Multi-criteria evaluation of energy recovery from urban wastewater sludges by anaerobic digestion and hydrothermal liquefaction

This study presents a multi-criteria evaluation of energy recovery from wastewater sludges by the traditional anaerobic digestion pathway versus the emergent hydrothermal liquefaction (HTL) pathway. The two pathways were assessed considering the following 12 criteria: process stability, volatile solids reduction, process time, energy recovery, energy yield, net energy yield, energy return on investment, net power yield, energy density of the resulting biofuel, potential for methane emissions, effluent quality, and technology readiness. Experimental results from HTL of primary and secondary sludges and literature data on mesophilic and thermophilic anaerobic digestion are used here to quantify the above criteria for assessment. Although the two anaerobic pathways ranked better than the HTL pathway on the basis of energy return on investment (5.5–19.8 vs. 1.5), when all the 12 criteria were taken into consideration by the multi-criteria decision making approach, the HTL pathway outranked both the anaerobic pathways with a preference ranking of + 0.44 compared to − 0.28 in anaerobic digestion. It is recommended that developing technologies for recovering energy from the solid residue of HTL, the biochar, the energy return on investment by the HTL pathway could be improved from 1.5 to 2.42.