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A new anaerobic digester system was built in our study, which integrated a solar collector and a greenhouse to provide heating requirement for maintaining mesophilic or thermophilic digestion.A comprehensive, fundamentally based mathematical model was developed that can predict the hearing energy requirements; net heat losses and solar energy gain for the system.The model accounts for heat losses through the digester wall and top cover material.Solar energy transmitted through a greenhouse in which the digester has been placed is also accounted for in this model.The model can be used to predict energy requirements and net solar energy gain for the digester system on a daily, monthly or yearly basis.The performance of the digester was investigated experimentally using water and the results indicate that the integration of a solar collector and greenhouse reduces the heat losses, therefore, the heating energy requirements for the digester placed inside the greenhouse as compared to the digester placed outside the greenhouse.It is found that the lowest energy requirement to operate the digester occurs during the hot summer month of July and the highest occurs during the cool spring or autumn month of April and October.The temperature of the digester system could be maintained in the range of 25-35℃ during spring and autumn days and 50-55℃ during summer days, which are close to mesophilic and thermophilic temperature respectively.