The potential of nature-based adaptation solution in municipal wastewater sector: willow planting systems as GHG emission reductants in Latvian villages

Nature-based solutions (NbS) are known to be important measures that can help reduce climate change effects while providing environmental, social and economic benefits.

This study presents one of the evaluated examples of mitigation and adaptation in the wastewater management sector: the potential of willow (Salix spp.) plantations in different regions of Latvia. They are considered to be cost-effective and highly efficient solutions for recovering nutrients in wastewater and also provide biomass that can be used for energy production.  

The particular study approximated the number of persons in households not connected to centralised wastewater treatment plants or using poor quality biological treatment plants in different regions of Latvia according to Latvia`s National Inventory Report under the UNFCCC Greenhouse Gas Emissions in Latvia from 1990 to 2022. Overall, 24% of private persons discharge inadequately treated domestic wastewater into the environment, accounting for 99.8% of methane emissions in municipal wastewater sector.

It is known that willow plantations are used for wastewater treatment in Denmark, Sweden and southern Finland (https://doi.org/10.1016/j.scitotenv.2020.138620), but their use in northern regions may be limited due to climatic conditions, as the efficiency of wastewater treatment decreases at low temperatures. Taking this into account, regions in Latvia where willow plantations would be more effective were initially assessed.  Overall, trends in climate parameters gave reason to believe that the western regions of Latvia are already suitable for the establishment of willow systems.

The IPCC (2006) methodology for calculating GHG emission reduction was used.  Main assumptions used in the evaluation of the implementation of the measures: assumption that all households without appropriate domestic wastewater treatment are connected to the system; assumption that biological treatment plants of adequate quality and efficiency are in place.  The willow system is designed to accumulate as well reduce N & P and their efficiency depends on correct operation. It should be noted that the system requirements depend on the water consumption and pollution load.

The cost of installing such systems in the first year will be the highest, but as the indicative lifetime of the system is 20 years, the long-term average cost could be around €440/tCO₂eq. Negative aspects or impacts as shown by studies  are most related to the cost of planning directly for biomass collection (on average 15 minutes mowing per 100 m²) as they should not be overgrown, to the approximately 12 hours of regular annual maintenance and to extreme rainfall events during which water levels have to be monitored.

From an adaptation point of view, there are several known positive aspects of willow planting, such as reducing flood risk. Willow plantations increase evaporation and slow down the spread of water in the floodplain. They also provide several ecosystem services, for example, they attract pollinators, supporting biodiversity, as well as improve the aesthetic value of the territory.