Geothermal energy for wastewater and sludge treatment: An exergoeconomic analysis

The highest economic and environmental costs of wastewater treatment plants are related to waste disposal, which is mainly sludge disposal, and energy supply. Such challenges are even more critical in islands, where there are many environmental, landscape, and geographical constraints that make the use of conventional technologies difficult. For this reason, a geothermal energy system for wastewater and sludge treatment, and power production is proposed. Such a system is assessed through an exergoeconomic analysis, performed in Engineering Equation Solver environment, that allows determining the exergoeconomic costs of geo-fluid, electricity production, and sludge drying. The sensitivity analyses, carried out to assess the effect of several design parameters, have highlighted that the geothermal source temperature significantly affects the system operation and consequently the exergoeconomic costs, which range 77–95 c€/kWhex for sludge treatment and between 4.8 and 6.6c€/kWhex for electricity production. Finally, multivariate optimization has been carried out to find the conditions that minimize the exergoeconomic costs of the system. The total hourly exergoeconomic cost of the system products, namely sludge and electricity, is minimized when the geothermal source temperature is equal to 110 °C and the 58.91% of the desiccant flow is recycled. Overall, this study outlines that an exergy-based economic analysis is fundamental to assess the economic viability of innovative integrated solutions based on renewables.