Energy sustainability of two parallel sewage sludge-to-energy pathways: Effect of sludge volatile solids content on net energy efficiency

Previously, we conducted a preliminary evaluation on two parallel sewage sludge-to-energy pathways from the perspective of energy conversion efficiency. One pathway combined anaerobic digestion with fast pyrolysis while its counterpart was simplified to only use the pyrolysis. In this study, their energetic performances were evaluated as a function of the volatile solids (VS) content and in terms of net energy efficiency. Both pathways, when used to convert sludge with higher VS content, can achieve higher net energy efficiency. The combined pathway could achieve higher net energy efficiency than the simplified pathway, but its relative advantage is not impressive when converting sludge with low VS content; for example, the difference in normalized net energy production between the two pathways was only 0.76 MJ/kg for sludge with 50% VS content.