Faecal sludge treatment by viscous heating : a paper presented at the second conference on developments in faecal sludge management in Durban, Sout...

Demonstrated is that viscous heating of faeces through an extruder works well. The first prototype, designed by computational fluid dynamics to obtain performance data over a range of operating conditions, was tested in the laboratory with simulant materials. The laboratory-scale unit is 10 cm long with a rotating central core and a space with the shell wall adjustable from 0.75 to 1.25 mm. Experiments with 5 Pascal-second viscosity simulants and the smallest spacing achieve 200°C within 3-4 minutes. Additional time to process the mass generated by an individual requires only a few additional seconds. At larger spacing the observed temperature is lower, but shear stress and pressure remain as microbial destruction mechanisms. If the unit is operated continuously the 10 cm size is sufficient to treat the mass produced by more than 1000 people. With the current geometry the simulant leaves the extruder as a moist, hot mass. While moist heat is effective in destroying microorganisms, water must escape to dry the solid. A possible design variation that dries the solid includes spreading the mass into a thin layer for water evaporation. This concept will be addressed prior to completion of Phase 1. A challenge is that spacing between the shell and the core is small and objects, gravel or sand in faeces will require screening prior to extrusion. Alternative geometries are being considered for high-volume sludge processing and pit extraction applications. Extrusion may also be integrated with other treatment technologies if parasite destruction would benefit the overall process. A second-generation prototype is under construction for presentation at the Reinvent the Toilet Fair that is simpler and easier to visualize in a small-scale toilet application. [authors abstract]