Batch dark fermentative hydrogen production from grass silage: The effect of inoculum, pH, temperature and VS ratio

Abstract

The potential for fermentative hydrogen (H 2) production from grass silage was evaluated in laboratory batch assays. First, two different inocula (from a dairy farm digester and digested sewage sludge) were studied with and without prior heat treatment and pH adjustment. Only the inoculum from the dairy farm digester produced H 2 from grass silage. Without heat treatment, methane (CH 4) was mainly produced, but heat treatment efficiently inhibited CH 4 production. pH adjustment to 6 further increased H 2 production. The effects of initial pH (4, 5 and 6), temperature (35, 55 and 70 ∘ C ) and the substrate to inoculum volatile solids (VS) ratio (henceforth VS ratio) (1:1; 1.5:1 and 2:1) on H 2 production from grass silage were evaluated with heat-treated dairy farm digester sludge as inoculum. Optimal pH was found to be between 5 and 6, while at pH 4 no H 2 was formed. The highest H 2 yield was achieved at 70 ∘ C . H 2 production also increased when the VS ratio was increased. However, the overall energy value of H 2 compared to that of CH 4 production remained low.