Anaerobic digestion in simple digesters depends on the temperature inside the digester, which is
influenced by external conditions [17]. Simple, efficient methods to increase the temperature inside
digesters could provide sustainable solutions to improve biogas production [12]. Two sources of
energy to heat the digester have been proposed: solar energy and energy from biogas combustion [14].
Covering the digester with a greenhouse can provide sufficient energy to heat the digester, as shown
for an Indian floating-dome digester [17,18]. The greenhouse increased the temperature during the
daytime but was only able to maintain an acceptable nighttime temperature when insulation was used
in the construction. In South America, greenhouse heating of a bag digester increased the slurry
temperature [19].
A fixed-dome digester heated by heat exchange with water has been designed in which heat is
generated by solar captors and circulated inside the digester during sunshine hours [20]. The
temperature of the slurry in this system can also be increased by mixing the slurry with additional hot
water from the solar panels, which is feasible because the slurry for simple digesters is generally
diluted with water to enhance through flow [21]. While this solution is interesting, it is important to
bear in mind that increasing the temperature may be achieved at the expense of increasing the volume
of the digester.