Polyhydroxyalkanoates and Crotonic Acid from Anaerobically Digested Sewage Sludge

A transdisciplinary approach that integrates thermochemical conversions and biological transformations for valorizing C-content and chemical energy of anaerobically digested sewage sludge and producing biopolymers (polyhydroxyalkanoates, PHA) and drop-in chemicals (crotonic acid) has been presented here. Hydrothermal carbonization has increased by 10-times the soluble COD of sewage sludge, allowing it to split 44-54% of its COD into an aqueous phase (HTCap) enriched in small fermentable organic molecules; the coupling of acidogenic fermentation and aerobic fermentation converted these substrates first into volatile fatty acids (20% of CODHTCap) and then into PHA (about 20% of CODVFA). The extraction of microbial biomass with dimethyl carbonate (DMC) allowed high quality PHA to be produced, with a high molecular weight (0.9 MDa) and a percentage of medium chain monomers (hydroxyvalerate and hydrohexanoate) close to 12%. The postextraction microbial biomass, still containing a relevant portion of "unextractable" PHA, was last treated at 300 °C to selectively convert the PHA-backbone into crotonic acid with a 42% yield on PHA content basis.