The production of value-added materials from waste stream-derived biomass as the feedstocks using waste activated sludge (WAS) as the biocatalyst is a promising strategy to establish sustainable wastewater treatment, transforming wastewater treatment plants (WWTPs) to biorefineries. As a key technology to realize this strategy, a series of researches have been performed on the production of polyhydroxyalkanoates (PHA), bioplastics, by WAS. It has been clarified that even native WASs possess a certain ability to produce PHA from a relatively wide range substrates like volatile fatty acids and sugars which can be contained in industrial waste streams. Further, application of the aerobic dynamic discharge process (ADD) can drastically improve the PHA-producing capability of WAS by specifically enriching PHA-accumulating microbes in a very short period (within 2 days) to achieve effective PHA production. Based on these research and development, PHA production in WWTPs is considered to be realized in near future.