How is organic loading defined for rotating biological contactors (RBCs)?

• A. Organic loading = lbs BOD per surface area per 1000 ft2.
• B. Organic loading = mg/L BOD per liter of wastewater.
• C. Organic loading = lbs BOD per cubic foot of media.
• D. Organic loading = pounds of TSS per 1000 ft2.

Answer: (A)

In an RBC, the amount of organic matter that the biofilm on the media can treat is determined by how much biodegradable material arrives per unit of surface area. That’s why organic loading is defined as pounds of BOD per thousand square feet of media surface area (often considered per day). This metric links the feed of organic matter directly to the available surface for microbial growth, so operators can keep loading within what the biofilm can efficiently carbon-oxidize, ensuring good treatment and avoiding overloading or clogging.

Expressing organic loading as mg/L BOD per liter of wastewater looks at concentration in the water rather than how much surface area the microbes have to work on, which is not the controlling factor for RBCs. Using pounds of BOD per cubic foot of media focuses on volume rather than the surface area, while RBCs rely on the surface area available for attachment and biofilm activity. Finally, tying loading to pounds of TSS changes the focus from biodegradable organics to total solids, which isn’t the parameter that governs the biological treatment in RBCs.