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In this research, PHA (polyhydroxyalkanoate) production system by activated sludge was studied. PHA behaves as carbon and energy storage material in bacteria. And PHA is a biodegradable plastic when extracted from bacteria. In this paper, the investigations from 3 aspects were reported; control of PHA composition, PHA production under coexistence of nitrogenous compounds, and influence of enrichment condition on PHA productivity. As results, it was possible to regulate PHA composition by utilizing acetate and propionate as carbon source and by regulating its composition. Nitrogenous compounds did not depress PHA productivities in the case of activated sludge, while nitrogenous compounds usually depress in general. PHA contents of MLSS were achieved up to 57% by using anaerobic-aerobic activated sludge. But microaerophilic-aerobic process could supply stably the activated sludge which accumulated PHA with high efficiency.

The main purposes of wastewater treatment systems are to remove organic pollutants, but it would be very attractive if there were a way to recover the organic pollutants as valuable organic materials. One of the possible ways to recover organic pollutants in wastewater is to convert them into polyhydroxyalkanoates (PHAs), which are biodegradable plastics. In this study, 18 activated sludge samples in 4 wastewater treatment plants (WWTPs) in Tokyo, Japan, were evaluated for their potential to produce PHAs by aerobic batch experiments with excess supply of acetate as the sole carbon source. The activated sludge samples tested had the capability to accumulate PHA up to 18.8% of dry cell weight on average, with the range of 6.0% to 29.5%. The results showed that the maximum PHA content was dependent on the influent more than on the operational conditions of the activated sludge, and that conventional activated sludge produced PHA as much as anaerobic-aerobic activated sludge did. The PHA content achieved in this study is still low, and further improvement is needed to put into practice the recovery process of organic pollutants as biodegradable plastics by activated sludge.

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