Modulation of sewage sludge microbiota for enhancing the transition from wastewater treatment plants into biorefineries in the circular economy era

Pollution demands urgent action, with waste reuse emerging as a critical strategy for sustainability. Wastewater treatment plants (WWTPs) play a key role in processing wastewater from residential, industrial, and commercial sources to ensure safe water discharge. However, WWTPs operate in a “linear” mode and that imply high energy and disposal costs. To address these challenges, the European Union promotes the principles of circular economy, envisioning WWTPs as biorefineries that recover valuable compounds from waste. For example, sewage sludge can yield volatile fatty acids (VFAs) and polyhydroxyalkanoates (PHAs), the latter being a sustainable alternative to petroleum-derived plastics. This study, part of the European “Achieving Wider Uptake of Water-Smart Solutions” project, examines how different operational conditions, such as pH, oxidizing agents like potassium permanganate, headspace volume, organic loading rate, and carbon-to-nitrogen ratio, affect microbial metabolism to enhance VFA and PHA production. The approach involves metagenomic DNA extraction, 16S rDNA sequencing, and gas chromatography analysis to assess microbial community changes and quantify VFA and PHA yields. Laboratory-scale batch reactors and a pilot plant on the University of Palermo campus simulate full-scale WWTP conditions, processing wastewater diverted from campus facilities. Metataxonomic analysis shows substantial shifts in sludge microbiota pre- and post-treatment, supporting improved VFA and PHA production. The sewage sludge microbiota primarily comprises Proteobacteria, Bacteroidetes, Firmicutes, and Verrucomicrobia, with minor Archaea presence. Increased VFA production was linked to higher Firmicutes levels, though some methanogenic archaea consumed these VFAs. PHA production was associated with a steady rise in Proteobacteria, particularly Betaproteobacteria and Rhodocyclaceae. Some isolated Proteobacteria strains demonstrated optimal PHA production, suggesting sewage sludge could yield bioplastic precursors. Treated wastewater (TWW), currently discharged into water bodies, could be reused for irrigationor for fertilization after enrichment of biochar and zeolite with nutrients.Further research examined the soil microbiota of plants irrigated using TWW added with wastewater recovered inorganic fertilizers, highlighting the presence of putative beneficial bacteria and the absence of possible pathogenic bacterial strains. One of the emergent contaminants to be considered in wastewater are antibiotic-resistant bacteria and DNA containing antibiotic resistance genes. This is consistent with the detection of resistance genes for vancomycin and tetracycline and 51.16% resistance to ampicillin and erythromycin of isolated bacteria. Althogether these findings increase our understanding of the role of WWTP microbiome modulation under various operational conditions and highlight the potential of exploiting sewage sludge and water resources within a circular economy framework.