Direct Lewis-Brønsted acid ethanolysis of sewage sludge for production of liquid fuels

Ethanolysis carried out under Lewis-Brønsted acid catalysis was investigated as a possible process to valorize the organic fraction of urban sewage sludge, with the aim of selectively obtaining liquid biofuels. In a single reactive step, the conversion of lipids into fatty acid ethyl esters, of carbohydrates into ethyl levulinate, furanic compounds and ethyl glycosides and of proteins into ethyl ester of amino acids was achieved. The optimization of reactive conditions was conducted using pure chemicals as model compounds. The effect of the co-presence of water was also considered. Then, real samples of sewage sludge (as dried and wet centrifuged samples) were reacted in ethanol in the presence of the appropriate combination of homogeneous Lewis-Brønsted acid catalysts, namely 1 %wt aluminium chloride hexahydrate and sulfuric acid respect to ethanol. After 6 h at 453 K, 99% of lipids and almost 60% of initial complex sugars were effectively converted into the abovementioned target products. Conversions and yields were quite similar to those obtained by reacting pure compounds singularly, confirming the robustness of the process and its applicability to differently composed sludge. At the end of the reaction, products were easily recovered and purified from the alcoholic phase, whereas only a very limited amount of solids remain as inert materials. Final refined biofuels have high calorific values (37 and 40 MJ kg−1) and actually represent the 68.5 and 59.2% of the initial energy content of starting sludge, respectively. This strategy combines valorization of the starting organic fraction of sewage sludge and a considerable reduction of final solid waste (in a stabilised form) to be disposed of. Finally, through a preliminary feasibility study, this acid ethanolysis resulted in a competitive alternative to the anaerobic digestion of mixed sewage sludge to obtain biofuels.