Results and discussion
3.1. Effect of VPW addition
Table 2 shows operational parameters for R1 and R2. Co-digestion
reactor (R2) achieved higher methane yields than those obtained
from R1 during the time of evaluation. Methane yield
values were improved 3 and 1.4 times when comparing values
obtained from R1 and R2 at HRTs of 25 and 15 d, respectively. Results
presented in this study are in good accordance with similar
researches regarding co-digestion of livestock residues with
VPW. In this context, Alvarez and Lidèn [12] obtained an improvement
in the methane yield in 1.5-folds when adding 47% VPW to
cattle and pig manure mixture. Callaghan et al. [13] achieved 1.6
times higher methane yields when adding 43% of VPW to cattle
slurry. VPW is composed by a high fraction of easily biodegradable