Plastic waste is an issue of global concern because of the environmental impact of its accumulation in waste management systems and ecosystems. Biodegradability was proposed as a solution to overcome this problem; however, most biodegradable plastics were designed to degrade under aerobic conditions, ideally fulfilled in a composting plant. These new plastics could arrive to anaerobic environments, purposely or frequently, because of their mismanagement at the end of their useful life. This review analyzes the behavior of biodegradable and conventional plastics under anaerobic conditions, specifically in anaerobic digestion systems and landfills. A review was performed in order to identify: (a) the environmental conditions found in anaerobic digestion processes and landfills, as well as the mechanisms for degradation in those environments; (b) the experimental methods used for the assessment of biodegradation in anaerobic conditions; and (c) the extent of the biodegradation process for different plastics. Results show a remarkable variability of the biodegradation rate depending on the type of plastic and experimental conditions, with clearly better performance in anaerobic digestion systems, where temperature, water content, and inoculum are strictly controlled. The majority of the studied plastics showed that thermophilic conditions increase degradation. It should not be assumed that plastics designed to be degraded aerobically will biodegrade under anaerobic conditions, and an exact match must be done between the specific plastics and the end of life options that they will face.
Anaerobic degradation (AD) of municipal solid waste (MSW) depends on the moisture and nutrient distribution within the bioreactor; these factors are affected by leachate recirculation. In this work, the effects of leachate recirculation of 15 to 120 by volume percentage (%V) on AD were studied. The ojectives of this study are as follows: a) to determine which recirculation rate provides the best conditions for accelerated anaerobic degradation (AD) and b) to determine the optimal range of recirculation rates for methane (CH 4 ) generation. In the first group of experiments, denoted as the exploratory range, laboratory-scale bioreactors (LSBs) were operated at leachate recirculation rates of 15, 30, 60 and 120% V. In a second group of experiments (denoted as narrowed range), a group of seven LSBs were operated at rates of 40, 60 and 80% V, and seven were employed as control, without recirculation. In this stage, LSBs were periodically dismantled to allow testing of the digested MSW. The AD rate was monitored for 201 days along with other variables, including the total volatile solids, holocellulose, lignin, organic carbon, total nitrogen and pH of the MSW matrix, the characteristics of the produced and recirculated leachates and CH 4 production rates. The results indicated that methane production during the methanogenic fermentation stage is directly correlated with the recycling rates. The 120%V recirculation rate was observed to cause washout in the waste matrix. The suggested range on the basis of CH 4 generation per liter of recirculated leachate was 30 to 40% V.
En México, 74% de los Residuos Sólidos Urbanos (RSU) generados se disponen en rellenos sanitarios (RESA), sin embargo, los reportes sobre las condiciones de operación de estos, así como de los tiraderos a cielo abierto (TCA) son escasos. El objetivo del estudio fue determinar la generación del metano como Gas de Efecto Invernadero en un RESA y un TCA. Para ello, se evaluaron las condiciones de operación, se caracterizaron los RSU confinados en temporadas de lluvias y estiaje y se determinó la generación de metano puntual y difusa. Con el fin de mejorar la operación del RESA bajo estudio, se detectaron ocho áreas de oportunidad y se documentaron los problemas ambientales del TCA. Si bien, el tipo de operación y la cantidad de materia biodegradable en los RSU (50.36% en RESA y 58.49% en TCA) influencian directamente la generación de metano por tonelada de RSU, en el TCA se incrementa en temporada de lluvias, mientras que en los pozos de venteo del RESA fue 15% más alto en temporada de estiaje, a causa de la recirculación de lixiviados durante la misma. Las emisiones superficiales de metano en el TCA fueron 97% más bajas que en el RESA, debido a su posible dispersión en los alrededores, lo cual es un riesgo potencial que requiere atención inmediata de las autoridades.
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