Effects of carbon to nitrogen ratio on oxygen mass transfer characteristics in wastewater and biofilms

Abstract

Systematic elucidation of the oxygen mass transfer (OMT) mechanism is the key to improving aeration and oxygenation efficiency. In this study, an aerobic fluidized bed biofilm reactor (AFBBR) was established to evaluate the OMT in the wastewater phase [macroscopic (reactor) scale] and diffusion kinetic parameters in the biofilm phase (microbiological scale) under different carbon-nitrogen(C:N) ratios. A model relating the oxygen concentration and the aerobic layer thickness of the biofilm was established based on oxygen mass balance theory and Fick's second law. The results showed that the average DO concentration decreased from 7.68 mg/L to 5.85 mg/L with increasing C:N ratio (from 4.79 to 13.94). The oxygen transfer rate (OTR) was consistent with the distribution of the total biomass (total suspended solids, TSS), which was the largest (9.74 ± 0.33 mg/(L·h) and 8178 ± 87 mg/L) under the highest C:N ratio (13.94). Additionally, the α-factor, as a key parameter for evaluating OMT efficiency, had an exponential correlation with chemical oxygen demand (CODcr). The oxygen concentration exhibited in a Gaussian distribution (0.9963 ≤R2 ≤0.9998) relative to the thickness of the aerobic layer. The mass transfer efficiency (ki) and diffusion coefficient (Di) of oxygen in the boundary layer both reached maxima [(8.1 ± 3.2)× 10−5 cm2/s and (2.0 ± 0.1)× 10−3 cm/s] under the condition of C:N = 9.49. However, under the lowest C:N ratio, the biofilm thickness (LF=2422 ± 411 µm) and oxygen diffusion coefficient (Db) both reached maxima. These results have important theoretical significance for improving OMT theory and reducing the energy consumption of the aeration process.