Last August I had chance to visit the city of Stockholm and hear about their biochar efforts. It is really impressive what they have succeeded within past five years or so, much thanks to the enthusiasm of Mr. Björn Embrén from City of Stockholm’s Landscaping Department.
It all started more than 15 years ago, when Björn noticed that trees perform poorly in compacted urban soils. It was costly to plant trees, fertilize and take care of them- and if they were not doing well, they had no landscaping value and they had to be replaced. On the other hand, there were trees growing on sides of rock walls or on top of old chimneys, where rarely any soil existed- so it was not soil that the roots needed but effective aeration (CO2 away and oxygen to the roots), moisture and nutrients.
They then started experimenting with structural soils, a planting holes with a layer of big, 3-7 cm diameter rocks in the bottom and soil washed between the rocks.
This was a success, and as the next step, in 2009, they switched to using
biochar instead of soil for all of their urban tree planting projects. They have found that the main advantage of biochar for urban tree
projects over other soil amendments is that it resists compression and
compaction. The crushed stones and
biochar mix results in an improved porosity in the soil (roughly 40%). Increased porosity facilitates better gas
exchange, permeability leading to improved root penetration.
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| Schematics from Björn Embrén. |
A further improvement on their methods has been to have the biochar and
stones delivered pre-mixed. They have found
that this saves up to 25% in time and labour costs. The preferred
biochar particle size of the biochar is between 1 – 10 mm and currently
they recommend approximately 2.25 cubic meters of biochar for each urban tree (likely mixed with rocks already, but still quite a lot!). This explains why loads of biochar from Europe have been shipped to Stockholm, mostly from Germany but also from RPK Hiili OY (Mikkeli, Finland).
In order to be truly sustainable, however, a goal of using locally
produced biochar from underutilized biomass was set. To attain this goal
the City of Stockholm initiated the
Stockholm Biochar Project
with the objective of producing both biochar and renewable energy using
urban green waste collected from municipal parks as well as city
residents. This particular biomass is often difficult to dispose of and
very much underutilized.
The Stockholm Biochar Project team succeeded in winning one of the
five coveted prizes of the 2014 Mayors Challenge funded by Bloomberg
Philanthropies and the EUROCITIES organization which netted them 1
million euros to be used to set up their initial pilot plant.
We visited the plant last summer and according to
latest news in the Biochar journal, it will produce its first tons of biochar within a few months from now, in summer 2016! The pilot
plant will start from producing 300 tonnes of biochar per year and at full
scale, will reach 1,500 tonnes per year. The heat generated during production will be added to a local or
district heating network. Biochar will be used both by city residents
and by city landscapers in public parks and urban tree beds. Probably we will soon see many other cities following the example and getting their green waste recycled, carbon fixed and trees happy!
Read more and see images at:
https://www.biochar-journal.org/en/ct/77-Planting-Urban-Trees-with-Biochar
