Under normal operating conditions, what color should activated sludge appear?

Activated sludge under normal operating conditions typically appears as dark chocolate brown. This coloration is indicative of a healthy and well-functioning biomass, which is composed of microorganisms actively breaking down organic matter in the wastewater. The brown color primarily results from the presence of bacterial cells and the organic material that has been metabolized.

The dark chocolate brown hue suggests that the microorganisms are thriving and that the system is efficiently treating wastewater. A deviation from this color, such as a light brown or yellowish gray, may indicate issues within the treatment process, such as poor settling characteristics or the presence of excessive inert solids. Dark green coloration may suggest the growth of algae or other specific conditions that do not align with the normal appearance of activated sludge, indicating a potential imbalance in the treatment process. Thus, the correct answer reflects the expected and healthy condition of activated sludge in an activated sludge wastewater treatment system.