Update, 13 November 2025: We have completed the clean-up and restoration work following the slurry leak at Mäntsälän Biovoima on 13 October 2025.
Our staff have cleaned the area surrounding the plant in accordance with the authorities’ instructions, and biogas production at the plant is returning to normal.
We warmly thank all our stakeholders and local residents for their cooperation and understanding while we dealt with the incident.
Update, 28 October 2025: A slurry leak occurred at our Mäntsälän Biovoima biogas production plant during the night of 13 October. The material that leaked from the tank is sanitised slurry, which does not pose a health hazard. It is commonly used, for example, as fertiliser on fields.
The leak was connected to the commissioning of a new bioreactor forming part of the plant expansion under construction. More specifically, the leak was caused by a failure in the new bioreactor’s outlet pipe.
Our staff immediately began cleaning the surrounding area and containing the effects of the slurry leak. Some of this work is still continuing. We are carrying out the clean-up in close cooperation with the authorities.
On the morning of 13 October, slurry from the tank reached a stream near our site that flows into the River Mustijoki. We first cleaned the area using a vacuum tanker, and then also removed slurry by replacing the topsoil. The aim was to eliminate odours and remove the nutrients contained in the slurry from the environment.
We apologise for any inconvenience caused by the situation and thank you for your understanding.
In brief:
- The leak was connected to the commissioning of the new bioreactor in the plant expansion under construction.
- Our staff are actively cleaning and restoring the surrounding area.
- We cleaned the area using methods including a vacuum tanker and topsoil removal to reduce odours and nutrients.
- The situation poses no danger to people.
- We will provide further updates as soon as more information is available.
Questions and answers about the slurry leak:
1. What happened and why?
During the night of 13 October 2025, sanitised slurry began leaking from a tank at Mäntsälän Biovoima. The leak was connected to the commissioning of a new bioreactor. Our staff detected the situation immediately and began containment measures. The leak was caused by a failure in the new bioreactor’s outlet pipe.
2. What is the leaked material? Is it dangerous?
The leaked material is sanitised slurry, which is not dangerous. It consists of organic raw materials that have been treated to make them safe for biogas production. The slurry contains no chemicals or bacteria.
3. Could the situation have posed a danger to people?
The situation has not posed a danger to people.
4. Will this harm the environment?
A small amount of the material entered a ditch that flows into the River Mustijoki. We initially cleaned the ditch using vacuum tankers and then removed its topsoil with an excavator. The slurry that leaked from the plant is used as organic fertiliser on fields and is rich in nutrients. We are assessing any temporary effects on river water quality in cooperation with the Centre for Economic Development, Transport and the Environment.
7. Can biogas be trusted as a form of production when something like this happens?
Yes. Biogas is a safe and environmentally friendly form of production. This was a highly exceptional incident that occurred during the testing phase of an expansion project, before actual production began. The incident does not affect the general safety or reliability of the process.
8. There have been odours in the area. When will these disappear?
We are cleaning up the slurry that leaked into the surrounding area so that the odours disappear as soon as possible.
Further information:
Maria Elojärvi, Communications
maria.elojarvi@aurisenergia.fi