High -rate activated sludge (HRAS) is wastewater treatment focusing on reducing greenhouse gas emissions via energy recovery from sludge. However, the effect of hydraulic retention time (HRT) on HRAS performance and redirection pathways has seldom been investigated. Also, the fractionation of extracellular polymeric substance (EPS) as loosely-bound (LB) and tightly-bound (TB) in HRAS sludge have not reported yet. This study aimed to comprehensively investigate the carbon redirection pathways and biogas recovery potential of the HRAS system, examining the separate effects of HRT and SRT. An HRT of 1.5 h and an SRT of 2 d, HRAS achieved the highest COD recovery, with 64.8 % removed and 60.5 % redirected as waste activated sludge (WAS) as the pseudo -steady average values. And WAS exhibited notably high LB -EPS, reaching 366.2 mg/g volatile suspended solids (VSS). Furthermore, the metabolism pathway for redirection resulted in biosorption 54.9 %, LB -EPS 34.0 %, TB-EPS 7.2 %, and cell growth 3.9 % based VSS. The biochemical methane potential of WAS was measured at 234.0 mL CH 4 / g COD, equivalent to 40.5 % of influent at HRT 1.5 h and SRT 2 d as optimal conditions. Notably, Family Carnobacteriaceae and Genus Trichococcus were emerged dominantly in HRAS. This study would be useful in the design and operation of HRAS for sustainable wastewater treatment and climate change mitigation.
