Glycerol load was varied from 0% to 70% of the total OLR, but the
concentration was dependent upon the primary sewage sludge,
which showed inconsistency. This altered the actual amounts of
organics available for the digestion process. Fig. 4 shows the
increasing glycerol loading and general primary load reducing
trend. It is also evident in this figure that the increased gas production
rate is due to the increased glycerol loading. After loading at
10 g of glycerol (70% OLR), the reactor became unstable and did
not produce biogas due to propionic acid accumulation which
became inhibitory.