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 (\IS\dS\N) and soluble phosphate (P\dO\d4) in the effluent were not significantly influenced. Their removal efficiencies were above 92 and 94%. Ammonia–nitrogen (N\dH\d3) removal efficiency increased from 36 to 83% and nitrate–nitrogen (N\dO\d3) increased from 1.7 to 2.9 mg/L. In biofilm, when the DO was 2.0 mg/L, the active autotrophic biomass (\IZ\dA\N) fraction was 15.7% (surface) to 12.9% (substratum). But when the DO was 0.5 mg/L, the \IZ\dA\N fraction became lower and the fraction was 6.2% (surface) to 3.5% (substratum). The fraction of active nonpoly-P heterotrophic biomass (\IZ\dH\N) in the biofilm did not vary significantly, the values were about 28–35%. \IZ\dI\N decreased as the DO increased. \IS\dS\N in the biofilm did not vary significantly and was maintained at about 2.0 mg/L. When DO increased, N\dO\d3 also increased, N\dH\d3 decreased from 13.1 to 1.8 mg/L in biofilm.