A Circular Economy Approach to Achieving Greener Cities, Better Urban Water Management & Net Zero: The Stockholm System
For many years the City of Stockholm has been pyrolysing its green waste to create bioenergy and biochar which is contributing to Net Zero and better urban water management while helping to create a greener, cooler city. The biochar is used along with compost and recycled rock to create structural soil which retains stormwater and nutrients and is used in municipal tree pits, raingardens, turf areas and other landscaping. This is providing multiple benefits including:
Healthy trees and larger tree canopies.
Increased drought tolerance and reduced irrigation demand.
Reduced stormwater runoff.
Improved stormwater quality and reduced eutrophication.
The Stockholm system is now being adopted by cities globally. This article discusses some opportunities for applying the Stockholm System to Australian cities and towns.
Figure 1 - Biochar provides multiple benefits which help cities and towns adapt to climate change
Adapting to a changing climate is providing local governments globally with multiple challenges. In addition to achieving Net Zero emissions and zero waste, local governments are responding to the effects of urban heat islands with plans to green and cool cities and towns. The same impervious surfaces which exacerbate the urban heat island effect are also exacerbating flooding. Hotter, drier conditions mean less water overall and higher plant water demand. Water is an essential ingredient to keeping cities and towns green, cool and liveable and needs to be better managed to address these problems.
Stockholm in Sweden has been addressing these issues for around 20 years with an evolving approach which initially used imported biochar in urban street tree plantings. They now produce biochar and bioenergy using an increasing number of pyrolysis plants fueled with municipal green waste. Their approach represents an example of what is now referred to as the Circular Economy, where materials or products are recycled, reused and regenerated in a sustainable or environmentally friendly way.
Figure 2 - Stockholm has multiple pyrolysis units producing biochar and bioenergy from tree prunings and grass clippings thereby contributing to a circular economy.
The Stockholm system
The approach developed by Stockholm uses pyrolysis plants to convert municipal green waste into biogas and biochar. The biogas (also called syngas) is used directly in local heat grids and/or to power turbines to produce electricity which is fed into the network thereby replacing fossil fuels that would otherwise be required, while biochar effectively captures carbon and avoids atmospheric release . The biochar is used in a variety ways including in street tree plantings, rain gardens and turf areas and in association with these, to manage stormwater. Prior to the installation of pyrolysis plants, Stockholm imported biochar from Germany for use in street tree pits. A grant of €1million from the Bloomberg Philanthropy in 2014 enabled the City to pilot their own pyrolysis plant and the production of biogas/biochar. The program has been so successful the City is developing a further 4 pyrolysis plants. The approach is also being adopted by numerous other cities globally. This video provides a good overview of the project.
The initial driver to use biochar was due to poor health and growth outcomes being experienced by street trees because of increasingly harsh urban environments. These were caused by factors such as soil compaction, poor water availability and poor nutrition. Following a visit to Germany the head of the City of Stockholm’s Landscaping Department, Bjorn Embren, designed a new type of tree pit containing structured soil , which comprises a mix of biochar, compost and rock aggregate (macadam). Using this approach it is reported that in some cases 6 year old trees planted in structured soils with biochar were five times larger than 30 year old trees using more traditional urban tree planting techniques.
Figure 3 - Stockholm has reported significant improvements in trees grown with biochar versus those without.
In addition to providing trees with a better growing environment the approach was also found to reduce stormwater runoff and improve stormwater quality. Consequentially it has also been used in the development of rain gardens throughout Stockholm. It is also used in other landscaped areas and garden beds as well as for turf playing fields. Other applications include private and public roof top and apartment gardens and vertical green walls. Now that the City of Stockholm can make their own biochar, they also provide it free to residents for use in home gardens.
Figure 4 - Raingardens and tree pits to retain and filter stormwater runoff while keeping water in the root zone.
Figure 5 - Stormwater runoff is reduced and water quality improved with the use of biochar
What is biochar?
Biochar is a type of charcoal that is produced by heating organic materials of biological origin, such as wood chips, grass clippings, manure, or wastewater solids, in a low-oxygen environment using a process called pyrolysis. This process breaks down the organic matter into a carbon-rich substance that can be used as a soil amendment.
Charcoal is a resistant form of carbon and is created naturally through the burning of forests and natural ecosystems. However, with clearing and intensification of land uses such as agriculture and urban development, its presence in soil has significantly diminished. Australia ranks third after the US and China as having the highest levels of soil carbon loss in the world. This is significant because soil stores 3 times more carbon than either the atmosphere or terrestrial vegetation and makes up two thirds of terrestrial carbon (SOE, 2021).
Biochar has a highly porous structure, which gives it a large surface area. The surface area of a teaspoon of biochar fines can be up to 1,000 square metres. This characteristic allows biochar to retain water (up to 5x its weight) and nutrients in the soil, making it an effective soil conditioner. It also provides a habitat for beneficial microorganisms, promoting soil health and fertility.
Figure 6 - Biochar provides a range of features and benefits
Apart from improving soil quality, biochar has several other benefits. It can remove carbon from the atmosphere, helping to mitigate climate change. The pyrolysis process sequesters carbon in a form that takes hundreds and potentially thousands of years to break down. Because of this, biochar is recognised as a negative emission technology (NET) by the International Panel on Climate Change (IPCC) and there are international market places where organisations, such as Microsoft, purchase carbon credits to offset their emissions.
When used as a soil amendment, biochar can enhance plant growth and reduce the need for synthetic/chemical fertilisers. Additionally, biochar can improve water quality by adsorbing pollutants and preventing their runoff into water bodies.
Biochar is a sustainable and environmentally friendly solution, as it uses organic waste materials that would otherwise be discarded, burned or left to decay, releasing greenhouse gases in the process. It has gained attention as a potential tool for sustainable agriculture and carbon sequestration, with ongoing research and development being undertaken to explore its full potential and localise its use.
Application to Australian cities and towns
While the Stockholm system has been adopted in a number of other major cities globally, it has not been adopted widely by local governments in Australia. Dubbo in regional New South Wales is one exception. Dubbo Regional Council uses biochar in plantings of street trees, planted largely to mitigate the urban heat island effect. Using a modified Stockholm method, the Council has reported significantly improved tree growth and a reduction in water use in the order of 70%. While the biochar used in Dubbo is not manufactured locally, the compost and rock aggregate used in the structured soil are both sourced nearby from recycled materials.
Perth, Western Australia’s capital, with its sandy soils and hot, dry conditions provides many opportunities to use biochar to enhance tree growth and canopy cover and reduce water use, using a circular, regenerative urbanism approach. In 2029 licensed groundwater allocations will be cut by 10% at a time when plant water demand is increasing. Many Councils are also seeking to increase tree canopy cover and the areas of green-blue spaces to cool increasingly hotter urban areas, which will require more water. They are also dealing with increased flooding events while seeking to identify new, non-potable water sources to replace increasing potable water use.
Figure 7 - Trees and landscaping with biochar and compost structural soils are replacing grass verges in Stockholm .
Increases in potable water use are also an issue because potable water is increasingly sourced from energy intensive desalination plants and groundwater replenishment plants powered by GHG emitting energy sources. These same energy sources pump potable water increasingly long distances to homes where around 40% is used to irrigate lawns and gardens.
Many cities in Australia are accommodating population growth with infill development and suburban greenfields development. Structure planning and development processes provide the ideal opportunity to integrate use of biochar, compost and recycled rock to enhance tree canopy cover, reduce water use and manage stormwater to create green, cool and climate resilient environments.
Figure 8 - Biochar is now being integrated into redevelopment areas such as the Stockholm Royal Seaport precinct.
By establishing pyrolysis plants to produce biochar and bioenergy from green waste, local governments can achieve multiple objectives including replacing greenhouse gas emitting energy sources, reducing the costs of disposing of green waste, sequestering carbon, creating greener and more climate resilient urban spaces, and managing water better. They can also derive an income on international markets by selling carbon offsets associated with creating the biochar. Further, they can provide opportunities for new and diversified businesses and jobs.
Towards a planned approach
The City of Stockholm system has evolved over many years of trial and error but the City has done a wonderful job of documenting the process, lessons learned, research outcomes and best practices. There is also a growing consortium of local governments such as the Carbon Neutral Cities Alliance which are funding initiatives such as the Center for Regenerative Solutions to develop local government focused resources to plan and implement bioenergy and biochar production systems.
Figure 9 - Over the last 10 years in Stockholm, significant areas of planting beds have been retrofitted with biochar.
Following initial raising of awareness and understanding of what biochar and pyrolysis are and their benefits, a key next step is to develop a business case for progressing an initiative and developing an implementation plan. It is possible to break down the evolution of the Stockholm Approach into a series of steps or treatments and then apply benefit:cost analysis to each to build the business case. This will be the subject of another article.
