Modeling a Combined Anaerobic/Anoxic Oxide and Rotating Biological Contactors Process under Dissolved Oxygen Variation by Using an Activated Sludge-Biofilm Hybrid Model

A hybrid model which incorporated a biofilm model into the general dynamic model was developed to predict the effluent quality of a combined activated sludge and biofilm process—Taiwan National Central University Process 1. The system was performed under three different dissolved oxygen (DO) conditions in the oxic tank, including 2.0, 1.0, and 0.5 mg/L. When the DO increased from 0.5 to 2.0 mg/L, the soluble biodegradable substrate $\left(S_S\right)$ and soluble phosphate $\left(P_{O4}\right)$ in the effluent were not significantly influenced. Their removal efficiencies were above 92 and 94%. Ammonia–nitrogen $\left(N_{H3}\right)$ removal efficiency increased from 36 to 83% and nitrate–nitrogen $\left(N_{O3}\right)$ increased from 1.7 to 2.9 mg/L. In biofilm, when the DO was 2.0 mg/L, the active autotrophic biomass $\left(Z_A\right)$ fraction was 15.7% (surface) to 12.9% (substratum). But when the DO was 0.5 mg/L, the $Z_A$ fraction became lower and the fraction was 6.2% (surface) to 3.5% (substratum). The fraction of active nonpoly-P heterotrophic biomass $\left(Z_H\right)$ in the biofilm did not vary significantly, the values were about 28–35%. $Z_I$ decreased as the DO increased. $S_S$ in the biofilm did not vary significantly and was maintained at about 2.0 mg/L. When DO increased, $N_{O3}$ also increased, $N_{H3}$ decreased from 13.1 to 1.8 mg/L in biofilm.