Geothermal energy for drying of wastewater sludge and electricity production

Waste treatment and disposal and electric energy production are crucial challenges in geographically disadvantaged areas, such as small islands, due to limited connection with mainland. Scarce land availability, environmental restrictions and tourism activity, that often characterize small islands, make difficult to adopt ordinary technical solutions, increasing these issues. Then, the most common strategy for waste disposal is shipping to the mainland, whereas electricity generation is based on the importation of fossil fuels for local production. Both these options make small islands strongly dependent on the mainland, and cause significant energetic, environmental and economic costs. For these reasons, the use of renewable energy sources for waste treatment and energy production is particularly attracting in small islands. In this work, geothermal energy at medium enthalpy is considered to produce heat for thermal drying of wastewater sludge and to power an Organic Rankine Cycle system for electric energy production. The analysis is carried out for the case study of a small Italian island. The geo-fluid, through an air-water heat exchanger, heats fresh air to produce the desiccant current for sludge drying, which is carried out by using a belt convective dryer, operating in the range of 90.0-180°C. The dryer is designed to achieve a final solids content of dry sludge higher than 90.0%. A fraction of the desiccant current exiting the dryer is recirculated in order to reduce thermal energy demand of the dryer and, at the same time, the flow rate of exhausts to be treated. Before reinjection, the geo-fluid powers a small-scale ORC system, designed to self-supply the proposed layout, providing electricity for the dryer and the geo-fluid pumps, and to produce electricity for the wastewater treatment facilities. An energy analysis of the proposed system is carried out through the software Aspen PLUS, and an economic and environmental model is developed to assess its profitability. This model estimates the economic and environmental benefits coming from sludge drying, which significantly decreases the amount of sludge to be transported and disposed, and from the use of a renewable energy source with respect to conventional fossil fuels, for sludge treatment and electric energy production.