Extreme weather events are increasingly putting human life to the test, especially in urban areas. Better preparing cities for climate change is the aim of blue-green infrastructure projects. Biochar can play a key role in preventing cities from flooding, heat waves and water pollution.

According to the German Federal Statistical Office, an estimated 4.6 billion of the 8 billion people worldwide lived in cities in mid-2023: this is 57% of the world’s population. Based on United Nations (UN) projections, the global urban population will increase to 5.2 billion people by 2030. Urban areas have become the center of people’s lives – and at the same time harbor climate-related risks that need to be addressed.

Due to global warming and the rise in temperatures, extreme weather events such as heat waves, droughts, flooding and storms are on the rise: According to studies by Munich RE analyzing the effects of global warming on weather-related natural catastrophes, the number of extreme weather events worldwide has doubled since the early 1990s. These weather extremes can become a huge threat to human life, especially in urban areas, destroying ecosystems, infrastructure and habitats. Recent flooding like in Porto Alegre (Brasil) and Southern Germany in spring of 2024 dramatically showed one of the main problems in densely populated areas: the water cycle in nature and in cities is different.

Urban water cycle is not designed for increasingly extreme weather conditions

In a natural ground cover, around half of the stormwater seeps away into the ground, while around 40 % evaporates with the help of plants. With 90% thus absorbed by nature, it leaves only 10% wasted as runoff water. With more sealed surfaces the natural water cycle is disturbed, leading to runoff water as high as 55% in cities, as studies show. Rainwater simply can’t seep away on sealed roofs, streets and squares, which makes flooding more likely.

The combination of heavy rains and extreme heat or drought makes this even worse: extremely dry and compacted soils can hardly absorb any water. During heavy rainfall, this leads to catastrophic flash floods.

Sustainable stormwater management aims to reduce stormwater runoff by tackling the problem as close to the source as possible, looking for ways to integrate nature-oriented water cycles in the city. Creating more natural water reservoirs and enabling evaporation is one of the main goals of blue-green infrastructure projects.

Blue-green infrastructure for smart future city planning

Blue-green infrastructure combines landscape architecture, hydrology and urban planning in a holistic way to better prepare cities for climate change and optimize their water cycle. It creates blue areas of the city, like lakes and ponds and green areas such as parks, city trees and green roofs to intercept water. This not only leads to better stormwater runoff to prevent floodings: Good vegetation and effective water systems are necessary to reduce pollution and cool down cities.

A more specific concept in the context of blue-green infrastructure projects is the so-called “sponge city”: cities designed like giant sponges so that rainwater is kept and absorbed where it falls and can drain away safely. The effects of climate change require rethinking urban living, and biochar has an important role to play here.

Urban greening in detail: how biochar can increase its positive impact

Let’s take a closer look at the various ways of making urban areas greener:

• Green roofs and terraces for air purification and to retain precipitation
• City trees and bioswales to keep the area cool and provide habitat for animals
• Parks and rainwater beds as water reservoirs and for more biodiversity

Biochar can be included in all these forms of urban greening. And there are many good reasons to do so because the positive effects of biochar work on several levels at once. The main benefits of biochar for urban greening lie in its ability as soil conditioner and to retain and conduct water.

Due to its porous structure, biochar acts like a sponge that can increase microbial diversity. It holds water and nutrients in the ground where the plant needs them and prevents them from being washed away. As a layer in roads, e.g. biochar can channel stormwater to supply nearby trees – thereby improving stormwater management and plant health at the same time. Included in soil substrates, these co-benefits of biochar can enhance urban greening measures:

• Included in green roofs and terraces, biochar ensures higher water storage capacity and water filtration, while at the same time a lower weight and lower nutrient leaching. Biochar additionally has an insulating effect and can make buildings more energy-efficient.
• City trees and bioswales also benefit from soil substrates with biochar: besides improving the nutrition retention capacity and filtering toxins in the water, it increases tree restoration for a longer plant survival. Adding a substrate enriched with biochar to city trees improves the water-holding capacity and oxygen availability – an important part of above-mentioned sponge city concepts.
• In city parks, biochar enriched soil substrates increase plant health and improve biodiversity as well as the microclimate, leading to higher evaporation and cooling.

Best practice: biochar for resilient city parks and climate-neutral buildings in Sweden

Novocarbo’s biochar has already shown a positive impact in various blue-green infrastructure projects. One example: Swedish construction giant Skanska used Novocarbo’s biochar to enhance the park “Nobelparken“ in the Swedish city of Lund – a natural oasis that combines the advantages of the city with the peacefulness of nature. Here, biochar was applied mixed with compost as a substrate and contributes to the growth and preservation of the 236 different plant species spread over 3.8 hectares.

Biochar also plays an important role in Skanska’s first climate-neutral office building in Hyllie, Malmö, that opened in 2023. The twelve-story building includes planted terraces utilizing biochar – contributing to water absorption, storm water treatment, biodiversity, low weight and the well-being of tenants.

The cooperation between Novocarbo and Skanska is a great example of integrating biochar and its soil conditioning benefits into holistic blue-green infrastructure projects for modern cities.

New research results: biochar can mitigate pollution of water bodies

Increased evaporation and cooling, better plant health, more biodiversity, higher water storage and reduction of stormwater runoff – the list of positive effects of biochar for urban greening is long. And there is even more potential, as new scientific findings show.

A research project by Priyanka Kudige Prakash for HafenCity University in Hamburg in cooperation with Novocarbo examined the effects of co-composted biochar on stormwater management systems. Stormwater runoff in cities brings pollutants into aquatic systems. The study took a closer look at trace organic contaminants (TrOCs), which are a serious threat to the biotic environment.

TrOCs like Benzotriazole are present in widely used materials in urban areas like car brake fluids, dishwasher detergents or paints. Since TrOCs are inefficiently removed in stormwater management systems due to their solubility and low sorption tendency, they are omnipresent in the water bodies harming the aquatic ecosystem and humans.

In an experiment, the research team tested to what extent co-composted biochar of Novocarbo with different physicochemical properties can remove selected trace organic contaminants in blue-green infrastructures. Therefore, they compared different substrate media with and without co-composted biochar in terms of their ability to retain the TrOCs and remove them from water. The results showed: the higher the concentration of biochar in substrates, the lower the contamination with selected TrOCs like Benzotriazole.

The tested substrate with 70% biochar mostly showed the best results with the fewest contamination with TrOCs, leading to the following conclusion: The higher the biochar content, the higher the microporosity or „sponginess“, the higher the sorption of TrOCs.

Harnessing the power of biochar for secure and healthy urban living

The message of these new scientific findings is clear: Amending substrate media for urban greening with co-composted biochar can reduce the pollution of rivers and groundwater. This turns biochar into an essential part of blue-green infrastructure and urban water management: it not only makes plants more resilient to extreme temperatures and optimizes the water cycle, but also keeps the water free from pollutants.

And there’s even more potential: Biochar is produced through pyrolyzing biogenic residues. In this carbonization process, a large amount of the carbon from the biomass is bound in the biochar. Once added to the soil the biochar acts like a carbon sink, storing the CO2 permanently. That’s a smart way of including carbon storage in urban vegetation, soils and buildings.

Biochar can help minimize the climate-related risks of urban areas and make them more secure and livable. Its ability to capture carbon from the atmosphere can furthermore help mitigate climate change and global warming and support cities on their way to net-zero. We at Novocarbo put all our efforts in further developing the potential of biochar as the „natural wonder“ it is in order to shape a sustainable and livable future.