Strategies to improve anaerobic digestion of wastes with especial attention to lignocellulosic substrates

Abstract

The energy demand increase and the generation of wastes is being the major problem regarding the next generation sustainability. Both problems can be corrected through the implementation of anaerobic digestion, a waste treatment technology able to produce electricity, heat and a fertilizer. The anaerobic co-digestion between two wastes with complementary characteristics has been widely studied to improve the methane production in anaerobic digesters. However, to increase the methane production from lignocelulosics substrates is still one of the main challenges of anaerobic digestion. Lignocelulosic components are a tridimensional structure between lignin, hemicellulose and cellulose, which bonds are extremely difficult to degrade by conventional anaerobic bacteria. Besides, those components can be found in a wide range of substrates such as municipal solid wastes, agro-wastes and energy crops.

In the following thesis, the increase of the economic viability of anaerobic digestion plants treating lignocelulosic materials has been studied. Initially, the transitory state while the co-substrate was changed in the anaerobic co-digestion between sewage sludge and fruit waste was studied. The stability of the reactors was not drastically affected when the co-substrate was changed, but, the use of a co-substrate with a high concentration of fibers did not improve the methane production too much. Secondly, in order to consider the valorization of lignocellulosic components through the production of by-products, the effect of these components on the municipal solid wastes anaerobic digestion performance was evaluated. When the paper waste was removed, the biodegradability of the feedstock increased allowing the specific methane production to increase. Nevertheless, the digester was more fragile against instabilities and the digestate quality decreased if short retention times are applied. Next, low-temperature and ultrasounds pretreatments, strategies that have not been used too much for the degradation of lignocellulosic components, were studied to increase the methane production during the anaerobic co-digestion of barley waste and pig manure. Low-temperature and ultrasound pretreatment increased the methane production in a 27 and 12% respectively but only the first one had a positive energy balance. Finally, rumen, a waste from the slaughterhouse industry was used as inoculum and as co-substrate to bring hydrolytic bacteria able to improve the degradation of Napier grass. The results showed that, when rumen is used as inoculum it need to be mixed with an inoculum with high buffer capacity and a co-substrate with alkalinity need to be used to avoid long start-up periods. The methane production only increased at the beginning and in a long-term, the microbial community was governed by the substrate and not by the rumen. However, rumen did not increase the methane production when it was used as a co-substrate because the digester conditions were not optimal for the activity of hydrolytic bacteria. All the experiments were carried out in the laboratory and the conclusions are considered a progress for the energy production through the use of lignocellulosic substrates.

El incremento de la demanda energética y la consecuente generación de residuos ponen en peligro la sostenibilidad del futuro. Es por eso que la digestión anaeróbica resulta ser una solución factible para mitigar el problema ya que se puede generar electricidad, calor y fertilizante a partir de los residuos orgánicos. El incremento de la producción de metano se consiguió a partir de la co-digestión de residuos con características complementarias. Aun así, no siempre se consigue extraer todo el potencial metanogénico de los residuos, especialmente, en el caso de los sustratos ligno-celulósicos. Los compuestos ligno¬celulósicos son estructuras complejos entre la lignina, la hemicelulosa y la celulosa con enlaces resistentes a la degradación microbiana que se encuentran en los residuos agro-industriales, los residuos municipales y los cultivos energéticos.

En la tesis, se ha buscado aumentar la viabilidad económica de las plantas de digestión anaeróbica que tratan residuos con componentes ligno-celulósicos. Se usaron distintas nuevas estrategias para aumentar la degradabilidad de la materia ligno-celulósica como los pretratamientos térmicos de baja temperatura y de ultrasonidos y la integración del rumen, un residuo de la industria cárnica, para aportar bacterias hidrolíticas. Para valorar la opción de separar estos componentes para la formación de sub-productos, se estudió su efecto sobre el rendimiento del digestor en términos de estabilidad, producción de metano y calidad del digerido para así poder implementar la producción se sub-productos.

Los estudios realizados en esta tesis doctoral se llevaron a cabo a escala de laboratorio y las conclusiones han supuesto un avance para el aprovechamiento energético de los residuos ligno-celulósicos en el futuro.