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Enhanced Biosolids Drying with a Solar Thermal Application

Domènec Jolis, Natalie Sierra

Covered, green-house type biosolids solar drying facilities provide a low-energy option with operational simplicity and reduced cost. However, their large footprint consequence of low water evaporation rates make them unattractive for large wastewater treatment plants in urban areas with limited available space. This project investigated whether recent advances made in solar thermal technology conferred sufficient benefit in water evaporation rates that solar drying of wastewater biosolids may be feasible. A demonstration solar drying chamber was constructed with warm air from a solar thermal panel being routed to the chamber to aid in evaporation. Experiments were conducted with water alone to measure water evaporation rates in a range of weather conditions and to develop a regression model for evaporation. Experiments were also conducted with digested, dewatered biosolids to measure evaporation rates when drying biosolids. Total solids concentration in biosolids samples reached 42.3% after 102 hours in the dryer. Data showed that evaporation rates strongly depend on the temperature inside the dryer chamber but also on biosolids mixing. Measured evaporation rates were more than twice those previously reported in the literature for solar dryers and imply that with an experimental setup optimized for mixing, humidity control and energy recovery, still higher rates could be achieved. If confirmed in larger scale demonstration projects, the results from this study would allow for compact solar dryers to be located in urban settings.