A biodigester is a large closed container. You put in plant material or animal waste. Bacteria work on it with little or no air, and they make a gas that is mostly methane plus carbon dioxide. People burn that gas for cooking, heat, or electricity. What is left can be used to feed the soil. Farms and some waste plants use them. It is a technical word; in ordinary talk people may just say “the methane tank” or “the biogas tank”. Though actually, digest here is not the digestion in your stomach. The “eating” is done by bacteria, in a tank, not by an animal. It is also not a compost heap in the open air — a biodigester is closed, on purpose, so the methane can be caught.

沼氣池；沼氣發酵槽（日常／農村說法）；生物消化槽（較字面的技術說法）