Sewage sludge as carbon source for polyhydroxyalkanoates: a holistic approach at pilot scale level

In the present study, polyhydroxyalkanoates (PHA) production by mixed microbial cultures (MMC) has been carried out using thermally pre-treated excess thickened waste activated sludge (WAS), applying the feast-famine approach at pilot scale. The preliminary results of WAS fermentation conducted both in mesophilic and thermophilic conditions, in combination with a thermal pre-treatment at 70 °C for 48 h, highlighted how the thermal hydrolysis has a crucial role for the solubilization of the chemical oxygen demand (COD), which allows a significant increase of the final volatile fatty acid (VFA) concentration (roughly 8.5 g CODVFA/L). Since thermophilic fermentation after thermal pre-treatment was the best performing condition, it has been applied at pilot scale in order to routinely produce VFA as precursors for the following PHA synthesis. The selection and enrichment of PHA-producing biomass was successfully established and maintained in a Sequencing Batch Reactor (SBR) during the whole experimentation period, under short hydraulic retention time (HRT; 2 days) and medium-low organic loading rate (OLR; 2.0–2.2 g COD/L d). At the end of the production process an average PHA content of 53 ± 3 %w/w was achieved. The polymer was finally extracted and recovered from the biomass using traditional chloroform and sodium hypochlorite extraction and then characterized for the quantification of thermal properties (Tm = 156.8–160.9 °C) and molecular weight (Mv = 396–405 kDa). A final overall mass balance, usually poorly reported in the literature, has been also assessed, resulting in an overall yield of 56 g PHA per kg of volatile solids (VS).