Microbiota structure shift under varying organic loading rates and their impact on polyhydroxyalkanoate production in wastewater treatment plants

The conversion of “linear” conventional wastewater treatment plants (WWTP) into “circular” biorefineries offers an eco-friendly and cost-effective method of extracting valuable resources from waste. Microbiome of sewage sludge (SS) enable the fermentation of organic contaminants, producing volatile fatty acids that can be further used to produce polyhydroxyalkanoates (PHAs)—essential bioplastic precursors that can replace petroleum-based plastics. Optimizing PHA production requires understanding the relationships between SS microbiota structure and the operational parameters. Thus, this study examines how organic loading rate (OLR) influences PHA production in a selection sequencing batch reactor (S-SBR) within a pilot-scale WWTP collecting wastewater from various facilities at the University of Palermo campus. The S-SBR, enriched with PHA-producing microorganisms, was fed by a synthetic VFA mixture under two OLR conditions and subjected to a feast-famine (F/F) cycle. The results demonstrated that high OLR level increased PHA yield and promoted the selection of bacteria involved in organic matter degradation and PHA production. Specifically, a OLR of 1.3 g COD L− 1 d− 1 resulted in PHA productivity reaching 20% of biomass content, compared to 15% at a lower OLR of 0.8 g COD L− 1 d− 1. Indeed, metaxonomics revealed structural changes in the SS microbiota, with higher OLR driving a selection for PHA-producing bacteria belonging to Proteobacteria phylum, mainly the Rhodocyclaceae family—whose members are known for PHA production capabilities—showing increased abundance in comparison to the starting and the lower OLR conditions. Thus, shifts in microbiota structure linked to OLR variation may account for the differences in PHA yields observed under realistic conditions, thereby supporting future research toward the development of full-scale biorefinery systems.