To handle increasing wastewater loads, sewage treatment plants are adopting more advanced treatment processes. However, many of these approaches require additional space and energy, highlighting the need for more efficient alternatives. Chemical-enhanced primary treatment (CEPT), which uses chemicals, instead of microorganisms, to promote flocculation and coagulation of sewage, has attracted significant attention for reducing energy consumption and operation costs in sewage treatment plants.
The sewage sludge produced during treatment can be further processed through pyrolysis, a high-temperature process that can reduce sludge volume, degrade pollutants, and produce value-added materials. However, biochar, which is product of pyrolysis, derived from CEPT sewage sludge (CS) differs from that produced by conventional biological treatment sewage sludge (BS). These differences can influence how heavy metals behave and remain stable in the biochar, potentially increasing environmental risks. Until now, limited information has been available on heavy metal behavior in CS-derived biochar.
To address this gap, a research team led by Professor Kitae Baek from the Department of Environment and Energy and Soil Environment Research Center, Jeonbuk National University, Republic of Korea, investigated the properties of heavy-metals in CS-derived biochar with those of conventional BS-derived biochar. Their study was made available online on December 18, 2025, and published in Volume 206 of Process Safety and Environmental Protection on January 15, 2026.
"While CEPT can reduce energy consumption, it is necessary to consider not only treatment performance but also the environmental impact of its byproducts. Our study clarifies the potential risks associated with CS-derived biochar and highlights the need for appropriate countermeasures as CEPT adoption increases," says Prof. Baek.
The researchers collected CS and BS samples from two sewage treatment plants in Hong Kong. Both sludge types were pyrolyzed at different temperatures, and the resulting biochars were analyzed to compare their heavy metal content and stability.
The biochar yield of CS ranged between 32.1% and 40.9%, notably lower than that of BS, which ranged between 43.9% and 75.2%. Heavy metal content analysis showed that a smaller proportion of heavy metals remained trapped in CS-derived biochar across all temperatures. This suggests that thermal treatment of CS can lead to secondary heavy metal pollution in the surrounding environment.
Further tests revealed that CS-derived biochar had lower heavy metal stability, especially at high temperatures above 800 °C. At such high temperatures the mobility of heavy metals also increased significantly and could be easily leached out, increasing environmental safety concerns. Based on these results, the researchers recommend using lower pyrolysis temperatures when treating CEPT sludge.
Interestingly, at an optimized temperature of 550 °C, heavy metals in both types of biochar showed long-term stability. This suggests that, when properly treated, CS-derived biochar can be safely used for applications such as soil amendment or fertilizer, similar to biochar from conventional sludge.
"Our findings show that an appropriate thermal treatment is necessary to enhance sustainability of the CEPT process. With proper sludge management, CEPT can support efficient sewage treatment while reducing carbon emissions and environmental impacts. This will have a long-term positive impact on people's lives and environmental conservation," concludes Prof. Baek.
