Sludge-based biochar tested in large-scale cultivation

Sewage sludge is a byproduct of municipal wastewater treatment plants that contains essential nutrients such as phosphorus and nitrogen, as well as organic matter that increases soil organic matter content. Therefore, it serves as a good fertilizer to spread on agricultural land. However, the sludge also contains contaminants, which has created uncertainty regarding the risks of using it.

A promising method to reduce contaminants in the sludge is to convert it into biochar. This is done through pyrolysis, meaning the sludge is heated to high temperatures in the absence of oxygen. The conversion to biochar also removes water, which reduces both transport requirements and the risk of soil compaction.

− Sludge-based biochar has the potential to become a safe and circular resource, but there is still a lack of practical knowledge about how the material performs in real-world applications and what conditions are required to develop a functioning market, says Sabina Braun, a researcher at the Department of Soil and Environment at SLU. She leads the field trials and demonstration crops in the project Sludge biochar – From Pollution Risk to a Safe Circular Resource.

Newly launched tests on Gotland 

The project has recently spread sludge-based biochar on its first large-scale demonstration crop on agricultural land in Hallfreda on Gotland. Several similar demonstration crops are planned in various locations across Sweden over the next two years.

− The purpose is to study how the spreading and handling of sludge-based biochar works in practice on a larger scale, and to investigate the effect of sludge-based biochar on crop emergence and yield levels, as well as on soil carbon content and available phosphorus, says Sabina Braun.

The demonstration crops will be complemented by eight field trials distributed across the country, where researchers will examine the effects of sludge-based biochar on both soil and crops in greater detail.

− What we hope to achieve with the project is to develop functioning circular solutions where resources from the wastewater system can be returned to society in a safe manner, while reducing the risk of spreading contaminants, says Sabina Braun.