Effect of organic solvents and acidic catalysts on biodiesel yields from primary sewage sludge, and characterization of fuel properties

Biodiesel is considered the most convenient biofuel, due to its direct use in existing combustion engines; however, its production is not economically optimal due to processes that utilize costly substrates, require high energy expenditure or achieve low biodiesel yields. We used primary sewage sludge for the production of biodiesel and compared yields at different temperatures (40, 50 and 60 °C) and with different organic solvents (chloroform, toluene and hexane), acid catalysts (HCl and H₂SO₄) and catalyst concentrations (3, 5 and 7%). A maximum of 18 wt% biodiesel was obtained using chloroform with 3% H₂SO₄ at 40 °C. The conversion efficiency of the primary sludge to biodiesel under these conditions was 86.4%. Based on the fatty acid methyl ester composition of the obtained biodiesel and measured fuel properties, it was deduced that fuel from primary sludge is more suitable for areas with warmer climates than ones that reach temperatures close to 0 °C. However, the fuel properties of the produced biodiesel were within the limits of accepted international standards. We conclude that production of biodiesel from primary sewage sludge under optimized conditions reduces the impact of the feedstock on the environment, decreases overall costs and produces fuel with acceptable properties.