The new paradigm in wastewater treatment aims at diverting as much organic matter as possible to anaerobic digestion to boost energy recovery (i.e. biogas production) and to decrease operational costs (i.e. aeration requirements and sludge generation). To achieve this, short sludge retention time (SRT) systems could be implemented in the first stage of the wastewater treatment plants (WWTPs) that include biological phosphorus removal. This enables efficient removal of both carbon and phosphorus. This paper focuses on the biochemical methane potential (BMP) that could be reached when using bio-P sludge with different polyhydroxyalkanoate (PHA) content obtained under different SRTs and under anaerobic and aerobic environments. The PHA content of the biomass increased from 3.15 to 9.80 mmol C/g VSS when the SRT was decreased from 15 to 5 days and, on average, anaerobic sludge contained 45% more PHA than the aerobic sludge. BMP was correlated with the PHA content, following an approximate linear trend: BMP (mL CH4/g VSS) = 240 + 15.3 * PHA (mmol C/g VSS), independently of SRT and the anaerobic or aerobic extraction. The maximum BMP obtained was 401 mL CH4/g VSS with the biomass with the highest PHA content (9.80 mmol C/g VSS). When operating at low SRT and purging from the anaerobic phase, 28% of the influent COD content could be recovered as methane. This represented up to 2.5 times more energy than in the more conventional scenario tested: operating at high SRT with conventional aerobic purge.