Direct carbon recovery from raw wastewater for bioenergy production by anaerobic digestion

Wastewater is rich in carbon, which is normally expressed as chemical oxygen demand (COD). In this study, carbon was recovered from raw wastewater by flocculation and then converted to bioenergy. Chitosan quaternary ammonium salt (HACC) demonstrated outstanding performance, achieving 75.1 % COD recovery from wastewater under the following conditions: an initial pH of 7, a dosage of 8 mg/L, rapid mixing for 2 min at 100 rpm, and slow mixing for 10 min at 60 rpm. The obtained sludge was subjected to anaerobic digestion for 45 days at 35.0 °C, with a methane yield of 317.2 mL CH₄/g VSS and a VSS degradation efficiency of 41.5 %. It demonstrates the feasibility of concentrating carbon sources from wastewater for energy recovery. A theoretical process integrating flocculation-based carbon recovery with microalgae-assisted nutrients removal was proposed, demonstrating significant potential for the reduction of energy consumption and GHG emissions. These findings highlight a promising strategy for energy-efficient wastewater valorization.