Under normal conditions, how long do solids remain in an anaerobic digester?

In an anaerobic digester, solids typically remain for a period of 15 to 20 days under normal operating conditions. This timeframe is crucial for the digestion process, where microorganisms break down organic material in the absence of oxygen. The retention time allows sufficient contact between the solids and the microbial population, promoting effective digestion and biogas production.

During this period, the anaerobic conditions maintain the activity of bacteria that process the waste material. The balance of time spent in the digester is important, as too short a retention time may not allow complete digestion, while too long could lead to complications such as the production of less biogas and potential accumulation of toxins.

In the context of the choices presented, the option indicating 15 to 20 days aligns with standard practice and scientific understanding of anaerobic digestion processes, making it the correct answer.