NOVELTY - The method comprises directing a wastewater containing volatile fatty acids (VFAs) into a wastewater treatment zone and a mixed culture biomass, treating the wastewater under anaerobic conditions, converting the organic compounds into the VFAs through acidogenic fermentation, selecting a glycogen accumulating organism (GAO) dominated biomass and causing the GAOs to proliferate and dominate over the other microorganisms in the biomass by subsequently feeding the VFAs to the biomass from a side stream and subjecting the biomass in a side stream polyhydroxyalkanoates (PHA) production zone.    USE - The method is useful for biologically treating wastewater and producing polyhydroxyalkanoates by selecting glycogen accumulating organisms having a high polyhydroxyalkanoates production potential in a mixed culture biomass.    ADVANTAGE - The method easily and economically produces high amount of polyhydroxyalkanoates with improved performance by effectively treating the wastewater without undue consumption of stored glycogen.    DETAILED DESCRIPTION - The method comprises directing a wastewater containing volatile fatty acids (VFAs) into a wastewater treatment zone and a mixed culture biomass, treating the wastewater under anaerobic conditions, converting the organic compounds into the VFAs through acidogenic fermentation, selecting a glycogen accumulating organism (GAO) dominated biomass and causing the GAOs to proliferate and dominate over the other microorganisms in the biomass by subsequently feeding the VFAs to the biomass from a side stream, subjecting the biomass in the side stream polyhydroxyalkanoates (PHA) production zone to alternating anaerobic and aerobic or anoxic treatment in the wastewater treatment zone, providing VFAs to the biomass during anaerobic treatment than during the aerobic or anoxic treatment such that during the anaerobic treatment and providing the VFAs to the biomass during anaerobic treatment at a ratio of 0.08-0.50 g of VFA per 1 g of total suspended solids (TSS) provided to the anaerobic treatment, accumulating the PHA in the GAO dominated biomass, and adding VFAs to the GAO dominated biomass in a PHA production zone from the wastewater such that the GAOs consume VFAs and produce PHA. The alternating anaerobic and aerobic or anoxic treatment produce the GAO dominated biomass having GAOs with relatively high levels of intracellularly stored glycogen. The GAOs in the GAO dominated biomass consume the VFAs and the intracellularly stored glycogen to produce PHA. The method further comprises directing the selected biomass to the side stream PHA production zone to a separator, separating the biomass from wastewater, feeding VFAs to the biomass in the PHA production zone, harvesting the GAO dominated biomass during an aerobic or anoxic treatment phase, and directing the GAO dominated biomass to a first separator and separating the GAO dominated biomass from the wastewater, and directing the GAO dominated biomass harvested to a side stream PHA production unit. The biomass is separated to a downstream processing zone. The VFAs are fed to the separated GAO dominated biomass. The biomass and wastewater are mixed to form mixed liquor. The subjected biomass produces the GAO dominated biomass having GAOs with 30% intracellularly stored glycogen under aerobic or anoxic conditions relative to the total suspended solids in the mixed liquor. The VFAs is fed to the separated biomass produces 45% of PHA relative to the TSS in the wastewater. The biological treatment of the wastewater includes removing biochemical oxygen demand (BOD) from the wastewater.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a glycogen accumulating organism selection process and a polyhydroxyalkanoates accumulation process into a wastewater treatment system.    Wastewater influent (12)    Biomass outlet (16)    Wastewater treatment reactor (22)    Chemical addition inlet (24)    Aeration inlet (26)    Gas outlet. (28)