Polyhydroxyalkanaote (PHA) is known to be temporarily stored by microorganisms in activated sludge especially in the anaerobic-aerobic processes. When PHA is extracted from activated sludge, it is a thermoplastic with the remarkable characteristics of biodegradability. We investigated the possibility of using activated sludge for the production of PHA, focusing on increasing the PHA content of activated sludge. Activated sludge from laboratory scale anaerobic-aerobic reactors accumulated PHA of around 20% under anaerobic conditions and up to 33% under aerobic conditions. In order to further increase the PHA content of activated sludge, we introduced the “microaerophilic-aerobic” process, where a limited amount of oxygen is supplied into the anaerobic zone of the anaerobic-aerobic process. Activated sludge acclimatized in the microaerophilic-aerobic process accumulated PHA of as much as 62% of sludge dry weight. The “microaerophilic-aerobic” process was demonstrated to be an effective process for the enrichment of PHA-accumulating microorganisms.

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