Food waste valorisation through anaerobic processes: thermal pretreatment or co-digestion?

Anaerobic digestion of Food Wastes (FW) was investigated in batch mesophilic tests. Scope of this work was to evaluate the efficiency of thermal pretreatment on the solubilisation degree of FW, alone or mixed with typical Mediterranean agro-wastes as olive husks (OH), and to evaluate the digestion enhancement potential of pretreatment and co-digestion strategy in terms of methane and hydrogen conversion rates of FW. Co-digestion was carried out by mixing FW and olive husks, untreated or thermal pre-treated. Thermal pretreatment was efficient in solubilising the organic material of FW (soluble COD increase up to 35%) and of the mixture FW and OH (up to +96%), in particular carbohydrates. After 30 days of digestion, an overall good anaerobic biodegradability of FW alone was observed, with methane yields in the range 0.350 – 0.592 Nm3/kgVSfed, overshadowing the effect of pretreatment, that affected positively only the hydrogenogenic phase. Moreover, also co-digestion with OH was a successful option for encouraging hydrogen conversion rate, which was found to be more than double compared to sole FW substrate digestion. Nevertheless, thermal pretreatment coupled with high sugars substrates as OH set the conditions for Maillard reactions occurrence, decreasing the conversion to methane.