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Anaerobic digestion of animal by-products: assessment of process limitations and opportunities in real pig slaughterhouses

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Universitat de Barcelona. Facultat de Química

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[eng] Meat industry and particularly pig slaughterhouses produce huge amounts of animal by- products not intended for human consumption. This residual stream appears as a potential raw material for anaerobic digestion and biogas production. The integration of biogas plant in pig slaughterhouses facilities led to double advantage: in one hand presents a sustainable alternative for animal by-products management as well as generate energy that can partially cover the high energy consumption levels in the slaughterhouses.
Even though animal by-products have been reported as high biogas potential feedstocks, their rich protein and lipid composition are associated to operational problems in the anaerobic digestion process. Thus, the general objective of the Thesis is to define and develop strategies to ensure high biogas production yield and process stability in the anaerobic digestion of animal by-products from pig slaughterhouses, considering the specific requirements of the slaughterhouses that produces the animal by-products, as well as the characteristics of the generated animal by-products.
To address this general objective, first a detailed study of a commercial pig slaughterhouse was carried out, by identifying the process activities, input, and output streams, as well as thermal and energy consumption levels. Having identified the suitable animal by-products fraction for biogas its biogas potential was evaluated, after a sample campaign in different periods of the year to ensure results representativity. In a first phase of biogas potential study in batch mode, besides the study of the biogas potential of each individual animal by-products fraction, several mixtures were created considering the percentage in which each fraction was generated in the commercial pig slaughterhouse. At this stage, two strategies were studied to promote biogas yield and kinetics: lipid fractions saponification and the suppression of the nitrogen rich fractions in the animal by-products mixtures. Lipid saponification resulted in the improvement of the kinetics parameters whereas nitrogen reduction led to higher biogas yields.
In a second phase of the study of anaerobic digestion of animal by-products, two strategies were addressed and applied in the continuous operation at laboratory scale: digestate solid fraction recirculation and hydrophobic membranes for nitrogen capture. To assess the effect of both strategies not only biogas production and the typical control process parameters were monitored, but also microbial dynamics. As a result of the hydrophobic membrane coupling, the nitrogen content in the reactor was reduced and biogas yield
 
increased. At microbiological level, the integration of the hydrophobic membrane in the system led also to an increasement in the methanogenic species directed involved in biogas production. On the other hand, solid fraction recirculation was associated to process instability, probably due to the accumulation of non-biodegradable compounds in the system.
After identified lipid and proteins as animal by-products mixtures components that could further negatively affect anaerobic digestion process, and emerging technology in this field, near infrared spectroscopy (NIRS) was studied to estimate lipid and protein content of the animal by-products mixtures used as substrates in biogas production. Furthermore, as innovative additional element, in this case a handheld NIR, was used instead the conventional benchtop device. After a calibration procedure, the handheld NIR was able to estimate lipid, protein, and moisture content in the animal by-products mixtures studied.
Finally, besides the study of the above-mentioned strategies and technologies, with the data collected, an energy balance was carried out to estimate the percentage of energy that the biogas produced from the studied animal by-products could cover in slaughterhouses, both considering an individual facility and at national level considering the annual pig slaughter data. The results revealed that the biogas produced could cover a high percentage of the energy requirements in pig slaughterhouses. Thus, as a conclusion, biogas plant can be coupled to pig slaughterhouse facilities and as a management alternative for animal by-products and on the other hand, to produce part of the energy required in the facility, reducing fossil fuel dependence, associated cost an greenhouses emission.

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[spa] La industria cárnica y los mataderos porcinos en particular generan grandes cantidades de subproductos animales no destinados a consumo humano. Esta corriente residual posee un atractivo potencial como materia prima para la digestión anaerobia y la producción de biogás. Sin embargo, aunque los subproductos animales han demostrado tener un alto potencial de producción de biogás, su composición, rica en proteínas y grasas, se asocia a problemas operativos. En este sentido, el objetivo principal de la presente tesis es definir y desarrollar estrategias que permitan asegurar altos rendimientos y estabilidad operativa en la digestión anaerobia de subproductos animales de origen porcino.
Para abordar este objetivo principal, inicialmente se llevó a cabo un estudio pormenorizado del funcionamiento de un matadero porcino. Una vez identificadas las fracciones de subproductos adecuadas para la producción de biogás, se procedió al análisis del potencial de biogás de cada una de ellas y además se crearon mezclas de estas fracciones atendiendo a la proporción en la que se generaban en el matadero de estudio. Para maximizar los rendimientos y la cinética de producción de biogás de estas mezclas en la operación en discontinuo, se aplicaron dos estrategias: la saponificación de las fracciones grasas y la supresión de las fracciones con alto contenido en nitrógeno como la sangre. En la producción de biogás en continuo, se estudiaron otras dos estrategias: recirculación de la fracción sólida del digestato y aplicación de membranas hidrofóbicas para la captura de nitrógeno. Adicionalmente, se calibró un dispositivo de espectroscopía de infrarrojo cercano (NIRS) para la determinación del contenido en grasa y proteína de mezclas de subproductos animales estudiados.
Finalmente, con los datos recabados, se realizó un balance energético cuyos resultados revelaron que el biogás podría cubrir un alto porcentaje de la demanda energética de los mataderos porcinos. Por lo tanto, se concluye que los mataderos porcinos son instalaciones susceptibles de integrarse con la producción de biogás para gestionar las corrientes residuales generadas y por otro lado, generar energía que cubra parte de la demanda energética, reduciendo así la dependencia con los combustibles fósiles, el coste económico y las emisiones asociadas.

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Anaerobic digestion of animal by-products: assessment of process limitations and opportunities in real pig slaughterhouses