Response of the mainstream anammox process to the biodegradable carbon sources in the granule-based systems: The difference in self-stratification of the microbial community

The inevitable introduction of biodegradable carbon sources (such as monosaccharides and volatile fatty acids) originating from pretreatment units might affect the performance of the mainstream anaerobic ammonium oxidation (anammox) process. Two model carbon sources (glucose and acetate) were selected to investigate their effects on granule-based anammox systems under mainstream conditions (70 mg total nitrogen (TN) L^-1, 15 °C). At a nitrogen loading rate of 2.87 ± 0.80 kg N m^-3 d^-1, a satisfactory effluent quality (TN < 10 mg L^-1) was achieved in the presence of glucose or acetate at a chemical oxygen demand (COD/N) ratio of 0.5. The contribution of anammox to nitrogen removal decreased with increasing COD/N ratio to 1.0 because the expression of anammox functional genes was inhibited, whereas the expression of denitrifying functional genes was promoted. However, the nitrogen removal efficiency of the two considered reactors was maintained above 80 %. Self-stratification of the microbial community along the reactor height facilitated a functional balance through the retention of anammox bacteria in granules but resulted in washout of denitrifying bacteria in flocs under a high-flow pattern. These findings highlighted the advantages of granule-based systems in the mainstream anammox process due to their inherent biomass self-segregation property and the need for the development of targeted biomass retention strategies.