Microbial evaluation of the viscous heater for commercial applications in faecal sludge treatment

Risk of infection from human pathogens by contact with untreated faecal sludge (FS) poses a threat to worker health. The aim of this study was to assess the commercial potential of a 1000 L h-1 viscous heater (VH) to minimise this risk as a component of a black soldier fly larvae (BSFL) production pipeline. Changes in source material properties during sludge processing, temperature stability and microbial treatment efficacy were evaluated. Inactivation of heterotrophic bacteria in FS treated in the VH was measured at 60 ℃ and 80 ℃. Approximately 1‑ to 3-log inactivation was observed, independent of residence time. Maintaining temperature stability proved difficult with variable sludge viscosity and manual control of flow rate and engine power. Adding operational controls based on effluent temperature would compensate for variable sludge properties. Preparing FS for treatment in the VH proved challenging due to the small particle size required and the large quantity of detritus present.