Chemical sewage sludge turns toxic above 800C - but a sweet spot exists at 550C

Hong Kong operates some of the busiest sewage treatment plants in Asia, and like many densely populated regions, it faces a problem of scale. Conventional biological treatment works well but demands space and energy that dense urban areas struggle to provide. An alternative called chemical-enhanced primary treatment, or CEPT, uses chemical flocculants and coagulants instead of microorganisms to separate solids from wastewater. It is cheaper, faster, and more compact. But CEPT produces a different kind of sludge - and what happens when you try to process that sludge has, until recently, been poorly understood.

A team led by Professor Kitae Baek at Jeonbuk National University in South Korea decided to find out. Their study, published in Process Safety and Environmental Protection, compared the behavior of heavy metals in biochar produced from CEPT sludge versus conventional biological sludge, and the results carry a clear warning - along with a practical solution.

Two sludges, different chemistry

Pyrolysis - heating material in the absence of oxygen - is a widely used method for reducing sewage sludge volume, breaking down pollutants, and producing biochar that can potentially be used as soil amendment or fertilizer. The process works well for conventional biological treatment sludge. But CEPT sludge has a fundamentally different chemical composition, loaded with the inorganic flocculants and coagulants used during treatment. Those chemicals change how heavy metals behave during and after pyrolysis.

The researchers collected both types of sludge from two sewage treatment plants in Hong Kong and pyrolyzed them across a range of temperatures. They then analyzed the resulting biochars for heavy metal content, stability, and leaching potential.

The temperature threshold

The first finding: CEPT sludge yields less biochar. Recovery rates ranged from 32.1% to 40.9%, compared with 43.9% to 75.2% for conventional sludge. More importantly, a smaller proportion of heavy metals remained trapped in the CEPT-derived biochar, suggesting that more metals were released into the surrounding environment during pyrolysis itself.

The second finding was temperature-dependent and more concerning. At high pyrolysis temperatures above 800 degrees Celsius, CEPT-derived biochar showed significantly lower heavy metal stability. The mobility of metals increased sharply, meaning they could be easily leached out by water or soil contact. This is precisely the kind of secondary pollution that makes environmental regulators nervous - you solve a wastewater problem only to create a soil and groundwater problem.

But at 550 degrees Celsius, the picture looked different. At this optimized temperature, heavy metals in both types of biochar showed long-term stability. The metals were effectively immobilized within the biochar matrix, reducing the risk of leaching to levels comparable to conventional sludge biochar.

A practical guideline for a growing practice

The implication is straightforward: temperature control is the key variable. CEPT is attractive for its efficiency and cost savings, and its adoption is increasing worldwide. But the sludge it produces requires more careful thermal processing than conventional sludge. Treating CEPT sludge at high temperatures - which might seem logical for maximizing pollutant destruction - actually increases environmental risk by mobilizing heavy metals.

At 550 degrees Celsius, the biochar produced from CEPT sludge can be safely used for the same applications as conventional biochar, including soil amendment and fertilizer. This finding provides plant operators and regulators with a specific, actionable parameter: keep pyrolysis temperatures moderate when processing chemical sludge.

Scaling up safely

The study has direct relevance for municipalities considering CEPT adoption. The technology's advantages - lower energy consumption, smaller physical footprint, reduced operating costs - are genuine. But those advantages evaporate if the sludge byproducts create downstream environmental contamination. By demonstrating that appropriate thermal treatment can neutralize the added risk, the research removes a potential obstacle to wider CEPT deployment.

There are limitations to note. The study examined sludge from two plants in Hong Kong, and the chemical composition of CEPT sludge varies depending on which flocculants and coagulants are used. Different chemical formulations may produce different heavy metal behaviors during pyrolysis. Broader validation across plants using different CEPT chemistries would strengthen the findings.

The research was published in January 2026 and offers a clear message for an industry navigating the tension between treatment efficiency and environmental safety: the choice is not between CEPT and environmental protection. With proper sludge management, both are achievable.