Propuesta para el diseño de un biodigestor anaeróbico empleando llantas usadas para mejorar el tratamiento de las excretas bovinas

Abstract

Anaerobic biodigestion is a process used to break down organic solid waste. The Taiwan biodigester is one of the most used technologies in rural areas, due to its low installation cost, however, it presents efficiency problems, so, to increase it, the use of alternative materials that abound in mass (used car tires), has been raised for its physical – chemical properties, this to stabilize the decomposition process of bovine excreta. Given the above, in the present project, the design of a biodigester was proposed as a system for the use of excreta from the livestock industry, using the tires used to improve the efficiency of the anaerobic biodigestion process. For this, 4 phases were established; (1) compilation and evaluation of the feasibility of four biodigestion alternatives, (2) characterization of the tires used in Colombia, (3) sizing of the biodigester and the use of tires in the biodigestion process, and (4) the formulation and financial evaluation of the project. Using the multiple – criteria technique TOPSIS, the Taiwan – type biodigester was chosen as the alternative with the greatest viability, to be used in the livestock industry and rural areas. In order to improve the biodigestion process, it was decided to use used tires, due to the good physical – chemical properties, such as the ease of growth of different microorganisms on the surface and transfer of heat energy to the biodigestion medium (3.61 Wh per tire). Additionally, by using the tires after completing their useful life cycle, an alternative for the use of this material is provided, reducing the problems of abandonment and stacking. According to the areas, with greater generation of bovine manure, whose temperatures on average favor the biodigestion process, a dimensioning was reached, based on variables such as manure, volume, HRT, size of the bag and trench; obtaining a biodigester with a volume of 8 m3 , with the capacity to introduce 9 car tires and transfer 32.51 Wh of heat energy to the environment. Once the biodigester was dimensioned, an investment of $ 2’288,606 pesos was calculated, related to materials, equipment, machinery and supplies; obtaining five – year profits (NPV) of $ 4’914,729, $ 4’047,055 and $ 3’335,662 pesos, with a discount rate of 10%, 15% and 20%, respectively, in addition to an internal rate of return (IRR) of 74.5% of the initial investment. With the fulfillment of the different phases of the project, the Taiwan biodigester was proposed as the best alternative for the treatment of bovine excreta, mainly because of its efficiency and cost – benefit ratio. It was found that, using the rubber of car tires, the biodigestion process can be improved, thanks to the properties of the material such as heat transfer, which keeps the internal temperature stable, and other properties (surface, porosity, water adsorption, neutral pH and high carbon content), favoring the growth of different microorganisms present in anaerobic biodigestion.