Mechanisms and Kinetics of COD Removal and Simultaneous Oxygen Consumption in a Rotating Biological Contactor Biofilm

A study on aerobic biofilm COD removal and simultaneous respiration is made using a Wazu respirationmeter unit. In a closed recirculating system completely submerged RBC biofilm samples have been examined in short-term batch experiments, using the corresponding waste water or dissolved acetate. The removal of dissolved organic substrate and the respiration were found to answer the ½-order/0-order kinetic model, whereas the removal of suspended solids COD (COD_s) showed 1-order kinetics. Under conditions usually met in practice the 0-order rates for dissolved COD(COD_d) removal and respiration were 30–40 gm⁻²d⁻¹ and 10–16 gm⁻²d⁻¹, respectively; in absence of organic substrate a certain background respiration was observed (3.5–9.0 gm⁻²d⁻¹). As a result of the 1-order kinetics over a wide concentration range, extremely high COD_s removal rates were observed even exceeding 300 gm⁻²d⁻¹. The difference between the maximum respiration and the COD_s removal rates is attributed to physical biofilm adsorption processes. Biofilm COD removal kinetics for waste water containing both COD_s and COD_d are discussed, illustrating that the kinetic order of the combined process and the reaction constant strongly depends on the COD_s/COD_d ratio. Conclusion is that a RBC biofilm loaded with a COD_s/COD_d mixture oxidizes some dissolved organic matter and acts as a catalyst in the coagulation and flocculation of fine suspended solids.