and irrigation channels.

Hydrothermal carbonization is increasingly used to convert sewage sludge into useful products. While the solid fraction can be reused as fuel or soil conditioner, the liquid fraction contains large amounts of dissolved organic matter, nitrogen, and phosphorus. This makes HAP attractive for agricultural reuse, but its ecological effects have remained poorly understood.

“Our goal was to understand whether this nutrient rich liquid can be safely returned to agricultural systems without undermining ecosystem functions,” said corresponding author Huifang Xie of Nanjing University of Science and Technology. “Periphyton offered an ideal model because it sits right at the soil water interface and responds quickly to environmental change.”

Using controlled laboratory microcosms, the team exposed periphyton communities to increasing concentrations of sludge derived HAP. They monitored water quality, nutrient removal, microbial diversity, metabolic functions, and interactions among organisms across different trophic levels.

The results showed that periphyton retained a surprising ability to remove pollutants even under high HAP exposure. At the highest concentration tested, the biofilm still removed up to 55 percent of chemical oxygen demand and 35 percent of ammonium nitrogen from the water. However, overall performance declined as HAP levels increased.

“Periphyton proved resilient, but that resilience came with tradeoffs,” said first author Xudong Zhong. “As HAP concentrations rose, the ecosystem became simpler and less multifunctional, even though basic nutrient removal continued.”

The study found that while overall microbial diversity remained stable, the composition of both bacterial and eukaryotic communities shifted significantly. These changes narrowed the ecological niche of many organisms and weakened the complex interaction networks that normally support multiple ecosystem functions at once.

In particular, high HAP inputs simplified cross trophic connections between bacteria and eukaryotes. Network analyses showed fewer interactions, lower connectivity, and reduced structural complexity. These changes were strongly linked to declines in ecosystem multifunctionality, a measure that integrates nutrient cycling, biomass production, and metabolic activity.

“At high concentrations, HAP does not wipe out the community, but it reorganizes it,” explained co author Bingyu Wang. “The system compensates by reshuffling functional roles, but the overall ecological capacity becomes more limited.”

The researchers also observed functional compensation strategies. Certain microbial groups increased their roles in nitrogen fixation and carbon metabolism, helping maintain nutrient removal despite environmental stress. This adaptive response highlights both the robustness and vulnerability of agroecosystem biofilms.

The findings provide important guidance for the sustainable reuse of sludge derived liquids in agriculture. Moderate application rates may allow nutrient recovery while minimizing ecological disruption, but excessive inputs could undermine long term ecosystem stability.

“This study emphasizes that recycling waste streams into agriculture requires more than nutrient accounting,” Xie said. “We need to consider how these inputs reshape living communities that support soil and water health.”

By revealing how cross trophic interactions mediate ecosystem responses to wastewater byproducts, the research offers a new framework for evaluating environmental risks and designing safer circular resource management strategies.

=== 

Journal Reference: Zhong X, Wang B, Sun K, Xie H, Tian W, et al. 2025. Sludge-derived hydrothermal carbonization aqueous phase regulates agro-ecosystem multifunctionality by affecting cross-trophic community in periphyton. Agricultural Ecology and Environment 1: e011  

https://www.maxapress.com/article/doi/10.48130/aee-0025-0012  

=== 

About Agricultural Ecology and Environment: 

Agricultural Ecology and Environment is a multidisciplinary platform for communicating advances in fundamental and applied research on the agroecological environment, focusing on the interactions between agroecosystems and the environment. It is dedicated to advancing the understanding of the complex interactions between agricultural practices and ecological systems. The journal aims to provide a comprehensive and cutting-edge forum for researchers, practitioners, policymakers, and stakeholders from diverse fields such as agronomy, ecology, environmental science, soil science, and sustainable development. 

Follow us on Facebook, X, and Bluesky.