Author(s): Maria Velmitskaya; Pekka Rossi; Maarit Liimatainen; Elisangela Heiderscheidt
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Abstract: Decentralised wastewater treatment facilities (on-site and small scale) play a vital role in Finland’s sanitation system, as roughly 15% of the population (approximately 800,000 people) reside in permanent residences that are not connected to centralised sewerage networks (Lapinlampi, 2021, as cited in Laukka et al., 2022). Additionally, as of 2024, Finland had around 495,000 leisure homes, most of which are not connected to municipal sewer networks, resulting in additional pressure on decentralised wastewater systems, particularly during the summer and holiday seasons (Statistics Finland, 2025). Although decentralised units are typically small in scale, they can have a considerable environmental impact. In regards to the release of contaminants with discharging water, research indicates that upgrading 20 malfunctioning wastewater treatment systems (serving 20–99 PE) in the Helsinki and Vantaa regions led to reductions in biological oxygen demand (BOD) and phosphorus (P) loads equivalent to those produced by a 10,000 PE wastewater treatment plant (Luodeslampi et al., 2019, as cited in Laukka et al., 2022). Moreover, one of the past projects at the University of Oulu (ON-SITE, Interreg Nord) revealed low compliance with current regulations for on-site wastewater treatment systems, even though the median age of the units studied was only 3.8 years (Kinnunen et al., 2023). While data on water-linked emissions are lacking, information on GHG emissions from decentralised facilities in Finland is non-existent. Current knowledge of GHG emissions from decentralised sanitation largely comes from a limited number of studies conducted in regions where these systems dominate - typically in developing countries with warm climates (e.g., Cheng et al., 2022). For climate impact assessment, IPCC default emission factors for sanitation are used, but these are widely recognised as having significant limitations (e.g., Ramirez-Melgarejo et al., 2020), which are even more pronounced when assessing sanitation in the Nordic countries, as they do not account for climatic conditions and different types of treatment systems.
Year: 2026