New insights into modeling two-step nitrification in activated sludge systems - The effects of initial biomass concentrations, comammox an heterotrophic activities

In this study, the conventional two-step nitrification model was extended with complete ammonia oxidation (comammox) and heterotrophic & nitrification on soluble microbial products. The data for model calibration/validation were collected at four long-tern 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 (MISS) 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 (p) for ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), comammox Nunospira, 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 mu(AOB )on the model calibration accuracy. All the examined model efficiency measures confirmed that the extended model was accurately calibrated and validated. The estimated p values were as follows: mu(AOB) = 0.38 +/- 0.005 d(-1), mu(NOB) = 0.20 +/- 0.01 d(-1), mu(CMX) = 0.20 +/- 0.01 d(-1), mu(HET) = 1.0 +/- 0.03 d(-1). For comparison, when using the conventional model, mu(NOB) and mu(AOB) increased respectively by 26 and 15 % (mu(AOB) = 0.48 +/- 0.02 d(-1) and mu(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.

Automated summary

This study extended the conventional two-step nitrification model by incorporating complete ammonia oxidation (comammox) and heterotrophic nitrification on soluble microbial products. A new calibration protocol was proposed and the impact of initial biomass concentrations and growth rates on model accuracy was investigated using response surface methodology. The extended model was accurately calibrated and validated, providing a better understanding of the effects of initial biomass composition on two-step nitrification modeling.