Carbon flows and biochar stability during co-pyrolysis of human faeces with wood biomass†

As non-sewered toilets are now the most commonly used sanitation facilities, the faecal sludge management (FSM) sector is starting to be recognised as an important actor for global carbon management. The development of systematic strategies to calculate avoided emissions and carbon storage opportunities is currently constrained by a lack of understanding of carbon flows during faecal sludge treatment. This study investigated carbon sequestration potential for faecal sludge treatment systems that involve co-pyrolysis of human faeces (HF) and wood biomass (WB) at different blending ratios HF : WB (100 : 0, 75 : 25, 50 : 50, 25 : 75, 0 : 100) and temperatures (450, 550, 650 °C). The systematic investigation of analytical biochar stability parameters and the quantification of carbon flows among pyrolysis products were carried out for the first time in the context of faecal sludge. The stability of the produced biochars was assessed based on their remaining volatility, carbon structure (H/C and O/C ratios, SEM and FTIR analyses) and oxidation resistance (chemical oxidation by H₂O₂ and thermal degradation by thermogravimetric analysis [R₅₀ index]). Overall, co-pyrolysis of HF and WB improved carbon fixation and biochar stability, enhancing carbon sequestration potential compared to pyrolysis of pure faecal feedstocks. Biochars produced from 50 : 50 HF : WB blends at 550 °C had the highest carbon retention (41.1%); this feedstock blending ratio corresponds to ∼30 g dry wood added in toilets as a cover material (per user per day), based on the expected daily excretion quantities. For these conditions, the H/C, O/C ratios, H₂O₂ oxidation and R₅₀ index values suggest that the produced biochars have developed an aromatic structure and are suitable for long-term carbon storage. The biochar characteristics were found to be more dependent on feedstock composition than pyrolysis temperature – provided that the temperature reached was sufficient to ensure completion of the main pyrolytic reactions (≥500 °C) – while carbon flows to the bio-oil and non-condensable gas fractions were significantly influenced by pyrolysis operational parameters (retention time and inert gas flow rate). The formation of CaCO₃ was observed via SEM/EDX and can be further investigated as a potential additional carbon storage mechanism in FSM. The findings of this research can be used to create a methodological dataset to inform carbon assessments and future modelling applications, paving the way towards the establishment of carbon-negative FSM.