Downstream processing and characterization of polyhydroxyalkanoates (PHAs) produced by mixed microbial culture (MMC) and organic urban waste as substrate

Abstract

The utilization of food waste and sewage sludge as organic substrate from urban context for the synthesis of microbial polyhydroxyalkanoates (PHAs) has been only recently investigated at pilot scale. Within this context, two stabilization methods have been found for preserving the amount of PHA intracellularly produced by open mixed microbial culture (MMC): thermal drying and wet acidification of the biomass at the end of PHA accumulation process. The extracted PHA from the two differently stabilized biomasses was then characterized with regard to chemical composition, molecular weight, and thermal properties. The polymer contained two types of monomers, namely 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) at a relative percentage of 93.0–79.8 and 7.0–20.2 w/w, respectively. PHA extracted from wet-acidified biomass had higher average molecular weights (Mw) of 370–424 kDa while PHA recovered from thermally stabilized dried biomass had a 3-fold lower Mw (on average). The PHA decomposition temperatures Td10% and Tdmax were in the range 260–268 °C and 269–303 °C, respectively, not dependent on the monomeric composition or molecular weight. Thermal properties such as melting temperature (Tm1 132–150 °C; Tm2 155–167 °C) and melting enthalpy (26–70 J/g) were quantified in a relatively broad range according to the different stabilization-extraction method and obtained composition.

Recovery of Polyhydroxyalkanoates From Single and Mixed Microbial Cultures: A Review

“… Villano et al (2014) treated MMC with NaClO for 3 and 24 h at rt and noticed that the effect of NaClO on the purity and the molecular weight of the extracted polymer is similar regardless of the reaction time; on the other hand, the temperature of the NaClO treatment inversely impacts both the recovery and the polydispersity index: the PHA recovery decreases from rt (100%) to 85°C (50%), whereas the polydispersity index increases (giving more heterogeneous polymers) if the treatment is performed at high temperature (values around 3) or rt (values of 4–10) ( Table 5 ). Recently, Lorini et al (2020) investigated the impact of two stabilization protocols applied at the end of the accumulation phase (thermal drying at 145°C for 30 min and at 70°C overnight, and wet acidification with H 2 SO 4 ) in combination with NaClO digestion (at rt overnight) for preserving the amount and the length of PHA produced by MMC. A wet-acidification pretreatment provides three-times higher molecular weights (0.4 MDa) than a thermal pre-treatment, a recovery of 96%, and a polymer purity of 90%.…”

Characterization of Polyhydroxyalkanoates Produced at Pilot Scale From Different Organic Wastes

“…Thus, in order to compare the melting behavior, the melting enthalpies (Δ H m ) of TV and C-samples, determined by DSC, are reported as a function of 3HV content in Figure 2 . Moreover, for a more complete evaluation of the 3HV content effects, results from a previous work ( Lorini et al, 2020 ) are reported, too. In this way, it was possible to have a wider range of 3HV percentage, specifically between 7.7 and 87% w/w.…”

“…More recently, disruption of NPCM was obtained by using non-ionic surfactants as pretreatment before PHA extraction with DMC ( Colombo et al, 2020 ) and by treating the biomass with ammonia solutions at high temperatures ( Burniol-Figols et al, 2020 ). In a previous study ( Lorini et al, 2020 ), NaClO digestion was investigated for PHA recovery from MMC produced at pilot scale. More in detail, PHA was produced in the pilot plant operating in Treviso, exploiting the same feedstock (OFMSW-WAS) and operative conditions and applying the same biomass stabilization methods described in this work.…”

“…In the view of MMC-PHA production at industrial scale and a subsequent market scenario, specific chemo-mechanical and thermal properties of the polymer need to be controlled. In a recent study, two types of stabilization method have been applied at pilot scale on PHA obtained from organic wastes, in order to preserve the amount of PHA produced after the accumulation step and its properties ( Lorini et al, 2020 ). The recovered polymer was extensively characterized and resulted in good thermal and chemical properties, while it showed variable molecular weight (between 100 and 450 kDa) dependent on the applied stabilization method.…”

“…The availability in literature regarding the chemo-mechanical and thermal characterization of MMC-PHAs is very limited, however, the published studies highlighted an increase in melting temperature ( T m ) for MMC-PHA respect to pure culture derived PHA ( Bengtsson et al, 2010b ) and a similar decomposition temperatures (270°C) to the commercial poly-3-hydroxybutyrate-co-3-hydroxyvalerate [P(3HB-co-3HV)] ( Carrasco et al, 2006 ). Furthermore, monomeric composition, i.e., 3HV content, showed to have effects on different polymer properties ( Arcos-Hernández et al, 2013 ; Lorini et al, 2020 ), such as crystallinity.…”