Digestate from biogas production is commonly utilised as a raw material for soil improvement products and growing media which are used in landscaping. The Sustainable Biogas project investigates, on a small scale, the types of nutrient runoff risks that may arise from the use of sewage sludge-derived digestates in landscaping. The review of runoff risks focuses in particular on the discharges of main nutrients, phosphorus and nitrogen, to surface water and soil. The monitoring also follows the levels of heavy metals and certain contaminants.

“There is not much research data available on the potential nutrient risk associated with landscaping. According to a report by the Envirogreen project (2010), phosphorus and nitrogen are leached, especially from black substrates that are still without vegetation. We hope that the monitoring to be carried out in the context of the Sustainable Biogas project will provide new information on the matter”, says project manager Anna Saarentaus from the John Nurminen Foundation.

The monitoring of runoff water is carried out in the Turku region in Paimio and Kaarina at conventional landscaping sites. The Paimio site is a middle lane of a road and it is to be greened with grass, while the Kaarina site involves the landscaping of an underpass of a pedestrian and bicycle way. The Paimio site was built at the end of 2020 and the Kaarina site in the spring of 2021 with the help of growing media made from the digestate of Gasum Oy’s Topinoja biogas plant.

“The pilot sites were selected in cooperation with the soil product manufacturer, contractors, the cities of Paimio and Kaarina and the ELY Centre. We are very happy that all stakeholders support the monitoring experiment”, says Sanna Tikander from the ELY Centre for Southwest Finland.