Wastewater Plant as a Power Plant: Sludge Co-Firing

Sewage sludge is an expensive disposal problem for many municipalities. But it is also an energy carrier: once dried, its heating value approaches that of lignite. The idea of co-firing this sludge in existing power plants is not new – but a recent study from China shows just how large the potential really is. More than 3,700 wastewater plants could there replace coal on a stable monthly basis, cut sludge volume by almost 80 percent and save around 15 million tonnes of CO₂ per year. For Germany, the question is how much of this is transferable – and where the limits lie.

Why sewage sludge is an energy carrier

Sewage sludge consists largely of organic matter – the remains of the microorganisms that clean the wastewater. After mechanical dewatering it has a dry solids content of about 20 to 25 percent. Its lower heating value is then 2.5 to 4.5 MJ per kilogram of wet sludge. The decisive step is drying: when the sludge is dried to 85 to 92 percent dry solids, the heating value rises to 10 to 13 MJ per kilogram of dry solids – roughly the energy content of lignite.

The catch is the drying itself. Evaporating water costs energy: around 2.8 to 3.6 MJ per kilogram of evaporated water. To bring sludge from 25 to 90 percent dry solids, you must expend about 7 to 9 MJ per kilogram of dry solids. Without waste heat from a neighbouring power plant or combined heat and power unit, the energy balance quickly becomes neutral or even negative. With waste-heat recovery, co-firing becomes economically attractive.

The Chinese study: numbers that impress

A study published in Nature Water in 2026 systematically examined the potential of co-firing. The researchers built a high-resolution database of 5,218 municipal wastewater plants and 1,990 thermal power plants in China and developed a monthly allocation framework that matches supply and demand of sludge and incineration capacity.

The results are remarkable:

• 3,735 wastewater plants (about 87 percent of capacity) can replace coal on a stable monthly basis – across 421 qualified power plants.
• Sludge volume drops by 79 percent (95% confidence interval 55–88 percent).
• 15.25 million tonnes of CO₂-equivalent per year are avoided – equivalent to 50 percent of the emissions of the participating plants.
• The median incremental profit is 369 million CNY per year (roughly 45–50 million euros).
• Rising fuel prices amplify the benefits: environmental gain +11 percent, economic gain +135 percent.

The network remains stable in the long term even as the coal sector shrinks, because the number of cooperating power plants expands by 35 to 67 percent.

Germany: mono-incineration instead of co-firing

In Germany the situation is different. Around 1.75 million tonnes of sewage sludge dry solids are produced each year. Of these, about 35 percent are incinerated and 48 percent used in agriculture. There are 28 dedicated mono-incineration plants with around 280,000 tonnes of dry solids per year, plus co-firing in some coal plants and waste-to-energy plants.

The decisive difference from China is the phosphorus recovery mandate. From 2029, plants above 50,000 population equivalents must recover at least 80 percent of the phosphorus from sewage sludge or sludge ash. Phosphorus is a critical raw material – and it can only be recovered economically from mono-incineration ash, where it is concentrated at 7 to 12 percent P₂O₅. In co-firing with coal, the phosphorus is diluted to below 3 percent, making recycling uneconomic.

That is why, in Germany, mono-incineration with downstream phosphorus recovery is often the more robust route – even though co-firing appears cheaper in the short term (60 to 120 euros per tonne of dry solids versus 250 to 380 euros for mono-incineration).

Economics: where the money is

Co-firing is not just a disposal option – it can become a revenue source:

• Electricity revenue: At 120 euros per MWh and around 0.9 MWh per tonne of dry solids, this yields about 108 euros per tonne.
• CO₂ avoidance: At a CO₂ price of 85 euros per tonne and substitution of hard coal (around 0.9 tonnes of CO₂ per tonne of sludge), this yields 70 to 80 euros per tonne.
• Capacity gap: Germany lacks around 400,000 tonnes of dry solids of incineration capacity – a market gap for investors and planners.

Scaling the Chinese figures to German volumes yields a theoretical CO₂ revenue potential of 120 to 150 million euros per year – minus logistics and drying costs.

The hurdles: logistics, pollutants, acceptance

However attractive the concept sounds, the obstacles are real:

• Logistics: Sewage sludge is hygroscopic and odorous. Transport over more than 50 kilometres costs 25 to 45 euros per tonne.
• Drying: Without waste-heat recovery, the energy balance is often negative.
• Pollutants: Mercury, cadmium, PCBs and dioxins limit the co-firing share. The TA Luft limits (0.03 mg/m³ mercury, 0.1 ng/m³ TE dioxins) are strict.
• Acceptance: Local resistance to "waste incineration" in coal plants is widespread. Permitting takes 18 to 36 months.

Conclusion

The wastewater plant as a power plant is more than a vision. The Chinese study shows with hard numbers that co-firing sewage sludge in power plants is a scalable, CO₂-saving and economically viable model. For Germany, co-firing makes sense where waste heat is available and phosphorus recovery is not jeopardised. In many cases, mono-incineration with phosphorus recovery remains the more robust route – but both paths turn an expensive disposal problem into an energy and revenue source.