Comparison of Thermophilic and Mesophilic One-Stage, Batch, High-Solids Anaerobic Digestion

Abstract

The concept of starting up a batch, high-solids anaerobic digester by simply flooding the bed with a pH-buffer solution was tested using a mixture of vegetable waste and wood chips as feedstock at mesophilic (38° C) and thermophilic (55° C) conditions. At both temperatures stable and balanced methanogenesis was rapidly established within four days and was sustained until substrate was exhausted. Methanogenesis was more rapidly initiated in the thermophilic digester than in the mesophilic digester. Acetic, propionic and butyric acids accumulated in the leachate of both digesters during the start-up of digestion of uninoculated batch of waste. Thereafter all acids were degraded; which was quicker in the thermophilic digester. The accumulation and degradation of these acids was slower in the mesophilic digester. These studies showed that inoculum for carrying out thermophilic and mesophilic anaerobic digestion is readily available within the waste and its activity for complete mineralization of organic matter can be enhanced and sustained by providing adequate alkalinity. By employing a process in which anaerobic digestion of subsequent batches of waste was carried out by flooding with leachate drained from the digestion of a previous batch of waste, the volatile organic acid accumulation was maintained low and 95% of the methane yield potential of the waste was produced in 11 days under thermophilic conditions as opposed to 27 days under mesophilic conditions.