Development of small biogas plants for the exploitation of fruits, vegetables and citrus residues.

An analysis of the availability of agro-industry residues as by-products was carried out at about 30 Producer Organizations (PO) distributed in all part of Italy. It has been found that residues exist in useful quantities in particular for the residues of the fourth-range fruit and vegetables, that is a result of selection, sorting, cutting (or peeling) and washing and packaging operations, Taking also into consideration the seasonal nature of the residuals themselves, it was found that all the PO could have the potential for a biogas plant with a power exceeding 10 kWe and some with potential of 20 kWe. From one PO, the biomasses (fruit and vegetables) potentially available to anaerobic digestion have been analyzed for the main chemical-physical characteristics (total solids, volatile solids, pH, C/N ratio). Biogas production tests have been carried out on twelve biomass, both in single and in codigestion between them, verifying that in 30-day laboratory tests it is possible to notice an increase in biogas productivity in codigestion test. Subsequently, an optimal proposal was drawn up for the selected OP for further study of the feasibility study for a 20 kWe plant. It is estimated that the agro-industry residues, in terms of quantity and seasonality, allow the processing of balanced feed mixes in terms of C/N and pH for the purposes of anaerobic digestion and a useful quantity of macroelements and microelements are present, although these last aspect could bring the digester under stress conditions in the long term. Taking into account the data obtained, it can be possible to draw proposal also for other PO that have sufficient residual quantities to power a 20 kWe biogas plant. The study conducted highlighted the potential of mini-scale plant solutions (20 - 22 kWe) for the anaerobic digestion of agro-industrial residues. In fact, although this size of plant is not normally on the market, that give the preference for larger sizes, the analysis showed the potential economic and environmental benefits achievable thanks to the anaerobic digestion of the residues of the agro-industry and also showed the good degradability characteristics of the matrices which can be started up under codigestion treatment. This plant dimension could favor a multiple number of PO and the size of 20 kWe could open up prospects also for small-sized oil mills, given the possibility of assessing the codigestion of the scraps of the mills with waste coming from fruit and vegetable PO.