Municipal wastewater sludge is usually treated as a costly by-product—something that must be dewatered, transported and ultimately disposed of. A new study suggests that, in China, this material could instead become a valuable industrial fuel. By coordinating wastewater treatment plants with coal-fired power stations on a monthly, plant-by-plant basis, researchers have identified a large-scale opportunity to reduce emissions, shrink sludge volumes and generate additional revenue, transforming two infrastructure challenges into a single circular-economy strategy.
The concept is known as sludge co-firing: dried or partially dried sewage sludge is blended with coal and burned in thermal-fired power plants. Because sludge contains organic matter and therefore chemical energy, it can replace a portion of the coal normally consumed by a boiler. Yet the practice is technically and economically difficult to scale. Sludge production varies across regions and seasons, while power plants have different combustion systems, operating schedules, fuel requirements and capacities. Transporting wet sludge over long distances can also consume significant energy and erase the environmental advantages of co-firing.
To examine whether these obstacles could be overcome, the researchers assembled a high-resolution national database covering 5,218 municipal wastewater treatment plants and 1,990 thermal-fired power plants across China. Rather than treating the country as a single supply and demand pool, they designed an allocation framework that works at the individual-plant level and updates the matching process monthly. The model considers where surplus sludge is generated, how much fuel each power plant can accept, whether facilities are technically compatible and whether delivery is economically practical.
This temporal detail is central to the study. A wastewater treatment plant may generate sludge continuously, but the quantity and moisture content can change with population patterns, industrial activity, weather and treatment conditions. Power plants, meanwhile, may operate at varying capacity throughout the year. A match that appears feasible using annual averages may fail during a particular month if sludge supply exceeds a plant’s co-firing capacity or if transportation costs become too high. Monthly optimization allows the network to respond to these fluctuations instead of relying on static, long-term averages.
The analysis found that surplus sludge from 3,735 wastewater treatment plants—representing approximately 87 percent of the treatment capacity examined—could be stably allocated to 421 qualified thermal-fired power plants. These facilities formed what the researchers describe as a symbiotic network, in which wastewater plants gain an outlet for residual sludge while power stations obtain a supplementary fuel that can displace part of their coal consumption. The result is not a universal recommendation for every plant, but a geographically and operationally selective system based on technical compatibility and recurring supply-demand balance.
According to the study, the optimized network could reduce total sludge volume by 79 percent, with an estimated range of 55 to 88 percent under the modeled uncertainty. This reduction is significant because sludge is difficult to handle primarily due to its high water content and the complex mixture of organic and inorganic materials it contains. Lowering the amount requiring landfilling, storage or other disposal routes could reduce pressure on municipal waste infrastructure. The environmental benefit would also extend beyond sludge management, because treatment and disposal operations themselves consume energy and can generate greenhouse-gas emissions.
The researchers estimate that the co-firing network could mitigate 15.25 million tonnes of carbon-dioxide equivalent annually, with a modeled range of 10.53 to 17.39 million tonnes. That amount corresponds to roughly half of the emissions attributed to the municipal wastewater treatment plants in the study. The climate benefit comes primarily from coal substitution and reduced sludge-related emissions, although its precise scale depends on factors such as sludge moisture, transport distance, combustion efficiency and the accounting method used to distinguish biogenic carbon from fossil carbon.
The proposal is also designed to produce an economic incentive rather than relying solely on environmental regulation. Across the modeled network, median incremental profits reached 369.41 million Chinese yuan per year, although outcomes varied widely, from approximately 4.18 million to 1.25 billion yuan annually. These gains may arise from avoided sludge disposal costs, reduced coal purchases and the value of integrating existing infrastructure. The study’s scenario analysis found that rising fuel prices could increase environmental benefits by 11 percent and economic benefits by 135 percent, making co-firing particularly attractive when coal becomes more expensive.
The findings also address a major concern about the future of the system: China’s coal-power sector is expected to contract as renewable energy and other low-carbon technologies expand. The researchers report that the long-term resilience of the co-firing network could nevertheless be maintained through a 35 to 67 percent expansion in collaborative thermal-fired power plants. In other words, the network would need to broaden its set of participating facilities as individual plants retire, reduce operating hours or become unavailable. This suggests that circular infrastructure may remain viable during an energy transition, provided planning anticipates changes in the power fleet rather than treating today’s plant network as permanent.
The study does not imply that sewage sludge can simply be burned in any boiler. Its feasibility depends on fuel preparation, moisture management, emissions controls, ash handling and the chemical composition of the sludge. Contaminants such as heavy metals and persistent compounds must be monitored because combustion can concentrate some pollutants in ash or create additional treatment requirements. Transportation is another decisive factor: wet sludge is expensive to move, so drying, dewatering and regional coordination will determine whether a proposed match produces a genuine carbon advantage. The plant-level framework is therefore important not only as a scheduling tool, but also as a way to identify where technical and economic conditions are strong enough for implementation.
By combining detailed infrastructure data with monthly optimization, the research offers a blueprint for linking sectors that are normally planned separately. Wastewater treatment is often managed as a sanitation service, while electricity generation is organized around fuel markets and grid demand. Their emissions, operating cycles and material flows, however, overlap in ways that conventional planning can miss. The study’s central message is that circular-economy strategies become more powerful when they are designed around real facilities, changing schedules and regional constraints. If validated through pilot projects and careful environmental monitoring, sludge co-firing could turn an expensive waste stream into a flexible resource—while providing a model for cross-sector industrial cooperation far beyond China.
Subject of Research: Municipal wastewater sludge co-firing with coal in thermal-fired power plants, including environmental, economic and network-allocation impacts.
Article Title: Unlocking sludge co-firing synergies between thermal power and wastewater treatment plants
Article References: Lin, J., Zhou, Q., Hu, L. et al. Unlocking sludge co-firing synergies between thermal power and wastewater treatment plants. Nat Water (2026). https://doi.org/10.1038/s44221-026-00697-8
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s44221-026-00697-8
Keywords: municipal wastewater sludge, sludge co-firing, thermal-fired power plants, coal substitution, circular economy, carbon emissions, wastewater treatment, China, energy transition, industrial symbiosis
