Aerobic granular sludge (AGS) is a new technology for biological treatment of wastewater. Simultaneous removal of organic matter, nitrogen and phosphorus is possible in an AGS reactor due to the large physical size of granules and their heterogeneous structure. While sufficient removal efficiencies of C and P have been achieved, N removal from municipal wastewaters using AGS still remains a challenge.
To address this knowledge gap, a model was developed in SUMO software to assess the mechanisms driving simultaneous nitrification and denitrification (SND) in a granular sludge reactor. Nitrification and denitrification can be accomplished in a single reactor due to coexistence of aerobic and anoxic zone in granules. Therefore, the objective of this master’s thesis is to evaluate the influence of oxygen concentration and anoxic volume in the granules on SND performance. Another objective is to evaluate the contribution of phosphorous accumulating organisms (PAO) on SND as PAOs are mainly responsible for the denitrification in AGS.
The granular sequenced batch reactor (SBR) module in SUMO was used to simulate mass transport and conversion processes occurring in the reactor. A low-strength influent with particulate organic carbon, which is typical for the municipal wastewater, was used in the study. A total COD concentration was 420 g_COD.m^-3. The model included growth and decay of nitrifiers, ordinary heterotrophs and PAO. Three case studies were analyzed with low, intermediate and high granulation. The simulations were performed under different airflow resulting in different dissolved oxygen concentration in the bulk.
The highest SND of 96% was achieved for the simulation with intermediate granulation and oxygen concentration in the bulk reaching 1.2 g_O2.m^-3. Complete nitrification occurred in this simulation. The fraction of anoxic volume in the granule reached 100% and remained high during most of the aerobic period, which is significantly higher than the anoxic volume in cases with low or high granulation. In case with intermediate granulation, high PHA concentrations are found in anoxic layers of the granule. Therefore, there is a sufficient amount of organic carbon for denitrification by PAOs. In cases with low and high granulation, most of the PHA was concentrated in the aerobic layers and was not available for denitrification by PAOs.
To conclude, this study shows that aerobic granular sludge can be an effective technology for treatment of municipal wastewater, achieving high nitrification and SND efficiencies. However, special consideration shall be paid to oxygen concentration, anoxic volume in the granule and organic carbon availability in the anoxic zone.