Co-pyrolysis of sewage sludge and coffee husk produces NPK-enriched biochar with improved carbon stability indicators

Sewage sludge (SS) is a nutrient-rich waste, but it typically yields biochars with high ash content, low K concentration, and limited C stability. In contrast, coffee husk (CH) is a lignocellulosic residue rich in K and organic carbon. This study evaluated how co-pyrolysis of SS and affects NPK enrichment and complementary indicators of C stability in the resulting biochars. SS, CH, and their mixtures at 80:20, 60:40, 40:60, and 20:80 (SS:CH, w/w) were pyrolyzed at 500 °C for 1 h under N 2 . The resulting biochars were characterized for yield, pH, electrical conductivity, proximate and elemental composition, nutrient contents, Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetric/derivative thermogravimetric analysis, and C stability indicators. Biochar yield decreased as the CH proportion increased, whereas pH, electrical conductivity, fixed carbon (FC), organic carbon, and K increased. In contrast, SS-rich biochars contained higher levels of P, S, Cu, Fe, and Zn. Co-pyrolysis produced biochars with total NPK contents ranging from 12.6 to 14.5%, with intermediate mixtures showing a more balanced N–P–K profile. Intermediate mixtures, particularly SS60CH40 and SS40CH60, represented a suitable compromise between NPK enrichment and improved carbonaceous characteristics compared with SSB. The contrasting responses among the thermostable fraction (TSF), remaining fixed carbon over time (CCy,t), C sequestration potential (CS), oxygen-to-carbon ratio (O/C), hydrogen-to-carbon ratio (H/C), recalcitrance index (R 50 ), and volatile matter/fixed carbon ratio (VM/FC) confirmed that C stability should be evaluated using combined indicators. Overall, SS–CH co-pyrolysis produced biochars with complementary nutrient and C-stability characteristics, and intermediate mixtures provided a suitable balance between NPK enrichment and improved carbonaceous properties.