What happens to the color of bio-film as organic matter decreases in a rotating biological contactor?

In a rotating biological contactor (RBC), the biofilm is composed of microorganisms that help in breaking down organic matter. As the concentration of organic material decreases, these microorganisms' food source diminishes, influencing the composition and appearance of the biofilm.

As organic matter declines, the biofilm tends to become clearer and lighter. This change occurs because the microorganisms that thrive on organic matter may die off or be washed away, leading to reduced biomass density. Consequently, the film becomes thinner and less pigmented, resulting in a lighter appearance. This phenomenon can also indicate a decrease in microbial activity, as the organisms are less capable of producing pigments when their food supply is limited.

The increased lightness and clarity of the biofilm can reflect the shift in microbial populations and their metabolic products, showcasing the dynamic nature of the biological treatment process. In contrast, darker or unchanged coloration would indicate a sustained or increased level of organic matter, which is not the case when organic matter levels drop.