Techno-Economic Assessment of the Redesign of a Wastewater Treatment Plant for the Coffee Industry: Implementation of Anaerobic Digestion

The coffee industry generates large volumes of wastewater with high organic loads, posing an environmental and economic challenge. Wastewater treatment plants do not eliminate the pollutant load, nor do they promote the revalorization of these residues. This manuscript evaluates the technical and economic feasibility of redesigning a treatment plant for a coffee production industry in Ecuador through the implementation of anaerobic digestion (AD). Tests were carried out in upflow anaerobic filter reactors under mesophilic and thermophilic conditions for 454 days. The results showed that the mesophilic regime achieved a methane yield of 200.5 NmL CH4 g-1COD and a COD removal of 64.1%, with greater stability than the thermophilic system. The data fit the modified Stover-Kincannon and Grau kinetic models (R2>95%). For the technological proposal, a hydraulic retention time of 7.3 days and an organic loading rate of 1.03 kg COD m-3d-1 were proposed. The economic evaluation showed an investment cost of USD 467,392.51 and annual operating costs exceeding revenues, requiring a minimum price of USD 171.11 per 60 kg bag to achieve a positive net present value of USD 212,469.35, an internal rate of return of 26.1%, and a payback period of 5.47 years. AD also reduces CO2 emissions by 17.9 tCO2 d-1, strengthening the environmental sustainability of the proposal. It is concluded that AD is a technology that can be technically, economically, and environmentally viable, improving the performance of the coffee industry and generating added value from highly polluting waste.