Mesophilic and thermophilic co-fermentation of cattle excreta and olive mill wastes in pilot anaerobic digesters

Cattle excreta and two-phase olive mill wastes (TPOMW) were codigested at a 3:1 ratio in two 75 L continuous stirred tank reactors at 37 °C and 55 °C to analyse their biogas production. The contribution of each residue to the total gas production at 37 °C was evaluated in reactors digesting either 3:1 excreta:water or 3:1 water:TPOMW. The mesophilic co-fermentation of cattle excreta with TPOMW at an organic loading rate (OLR) of 5.5 g COD L−1 d−1 rendered 1096 mL biogas L−1 sludge d−1. This was 337% higher than that of excreta alone. The methane yield resulting from the codigestion was 179 L CH4 kg−1 VS loaded, of which 42% was attributed to the quarter of the reactor corresponding to TPOMW. Under thermophilic conditions, the codigestion yielded 17.3% more methane than mesophilically. In the reactor digesting TPOMW alone (OLR = 3.8 g COD L−1 d−1) the ratio VFA/alkalinity exceeded 0.8 after 21 d, leading to its acidification and inhibition of methanogenesis. Farm-scale digestion of animal excreta and TPOMW should be promoted in Mediterranean countries as an environmentally sound option for waste recycling and renewable energy production.