Sludge Pasteurization with Solar Water Heaters – System Optimization

This thesis addresses unsafe sludge management in low-resource settings by further developing a decentralized, solar-powered pasteurization unit for digestate from school-based anaerobic biogas digesters in Nakuru, Kenya. The unit used a solar water heater as a gravity-fed treatment chamber built from locally available materials. Field experiments during the rainy season assessed how heat exchanger redesign, nighttime flow closure, and tank insulation affected thermal performance and pathogen inactivation, while raw and treated sludge were analyzed separately for microbiological and chemical properties. The redesigned heat exchanger increased operating temperatures by up to 8 °C per day, and the 150-liter pasteurizer achieved non-detectable E. coli and total coliforms at a 3 L/h flow rate under moderate irradiance, though performance dropped during consecutive cloudy days.