Suitability of fecal sludge from composting toilets as feedstock for carbonization

Composting toilets provide a promising decentralized alternative to sewer based sanitation systems. However, the valorization of fecal sludge from composting toilets (CFS) by conventional technologies is limited. In this study, we evaluated carbonization as an alternative valorization route for CFS, providing a time-efficient, decentral applicable and hygienically safe treatment option. CFS was carbonized using either pyrolysis (at 500 °C for 1 h) or hydrothermal carbonization (HTC, at 200 °C for 4 h). CFS exhibits promising feedstock characteristics, such as high total solids (TS, 21 ± 9%) and low ash content (9 ± 2%_TS). HTC of CFS exhibits high char yields (75 ± 7%) while pyrolysis char from CFS exhibits elevated higher heating values (25 ± 2 MJ kg⁻¹) and low ash content (24 ± 5%_TS) compared to chars produced from fecal sludge (FS) from other sources. Further calculations showed that FS with low TS amended with sawdust can be used as inexpensive feedstock to achieve energy self-sufficient pyrolysis operation.