Abstract: Anaerobic digestion of the liquid fraction (LF) of dairy manure was studied in lab scale in a high-load anaerobic reactor (upflow anaerobic sludge blanket (UASB)) operated at 25 °C. The LF was obtained by a separation process in a pilot plant consisting of screw pressing and centrifugation enhanced by flocculation in a pilot plant. The separation process produced a LF free of suspended solids (SS), whose supernatant chemical oxygen demand (COD) and COD due to volatile fatty acids (VFA) were 93.7% and 71.6% of the total COD (CODT) respectively, with an anaerobic biodegradability of 84%. The UASB reactor processing the LF exhibited a significantly higher volumetric methane production than conventional complete mixing reactors treating manure since UASB reactor required low hydraulic retention time (HRT) due to the characteristics of the LF. At HRTs between 1 and 2 days, COD removal efficiencies higher than 80% were achieved without VFA accumulation in the effluent. A minimum HRT of 0.65 day was performed with a corresponding organic loading rate (OLR) of 34.8 g [COD] l-1 d-1, reaching a volumetric methane production of 8.4 l [CH4] l-1 d-1.

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