Sludge Retention Time Is Associated with EPS and Microbial Community Shifts That Enhance Carbon Capture in a Contact Stabilization Process Treating Synthetic Food-Processing Wastewater

A considerable portion of influent organic carbon is lost as CO2 in conventional activated sludge processes, diminishing the resource potential of wastewater. To clarify how sludge retention time (SRT) is associated with carbon capture in a contact stabilization (CS) process treating synthetic food-processing wastewater (COD ≈ 800 mg·L−1), a CS system was operated at SRTs of 1, 4, and 7 d. Extracellular polymeric substances (EPS), microbial community structure, removal of different COD fractions, and carbon flow distribution were systematically analyzed. Results indicated that SRT was significantly associated with EPS organization, microbial cooperation, and carbon partitioning. At SRT = 4 d, the system achieved the best carbon capture performance, with total EPS content reaching 43.14 mg·gVSS−1 and a more pronounced TB-EPS-dominated structure. Polysaccharides accumulated during the contact phase (38.42 mg·gVSS−1) and were effectively utilized during the stabilization phase (a reduction of 48.9%). Bacteroidota and key functional genera, including Flavobacterium, Zoogloea, and Acidovorax, were enriched. Fractionated COD analysis showed that an intermediate SRT promoted coordinated removal and sludge-phase transfer of particulate, colloidal, and soluble organics. Carbon flow analysis indicated that SRT = 4 d achieved the highest sludge carbon recovery (31.1%) with lower oxidation loss (31.9%). Overall, an appropriate SRT coincided with enhanced carbon capture and redirection by optimizing EPS structure, microbial community assembly, and carbon metabolic allocation.