Investigation of hydrodynamic and mass transfer characteristics in a draft tube spouted bed bioreactor (DTSBBR) for simultaneous COD and nitrate removal.

Because interactions among microbial growth kinetics, biofilm formation, hydrodynamic conditions, and mass transfer constraints frequently limit process stability and scale-up, the simultaneous removal of chemical oxygen demand (COD) and nitrate from wastewater remains a significant challenge in biological treatment systems. The hydrodynamic and mass transfer characteristics of a Draft Tube Spouted Bed Bioreactor (DTSBBR) were examined for the simultaneous removal of chemical oxygen demand (COD) and nitrate from synthetic wastewater under different operating parameters. In a biofilm-assisted reactor, performance was assessed at dilution rates (0.6, 0.9, and 1.2/h) and granular activated carbon (GAC) loads (10, 20, and 30 g). The biofilm thickness, substrate diffusion, external mass transfer coefficient, Reynolds number (Re), Schmidt number (Sc), and Sherwood number (Sh) were precisely studied and evaluated under the influence of reactor hydrodynamic studies. The COD elimination confirmed higher mass transfer capability compared to the nitrate values because of diffusion-limited denitrification kinetics, with Sherwood numbers ranging from 4.60 to 7.10 and mass transfer coefficients between 0.66*10-6 and 1.04*10-6 m/s. The empirical correlations Sh = 0.86 Re 0.52 Sc 0.33 for COD and Sh = 0.76 Re 0.50 Sc 0.33 for nitrate were obtained by diffusion-controlled substrate transport and substantial hydrodynamic dependence. The Monod kinetic experiments showed µmax values of 0.25/h for COD and 0.23/h for nitrate, with a constant Ks of 25 mg/L for COD and 14 mg/L for nitrate. The obtained results confirmed that DTSBBR provided consistent simultaneous organic and nitrogen removal, enhanced biofilm activity, and efficient substrate transport, demonstrating its applicability for advanced biological wastewater treatment applications.