Validating the Colloid model to optimise the design and operation of both moving-bed biofilm reactor and integrated fixed-film activated sludge systems

The paper presents a systematic study of simulations, using a previously calibrated Colloid model, from which it was found that: (i) for pure moving-bed biofilm reactor (MBBR) processes with tertiary nitrification conditions (no influent chemical oxygen demand (COD)), dissolved oxygen = 5 mg/L and residual NH₄-N > 4 mgN/L, a nitrification rate of 1.2 gN/(m²d) was obtained at 10 °C. This rate decreases sharply when residual NH₄-N is lower than 2 mgN/L, (ii) for MBBR systems with predenitrification–nitrification zones and COD in the influent (soluble and particulate), the nitrification rate (0.6 gN/(m²d)) is half of that in tertiary nitrification due to the effect of influent colloidal X_S (particulate slowly biodegradable COD) and (iii) for integrated fixed-film activated sludge (IFAS) processes the nitrification rate in the biofilm (0.72 gN/(m²d)) is 20% higher than for the pure MBBR due to the lower effect of influent X_S since it is adsorbed onto flocs. However, it is still 40% lower than the tertiary nitrification rate. In the IFAS, the fraction of the nitrification rate in suspension ranges from 10 to 70% when the aerobic solids retention time varies from 1.4 to 6 days.