Study on microbial characteristics of the ultra-short sludge retention time activated sludge system

• The USSAS system had lower biodegradation capacity and higher biosorption capacity; • Compared with the CAS, the microbial species diversity in the USSAS system was smaller; • There is a positive correlation between SRT and microbial community diversity; • 2 h is the critical point of HRT for functional-dominated or stability-dominated system. As an enhanced process for high-rate activated sludge (HRAS) systems, the ultra-short sludge retention time (SRT) activated sludge (USSAS) process can achieve high efficiency carbon separation, energy savings and resource recovery potential, this study deeply investigated the microbial community characteristics and the microbial kinetic behavior of activated sludge of the USSAS system. The results showed that, compared with conventional activated sludge (CAS), the USSAS system can screen microorganisms with different functions and generation cycles. The microbial community diversity in the USSAS system was 80% of that in CAS, and the dominant bacteria were mostly microorganisms with short generation cycles and filamentous bacteria that can cause instability such as sludge bulking. Additionally, HRT=2 h is the critical point of the system from stability to functionality. In the USSAS system, with increasing SRT, the specific oxygen uptake rate gradually increased, the activity of sludge was enhanced, and the maximum activity was observed at the SRT of 3 d. Additionally, the values of sludge observed yield (Y obs ), heterotrophic bacteria yield coefficient (Y H ) and heterotrophic bacteria attenuation coefficient (K dH ) all increased with SRT and were all higher than those at CAS, the microorganisms in the USSAS system renewed faster, and the heterotrophic bacteria that can utilize organic matter rapidly can dominate the system and multiply rapidly. However, the maximum specific removal rate of COD (V mS ) was lower than CAS, means the USSAS system had a lower biodegradation capacity and a higher biosorption capacity.