NOVELTY - Polyhydroxyalkanoate (PHA) is produced which involves: (a) supplying a stream of wastewater comprising readily biodegradable chemical oxygen demand (COD) (RBCOD) to a first reactor containing activated sludge comprising microorganisms capable of accumulating PHA; (b) transferring part of the activated sludge comprising PHA from the first reactor to a second reactor and retaining part of the activated sludge comprising PHA in the first reactor; and (c) supplying a stream of wastewater comprising RBCOD to the first reactor containing part of the activated sludge comprising PHA.    USE - The process is useful for producing PHA (claimed). Generally, it can be used for producing a microbial storage compound using microorganisms capable of accumulating microbial storage compound.    ADVANTAGE - The feast phase of the selection step and the accumulation step can be carried out in a continuous reactor, where wastewater with substrate is continuously fed to the reactor and treated wastewater may be continuously withdrawn from the reactor, without the typical disadvantages of a continuous process such as a distributed retention time of the bio mass (sludge). By discharging the sludge from the first reactor after accumulation step (c) and by discharging the sludge from the second reactor after famine phase (e), the biomass retention times in each of the steps (a), (c) and (e) are narrowly defined. Thus, the process combines the advantages of a sequencing batch reactor with the advantages of a continuous reactor, i.e. no need to transport large volumes of liquid in a relatively short period of time and no requirement for large pumps and buffer volume capacity. A further advantage is that no separate accumulation reactor is needed.    DETAILED DESCRIPTION - Polyhydroxyalkanoate (PHA) is produced which involves: (a) supplying a stream of wastewater comprising readily biodegradable chemical oxygen demand (COD) (RBCOD) to a first reactor containing activated sludge comprising microorganisms capable of accumulating PHA and subjecting the sludge in the first reactor to PHA-accumulating reaction conditions during a first period of 0.25-5 hours, where the conditions comprise the presence of dissolved oxygen and the presence of at least 10 mg/L RBCOD in the mixture, to obtain activated sludge comprising PHA at a level of at least 30 wt.% based on the dry weight of the sludge; (b) transferring part of the activated sludge comprising PHA from the first reactor to a second reactor and retaining part of the activated sludge comprising PHA in the first reactor; (c) supplying a stream of wastewater comprising RBCOD to the first reactor containing part of the activated sludge comprising PHA and subjecting the sludge to PHA-accumulating conditions during a second period of 2-24 hours, where the PHA-accumulating conditions comprise the presence of dissolved oxygen and the presence of at least 10 mg/L RBCOD, to obtain activated sludge that is further enriched in PHA to a level of at least 60 wt.% based on the dry weight of the sludge, and treated wastewater; (d) withdrawing the activated sludge that is further enriched in PHA from the first reactor as a product stream; (e) subjecting the activated sludge comprising PHA in the second reactor in a liquid reaction medium to famine reaction conditions during a third period of time of 2-24 hours, the famine reaction conditions comprising the presence of dissolved oxygen, to obtain activated sludge enriched in microorganisms capable of accumulating PHA; and (f) transferring the activated sludge enriched in microorganisms capable of accumulating PHA obtained in step (e) to the first reactor to form the activated sludge comprising micro-organisms capable of accumulating PHA in step (a).