Biochars are carbon-rich soil additives derived from agricultural and other plant waste that could enhance crop productivity (see Nature 517, 258–260; 2015). We suggest that biochars could also be produced from human sewage — an underutilized resource that is rich in soil nutrients and carbon.

Sanitation problems in developing regions would be alleviated by diverting sewage solids into producing biochar, made by thermal conversion in sealed containers. This might even offset the need to install conventional sewage-treatment infrastructure with its higher construction and operation costs.