A method for promoting the accumulation of polyhydroxyxalkanoates (PHAs) in biomass, wherein the biomass is contained within mixed liquor, said method comprising: directing the biomass into a first zone and thereafter directing the biomass to a second zone; wherein the second zone includes a volume of mixed liquor less than the volume of mixed liquor in the first zone; feeding an organic carbon-containing substrate to the biomass by intermittently supplying the substrate into the second zone including the biomass over a period of time; wherein the substrate contains readily biodegradable chemical oxygen demand (RBCOD); wherein the substrate is fed to the biomass in three or more pulses and wherein the frequency of the pulses is such that the respiration rate of the biomass does not drop more than 70% relative to the maximum respiration rate achieved by the biomass in response to the most recent pulse fed; and wherein the concentration of substrate in the second zone is higher than the concentration of the substrate in the first zone; controlling the frequency of the intermittent supply and amount of the intermittent supply of the substrate to the biomass such that the average molecular weight of the PHAs produced is at least 400,000 g/mole; wherein an online process monitoring is employed with UV/VIS spectroscopy, conductivity, or pH, for controlling substrate feeding rate or pulse feeding frequency; wherein the frequency of feeding the substrate to the biomass is controlled by sensing the dissolved oxygen concentration in the mixed liquor and triggering the feeding of the substrate to the biomass in response to an increase in dissolved oxygen concentration, over a selected time, that is greater than selected threshold value; wherein the concentration of RBCOD in the biomass-substrate mixture is controlled, such that, during PHA accumulation of RBCOD in the biomass-substrate mixture, it is generally maintained between 1,000 mg-COD/L and 10 mg-COD/L. directing the biomass into a first zone and thereafter directing the biomass to a second zone; wherein the second zone includes a volume of mixed liquor less than the volume of mixed liquor in the first zone; feeding an organic carbon-containing substrate to the biomass by intermittently supplying the substrate into the second zone including the biomass over a period of time; wherein the substrate contains readily biodegradable chemical oxygen demand (RBCOD); wherein the substrate is fed to the biomass in three or more pulses and wherein the frequency of the pulses is such that the respiration rate of the biomass does not drop more than 70% relative to the maximum respiration rate achieved by the biomass in response to the most recent pulse fed; and wherein the concentration of substrate in the second zone is higher than the concentration of the substrate in the first zone; controlling the frequency of the intermittent supply and amount of the intermittent supply of the substrate to the biomass such that the average molecular weight of the PHAs produced is at least 400,000 g/mole; wherein an online process monitoring is employed with UV/VIS spectroscopy, conductivity, or pH, for controlling substrate feeding rate or pulse feeding frequency; wherein the frequency of feeding the substrate to the biomass is controlled by sensing the dissolved oxygen concentration in the mixed liquor and triggering the feeding of the substrate to the biomass in response to an increase in dissolved oxygen concentration, over a selected time, that is greater than selected threshold value; wherein the concentration of RBCOD in the biomass-substrate mixture is controlled, such that, during PHA accumulation of RBCOD in the biomass-substrate mixture, it is generally maintained between 1,000 mg-COD/L and 10 mg-COD/L.