Microwave-Induced Pyrolysis for Production of Sustainable Biodiesel from Waste Sludges

Purpose Disposal of sewage sludge is becoming one of the most important issues in wastewater treatment: the 2005 EU sludge production was estimated at 9.5 million tons dry weight, up 54 % in 10 years. Sludge disposal costs may constitute 30-50 % of the total operation cost of WWTPs. Sewage sludge still contains resources that may be put to use, like nutrients and energy, that can be recovered through a variety of approaches. MethodsPyrolysis is a thermochemical decomposition process occurring by applying heat in absence of oxidizing agents; this way, chemical bonds are broken, with formation of simpler molecules, producing solid, gaseous and oil fractions that can be used as fuel or as prime material for chemical products. By applying microwave heating to achieve pyrolysis to the sludge under treatment, the more desirable production oil and gas fractions is increased and the residual solid fraction decreases. Results Microwave-assisted pyrolysis of urban waste sludge was applied for the production of oil and (Syn)gas, that were afterwards characterized and compared to mainstream alternative fuels (biodiesels). An energetic estimate of the process is performed and described. A key factor in deciding the positivity of the energetic balance (and therefore the economic validity of the process) is the correct tuning between microwave source and treated sample. Conclusions Based on our tests, the overall energy balance of the process is not always immediately favorable, however, by appropriately tuning the wave source and minimizing reflected radiation (lost energy) an energy-positive recovery of resources on one side, and considerable reduction of sludge volumes, on the other, may be achieved.