Techno-economic analysis of wastewater sludge gasification: A decentralized urban perspective

The successful management of wastewater sludge for small-scale, urban wastewater treatment plants, (WWTPs), faces several financial and environmental challenges. Common management strategies stabilize sludge for land disposal by microbial processes or heat. Such approaches require large footprint processing facilities or high energy costs. A new approach considers converting sludge to fuel which can be used to produce electricity on-site. This work evaluated several thermochemical conversion (TCC) technologies from the perspective of small urban WWTPs. Among TCC technologies, air-blown gasification was found to be the most suitable approach. A gasification-based generating system was designed and simulated in ASPEN Plus® to determine net electrical and thermal outputs. A technical analysis determined that such a system can be built using currently available technologies. Air-blown gasification was found to convert sludge to electricity with an efficiency greater than 17%, about triple the efficiency of electricity generation using anaerobic digester gas. This level of electricity production can offset up to 1/3 of the electrical demands of a typical WWTP. Finally, an economic analysis concluded that a gasification-based power system can be economically feasible for WWTPs with raw sewage flows above 0.093 m3/s (2.1 million gallons per day), providing a profit of up to $3.5 million over an alternative, thermal drying and landfill disposal.