Smouldering combustion as a treatment technology for faeces: Exploring the parameter space

•Faeces mixed with sand can be smouldered in a self-sustaining process.•This process achieves elimination of biological hazards.•A robust self-sustaining region for different parameters is identified.•Is promising as the basis for a new, energy efficient waste treatment approach.

The poor management of human excreta in developing countries is among the most prominent global issues due to its negative impact on public health. This work demonstrates for the first time that self-sustaining smouldering of faeces mixed with sand is a feasible alternative to incineration for rapid destruction of waste. Self-sustaining smouldering requires minimal energy input and pre-drying of faeces compared to incineration. This process ensures the elimination of biological hazards via long residence times (>20 min) at high temperatures (>400 °C). Surrogate faeces which exhibits similar energetic, thermal, and mechanical properties to real faeces are used in this study. The parameters controlling the combustion process including moisture content, airflow rate, and sand-to-faeces ratio are mapped to establish the range of conditions where self-sustaining smouldering of faeces can be achieved. Experiments were conducted within the ranges 0–75% for moisture content, 7–108 g/min for airflow rate and 2.75–11.9 g/g for sand-to-faeces (wet basis) ratio. Preliminary validation of the parameter space is done using real dog faeces. In this work, the parameter space defining the range of conditions where self-sustaining smouldering occurs is mapped. Results show successful self-sustaining smouldering of faeces for moisture contents of up to 60%, airflow ranging from 10 to 100 g/min, and wet sand-to-faeces ratio greater than 3.25. This proof-of-concept for a smouldering reactor to treat human solid waste demonstrates that smouldering of faeces could be the basis for a new, energy efficient waste treatment approach.