When biodigesters operate, they emit heat from the bioreactions that take place and since the reactor walls are made of high-density plastic material say high-density polyethylene (HDPE), you could use a coil around the tank walls with water flowing through the coils. Depending on the desired temperature, water flow rate through the coil could vary to keep the set temperature inside the biodigester.

Hope this helps answer your question.

Professor Yehia Khalil, Yale University, USA

Fellow of the University of Oxford, UK

I would like to suggest reads the following papers:

1-https://www.standort-ludwigshafen.basf.de/group/corporate/site...

2- https://energypedia.info/wiki/Electricity_Generation_from_Biogas

thanks Mr Yehia F. Khalil for your candid suggestion.