Municipal wastewater treatment plants are a ubiquitous source of microbial biomass for PHA production. Technological feasibility of directly using municipal activated sludge (WAS) for a PHA accumulation bioprocess is demonstrated in the literature. However, PHA contents and yields may be lower due to a coexistence of PHA-storing and non-PHA-storing microorganisms in WAS. This work focused on metabolic principles for stimulating selective growth of PHA-storing microorganisms during a PHA accumulation bioprocess to enhance PHA productivity. Two model substrates, butyrate and acetate, were used to evaluate conditions and principles that may regulate this selective growth response. Conditions promoting selective growth of the PHA storers were consistently observed in the fed-batch PHA production process fed with butyrate. Productivity was increased to 4 times more PHA produced over 48 h, wherein PHA contents and average yields were improved from 0.39 gPHA/gVSS and 0.25 gCODPHA/gCOD to 0.61 gPHA/gVSS and 0.47 gCODPHA/gCOD, respectively. Respiration monitoring and mass balances, with metabolic modelling, suggest that expected underlying differences in ATP yields between two tested substrates are the main mechanistic drivers of the observed selective growth. This study proposed that if conditions are created such that ATP is produced in sufficient excess of the demands for PHA storage and cell maintenance, then those PHA storers will further grow concurrently. The selective microbial growth allows extra conversion of substrates into PHA. These principles, concerning the metabolic basis of the PHA production pathway, provide a foundation that can be applied to a range of substrates or substrate mixtures for enhanced PHA accumulation.