From waste activated sludge to polyhydroxyalkanoate: Insights from a membrane-based enrichment process

Polyhydroxyalkanoate (PHA) production is a promising technology fostering the spread of the circular bio-economy approach. However, the environmental implication of the process is usually neglected. This paper shows the results of a membrane-based PHA production pilot plant fed with no-pretreated waste activated sludge (WAS). The system was monitored for effluent water quality, nitrous oxide (N2O), and PHA production by dynamic accumulation over a long-term period to assess the consistency of the results over several fluctuations. The experimental study was characterized by three C/N ratios of 9, 4.5, and 4 g COD/g N. The system achieved a stable and high removal efficiency for carbon and nitrogen (96.3 ± 2.6 % and 89.9 ± 6.7 %, respectively), despite the only legislation limit respected being the biological oxygen demand concentration discharge limits imposed by 2020/741/EU. Low N2O gaseous and liquid concentrations were achieved over the 200-day experimental period, never exceeding 0.52 mg N2O-N/L. Despite the high concentration, the N2O emission factor accounted for only 0.21 ± 0.14 % of the influent nitrogen. Finally, the system produced an average of 36.3 ± 1.8 % g PHA/g VSS with a storage yield of up to 0.42 g CODPHA/g CODVFA. The system revealed a high stability over a long-term experimental period, achieving a considerable amount of PHA while maintaining a low N2O emission. Promising effluent water quality was achieved, highlighting the potential of applying the water reuse practices.