Faecal sludge pyrolysis as a circular economic approach to waste management and nutrient recovery.

The disposal of faecal sludge from non-networked sanitation amenities leads to contamination of the surrounding environment and increasing public health problems across developing countries. Permanent solutions to deal with faecal sludge are required to solve the sanitation crisis and achieve the Sustainable Development Goal (SDG) 6 "ensure availability and sustainable management of water and sanitation for all" by 2030. Full-scale pyrolysis of faecal sludge in developing countries is fast becoming a safe and long-term option. Pyrolysis not only eliminates pathogens within the sludge but produces biochar as an end product which has the potential as a soil amendment to increase crop yield. In general, faecal sludge biochars have high pH values, high ash contents, and high macro-and micronutrient concentrations. Compared to biochar from lignocellulosic materials, faecal sludge biochar contains less carbon and exhibits lower porosities, and lower surface areas. However, evaluating the properties of faecal sludge biochar is difficult due to the different technologies used in collection, storage, and transportation of the feedstock. Differences in faecal sludge characteristics based on location, climate, age of the sludge, type of sanitation technology and seasonality are also factors in determining the properties of faecal sludge biochars. These factors contribute to the difficulty in describing faecal sludge biochar properties in general terms, and there is an argument to be made that characteristics of large-scale faecal sludge biochar should be determined on a case-by-case basis. The conclusion of this review is that future research should concentrate on short-term and long-term field studies of faecal sludge biochar application to different soil types.