California Wastewater Treatment Operator Practice Exam

What is the differnce between the sludge from a primary clarifier and a secondary clarifier?

1. Primary sludge is lighter than secondary

2. Primary sludge is denser than secondary

3. Primary sludge is more odorous than secondary

4. Primary sludge is more hazardous than secondary

The correct answer is: Primary sludge is denser than secondary

The distinction between the sludge from a primary clarifier and that from a secondary clarifier primarily relates to their composition and density. Primary sludge, which accumulates during the initial sedimentation step of wastewater treatment, typically consists of the heavier particles, including organic and inorganic solids that can settle out of the wastewater. This sludge tends to be denser because it has not undergone significant biological breakdown. In contrast, secondary sludge, also known as activated sludge, results from biological processes where microorganisms consume organic matter in the wastewater. This sludge is often less dense due to the presence of living microorganisms and the breakdown of organic material, which can create a fluffier, more buoyant structure. Thus, the correct choice highlights that primary sludge has a higher density compared to secondary sludge, reflecting its composition of settled solids versus microbial biomass. Understanding this difference is crucial for operators as it influences subsequent treatment processes and sludge management strategies.