Wastewater Treatment Operator Certification Practice Test

What is the difference between the settling rate of activated sludge and raw sludge?

1. Activated sludge settles slightly faster than raw sludge

2. Activated sludge settles at the same rate as raw sludge

3. Activated sludge settles slower than raw sludge

4. Activated sludge settles much faster than raw sludge

The correct answer is: Activated sludge settles slower than raw sludge

The settling rate of activated sludge in wastewater treatment systems is influenced by its composition and the biological activity within it. Activated sludge is characterized by a high concentration of microorganisms that have been cultivated to effectively break down organic materials. This flocculent material tends to have a lower specific gravity, allowing it to remain suspended in the water more easily, while still providing effective treatment. On the other hand, raw sludge, which is often a mixture of solids including larger particulates and less biologically active material, typically has a higher density and can settle more quickly under gravity. Raw sludge tends to have less biological agglomeration and may contain a greater volume of solid particles, leading to faster settling rates compared to the lighter flocs of activated sludge. Understanding the behavior of these two types of sludge is crucial for proper management in wastewater treatment processes. Activated sludge, due to its slower settling rate, requires careful handling to avoid issues such as poor separation during the clarification process and potential backflow into biological reactors.