Anaerobic co-digestion of agricultural residues produced in Southern Greece during the spring/summer season

Anaerobic digestion (AD) is a well-known environmentally friendly technology, that even though has been investigated extensively, in Greece there is still a lack of experimental knowledge on locally available substrates' valorisation. This study aimed to assess a continuous one-stage AD system for the valorization of Southern Greece's spring/summer residual biomass. The most suitable substrate in Southern Greece during the spring/ summer season is 10% corn silage, 14% cattle manure, 66% unsuitable for human consumption watermelons, and 10% tomato processing residues, in terms of methane yield maximization during anaerobic co-digestion. Regarding the system operational conditions, the hydraulic retention time (HRT) was set at 20 d, while the initial organic loading rate (OLR) of 2 g COD/(L center dot d) was applied, which was increased gradually, up to 7 g COD/ (L center dot d). The reactor operated successfully for the applied OLRs 2-5 g COD/(L center dot d), while a further increase led to inhibition phenomena. Even if propionate accumulated up to 5 g/L, it successfully coped with the correction of alkalinity levels (up to 6.4 g CaCO3/L). The system's efficient operation paves the way for residual biomass treatment in one-stage systems, without substrate dilution, at high OLRs, that can be applied in countries characterized by a surplus of the aforementioned residues.