Multicriteria analysis of sewage sludge-based biodiesel production

There is increasing attention on developing efficient processes including circular economy principles, and obtaining fuels from wastewater treatment feedstocks is among the most promising. As a wastewater treatment byproduct, sewage sludge is a source of lipids that can be converted to biodiesel in a transesterification process. Economic and environmental analysis have been applied to a 60 m³/h sewage sludge plant, exploring 32 process alternatives. Using solvent extraction from wet sewage sludge, the high cost associated with the drying step is skipped. The wet alternatives with low amounts of solvent and acid usage depicted higher performance compared to the dry ones. Incorporating additional extraction stages increases both the financial gains and environmental impacts. As a result, a multicriteria analysis is implemented to ascertain the optimum process based on different priorities. The case with 0.5:1 (v/v) of hexane to biomass ratio, 3-stage extractor, 60 min residence time and pH 4 was the optimum alternative in most criteria.