Use of modeling to aid design of a two-phase grass digestion system

A sequentially fed leach bed system coupled with a leachate holding tank and an Upflow Anaerobic Sludge Blanket (UASB) was modeled based on 310 d of grass silage digestion with the goal of generating specific design instruction. The model suggests the hydrolysis rate is proportional to the sprinkling rate and retention time. It suggests that raising the sprinkling rate by a third (from 600 L/d to 800 L/d) increases the volatile solids destruction from 70% to 80% for a retention time of 30 d yielding 370 L CH4/kg VS. The volume of the leachate holding tank has a minimal influence on methane production (reducing its volume by a factor of 2 reduces methane yield by 1%). The model suggests that for a constant sprinkling rate, shorter retention time increases daily methane production, but lowers specific methane yield (L CH4/kg VS). Longer retention time increases methane content in the biogas.