Impact of Chemical Oxygen Demand/Total Nitrogen Ratio on Shifting Autotrophic Partial Nitrification to Heterotrophic Nitrification and Aerobic Denitrification in High-Strength Ammonium Wastewater Treatment

Partial nitrification (PN) is an effective process for treating high-strength ammonium wastewater with a low COD/N (chemical oxygen demand/total nitrogen) ratio; this is because the cooperative interaction with denitrification or anammox can result in a reduction in aeration costs of approximately 25% and a reduction in the use of organic sources during biological nitrogen removal of 40%. However, the key functional microorganisms in the partial nitrification (PN) process are ammonia-oxidizing bacteria (AOB), which are autotrophic microorganisms that are influenced by carbon sources. Therefore, the COD/N ratio affects the performance of the PN process when treating high-strength ammonium wastewater. In this study, five sequence batch reactors were constructed and operated for 42 days; they were fed with synthetic high-strength ammonium wastewater (500 mg/L) with various COD/N ratios (at 0, 0.5, 1, 2, and 4). The results suggested that the PN process could be accomplished at COD/N ratios of 0 and 0.5, but its performance decreased significantly when the COD/N ratio increased to 1 due to the occurrence of simultaneous nitrification and denitrification. The AOB could not compete with the heterotrophic bacteria; as the COD/N ratios increased, the abundance of Nitrosomonas (a genus of autotrophic AOB) decreased, and it was not detected at COD/N ratios of 2 and 4. Instead, the heterotrophic nitrification and heterotrophic denitrification (HNAD) bacteria appeared, and their relative abundance increased when the COD/N ratios increased from 1 to 4.