Microbial Poly-β-hydroxyalkanoate Accumulation from Wastewater Sludge for Plastic Production

Treated wastewater sludge may contain harmful compounds that prevent its use as fertilizer. However, wastewater sludge carbon could be used to produce biodegradable plastic material called polydroxyalkanoates (PHAs), which could replace the traditional plastics made from fossil raw materials to reduce carbon dioxide emissions. This takes place in a two-step process. First, the sludge is hydrolysed into volatile fatty acids (VFAs), and then in the second step, bacteria enriched from the activated sludge convert the VFAs into PHAs in an aerated bioreactor. For the hydrolysis, pH values of 9, 7, 5, and no pH adjustment were tested in high volatile solid (VS) and high fixed solid (FS) bioreactors. Based on the VFAs yield, pH 7 was chosen for the sludge hydrolysis experiments. Three types of sludge (sludge after nitrogen removal, sludge for biogas production, and dewatered sludge) were hydrolysed at pH 7. The sludge intended for biogas production produced the maximum amount of VFAs (8861 ± 323 mg/L). In PHA accumulation, the feast-and-famine process was not suitable for converting VFAs to PHAs in the sludge with chemical phosphorus precipitation, but fermenter aeration had to be continued to cultivate PHA-accumulating microorganisms. The final PHA concentration was 80% of volatile solids.