The maximum gain in gas production, the difference between
the primary and the co-digested glycerol and primary, was
observed in the 4–9 g of glycerol loading range representing 45–
69% of OLR. The overall maximum benefit of glycerol addition
was observed at 7 g of glycerol loading (64% glycerol in OLR as
shown in Table 2) due to enhanced methane content as compared
to primary alone. This maximum benefit is found by considering
the balance between the amounts of gas produced (Fig. 2), the
methane content of the produced gas (Fig. 3) and the amount of
glycerol required to obtain this enrichment. The methane content
was variable, but after the startup instabilities, there was an
upward trend in biogas content from 5 g of glycerol loading to
10 g as shown in Fig. 4. It is important to note that variations in
primary sludge, a product of weather conditions, TS/VS and other
WWTP issues were significant and showed an effect on the gas production
throughout the experiment. The maximum benefit range
was determined to be between 25% glycerol OLR to 60%.