Efecto del control de pH, temperatura y adición de nitrógeno sobre la digestión anaerobia de residuos hortícolas

Abstract

El objetivo principal del estudio fue aumentar la producción del biogás y el contenido de metano en un proceso de digestión anaerobia (DA) con residuos de la Central de Abastos de la Ciudad de México (CACM). Estos residuos contienen menos de 10% materia orgánica (MO) y pH cercano a 4, por lo tanto sus características ácidas hacen necesario el control del pH. La primera etapa comprendió la evaluación de la inoculación en condiciones naturales, dando como resultado que el sistema inoculado presentara mayor degradación de la MO (75%), pero el pH en ambos sistemas, se acidificó (3.4 y 3.5 respectivamente). Tomando como base los resultados de la primera etapa, se planteó la siguiente fase que correspondió a la adición de sales amortiguadoras con y sin inoculación, demostrando que el control de pH más la adición del inóculo favoreció el proceso ya que fue el sistema con mayor la producción de biogás 11.3 L (Litros) de biogás/ Kg (Kilogramo) Residuos(R) y donde se detectó metano (10%). La tercera etapa experimental correspondió a la evaluación de la temperatura en el intervalo mesofílico y ajuste de la relación carbono:nitrógeno (C:N), mostrando el mejor resultado de rendimiento a temperatura de 30ºC en el sistema inoculado con control de pH y nitrógeno, ya que se alcanzó un 0.42 m3 (metros cúbicos) de biogás/ Kg de MO. Posteriormente en la cuarta etapa experimental se llevó a cabo la sustitución de las sales de nitrógeno por los residuos cárnicos a 30ºC con y sin inóculo. Lo anterior aumentó notablemente el rendimiento obteniendo 1.42 m3 de biogás/ Kg de MO en el sistema inoculado. Finalmente se evaluó la co-digestión en el biorreactor anaerobio de 30 L de capacidad, donde con una relación (50:50) de residuos vegetales:cárnicos, se obtuvo 30% de metano en el biogás, equivalente a una productividad de 0.53 m3 de biogás/ Kg de MO por día.

In Mexico City’s 13,250 ton of waste are produced daily, 42% correspond to organic fraction. The organic fraction of the municipal solid waste (FMSW) may contain food, yard waste or paper in varying concentrations, sizes, and composition. Fruit and vegetable waste (FVW) are produced in large quantities (750 ton/day in the Central de Abastos) and constitute a source of nuisance in landfills because their high bio degradability. In the Central de Abastos these products are sold in different sections for this reason some of the organic waste products are usually source-separated, which constitute the ideal waste stream for an AD plant. These high volumes of organic waste are disposed in Bordo Poniente sanitary landfill. Due to the limited landfill capacity and the exhaustion of places to dispose the solid waste, the use and application of efficient technologies for the volume reduction are becoming increasingly necessary. The anaerobic treatment of organic waste can contribute to the reduction in the volume of refuse simultaneously providing regenerative energy in the form of biogas. The process of anaerobic digestion (AD) employs specialized bacteria to break down organic waste, converting it to a stable solid and biogas, a mixture of carbon dioxide and methane. Several factors affect the rate of digestion and biogas production; pH, temperature, C/N ratio, retention time, organic loading rate (OLR), bacterial competition, nutrient content, the presence of toxicants and solids content. The successful in the treatment of the FVW in an anaerobic process depend on various factors as; the control of its high acid conditions, and its poor nitrogen content. The vegetable waste contain high amounts of water and an acid pH, which represent no favorable conditions for the anaerobic conditions, for this reason the first experiment was conducted to evaluated the effect of control the pH to favored the anaerobic digestion process. The inoculum addition was also tested. When cow manure was added as inoculum to the FVW a 75% of the organic matter was consumed, while only a 26% of the initial OM was depleted when the vegetable residues mixture where digested under anaerobic conditions, during a same time period. Based in the preliminary results two experimental sets were simultaneously assessed, the inoculated system with pH control (by buffering salts addition) attained the maximum biogas production (11.8 L/KgTS) in approximately 15 days, while 82% of the initial OM was degraded, and the pH was maintained around 6.5, 10% of methane was detected. During the next set of experiments different conditions were evaluated in a mesophilic temperature range. The conditions which enhanced the biogas production were the control of the pH, the nitrogen addition and the inoculation of the FVW. The highest biogas yields (0.27 and 0.23 m3/kgOM) were reached combining the inoculation of the FVW, the nitrogen addition and the pH control, all these parameters allows the optimal conditions for the anaerobic process. The co-digestion of the FVW and meat residues allows obtaining a biogas yield of 1.42 m3/kgOM. An AD system of 30L of total volume was efficiently operated under the co-digestion condition, over 83 days.