Technoeconomic Analysis for Renewable Energy Development Using Anaerobic Digestion and Pyrolysis at a Water Resource Recovery Facility

Abstract

The objective of this research was to develop an efficient method to compare approaches to convert biosolids from water resource recovery facilities (WRRFs) to renewable energy. The emphasis was on collecting data to conduct a preliminary technoeconomic analyses to determine whether a site-specific strategy warranted further study. A case study using the Great Lakes Water Authority (GLWA) WRRF examined four general strategies: (1) anaerobic digestion of biosolids, (2) anaerobic codigestion of biosolids and processed food waste, (3) pyrolysis of dried biosolids, and (4) anaerobic digestion of biosolids followed by pyrolysis. Biogas assays were conducted to evaluate biogas production potential to select the best feedstocks. Assays were also conducted to examine pretreatment using thermal hydrolysis, sonication, and enzyme addition. None were found to be advantageous. Pilot-scale digesters were operated to test the reactor stability of a high volume of cosubstrate and obtain the design data needed for the technoeconomic analysis. Pyrolysis data were obtained using dried, pelletized GLWA WRRF biosolids, which currently processes approximately 50% of the biosolids. The optimal pyrolysis temperature was identified as 340°C by producing a differential thermogravimetry curve using a thermogravimetric analyzer. Elemental analyses were performed on each biochar sample to provide the data needed for energy modeling. Pyrolysis produced greater net energy, assuming the feedstock was already dry, but was less economical than anaerobic digestion due to its high annual operating expenses. If drying the biosolids was included in the energy analysis, then pyrolysis would be net energy negative for this case study. Integrated anaerobic digestion and pyrolysis achieved the highest energy efficiency at 69.7% but was less economically feasibility because of the highest annual operating cost. Anaerobic codigestion with the cosubstrate had a higher capital investment and operating cost than only digesting biosolids due to the larger digester. However, significantly more energy was produced, resulting in the lowest overall energy cost.