Which characteristic best describes activated sludge?

Activated sludge is a mass of microorganisms that are used in the biological treatment of wastewater. The correct choice highlights that activated sludge is typically brown in color, which is due to the presence of organic matter and the microbial population that thrives in the aeration tank of a wastewater treatment plant.

The presence of some dissolved oxygen is also an important aspect of activated sludge. During the aeration process, oxygen is introduced to facilitate the growth of aerobic bacteria that break down organic pollutants in the wastewater. This aerobic process is crucial for efficient treatment and is a defining feature of the activated sludge process, as these microorganisms rely on oxygen to metabolize waste materials.

The other options describe characteristics that do not align with the typical observations of activated sludge. For instance, the description of activated sludge as blue or beige in color is misleading, as this does not accurately represent its usual appearance or the biological activity within it. Likewise, while dissolved oxygen levels are vital, the specific amount can vary based on operational conditions in the treatment process. However, the presence of some dissolved oxygen is essential for the bacteria to perform effectively.