Role of Leaching and Hydrolysis in a Two-Phase Grass Digestion System

Grassland is ubiquitous in Ireland, covering over 91 A) of agricultural land. Grass biomethane has shown to be a sustainable biofuel with a very strong energy balance. Anaerobic digestion is a mature technology, particularly wet continuous digestion. However, the retention periods for grass digestion are relatively long, typically over 60 days. Recently, dry batch digestion has become quiet prevalent; retention times are lower, at about 30 days, but because half of the feedstock is left in the digester for a second cycle as an innoculum, the actual retention time is of the order of 45 days. A methodology that is at the development stage is a two-stage system. The first stage is a dry batch leaching stage (hydrolysis and acidogenesis). The leachate produced is treated in an upflow anaerobic sludge blanket (UASB), where methanogenisis occurs. This should allow for the shorter retention times of the dry batch process because there is no need to leave half of the feedstock in the digester as an innoculum for a second cycle. This paper concerns itself with the leaching process. Flow should it be carried out? What recirculation rate should be used? Should the grass silage be from a pit (ca. 20% dry solids) or from a bale (ca. 30% dry solids)? Should the grass silage be flooded or sprinkled? An experimental process was set up that allowed for four scenarios. These scenarios included sprinkling and flooding of pit silage and bale silage. The results of the analysis were used to generate a model that predicted the application of the leach beds with a UASB. The results suggested that sprinkling of bale silage was the preferable option. It suggested that, with a 40 day retention time, gas production of 0.4 m(3) of CH4/kg of volatile solids added could be achieved. This would be a similar value to a wet continuous system operating at a 60 day retention time and more efficient than a one-stage dry batch process.