Case study of potential production of Renewable Energy Sources (RES) from livestock wastes in Mediterranean islands

The aim of this work is to evaluate the potential production of Renewable Energy Sources (RES) from livestock wastes that are commonly found on Mediterranean islands, by using the island of Malta as a case study. Organic wastes in the form of livestock manure and slurry, as well as other by-products originating from the food transformation industry in the making of alcoholic beverages and the processing of olives, tomatoes and other streams, if mixed in such a way to achieve a correct C/N ratio, can be subjected to anaerobic codigestion for the production of biogas and digestate. Biogas can be further transformed into biomethane, a fuel that can be used to power surface transportation and agricultural machines, producing heat through burners, or co-generating electric and thermal energy through Combined Heat and Power (CHP) plants. Moreover, the digestate, if treated and transformed into a dry form, can be applied to soils as biofertiliser. Anaerobic co-digestion process can be compatible with the zero km / zero landfill concept. The statistical data on livestock populations and herd sizes published by the National Statistics office of Malta were evaluated so as to compute the Maltese potential production of biogas and, indirectly, biomethane or electric and thermal energy. The results indicate that the Maltese potential production of biogas is 17, 942, 115 m3, from which 10, 092, 230 m3of biomethane could be extracted or 1974 MWh ca. of electric energy and 2072 MWh ca. of thermal energy could be generated. Instead, the application of the obtained digestate would be limited by: 1) Nitrates Directive (maximum rate of 170 kg ha-1); 2) fertiliser plans; 3) soil properties such as pH (in the calcareous soils, very common in Malta, differently from acid ones, the nutrients included in the digestate tend to generate insoluble compounds, instead of being absorbed by plant roots); 4) window for application (March-October); 5) local laws that deal with the application of fertilisers (this may be a restriction for the use of wet digestate in Malta, since fertilisers having dry matter less than 30% cannot be applied to Maltese soils). This work demonstrates that the production of RES, i.e. biomethane, electric and thermal energy, from organic wastes can also have a significant added value, because the potential additional income would include that derived from the sale of biomethane or electric and thermal energy. Furthermore, it should be quantified also the income from the savings obtained by replacing chemical fertilisers with digestate.