Biomass segregation in the granules and flocs affects the role of heterotrophic bacteria in the ANAMMOX process

The autotrophic nitrogen removal could be achieved in the one-stage or two-stage PN-ANAMMOX (Partial nitrification -anaerobic ammonium oxidation) systems. A number of researches were carried out to study the impact of HET (heterotrophic bacteria) on the one-stage PN-ANAMMOX system. It is still not clear about the role of HET in the ANAMMOX process of the two-stage system. The granule sludge system used for the ANAMMOX process also included the flocs, resulting in the segregation of biomass between the flocs and granules. How the HET is segregated between the flocs and granules is still unknown. In this study, we examined the role of HET in the two-stage system ANAMMOX process and how the biomass segregation affects its role. The increase of floc SRT (Sludge Retention Time) from 7 h (the same as HRT (Hydraulic Retention Time)) to 30 d increased the floc/granule ratio by 150-200%, depending on the influent COD concentration. The biomass segregation affected the role of heterotrophic bacteria in the ANAMMOX process of the two-stage PN-ANAMMOX system. When there was no biomass segregation (granules alone), the heterotrophic bacteria negatively impacted the TN removal as the presence of HET within the granules reduced the number of ANAMMOX bacteria, reducing the nitrogen removal capacity. Under biomass floc/granule segregation conditions, the HET predominantly resided in the flocs and increased nitrogen removal by providing additional nitrite through the denitrification process. Compared to the TN removal without biomass segregation, the segregation of HET between the floc and granules caused 3% to 40% more TN removal depending on the influent COD concentration.