Enhancing Waste-Activated Sludge Valorisation Through Sequential Recovery of Polyhydroxyalkanoates and Extracellular Polymeric Substances

This study presents a coupled route for recovering intracellular polyhydroxyalkanoates (PHA) and extracellular polymeric substances (EPS) from waste-activated sludge. A direct accumulation approach was applied without prior microbial enrichment, using biomass collected from two membrane bioreactor (MBR) configurations: a conventional activated sludge MBR and a microalgae-sludge MBR. Both systems achieved comparable PHA accumulation kinetics and final contents, reaching approximately 31% (w/w) of volatile solids within 72 h. PHA was successfully extracted using a single-step process with the bio-based solvent 2-methyltetrahydrofuran, yielding recovery efficiencies of 62–66% and purities of around 65%, independent of sludge origin. The EPS were recovered from the acidified supernatant generated during biomass stabilisation, enabling partial precipitation and subsequent hydrogel formation, particularly for EPS derived from conventional sludge. The presence of microalgae significantly increased the overall EPS content but altered the polymer composition, thereby affecting hydrogel stability. The results demonstrate that sequential recovery of PHA and EPS is technically feasible and economically viable (< 0.5 € kg-1 PHA-EPS) within a single process train, without compromising PHA extraction performance. Further, results offer clear potential to improve the economic and resource efficiency of wastewater-based biopolymer production.