What percentage of solids is typical for good primary sludge withdrawn from a primary settling tank?

The correct choice is based on the typical characteristics of primary sludge obtained from primary settling tanks in wastewater treatment processes. Good primary sludge usually contains a solid concentration ranging from 4 to 6%. This concentration reflects an optimal balance where the sedimentation process effectively removes settleable solids, while ensuring that the remaining solids have sufficient concentration for subsequent treatment processes.

In primary settling, the goal is to separate settled solids from the liquid effluent. The 4 to 6% range indicates that the sludge has an adequate amount of solids, allowing for efficient handling and processing in further treatment stages. A higher percentage of solids might indicate that either the settling tank operated longer than necessary or that too much solids were allowed to settle, which could affect downstream processes. Conversely, a lower percentage would suggest ineffective settling, where not enough solids are removed.

Ultimately, understanding this solid concentration is crucial for operators and engineers in ensuring that the treatment plant functions optimally and meets regulatory standards.