The impact of biofilm thickness-restraint and carrier type on attached growth system performance, solids characteristics and settleability

Abstract

The moving bed biofilm reactor (MBBR) technology is a proven standalone and add-on technology for carbon and nutrient removal from municipal wastewaters. The key challenge of the carbon removal MBBR technology is the production of poor settling biological solids and the need for intense solid separation methods. This study investigates the effect of carrier type and biofilm thickness-restraint on MBBR system performance, biofilm thickness, solids production, detachment rate, solids characteristics and settleability. Two new emerging “thickness-restraint” carriers, AnoxK™ Z-200 and Z-400 (allowing for 200 and 400 μm maximum biofilm thickness, respectively), are compared to the conventional AnoxK™ K5 carrier at BOD loading rates of 6 g-sBOD m⁻² d⁻¹. The obtained results indicate that carrier type has a significant effect on MBBR carbonaceous removal, biofilm thickness, detachment and solids production. The K5 carrier MBBR system demonstrated statistically significant higher carbonaceous removal rates of 3.8 ± 0.3 g-sBOD m⁻² d⁻¹, higher biofilm thickness (281.1 ± 8.7 μm), lower solids production (7.7 ± 3.2 mg-TSS L⁻¹) and greater stability with respect to the detachment rate compared to the two Z-carriers. Particle size distribution analysis demonstrates a higher percentage of small particles in Z-carrier system effluent and hence significantly lower solids settling efficiency. Therefore, the K5 carrier produced solids with improved settling characteristics compared to Z-carriers. No significant difference was observed in removal efficiency, solids production, detachment rate, particle characteristics and settling behaviour when comparing the Z-200 to the Z-400, indicating that biofilm thickness-restraint carrier design was not a controlling factor in the settling potential of produced solids.