Secondary biological treatment designed to remove nutrients and dissolved organic material from the wastewater is provided by a rotating biological contactor (RBC) / activated sludge hybrid system. The RBC component is referred to as a “fixed film” process because microorganisms growing on discs of partially submerged plastic media rotate slowly (1-3 rpm) through the wastewater to provide treatment. The activated sludge component is considered a “suspended growth” process because the microorganisms performing the biological treatment are mixed in the wastewater solution by diffused air, with also provides dissolved oxygen needed to maintain aerobic conditions. KSD adopted the hybrid approach to provide higher levels of nutrient removal and biological treatment required by stricter effluent discharge standards.
The “mixed liquor”, a term used to describe wastewater solution containing biomass grown for secondary treatment, then flows to secondary clarifiers.