In a train of rotating biological contactor units, what would be the color order if the first unit shows gray and the second and third units show reddish brown?

In a rotating biological contactor (RBC) system, the color of the biological growth on the media can indicate the condition and metabolic activities of the microorganisms present. Typically, gray colors often suggest the presence of aerobic bacteria that are actively consuming organic material. On the other hand, reddish brown colors typically indicate the presence of iron-oxidizing bacteria, which thrive in anaerobic or reduced conditions and can occur in subsequent units due to a changing environment as wastewater progresses through the system.

Given that the first unit is showing gray, it suggests a healthy aerobic environment where the appropriate bacteria are thriving. The reddish brown colors in both the second and third units indicate that, as the wastewater progresses, conditions may change to become more favorable for iron-oxidizing bacteria. Thus, if the first unit is gray and the subsequent units are reddish brown, it shows a transition from aerobic to possibly more anaerobic conditions, leading to the growth of different microbial populations.

Thus, the color order reflecting the described situation accurately is: gray for the first unit, and reddish brown for both the second and third units. This reflects the shift in microbial communities that can occur as wastewater moves through the RBC system.