Anaerobic Treatment of the Liquid Fraction of Food Waste in a Hybrid Reactor with Spatially Structured Biomass: Process Performance and Microbial Community Dynamics

This study investigated the performance of a hybrid upflow anaerobic reactor (HUAR), integrating suspended and fixed biomass, for the treatment of the liquid fraction of food waste (LFFW), a nutrient-rich and highly organic strength stream generated during municipal organic waste handling. The reactor was fed with LFFW diluted to 10, 15, 30, 50, and 75% (v/v), corresponding to organic loading rates (OLRs) of 2.9, 4.1, 7.4, 11.5, and 19.2 kg COD/(m3·d), respectively, at a constant hydraulic retention time of 48 h. COD removal efficiency increased with OLRs, reaching 81% at the highest LFFW content, while methane content in biogas rose from 52 to 73%. Process stability was maintained throughout the reactor operation, with pH, alkalinity, and VFA/TA ratios remaining within optimal ranges (<0.4), and intermediate VFA accumulation was effectively buffered. Microbial community profiling revealed microbial functional stratification between suspended and attached biomass. Bacteria dominated the suspended phase (76%), driving hydrolysis and acidogenesis, while the biofilm zone had a higher proportion of methanogenic Archaea (35%), favoring methane production. The attached biomass was enriched in Methanothrix and Methanosarcina, along with syntrophic bacteria such as Macellibacteroides, Aminivibrio, and Cloacibacillus. The dual-compartment design promoted stable operation and efficient conversion of organics to methane even at high organic loading rates. These results demonstrate that the HUAR is a robust and high-performance system for anaerobic treatment of the liquid fraction of food waste, offering promising opportunities for integration into circular economy strategies and decentralized waste-to-energy solutions