Optimization of biogas productivity in lab-scale by response surface methodology

Optimization of canola residues in co-digestion with cattle manure for enhanced biogas yield was carried out using response surface methodology (RSM) and Box - Behnken design of the experiment. The effects of inoculum, total solids (TS), temperature (psychrophilic, mesophilic and thermophilic conditions) and stirring time (duration) in three levels were investigated. After applying of thermo mechanical pretreatment, 29 tests were performed. The experimental results showed that the temperature variable had the greatest effect in the model. The optimum degree of variables achieved in anaerobic digestion was 52.49 °C of temperature, 3.12 min day−1 of stirring time, 7.02% TS of substrate working volume and 22.17% inoculum (22.17% TS) with a methane yield of 403.63 L kg−1 in thermophilic conditions. The optimum degree of variables was: stirring time 3.57 min day−1, 7.41% TS of substrate working volume, 26.26% inoculum (26.26% TS) and temperature at 40.36 °C with a methane yield of 376.76 L kg−1VS in mesophilic conditions. The high value of R2 (0.9983) showed that the model can efficiently be applied to prediction of methane production from the co-digestion of cattle manure and canola residues. Both mesophilic and thermophilic conditions were suitable in terms of energy production, however, the mesophilic conditions showed more positive energy balance and higher Net Energy Gain (NEG) than the thermophilic conditions.