Extraction of Municipal Sewage Sludge Lipids Using Supercritical CO₂for Biodiesel Production: Mathematical and Kinetics Modeling

Demand for determining renewable lipids feedstock for the production of biodiesel is increasing with the rapid depletion of petroleum diesel. The present study was conducted to assess the feasibility of utilizing municipal sewage sludge (MSS) as a potential lipids feedstock for biodiesel production. The lipids' extraction and separation from MSS were conducted using supercritical CO2 (scCO2) with varying treatment time (15-120 min), temperature (30-80°C), pressure (10-50 MPa), and addition of cosolvents (1-10 wt.%). The modified Gompertz equation and Arrhenius equation were employed to evaluate lipids' extraction and kinetics behavior from municipal sewage sludge using scCO2. About 27% of lipids were extracted from MSS with scCO2 at a temperature of 60°C, pressure of 30 MPa, treatment time of 60 min, and 5 wt.% of ethanol (EtOH) as cosolvent. The modified Gompertz equation was adequately fitted with experimental data of the lipids' extraction from MSS using scCO2. The kinetics properties analyses revealed that the scCO2 extraction technology was highly dependent on pressure than the temperature for the extraction of the lipids from MSS. The physicochemical characteristics and fatty acid contents of the scCO2 extracted lipids from MSS and sewage sludge biodiesel were determined using a variety of analytical techniques. The physicochemical properties of the sewage sludge biodiesel were compared with the international standard specifications of biodiesel, such as the American Society for Testing and Materials specifications for diesel fuel (ASTM D6751) and European Standard (EN 14214) specifications.