In recent years, the attention to organic wastes as a new feedstock for the production of PHA is increased based on the idea that this is the only way to reduce their high cost and to increase their sustainability. For this reason, a pilot platform has been designed to produce PHA from the source-sorted organic fraction of municipal solid waste (OFMSW). The cost-effective system consists in the three-steps anaerobic-aerobic process: the fermentation step was performed in a 200 L CSTR (OLR 20.0 kgVS/m3.d, HRT 3.3 d) in thermophilic conditions (55°C). The VFA-rich mixture was used for the two following aerobic steps: a first sequencing batch reactor (SBR, 100 L) for the production of functionalized PHA-producing biomass (mixed consortium, MMC), and a second batch reactor (50 L) for production of PHA and its maximization inside cell walls. The SBR was operated at 1.2-2.5 kgCOD/m3.d as OLR, and HRT of 1 d (equal to SRT); dynamic feeding regime (feast-famine) was used for consortium selection.
The fermentation conditions allowed obtaining a significant COD conversion into VFA. Acetic, propionic and butyric acids accounted for more than 80% of total VFA and the relative yield was 0.45 ± 0.03 CODVFA/VS. The necessary buffering capacity was provided by anaerobic digestate addition, which increased the system alkalinity (2.8 ± 0.2 gCaCO3/L at pH 5.75). In SBR, a strong selective pressure favouring PHA-accumulating organisms was easily reached, as demonstrated by the short feast-to-cycle length ratio (often below 0.12). In the second aerobic accumulation reactor, the biomass was able to accumulate up to 49% gPHA/gVSS. Both aerobic steps were performed without temperature (T) control, avoiding additional consumption of energy; the applied OLR was chosen based on biomass kinetic in order to have a constantly short feast phase and a stable selection/enrichment of PHA-accumulating consortium.