Generally, co-digestion scenarios with high proportion of energy crops (Base SS-1 and Base LS-1) had relatively higher
environmental impacts as would be expected (Table 2). For example, the data for small-scale biogas plants showed that POF impact value was nearly twice as high for Base SS-1 (0.63 kg NMVOC), compared to base case Base SS (0.34 kg NMVOC). Analysis of single feedstock scenarios already showed (Table 1), that POF impact potential was 60% and 125% higher for grass silage and whole wheat plant silage, respectively, compared to corn silage (0.67 kg NMVOC); hence, corn silage feedstock was more environmentally compatible. Co-digesting cattle manure with 35% corn silage, 18% grass silage and 22% whole wheat plant silage (Base LS-1) incurred 7 times higher POF impacts compared to digesting mainlyMSWand food industry residues (Base LS). Results in Table 1
already showed that residues from food industry incurred lower
POF impacts of between 0.15 (grease separator sludge) and 0.30 kg
NMVOC (pomace), in contrast to energy crops with 0.67 (corn
silage) up to 1.51 kg NMVOC (whole wheat plant silage).