Polyhydroxyalkanoates production with mixed microbial cultures: from culture selection to polymer recovery in a high-rate continuous process

Polyhydroxyalkanoates (PHA) production with mixed microbial cultures (MMC) has been investigated by means of a sequential process involving three different stages, consisting of a lab-scale sequencing batch reactor for MMC selection, a PHA accumulation reactor and a polymer extraction reactor. All stages were performed under continuous operation for at least 4 months to check the overall process robustness as well as the related variability of polymer composition and properties. By operating both biological stages at high organic loads (8.5 and 29.1 gCOD/L d, respectively) with a synthetic mixture of acetic and propionic acid, it was possible to continuously produce PHA at 1.43 g/L d with stable performance (overall, the storage yield was 0.18 COD/COD). To identify the optimal operating conditions of the extraction reactor, two digestion solutions have been tested, NaOH (1 M) and NaClO (5% active Cl-2). The latter resulted in the best performance both in terms of yield of polymer recovery (around 100%, w/w) and purity (more than 90% of PHA content in the residual solids, on a weight basis). In spite of the stable operating conditions and performance, a large variation was observed for the HV content, ranging between 4 and 20 (%, w/w) for daily samples after accumulation and between 9 and 13 (%, w/w) for weekly average samples after extraction and lyophilization. The molecular weight of the produced polymer ranged between 3.4 x 10(5) and 5.4 x 10(5) g/mol with a large polydispersity index. By contrast, TGA and DSC analysis showed that the thermal polymer behavior did not substantially change over time, although it was strongly affected by the extraction agent used (NaClO or NaOH).