Torrefaction is explored as a thermal treatment process for solid human excreta to produce pathogen-free biochar while preventing harmful gas emissions. A non-sewered sanitation system (NSSS) was developed by Cranfield University with funding from the Gates Foundation. The torrefaction process was integrated in the solid processing part of the system, which is being tested in compliance with ISO30500 standard. Systematic testing of operational parameters revealed that the resulting biochar materials exhibited distinct physical, chemical, and microbial properties. Capturing this variability, a new rheology chart was developed and presented to classify the biochar from Rheology State (RhS) 0 to 7. The chart grades the product based on colour tone (light brown to black), increasing hydrophobicity, lower moisture content, and higher fixed carbon content. The scale also indicates pathogen reduction (i.e. Escherichia coli).
This rheology chart reduces the need for repeated, detailed characterisation in future studies and will be useful for various applications. It will also help optimise operational parameters according to specific application requirements.