Thermal Exchanges in a Covered Lagoon Biodigester Treating Pig Farm Effluent Heated by Solar Energy

Abstract

Covered lagoon biodigesters are widely used in Brazil for treatment of agro-industrial effluents; however, under natural conditions, they operate at temperatures below the ideal. Thus, external sources for heating the effluent can enhance reactor performance and optimize thermal exchanges between the biodigester and the environment. This study aimed to evaluate the thermal exchanges in the internal and external environments of a covered lagoon biodigester from the influence of effluent heating through solar energy. Mathematical modeling (EnergyPlus software) was used as a tool to simulate thermal exchanges and obtain heat transfer rates. To do so, two scenarios were considered: with and without heated effluent. The results showed that solar radiation is the primary source of heating for anaerobic reactors and that the high thermal inertia of the soil contributes to a small variation in temperature of the resident biomass over the course of the day, even in the scenario with heated effluent. The temperature of the resident biomass reached and stabilized at 30°C, even after thermal exchanges with biogas and soil, in both hot and cold periods when heating was applied.