An in-depth analysis of biogas production from locally agro-industrial by-products and residues. An Italian case.

The aim of this study is to evaluate the most suitable operating conditions for anaerobic co-digestion processes with high replacement rates (64% on average) of dedicated crops (silage of maize and triticale) with residual biomass, represented by a mixture of whey (scotta), pitted olive pomace, olive mill wastewater, and sheep manure. Experimental trials were first carried out at pilot plant scale and then at industrial plant scale. Univariate and multivariate statistical analysis demonstrates that almost 70% of the dedicated crops, ordinarily used in industrial scale anaerobic digestion plants, can be replaced by a mixture of residual biomass, available on the territory, without a reduction in performance. Compared to the industrial plant, the pilot plant shows much higher values of biogas production rate and specific production rates for both biogas (about 0.64–0.86 Nm3/kgVS vs 0.35 Nm3/kgVS) and methane (about 0.30–0.43 Nm3/kgVS vs 0.20 Nm3/kgVS). With reference to a case-study of the Sardinia island, the use of all available biomass residues in existing and new anaerobic digestion plants can provide a production of more than 160 million Nm3/y of biogas, that corresponds to about 87 millions Nm3/y of biomethane, that is about 20% of the forecasted natural gas demand of the island.