Tennessee Wastewater Operator Practice Exam

How do solids leave the digesters?

1. Evaporation methods

2. Sludge withdrawal and decanting

3. Filtration techniques

4. Gravity separation

The correct answer is: Sludge withdrawal and decanting

Solids leave digesters primarily through sludge withdrawal and decanting. In the context of wastewater treatment, digesters are designed to break down organic matter in sludge through anaerobic processes, allowing for a reduction in the volume of the solids and stabilizing the remaining material. The process involves withdrawing settled sludge from the bottom of the digester, commonly referred to as sludge withdrawal. This removes the digested solids that have accumulated over time. Decanting, on the other hand, involves the separation of liquid from solids. After digestion, the mixture contains both treated liquid and residual solids. Decanting allows the clear liquid on top to be removed while leaving behind heavier, undigested solids. This method is essential for effectively managing the byproducts of the digestion process. Other potential methods such as evaporation techniques, filtration, and gravity separation do play roles in wastewater treatment but are not the primary means by which solids exit a digester. Evaporation would typically be ineffective for large volumes of sludge, filtration is generally employed in different stages of treatment rather than in digesters, and gravity separation is more relevant to the clarification stages rather than the direct removal of solids from digester environments.