Understanding biofilm diffusion profiles and microbial activities to optimize integrated fixed-film activated sludge process

• Physical properties were more sensitive than kinetics in biofilm modeling.
• Ammonia limited diffusion in biofilm supported aeration optimization in MBBR.
• Simplified respirometric test revealed variation of biomass activity over long time.
• The integrated methods in this study showed great potential in engineering practice.

The integrated fixed-film activated sludge (IFAS) process is widely used in wastewater treatment plants (WWTPs) of China to ensure complete nitrification. However, high aeration intensity in this process requires optimization to reduce its energy footprint. Moreover, understanding microbial activity is also critical for process operation, especially for the biofilm on the carriers which play an important role for nitrogen removal. Here, we integrated process modeling, biomass activity and microbial community analysis in a full-scale IFAS plant, in order to investigate and optimize the process operation for better performance especially in cold climate. The field surveys indicated complete nitrification in the IFAS process, with biofilm contributed two thirds of it. Parameter sensitivity analysis also emphasized the importance of biofilm diffusion profile for process operation, and ammonia limited diffusion profile implied the bulk dissolved oxygen (DO) can be reduced from 8 mg/L to 4 mg/L. Furthermore, simplified respirometric tests revealed the variation of biomass activity over long time, proving attached biomass had much higher nitrification activity than suspended biomass, with almost no change after DO dropped to 4 mg/L. Meanwhile, the microbial community by 16S rDNA clone library also indicated the accumulation of abundant nitrifiers in the biofilm, supporting the process optimization. The integrated methods of process modeling, biomass activity evaluation and microbial community analysis showed great potential in engineering practice for process optimization.