The Utilization of Activated Sludge Polyhydroxyalkanoates for the Production of Biodegradable Plastics

Sequencing batch reactor (SBR) systems were used for the development of a system and operating procedures for the high production and storage of polyhydroxyalkanoates (PHAs) by wastewater treatment (activated sludge) bacterial cultures. The systems were maintained at a nominal hydraulic retention time (HRT) of 10 hours, and a temperature of 20°C. The solids retention time (SRT) was maintained at 10 days during the first experiment, but a two-stage bioprocess approach that had a growth phase and a separate PHA accumulation phase was used for subsequent experiments. After the initial experiment, the system was fed a 660 mg/L mixture of acetate and propionate as the sole sources of organic carbon. Whereas the initial experiment with microaerophilic (MAA)/aerobic (AE) cycling was unsuccessful, the two-stage bioprocess approach proved to be a successful strategy for PHA production by activated sludge biomass. Unbalanced growth conditions stimulated massive production of PHA. Operating conditions had a significant effect on PHA production and the composition of the accumulated copolymer. Fully AE conditions with N&P limitations were the optimum conditions for PHA production using activated sludge biomass. Maximum cellular PHA accumulation of 70%TSS was obtained under fully AE conditions with multiple periods of simultaneous nitrogen (N) and phosphorus (P) limitations. Surprisingly, the relative amounts of the PHA copolymers formed, i.e., polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV) were different under different operating conditions, even though the types and amounts of volatile fatty acids (VFAs) in the feed were the same. The results indicate that activated sludge biomass has considerable potential