New insights in partial nitrification were revealed by using a model based approach to study the synergetic impact of various factors on partial nitrification. The traditional argument for using low DO (dissolved oxygen) in achieving partial nitrification is that AOB (ammonium oxidizing bacteria) has a higher DO affinity than NOB (nitrite oxidizing bacteria). However, the AOB was found to have a lower DO affinity (DO half saturation constant KO,AOB = 1.0 mg L−1) than NOB (KO,NOB = 0.35 mg L−1) in the nitrification process in this study. The rational for using a low DO concentration in this study was to slow down the ammonium removal, therefore increasing the FA (free ammonia) concentration for the inhibition of NOB. The optimal operational strategy for partial nitrification start-up was identified by the model based approach to be at a sludge retention time of 10 days, DO at 1.0 mg L−1 and pH at 8.2. The results indicated that a nitrite accumulation rate above 80% and complete ammonium removal can be achieved within 10 days of the start-up period by applying the model based optimization results.

Activated sludge is a complex mixture of solid, colloidal and soluble components. Their respective contributions to membrane fouling process in membrane bioreactor (MBR) have been investigated by the fractioning method. The synergistic effects among these components on membrane fouling are neglected by using the method. A mathematical model was developed to simulate fouling that included the mixed liquor suspended solids (MLSS), soluble and colloidal components, the activated sludge floc size distribution (FSD) and aeration intensity. The membrane fouling was divided into cake layer formation and pore fouling. The cake layer is assumed to be formatted by MLSS and consolidated by the entrapment of colloidal components, resulting in the decreasing in cake porosity and increasing in specific cake resistance. The addition of activated sludge FSD in the model development allows the probability and deposition rate of individual solid sizes to be considered. The developed model was calibrated and validated using experimental data from different aeration intensities. It provides a valuable tool for analyzing the synergistic fouling effects from solids, colloidal and soluble components within activated sludge.Highlights► Fouling model developed based on the pore fouling and cake formation. ► The synergic effect of solid, colloidal and soluble components on fouling. ► A layered cake structure and vertical distribution of cake porosity. ► Colloidal entrapment within cake layers.

•Partial nitrification was closely associated to the specific biomass NH4+ load.•A low SRT was needed to maintain high specific biomass NH4+ load.•FA inhibition of NOB was insignificant for partial nitrification.•Increased AOB activity was the main reason for partial nitrification.Partial nitrification is usually more easily achieved for high NH4+ concentration wastewater. In this study, a two-compartment aerobic nitrification reactor was used to raise the NH4+ concentration in the first aerobic compartment and to enhance partial nitrification for low NH4+ concentration wastewater. In addition to the regular sludge wastage to maintain sludge retention time (SRT), extra sludge wastage was used to increase the specific biomass NH4+ load. The experimental results indicated that partial nitrification was closely associated to the specific biomass NH4+ load. The modelling results indicated that a low SRT was needed to maintain the low biomass concentration and high specific biomass NH4+ load. The increased AOB (ammonium oxidizing bacteria) activity, instead of the free ammonia (FA) inhibition of NOB (nitrite oxidizing bacteria) was identified to be the main mechanism for partial nitrification in low NH4+ concentration wastewater treatment with high specific biomass NH4+ load.

The successful operation of partial nitrification relies on the optimal condition in which the ammonia oxidizing bacteria (AOB) is favored over nitrite oxidizing bacteria (NOB). The fluctuation of influent load means the optimal condition varies dynamically. The routine monitoring such as dissolved oxygen (DO), nitrite and ammonia concentration is usually used for the system optimization. The sensors used for these routine monitoring are usually submerged in a hostile activated sludge environment and are subjected to sensor fouling. In this paper, the non-invasive off-gas nitrous oxide (N2O) monitoring was used for the optimization of partial nitrification. An empirical N2O emission model and a two-step nitrification activated sludge model No. 1 (ASM1) were used for the optimization study based on the concept of model predictive control. The results indicated that the non-invasive N2O monitoring can be used as an alternative to the nitrite and ammonia sensor for the optimization of partial nitrification process.

•N2O emission monitoring was used for the partial nitrification optimization.•The off-gas “dry” sensor of N2O emission was used.•partial nitrification can be improved by N2O emission monitoring.The successful operation of partial nitrification relies on creating the optimal condition in which the ammonium oxidizing bacteria (AOB) is favoured and nitrite oxidizing bacteria (NOB) is inhibited. The fluctuation of influent load means the optimal condition varies dynamically. The routine monitoring such as dissolved oxygen (DO), nitrite and ammonium concentration is usually used for the system optimization. The sensors used for these routine monitoring are usually submerged in a hostile activated sludge environment and are subjected to sensor fouling. In this paper, the non-invasive off-gas nitrous oxide (N2O) monitoring was used for the optimization of partial nitrification in a laboratory scale bioreactor. The results indicated that N2O emission can be used as a quantitative tool for the optimization of partial nitrification process. The N2O emission data was used as feedback information to derive a more accurate model, which can then be used in the partial nitrification optimization.

•Model predictive control for partial nitrification was developed.•Aeration and carbon source addition can be saved by MPC.•Partial and complete nitrification control can be switched by using different set-point value for DO.The shortage of easily degradable carbon source in the denitrification process limits the biological nitrogen removal efficiency. Partial nitrification has shown to be an attractive technology due its savings in aeration and external carbon source addition cost in the biological nitrogen removal. In this article, a model predictive control method was proposed to optimize the aeration and the external carbon source addition under the disturbance of influent flow rate and nitrogen load. The results indicated that 56% saving in external carbon source addition and 26% saving in aeration cost can be achieved by the model predictive control method applied in the partial nitrification, in comparison to the complete nitrification.

•The measured values of KO,AOB and KO,NOB were low under small floc diameter and low nitrifiers enrichment condition.•The nitrifiers enrichment and floc diameter had higher impact on the KO,AOB measurement than on the KO,NOB measurement.•The results explained the conflicting reports on the measured KO,AOB and KO,NOB values in the literatures.It has been recognized that the size of activated sludge floc affected the measurement of substrate half-saturation value. The impact of bacteria enrichment on the half-saturation value measurement has not been studied. This study highlighted the importance of bacteria enrichment on the evaluation of the half-saturation values. Due to the critical importance of oxygen half-saturation value in achieving partial nitrification, the impact of floc diameter and nitrifier enrichment on the measurement of oxygen half-saturation value for AOB (KO,AOB) and NOB (KO,NOB) was evaluated both in experiment and in mathematical simulation. The experiment results showed low measured KO,AOB and KO,NOB values under small floc diameter and low nitrifiers concentration condition. The increase in floc diameter and nitrifiers enrichment caused a significant increase in the measured KO,AOB, while only slight increase in KO,NOB was noticed. The simulation results were consistent with the experiment results and indicated that the nitrifiers concentration and the floc diameter had higher impact on the KO,AOB measurement than on the KO,NOB measurement. The results could explain the conflicting reports on the measured KO,AOB and KO,NOB values.Download high-res image (121KB)Download full-size image