Model-Based Evaluation of Denitrifying P Removal in a Two-Sludge System

The anaerobic-anoxic-nitrification (A2N) process has been proved as a two-sludge system for denitrifying dephosphatation, based on denitrifying phosphorus-removing bacteria. Optimization of biological processes traditionally occurs by pilot-scale experiments. However, mathematical modeling may minimize pilot-scale experiments although it cannot completely predict or replace a reality. For this reason, an activated sludge model in AQUASIM 2.0 simulated the A2N process. The used model originated from the Delft model for bio-P removal and activated sludge model No. 2 for COD and N removal. The model was first verified on its description of a laboratory A2N system. The following simulations confirmed the experimental conclusion that an influent biodegradable COD:N = 3.4 mg COD/mg N (biodegradable COD:P = 26.7 mg COD/mg P) was an optimal ratio to realize full nitrification and complete denitrifying dephosphatation. Simulations also demonstrated the behavior of the A2N process under the different COD/N, COD/P, solids retention time (days), temperature, and split ratios. Simulations indicated that the COD/NO-3 was an actual governing factor in denitrifying dephosphatation and that in practice a short aerobic period following the anoxic phase was needed to obtain a stable system.