This work explores the combined application of biological nitrogen removal and polyhydroxyalkanoate (PHA) synthesis within the operational framework of a wastewater treatment plant (WWTP), employing waste-activated sludge as both the microbial inoculum and carbon source. Consistent and high-yield PHA production was achieved through a membrane-assisted microbial enrichment strategy, carried out in alternating aerobic and anoxic conditions, comprising a nitritation sequencing batch reactor (N-SBR), a membrane-based selection reactor (S-SBR), and a continuously operated PHA accumulation unit (A-SBR). The A-SBR reached 40–44 % w/w of PHA for about 70 days, while the storage yield was within 0.32–0.53 g COD_PHA g⁻¹ COD_VFA. The system maintained high efficiency despite the C/N variation around 2 and 6 g COD g⁻¹ N. Nitrous oxide (N₂O) was monitored to assess the direct greenhouse gas (GHG) emissions. The N-SBR achieved the peak concentration of 0.62 ± 0.08 mg N₂O-N L⁻¹ during period I (C/N 2), while the highest emission factor of 0.49 ± 0.08 % was reached during period IV (C/N 6). This research highlights the benefits of integrating PHA production into WWTP operations, which involves recovering resources while meeting stricter nutrient removal and environmental impact standards. Future work should focus on optimizing nutrient removal and mitigating GHG emissions to fully meet the requirements of evolving urban wastewater treatment regulations.

This study compares two membrane bioreactor-based enrichment strategies to produce polyhydroxyalkanoates (PHAs) from domestic waste-activated sludge. The aerobic dynamic feeding was implemented in layout 1 while layout 2 employs an aerobic/anoxic enrichment adopting an additional nitritation reactor. Both systems achieved around 38 % w/w of PHA with storage yields of 0.28–0.42 and 0.35–0.53 gCOD_PHA/gCOD_VFA for layouts 1 and 2, respectively. Layout 2 demonstrated an average N removal efficiency of 88.8 ± 3.9 %, slightly higher than layout 1 (82.7 ± 9.9 %). However, layout 2 showed greater nitrous oxide (N₂O) emissions, averaging 0.7 ± 0.2 mg N₂O-N/L almost doubling layout 1 (0.4 ± 0.1 mg N₂O-N/L). Additionally, layout 2 exhibited a 42 % increase in carbon footprint compared to layout 1, reaching 10.2 kg CO₂/day. This research highlights the high potential and drawbacks of the AE/AN enrichment strategy for integrating PHA production into wastewater treatment plant operations.

Polyhydroxyalkanoates (PHA) can be produced using fermentation products of an excess sewage sludge fermentation process. An efficient method to enrich a PHA-producing community is an aerobic-feast/anoxic-famine enrichment strategy. The effect of different carbon to nitrogen (C/N) feed ratios of 1, 2 and 3.5 g COD/g N on the process performance was studied. The study was executed on a pilot plant scale using fermented waste activated sludge as the organic carbon source. The system's performance was monitored in terms of removing contaminants, producing PHA, and reducing N2O emissions. The results indicated that a lower C/N ratio results in lower PHA production, with PHA content in the sludge of 20, 24 and 36 % w/w for C/N ratios of 1, 2 and 3.5 g COD/g N, respectively. At the lowest C/N ratio, the highest nitrite accumulation rate (77 %), nitrification efficiency (89 %) and denitrification efficiency (89 %) were observed, but the N2O production was also the highest (0.77 mg N2O-N/L). The long-term comprehensive monitoring carried out in this study revealed high carbon and ammonia removal efficiencies (never below 80 %) despite the C/N shifts and high COD and ammonia concentrations. At the same time, the system showed relatively low PHA production and high environmental impact in terms of high gaseous N2O emission. These findings question the sustainability of the aerobic-feast/anoxic-famine enrichment strategy for PHA production in full-scale plants.

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 COD_PHA/g COD_VFA. 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.