Biodiesel from municipal sludge: a new approach for a more efficient application

The disposal of municipal sludge represents a relevant environmental problem in the field of wastewater treatment. As a matter of fact, 80 g of solids are produced per equivalent habitant per day. In detail, 60% comes from the primary treatment and 40% from the secondary one. The sludge collected from these two different steps is made of two very different kind of biomasses: the primary sludge, is basically constituted by a large organic component (cellulose, lipids and proteins) and a minor inert one, while the secondary sludge is made of the biomass produced during the demolition of the soluble BOD. Such differences are going to open new distinguished uses for the two kinds of sludge [1]. Actually the production of secondary sludge can be drastically reduced by a new technology whose acronym is SBBGR[2] thus limiting the problem to the disposal of primary sludge. In the last years, there have been some examples of sludge valorisation, in particular of its lipid fraction, to afford Fatty Acids Methyl Esters (FAMEs, namely biodiesel). The common approach used consists into working directly on a dry sludge with methanol and sulphuric acid as catalyst obtaining 11-14% of FAMEs from primary sludge and only 2% from secondary sludge [3]. Such an approach requires a high energy demand to obtain the dry sludge as starting raw material for the alcoholysis process, from the dewatered sludge, the real product obtained from the sludge treatment in a wastewater plant. In this work we present an inedited approach in which the starting raw material is the dewatered sludge (TSS = 20%) coming from the municipal wastewater treatment plant of Bari West (a Soutern Italy town). Two different methods have been used and compared: 1) the direct alcohlolysis of the dewatered sludge and 2) the preliminary extraction with hexane with the subsequent alcoholysis of the oily residue. In both cases the energy demand per kilo of the final product result to be lower than the energy required with the common approach. From a chemical point of view, a detailed analysis of the starting system has been carried out evidencing that together with the glycerides, also free fatty acids (FFAs) and soaps are present in the sludge. For this reason, the trans-esterification of glycerides is not the only reaction involved in the production of FAMEs, but direct esterification of FFAs and the splitting of soaps into the relevant FFAs are also involved in the whole process. Sulphuric acid plays a key role in the overall process as catalyst not only for the trans-esterification of glycerides (as already mentioned) [2] but even for the direct esterification of FFAs in addition to work as strong acid for producing new FFAs from the soaps. The best results in term of yields of FAMEs (14% of the dry solids) have been obtained with the approach of hexane extraction operated directly on the dewatered sludge, with a subsequent methanolysis catalysed by sulphuric acid at 5%w. With such an approach it is possible to extract from the sludge, also other high value added compounds such as sterols, phthalates and Linear-Alkyl-Benzene [4]. In conclusion, the primary sludge could be considered not only as a potential source of biofuels, but also as a possible source of several chemicals of higher value. References [1] G. Mininni, C.M. Braguglia, R. Ramadori, M.C. Tomei Water Science & Technology 2004, 50, 9, 145-153. [2] C. Di Iaconi, M. De Sanctis, S. Rossetti, R. Ramadori Water Research 2010, 44, 1825-1832. [3] a) D. M. Kargbo Energy Fuels 2010, 24, 2791-2794; b) M. N. Siddiquee, S. Rohani Fuel Processing Technology 2011, 92, 2241-2251; c) A. Mondala, K. Liang, H. Toghiani, R. Hernandez, T. French Bioresource Technology 2009, 100, 1203- 1210. [4] E. Jarde´, L. Mansuy, P. Faure Water Research 2005, 39, 1215-1232.