Increased loading rates and specific methane yields facilitated by digesting grass silage at thermophilic rather than mesophilic temperatures

This study was conducted to advance the understanding of thermophilic grass digestion. Late harvested grass silage was fermented at thermophilic conditions at increasing organic loading rates (OLR). Stable digestion took place at an OLR between 3 and 4 g VS L^-1 d^-1. This enabled specific methane yields (SMY) as high as 405 L CH₄ kg VS^-1. An accumulation of volatile fatty acids (VFA), accompanied by a gradual deterioration of pH, FOS/TAC (ratio of VFA to alkalinity) arose at an OLR between 5 and 7 g VS L^-1 d^-1, yet inhibition did not occur. SMY decreased with reduced retention time ranging between 336 and 358 L CH₄ kg VS^-1 at OLR 7 and 5 g VS L^-1 d^-1 respectively. The biomethane efficiencies remained high (92-103%) at corresponding retention times. Comparative results indicated a superior performance with respect to higher loading and SMY as compared with mesophilic conditions.