Wastewater ammonia removal using an integrated fixed-film activated sludge-sequencing batch biofilm reactor (IFAS-SBR): Comparison of suspended flocs and attached biofilm

• IFAS-SBR was robust in removing COD and ammonium under different organic loading.
• Bacterial communities in attached biofilm and suspended flocs responded differently to organic loading changes.
• C/N ratio had negative impact on nitrification rate.
• EPS from Biofilm and suspended flocs showed different characteristics in response to organic loading changes.

Integrated fixed-film activated sludge (IFAS) that combines the activated sludge (suspended flocs) and attached biofilm has been applied in municipal wastewater treatment to promote nitrification. This study investigated the changes in reactor performance and bacterial community structure in response to different organic loadings (500, 250, 150 mg/L COD) in an integrated fixed-film activated sludge (IFAS) reactor operated under sequencing batch mode. Changes of bacterial population in the suspended flocs and attached biofilm were compared. Biomass in the suspended flocs and in biofilm decreased as the organic loading decreased. The specific nitrification rate was highest when COD was only 150 mg/L in the feeding, which highlighted the effects of competition between heterotrophic bacteria and autotrophic bacteria over oxygen and space. Besides, lack of carbon sources limited denitrification activities. The percentages of nitrifying related genes over total genes were higher in biofilms as compared to those in activated sludge flocs, indicating biofilm was a more favorable habitat for nitrifiers. In addition, extracellular polymeric substance (EPS) in biofilm and suspended flocs changed differently in response to the organic loading changes, which might also be related to the microbial population changes.