Two-stage anaerobic digestion of sugar beet silage: The effect of the pH-value on process parameters and process efficiency

•Effect of acidification pH on two-stage AD of sugar beet silage was investigated.•Instead of a buffer, effluent refeeding was used for pH control.•The highest specific methane yield was achieved at pH 6.

The study investigated the influence of the target pH-values 4.5, 5, 5.5 and 6 in the acidification reactor on process parameters, such as substrate-specific methane yield and the intermediates, in the two-stage anaerobic digestion of sugar beet silage. The total specific methane yield (Nl kg−1 COD d−1) increased with an increase in the pH (pH 4.5: 140.58 ± 70.08, pH 5: 181.21 ± 55.71, pH 5.5: 218.32 ± 51.01, pH 6: 256.47 ± 28.78). The pH-value also had an effect on the dominant intermediate in hydrolysate. At the pH-value of 4.5, almost no acidification and microbial activity was observed. At pH 5 and 5.5, butyric acid production dominated, guided by H2 production. At pH 6 acetic acid was the main product. The absence of H2 and the highest SMY makes it favorable under practical aspects.