Sunken logs work as a bio-purification system

Structures of sunken logs can be used to purify the runoff from forest ditches by slowing down the flow of water and restraining the flow of nutrients brought by flood waters that would otherwise end up in waterways.

Wooden structures built in forest ditches are gradually covered by bacteria, algae and mycelia, which attract other species. In turn, these microbes are eaten by frogs, insects and fish further along in the food chain. As a result, the amount of nutrients dissolved in the water is reduced.

This topic was studied in the PuuMaVesi project, run by the Finnish Environment Institute, which also involved Metsä Group as a partner. In field tests, sunken log structures were shown to reduce solids by nearly 80 per cent, and nutrient and humus content by 40 per cent, during flood periods. The diversity of benthic organisms also increased considerably.

Nature’s own purification method works efficiently, and adding sunken logs into sedimentation basins and gullies is a cost-effective and easy solution. The materials can be found on the spot in the forest, making the method easy to apply to the planning and implementation of drainage, felling and waterway restoration. The need for restraining nutrients will increase due to the milder and wetter winters.

Bundles of wood purify the runoff from drainage areas

On the main estate of the Vihtori Peltonen-Lihasula foundation in Eerola, Kangasala, bundles of wood were placed in water to purify runoff flowing through the sedimentation basin of the old drainage area. Here, the ditches of some five hectares of peatland were cleared considerably more than twenty years ago in connection with mounding following regeneration felling.

“The runoff first flows along the ditch into Säynäjärvi and from there into Vesijärvi. The shores of both lakes have summer cottages. With this project, we aim to further reduce the nutrient input caused by the drainage area,” says forest manager Kaj Karlsson, who is chair of the foundation’s board of directors. The foundation is Metsä Group’s forest asset management customer, and forest specialist Risto Ojanen has been cooperating with Karlsson since 2007.

On the site, a forest worker first cut down trees between the runoff ditch and the forest road. An excavator then dug out the trench where the bundles of wood were to be placed. According to Ojanen, the trench was widened only enough to create a small basin for the bundles. “We tried to leave the surroundings as untouched as possible otherwise.”


New service to be included in drainage water protection

Metsä Group is collecting experiences from the project concerning the use of wood for water purification. Other similar trials are also underway. The Lihasula foundation and Metsä Group have agreed that the Kangasala site can also be used for training and visits.

The goal is to provide the method as a service to forest owners who are carrying out drainage repair in their forest and would like to purify the runoff from old drainage areas. The new service entered the development stage in late 2020, and the method could be introduced as part of the water protection measures related to drainage.

Read more about the PuuMaVesi project and its results at

This text is based on articles published in Metsä Group’s Viesti magazine, issues 1/2020 and 4/2020.