This work investigates the start-up phase of anaerobic digestion in wet, semi-dry and dry conditions of rice straw analysing the role of volatile fatty acid (VFA) production on process kinetics. Methane production yields and biodegradation kinetics in reactors operated under wet semi-dry and dry conditions were investigated. The experimental results showed a reduction in the specific final methane production yield of 57% and 63% in, respectively, semi-dry (TS = 14.8%) and dry (TS = 23.4%) conditions compared to wet (TS = 4.8%) conditions. The total VFA concentration and speciation are proposed as indicators of process development at different total solids content. High VFA concentrations were found in dry conditions, with a maximum total VFA concentration of 2110 mg/kg in dry conditions, 930 mg/kg in semi-dry conditions and 180 mg/kg in wet conditions.
Keywords: Total solid; dry anaerobic digestion; rice straw; volatile fatty acids.