Stockholm's Biochar Project — Turning Park Waste into Urban Carbon Storage and Better Tree Soil

Stockholm's parks and gardens generate large volumes of woody green waste. Rather than composting it or burning it purely for energy, the city - through its municipal water-and-waste utility, backed by a 2014 Bloomberg Philanthropies Mayors Challenge award - built pyrolysis plants that convert the waste into biochar, a stable, high-carbon charcoal-like material, while also recovering heat for the district-heating network.
C40 Cities' 'Cities100' case study reports a target of roughly 7,000 tonnes of biochar a year, later described elsewhere as up to 8,000 tonnes across five facilities, sequestering an estimated 25,000-25,200 tonnes of CO2 annually - equivalent to removing several thousand cars from the road - while producing around 25,000 MWh of heat, enough for several hundred apartments. The biochar is sold to city boroughs and mixed with compost and recycled rock into 'structural soil' placed in tree pits and rain gardens; the utility's planning and development director told journalists in 2019 that tested biochar quality was high enough to fetch a premium price, and the structural soil is designed to improve water and oxygen penetration to street-tree roots.
The tonnage and CO2 figures available are the programme's own operating targets, repeated across C40 and city-linked sources, rather than independently audited annual production totals or third-party-certified carbon-removal volumes; C-NAPSE scores the carbon and water-soil dimensions to reflect a credible but self-reported, target-based evidence base.

Read the full analysis: https://bloombergcities.jhu.edu/news/stockholm-biochar-project-mayors-challenge-winner-opens-its-first-plant-bloomberg