Pyrolysis of faecal sludge and biomass waste for resource recovery in Kampala, Uganda

Abstract

Globally, the rapid increase in non-sewered sanitation services is leading to the accumulation of large quantities of faecal sludge (FS) that need to be safely collected and treated. Pyrolysis is a promising technology for FS sterilisation and resource recovery, however, there is still limited knowledge on the properties and recovery potential of FS chars produced at scale. This study assessed the agricultural and solid fuel value of chars produced at an operating treatment plant in Uganda, that treats FS (from pit latrines and septic tanks) and local biomass waste (sawdust and bagasse). Results were compared with findings for laboratory-prepared excreta chars (from mixed or separated faeces and urine) to identify optimisation pathways via sanitation source control. The phosphorus content of FS chars was promising (4% P w/w), but nitrogen and potassium levels were relatively low compared to typical fertiliser requirements. Feedstocks from urine-diverting toilets could enable further nitrogen recovery from urine and maximise the total nutrient recovery potential. Heavy metal levels were below threshold values published in Uganda, although a need for regulatory guidelines specific to char-based fertilisers was identified. Outlier values were observed, highlighting the importance of regular quality control testing. Solid fuel briquettes prepared from carbonised FS and biomass waste were incorporated into the local market, mainly due to their slow burning properties and affordability, and despite their low calorific value compared to commercial standards (HHV = 12.5–16 MJ kg⁻¹). The high ash content of FS chars (∼70% w/w) was the limiting factor for improved briquette quality, hence source control to limit inorganic contaminants (e.g. lining latrines) and urine diversion to separate organic and inorganic excreta streams were identified as suitable interventions to maximise the energy value of FS-derived briquettes (HHV = 20–22 MJ kg⁻¹ possible for outputs of source-separating toilets mixed with biomass waste). This research provides novel field-based insights into FS pyrolysis in low-income settings, highlighting the importance of both strategic sanitation design and improved treatment efficiency to maximise resource recovery at scale.