Análise econômica e energética do aproveitamento do lodo de esgoto sanitário considerando o uso nos sistemas de geração de energia

Abstract

Periodically, various types of waste are generated and disposed of in landfills. Among these, sewage sludge stands out, representing problems from both an environmental and financial standpoint for sanitation companies. In this context, the present study proposed an analysis of the economic and energy viability of utilizing sewage sludge, filling the gap in integrated studies between these analyses. Pellets were produced with different proportions of sludge and Pinus wood dust (0%, 25%, 50%, 75%, and 100% w/w) using a Rotex Master pellet mill (1,500 kg·h⁻¹). The samples were characterized in terms of moisture, ash content, volatile matter, fixed carbon, calorific value, elemental composition (TXRF), thermal stability (TGA), and pyrolysis gas emission (EGA-FTIR). The results showed moisture contents between 1.89 and 6.76%, according to NBR ISO 18134-2:2015. Ash content increased proportionally to sludge content (0.85% to 29.96%), while the higher heating value (HHV) ranged from 14.447 to 19.006 MJ·kg⁻¹. Thermogravimetric analyses showed greater thermal stability for pellets with a higher proportion of sludge, with mass losses of 39% to 75% between 25 and 600°C. Formulations with up to 50% sludge showed energy performance compatible with ISO 18125:2017 (≥16.56 MJ·kg⁻¹). Thermogravimetric and spectroscopic analyses showed stable thermal behavior, predominant release of CO₂, CH₄, and oxygenated compounds, and absence of toxic gases at critical concentrations. For the Biomass Quality Index (BQI) calculations, pellets with sewage sludge in their composition presented an index outside the standard limit due to the large amount of ash, zinc, and copper in their composition. For the economic feasibility analysis, all types of pellets produced from sewage sludge show favorable returns on investment and economic indicators when compared to those made from wood dust, as the cost decreases as a greater amount of sludge is included in their composition, the Net Present Value (NPV) also increases, and the payback period decreases, indicating an increasing viability of the investment. The Levelized Cost of Pellets (LCOP) per ton of pellet (R$/ton) ranged from R$ 614.44/ton for pellets with 25% sludge (15% discount rate) to a minimum of R$ 296.31/ton for pellets with 50% sludge (5% discount rate). Pellets using pure wood showed an LCOP between R$ 882.99 and R$ 888.51/ton. Thus, the incorporation of up to 50% sewage sludge in biomass pellets proved viable, preserving adequate energy properties and being economically feasible despite the excessive amount of ash and some metals, since the inclusion of sludge in the production process reduces the cost of raw materials and eliminates the cost of disposal in landfills.