From waste to energy: a closed-loop two-phase anaerobic digestion system for sustainable dairy wastewater management

The treatment of dairy wastewater (DW), characterized by high organic load and lipid/protein content, remains challenging due to the energy-intensive nature of aerobic processes and instability of anaerobic methods. This study developed a self-regulating two-phase anaerobic digestion (TPAD) system integrating an anaerobic baffled reactor (ABR) with an up-flow anaerobic sludge blanket (UASB) reactor. Sequential phase separation in the ABR enables microbial self-organization for staged lipid adsorption, protein denaturation, and hydrolysis-acidification, ensuring stable UASB input. Laboratory-scale operation achieved exceptional chemical oxygen demand (COD) removal (97.06-99.01%). Full-scale implementations across three Chinese provinces demonstrated robust performance, with COD removal of 78.13-93.46%, high methane content (83.20-83.94%), sludge reduction >75.00%, and reductions in energy consumption (64.71-85.03%) and greenhouse gas emissions (88.01-97.09%) compared to conventional systems. Microbial analysis confirmed functional spatial divergence. The TPAD system presents a regionally-proven, versatile, and scalable solution to transform DW management from a disposal cost into a biogas-generating process.