Recovery of bioplastics from activated sludge wastewater treatment process

Abstract

Polyhydroxyalkanoates (PHAs) are polyesters of hydroxyalkanoates (HAs) synthesized by numerous bacteria as intracellular carbon and energy storage compounds and accumulated as granules in the cytoplasm of cells. More than 80 HAs have been detected as constituents of PHAs, which allow these thermoplastic materials to have various mechanical properties resembling hard crystalline polymer or elastic rubber depending on the incorporated monomeric units. Even though PHAs have been recognized as good candidates for biodegradable plastics, their high price compared with conventional plastics has limited their use in a wide range of applications. Recently, much effort has been spent in improvements in fermentation technology and the development of bacterial strains that more efficiently synthesize PHAs to bring the costs down to make PHAs more competitive. In this study, survey of plastics waste generation in Hong Kong was reviewed and current plastics waste management options were evaluated. It is argued that the current and proposed management strategy adopted does not help to handle the environmental problems relating to plastic wastes effectively. Thus, it is necessary to develop an economical process to produce PHAs as environmentally-friendly substitutes for conventional plastics. In this study, activated sludge bacteria from a conventional wastewater treatment process were induced to accumulate PHAs under different carbon-nitrogen (C:N) ratios. As the C:N ratio increased from 24 to 144, specific polymer yield increased to a maximum of 0.27g of polymer per gram of dry cell mass while specific growth yield decreased. The highest overall polymer production yield of 0.09 g of polymer per gram of carbonaceous substrate consumed was achieved using a C:N ratio of 96, without significantly affecting the organic treatment efficiency in the wastewater treatment system. In addition, an intermittent nitrogen feeding program was established to optimize the volumetric PHA productivity of the process. The optimal overall polymer production yield of 0.11 g polymer per gram of carbonaceous substrate consumed was achieved under C:N ratio of 96 by feeding inorganic nitrogen in the reactor liquor once every four cycles. At the same time, the amount of excess sludge generated from the wastewater treatment process was reduced 23 %. Moreover, the composition of polymer accumulated was dependent on the valeric acid content in the feed. Copolymer poly(3-hydroxbutyrate-co-3-hydronxyvaleerate) [P(3HB-co-3HV)] was produced in the presence of valeric acid. The 3-hydroxyvalerate (3HV) mole fraction in the copolymer was linearly related to valeric content in the feed, which reached a maximum of 43 % when valeric acid was used as the sole carbon source. When the 3HV units in the polymer increased from 0 to 43 mol%, the melting temperature decreased from 177.6 to 99.0 C. Thus, the composition of the copolymers, and hence the physical and mechanical properties, could be controlled by manipulating the influent organic compositions.