TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Method: Producing the PHA-rich-biomass from open mixed cultures further involves supplying the adjusted feed to the stimulating zone at a rate that is proportional to the demand created by the biomass respiration rate. The feed is blended from at least two distinct sources, to provide a selected concentration of nitrogen with respect to RBCOD in the feed supplied to the stimulation zones. The feed is blended from at least two distinct sources, to provide a selected concentration of phosphorus, with respect to RBCOD in the feed supplied to the stimulation zones. The method involves measuring the phosphorus, nitrogen, and/or RBCOD concentrations in, and/or supplied to, the fed-batch reactor and in response to the measured concentrations, adjusting the concentration of nitrogen, phosphorus and/or RBCOD in the feed supplied to the stimulation zones. The method also involves measuring the biomass PHA content and/or the time rate of change of the biomass PHA content for the biomass in the fed-batch reactor and in response to the measured content or changes in content with time, adjusting the concentration of nitrogen, phosphorus and/or RBCOD established in the feed supplied to the stimulation zones; measuring the biomass respiration rate and/or the time rate of change of the biomass respiration rate for the biomass in the fed-batch reactor, and in response to the measured respiration rate or respiration rate changes in time, adjusting the concentration of nitrogen, phosphorus or RBCOD in the feed supplied to the stimulation zones; and measuring the consumption rate of COD and/or the time rate of change of the COD consumption rate by the biomass in the fed-batch reactor, and in response to the measured COD consumption rate or the COD consumption rate changes in time, adjusting the concentration of nitrogen, phosphorus or RBCOD in the feed supplied to the stimulation zones. The fed-batch reactor includes at least two tanks with at least one tank including the stimulation zone and at least one tank including the maintenance zone and where the feed to the fed-batch reactor is directed to the tank that includes the stimulating zone. The volume of the stimulation zone is sufficiently large to provide the biomass disposed to the stimulation zone a hydraulic retention time of at least 20 seconds. The volume of the stimulation zone is integrated into the volume of the maintenance zone, but where the volume of the stimulation zone is at least partly separated from the maintenance zone by a physical structure. The method also involves biologically treating a wastewater stream in a biological wastewater treatment system and separating the mixed liquor and biomass from the wastewater stream and directing the mixed liquor and bio ass to the fed-batch reactor; continuing to circulate the mixed liquor containing the biomass back and forth between the stimulating zone and the maintenance zone and continuing to maintain the phosphorus and nitrogen to RBCOD ratios within the range until the PHA content of the biomass is, on average, greater than 0.40 g-PHA/g-VSS; and harvesting at least a fraction of the biomass from the process during the fed-batch operation and before the batch process terminates. Stimulating the biomass respiration rate is such that the biomass is stimulated, on average, greater than 70% of its extant maximum respiration rate. The biomass is harvested from a biological wastewater treatment process prior to being directed to the fed-batch reactor and where the PHA content of the harvested biomass is less than 10% of the dry biomass weight. The method involves pre-treating the mixed liquor prior to directing the mixed liquor into the fed-batch reactor where pre-treating the mixed liquor includes thickening the mixed liquor so as to reduce the concentrations of phosphorus and nitrogen in the mixed liquor directed to the fed-batch reactor. After thickening the mixed liquor, adding a diluent to the mixed liquor to further reduce the concentrations of phosphorus and nitrogen in the mixed liquor. Alternatively, the feed on demand fed-batch method for producing a PHA-rich-biomass from open mixed cultures involves: directing mixed liquor containing biomass into a fed-batch reactor including at least one biomass stimulating zone and at least one biomass maintenance zone; directing a feed into the fed-batch reactor where the feed contains RBCOD along with bioavailable N and P; intermittently and repeatedly stimulating the respiration rate of at least a portion of the biomass by directing the feed into the fed-batch reactor and exposing the biomass to a relatively high concentration of RBCOD in the stimulating zone; after the biomass has been exposed to the relatively high concentration of biomass in the stimulating zone, transferring the biomass to the maintenance zone where the biomass is exposed to a relatively low concentration of RBCOD; circulating the biomass back and forth between the stimulating zone and the maintenance zone; and controlling the N or P concentration relative to the RBCOD in the feed directed into the fed-batch reactor by controlling the N to RBCOD ratio or the P to RBCOD ratio in the feed. The method involves controlling the N and P concentrations relative to the RBCOD in the feed by adjusting the concentration of the bioavailable N relative to the concentration of RBCOD to yield an adjusted feed with an average N to RBCOD ratio within a range of 2-15 mg-N/g-RBCOD and the concentration of bioavailable P relative to the concentration of RBCOD to yield a feed having an average P to RBCOD ratio within the range of 0.5-3 mg-P/g-RBCOD; measuring the N and P concentrations in the mixed liquor in the fed-batch reactor and adjusting the N to RBCOD ratio or the P to RBCOD ratio in the feed to yield selected ratios within a pre-determined range; and measuring signals that infer changes in the biomass PHA content, biomass respiration rate and/or the COD consumption rate in the fed batch reactor and adjusting the N to RBCOD ratio or the P to RBCOD ratio in the feed to yield selected ratios within a pre-determined range based on one or more of these signals. The sensors are utilized to measure N, P and RBCOD in the mixed liquor in the fed-batch reactor or in the feed and the sensors are operative to direct data to a controller where the data is representative of the concentrations of P, N, and/or RBCOD, and where the controller is operative to control the ratio of N to RBCOD and P to RBCOD such that on average the ratios fall within a pre-selected range. Also, the sensors are utilized to measure or infer proportional changes in biomass PHA content, biomass respiration rate, and/or COD consumption rate and the sensors are operative to direct data to a controller where the data is representative of the concentrations of P, N, and/or RBCOD in the feed supplies, and where the controller is operative to control the ratio of N to RBCOD and P to RBCOD such that on average the ratios fall within a pre-selected range. The method involves controlling the N concentration relative to the RBCOD in the feed by adjusting the concentration of the bioavailable N relative to the concentration of RBCOD to yield an adjusted feed with an average N to RBCOD ratio within a range of 2-15 mg-N/g-RBCOD. The method involves controlling the P concentration relative to the RBCOD in the feed by adjusting the concentration of bioavailable P relative to the concentration of RBCOD to yield a feed haying an average P to RBCOD ratio within the range of 0.5-3 mg-P/g-RBCOD; measuring the biomass PHA content and/or the time rate of change of the biomass PHA content for the biomass in the fed-batch reactor and in response to the measured content or changes in content with time, adjusting the concentration of nitrogen, phosphorus and/or RBCOD established in the feed supplied to the stimulation zones; and measuring the biomass respiration rate and/or the time rate of change of the biomass respiration rate for the biomass in the fed-batch reactor, and in response to the measured respiration rate or respiration rate changes in time, adjusting the concentration of nitrogen, phosphorus or RBCOD in the feed supplied to the stimulation zones. The method involves continuing to circulate the mixed liquor containing the biomass back and forth between the stimulating zone and the maintenance zone and continuing to maintain the phosphorus and nitrogen to RBCOD ratios within the range until the PHA content of the biomass is, on average, greater than 0.40 g-PHA/g-VSS. Stimulating the biomass respiration rate is such that the biomass is stimulated, on average, to greater than 70% of its extant maximum respiration rate. EXAMPLE - No suitable example given.