New insights into modeling two-step nitrification in activated sludge systems - The effects of initial biomass...

In this study, the conventional two-step nitrification model was extended with complete ammonia oxidation (comammox) and heterotrophic denitrification on soluble microbial products. The data for model calibration/validation were collected at four long-term washout experiments when the solid retention time (SRT) and hydraulic retention time (HRT) were progressively reduced from 4 d to 1 d, with mixed liquor suspended solids (MLSS) of approximately 2000 mg/L at the start of each trial. A new calibration protocol was proposed by including a systematic calculation of the initial biomass concentrations and microbial relationships as the calibration targets. Moreover, the impact assessment of initial biomass concentrations (X) and maximum growth rates (μ) for ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), comammox Nitrospira, and heterotrophs on the calibration accuracy were investigated using the response surface methodology (RSM). The RSM results revealed the strongest interaction of X_AOB and μ_AOB on the model calibration accuracy. All the examined model efficiency measures confirmed that the extended model was accurately calibrated and validated. The estimated μ values were as follows: μ_AOB = 0.38 ± 0.005 d^-1, μ_NOB = 0.20 ± 0.01 d^-1, μ_CMX = 0.20 ± 0.01 d^-1, μ_HET = 1.0 ± 0.03 d^-1. For comparison, when using the conventional model, μ_AOB and μ_NOB increased respectively by 26 and 15 % (μ_AOB = 0.48 ± 0.02 d^-1 and μ_NOB = 0.23 ± 0.005 d^-1). This study provides better understanding of the effects of the initial biomass composition and the accompanying processes (comammox and heterotrophic denitrification) on modeling two-step nitrification.