Effects of a Chemically Enhanced High-Rate Activated Sludge Process Using Polyferric Sulfate on Organic Matter Recovery and Methane Production

This study evaluated the chemically enhanced high-rate activated sludge (CE-HRAS) process utilizing polyferric sulfate (PFS), a coagulant gaining popularity in wastewater treatment plants owing to its low corrosivity toward metal equipment, to maximize energy recovery from municipal wastewater. Comparative bench-scale tests using actual wastewater were conducted under practical conditions at 17–26 °C. Although the soluble chemical oxygen demand (COD) removal rate remained comparable to that of the high-rate activated sludge (HRAS) process, PFS addition significantly improved suspended and colloidal COD removal. The CE-HRAS process achieved an organic matter recovery rate of 43–58% relative to the influent organic load, 2.0 to 3.2 times higher than that of the HRAS process. However, the methane conversion ratio of CE-HRAS sludge in the medium-temperature period was only 0.37 times that of HRAS sludge due to anaerobic digestion suppression. Consequently, the methane recovery rate relative to the influent organic load was virtually equivalent between the HRAS and CE-HRAS processes (0.11 ± 0.01 and 0.12 ± 0.00 g-COD-CH4/g-COD, respectively), demonstrating that enhanced carbon capture does not inherently translate into increased biomethane yield. Therefore, future strategies must focus on optimizing coagulant dosages or developing integrated downstream processes to mitigate digestion inhibition and maximize energy recovery.