Tennessee Wastewater Operator Practice Exam

Solids leave an anaerobic digester primarily through which process?

1. Natural sedimentation

2. Centrifugation

3. Sludge withdrawal and decanting

4. Evaporation

The correct answer is: Sludge withdrawal and decanting

The primary process by which solids leave an anaerobic digester is through sludge withdrawal and decanting. In anaerobic digestion, organic material is broken down by microorganisms in the absence of oxygen, producing biogas and leaving behind digested solids, often referred to as sludge. Sludge withdrawal involves removing this accumulated sludge from the digester, which is a critical step in maintaining the system's efficiency and preventing the buildup of solids that could hinder the digestion process. Decanting is often used after sludge withdrawal to separate the liquid phase from the solids, allowing for the recovery of effluent while retaining a portion of the solids for further treatment or processing. Natural sedimentation and centrifugation are processes that can assist in the separation of solids in other contexts, such as in secondary clarification or when further processing sludge. However, they are not the primary mechanisms by which solids exit an anaerobic digester. Evaporation does not play a significant role in the solid removal process within the digester system. Therefore, the process of sludge withdrawal and decanting is integral to the operation of anaerobic digesters, ensuring that solids are effectively managed within the treatment process.