Modeling long term nutrient removal in a sequencing batch reactor

A modified version of IAWQ activated sludge model 2 (ASM 2) was developed to address the long-term dynamic behavior of nutrients in a sequencing batch reactor (SBR) activated sludge process. Experimental data was obtained from a long-term experimental work carried out in a 100-l bench scale SBR. Changes in TOC, NH4-N, NO3-N, NO2-N and PO4-P could be reliably predicted after the model parameters were adapted to the SBR conditions. Better phosphorus dynamics were achieved by considering the sub-model of denitrification by phosphorus accumulating organisms (PAO). Long-term simulations were carried out with the calibrated model to investigate the behavior of N and PO4-P under disturbed loading conditions. After attaining sufficient biological phosphorus removal activities in the sludge, influent TOC concentration was stepwise decreased and increased for 5 weeks. The decrease in organic loading caused the deterioration of biological phosphorus removal with simultaneous increase in effluent NO3-N concentration. Subsequently increasing the organic loading restored the original effluent conditions. The model simulation predictions well match with the experimental results under disturbed organic loading conditions. The simulated results implied that the deteriorating phenomena of phosphorus removal can be explained by two mechanisms; poor P-uptake by PAO and washout of PAO itself.