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At the sewage treatment plant in Haßfurt, Bavaria, the local authority plans to thermally recycle sewage sludge directly on site in future. © Matthias Schnell, TEER, RWTH Aachen
At the sewage treatment plant in Haßfurt, Bavaria, the local authority plans to thermally recycle sewage sludge directly on site in future.

Haßfurt sewage treatment plant
Saves costs, protects the environment: How even small plants can recycle sewage sludge themselves

12.08.2025 | Updated on: 05.09.2025

When sewage sludge is recycled on site, this not only saves costs but also protects the environment. Natural gas is saved, and electricity and heat are used directly. The VerKlär² project has tested how this can also be achieved in small plants, enabling them to become energy self-sufficient.

Until now, sewage sludge has mainly been recycled in larger sewage treatment plants. Smaller plants have to dispose of their sewage sludge elsewhere, which is a costly and environmentally unfriendly undertaking, especially in rural areas without a large city nearby. The VerKlär² joint project, launched in March 2021 and funded by the Federal Ministry for Economic Affairs and Energy, aims to change this. The focus is on a sewage treatment plant in Haßfurt, Bavaria, south of Würzburg.

"We were lucky that the sewage treatment plant in Haßfurt is already a very innovative facility. The city, as the operator, has been trying for some time to optimise the energy balance of the entire plant – and started thinking early on about how it could organise sewage sludge recycling in the future", says Matthias Schnell from the Teaching and Research Group for Thermal Processes and Emission Reduction in Waste Management and Recycling (TEER) at RWTH Aachen University.

What is sewage sludge?

When wastewater is treated in sewage treatment plants, the purified wastewater is not only discharged into rivers and other bodies of water, but a waste product is also produced: sewage sludge. This sludge is, so to speak, a sink for pollutants – and is unavoidable in wastewater treatment. In addition to water, sewage sludge consists of various solid organic and inorganic substances. These include nutrients such as phosphorus, which is used as a fertiliser in agriculture. However, it also contains pollutants such as heavy metals, drug residues and hormone-active substances that can contaminate soil and groundwater.

"That is why it makes sense not to spread sewage sludge on fields, but to recycle it thermally. The high temperatures and the addition of air cause the organic pollutants to oxidise and thus be destroyed or rendered harmless", explains Matthias Schnell. Sewage sludge can also be used to generate electricity and heat. Recycling is therefore a relevant aspect of the circular economy, both in terms of conserving resources and reducing waste and protecting the environment.

Until recently, sewage sludge – mainly from smaller, rural plants – was spread on fields like manure. However, this practice is increasingly being phased out. The amendment to the Sewage Sludge Ordinance in 2017 stipulates that larger sewage treatment plants with a capacity of more than 100,000 population equivalents will no longer be allowed to spread sewage sludge in agriculture from 2029. The next smallest plant category with more than 50,000 population equivalents will follow in 2032.

In addition, there is an obligation to recover nutrients such as phosphorus and the target of thermally recycling sewage sludge or using it for other purposes. For plants with a population equivalent of less than 50,000, agricultural use will remain permitted for the time being. However, researcher Matthias Schnell explains: "Many municipalities and federal states have voluntarily decided to discontinue use in agriculture in the near future."

The sewage treatment plant in Haßfurt is one of these relatively small plants with a capacity of 27,500 population equivalents – so far without its own drainage, drying or incineration facilities. Instead, it has digestion towers. "Until now, the sewage sludge has been digested on site to produce biogas. This can then be used locally to generate electricity and heat." But then the idea arose in Haßfurt to keep sewage sludge recycling completely in-house and use the energy locally. Contact was established with RWTH Aachen University, where a team led by Matthias Schnell was working on the development of a prototype for incinerating sewage sludge on a very small scale on site.

Challenges at the start of the project

"Things were a little bumpy at the beginning because we started right in the middle of the coronavirus pandemic. Restricted laboratory hours set us back a year with our experiments", says Schnell. But the war in Ukraine, cost increases and calculations that were no longer viable – for example, for components for exhaust gas cleaning and measurement technology – also slowed down the project.

Once the pandemic was over, the researchers were able to start testing the prototype – with success: "We were able to show that we can burn sewage sludge very well in the swirl combustion system we used, while producing very low carbon monoxide and nitrogen oxide emissions", explains Schnell. The exhaust gas cleaning system planned for Haßfurt was also replicated in a similar form at RWTH Aachen University. "We were able to prove that it is feasible to comply with all limit values using a small combustion plant." A few areas for optimisation emerged during trial operation, for example in the plant design and control system.

swirl combustion

How the swirl combustion system works at the Haßfurt sewage treatment plant © Matthias Schnell, TEER, RWTH Aachen

The swirl combustion used is an in-house development by TEER and the head of the teaching and research department, Prof. Dr.-Ing. Peter Quicker, and has not yet been used on the market. There are currently three prototypes in addition to the pilot plant in Haßfurt. Researcher Schnell explains: "The staged combustion in two combustion chambers offers the advantage that combustion can take place at comparatively low temperatures in the first zone. This is because sewage sludge has a relatively high ash content. This begins to melt at low temperatures, forming agglomerates or slag. This is often a problem in incineration plants, but we are able to control it well with the staged combustion process."

In addition, the nitrogen oxide emissions produced during the incineration of sewage sludge can be significantly reduced in the primary stage of the swirl combustion process. The nitrogen oxides formed can be broken down in the main combustion chamber with the aid of reducing components in the staged combustion process. This eliminates the need for additional agents such as urea or ammonia water to reduce nitrogen oxide.

In large-scale technology, stationary fluidised beds are primarily used. "In contrast, we do not use a sand bed in the swirl combustion process. While this offers the advantage of compensating for larger fluctuations in calorific value in large-scale technology, it quickly becomes uneconomical on a smaller scale. Especially when the plant has to be shut down", explains Schnell. With swirl combustion, the combustion process can be scaled down to such an extent that economic efficiency is maintained.

Continuous operation with sewage sludge utilisation possible

At the beginning of 2025, the plant, including exhaust gas cleaning and emission technology, was then set up in Haßfurt. Since its inauguration in April, the plant has been running in trial operation: the purchased exhaust gas cleaning system fits well with the combustion system. The aim now is to reliably comply with the limit value for nitrogen oxide emissions and, if necessary, to reduce it further in the future. The integration of heat into the sewage treatment plant is also working satisfactorily: the heat generated during combustion can be used in its entirety to fill buffer storage tanks and heat the digestion tower and operating buildings.

"Everything is working very well. Of course, we are always looking for ways to optimise things", says Schnell. This includes, for example, redesigning the motors for the ash screws or expanding the blower to provide more secondary air. Due to delays in the approval process, the drying system is still missing. "So we're not quite where we want to be yet. But these are not serious problems, we'll get everything sorted out",  says Schnell. Approval for continuous operation is still pending, but Schnell is optimistic that it will not fail on the technical side: "All project partners are convinced that we can transfer the plant to continuous operation and that it can then continue to be used as planned."

In Haßfurt, the advantages of on-site sewage sludge recycling are clear: transport routes and costs would be saved and environmental and traffic pollution would be reduced, according to Günther Werner, mayor of Haßfurt. "This has a positive effect both for residents and for the town of Haßfurt as the authority responsible for the access roads", says Werner.

How the project not only reduces costs but also contributes to climate protection

  • Using the heat locally saves disposal costs and transport emissions.
  • Heating without fossil natural gas makes the plant independent of price fluctuations on the natural gas market and points towards energy-neutral operation of the sewage treatment plant.
  • If, in addition, digester gas is produced and a PV system is operated, a large part of the plant's electricity requirements can be covered by running the sewage gas CHP plant in electricity-driven mode.
  • The bottom line is that an independent cycle for on-site sewage sludge utilisation can be created without the use of external energy, i.e. energy-self-sufficient operation of the sewage treatment plant.
  • Surplus heat can be fed into the heating network in future.

Other sewage treatment plant operators are showing interest

And interest in the plant extends beyond Haßfurt: "We have already received many enquiries about the plant, not only from Germany, but also from Austria", says Schnell. The approach is very interesting for smaller plants without a large city nearby. Mayor Werner also says: "We can definitely recommend other municipalities to participate in such a research project as project partners in order to achieve innovative goals and further developments more easily."

Engineering scientist Schnell sees further research needed, among other things, to test sewage sludge with different compositions that differ from that in Haßfurt. Emission reduction could also be further optimised by further adjusting the combustion process: "We currently use primary and secondary air. Perhaps this can be further graded."

The issue of phosphorus recovery is also still open. "Because we are now incinerating the sewage sludge, we will also be obliged to recover the phosphorus from the ash from 2029 onwards. There are approaches to optimising sewage sludge ash during incineration so that it can potentially be used directly as fertiliser", explains Schnell. Heavy metal limits must be complied with, as well as limits for the availability of phosphorus.

The city is optimistic that the project will continue to pay off in the future. Although a final statement cannot be made at this stage, it can be assumed that "pilot plants will generally take longer to become profitable". However, Mayor Werner emphasises: "In the long term, we are confident that the cost-benefit analysis will remain positive once the heat generated can be fully utilised in conjunction with other municipalities." (uj)