Solar Pasteurisation for Anaerobic Digestion Sludge

Anaerobic digestion is a waste-valorization process that converts human waste into biogas and fecal sludge as a by-product. The resulting sludge contains fecal matter and pathogens responsible for a variety of diseases, with diarrhea being particularly prevalent in Sub-Saharan Africa. To reduce the risk of infection from the waste stream, this sludge must be sanitized before disposal. Solar pasteurization is a sanitation technology that utilizes solar energy to heat sludge beyond the survival threshold of pathogens. Effective pasteurization requires both high temperatures and sufficient holding time to eliminate microorganisms. This study explores the adaptation of an off-the- shelf solar water heater, available in Kenya, to pasteurize anaerobic digester sludge derived from human waste. The pasteurizer operates with a convection loop to evenly distribute temperatures within the system, a heat exchanger to recover heat from the treated sludge, and a thermostatic valve set to 78 °C to regulate the temperature at which the sludge exits the system. During fieldwork in Nairobi, conducted in the winter months with low solar irradiance, the valve’s temperature setpoint was not reached, preventing the collection of fully processed sludge samples. However, by analyzing samples from both the anaerobic digester and the main tank of the pasteurizer, it was found that the system could achieve low-temperature pasteurization, resulting in a 99 % reduction in E. coli concentrations. A basic Python model was developed to simulate pasteurizer operation using the temperature and solar irradiance data collected in Nairobi. Simulations were also conducted using irradiance data from Narok and Homa Bay, focusing on the effects of three control parameters: the thermostatic valve setpoint, solar collector area, and heat exchanger efficiency. From this analysis, it was determined that the solar collector area and the thermostatic valve setpoint are the most effective levers for improving system performance. Increasing the collector area enhances productivity (i.e., the volume of sludge processed), while raising the temperature setpoint of the thermostatic valve extends the holding time, which is crucial for thorough pasteurization. However, productivity and holding time are inversely proportional – increasing one typically reduces the other. What makes these two parameters effective is that they allow for improving one target metric without severely compromising the other, making them ideal control levers for optimizing system performance. After one month of operation, maintenance revealed significant corrosion of the sacrificial anode inside the main tank. To prevent further degradation of components from sludge exposure, a two-cycle heating system is proposed, where water would act as the working and heating fluid for sludge pasteurization. When implemented in the right location, a solar pasteurizer system could offer improved sanitation at a lower cost and with less space required than conventional post-treatment methods.