Anaerobic digestion of agricultural wastesstudies of temperature, co-digestion and potential inhibitors

Resumen

The high global generation of residue that derives from the activity of the agricultural sector makes of its efficient treatment an issue of capital importance. The treatment of wastes through biological pathways, such as anaerobic digestion (AD), is increasingly being encouraged. However, a successful performance of AD is yet to be fully achieved. AD is a microbiological process that takes place in the absence of oxygen leading to organic substrate consumption and biogas production. The waste valorization is thus twofold: energetic, as biogas combustion can be used for electricity or heat production; and agricultural, due to the feasible production of compost from the waste after AD. The agricultural wastes herein employed were chosen attending to their worldwide relevance and their role in the economic activity of Navarre (Spain). These are: artichoke, asparagus, carrot, bean, pea, green bean, cabbage, sloe and malt. Additionally, manure was assessed as result of the international stay in the Technical University of Denmark. As temperature has shown to be a determinant condition in the process, this project has covered different operation temperatures, namely mesophilic (35 °C), intermediate (42 °C) and thermophilic (55 °C) ranges. Also, anaerobic co-digestion (AcoD) of several types of agricultural wastes has been studied throughout the whole study. Firstly, as preliminary assessments, the use of a nutrient solution in AcoD was evaluated. Results clearly showed that the use of it was beneficial for the performance of the reactor and, therefore, was implemented in the following studies. The next step was the AD of lignocellulosic substrates, artichoke and asparagus, whose composition makes of them little accessible for the microbial community. Single-stage (at the three aforementioned temperatures) and two-phased anaerobic digestion (TPAD; thermophilic and mesophilic phases) were compared. The following stage of the project has been to approach the microbiology underlying the anaerobic process. To this aim, bacterial identification of AD reactors treating thermally pretreated and non-pretreated substrates (sloe and malt) was performed. Further microbiological studies based on 16S rRNA were carried out on continuous anaerobic reactors treating manure and subjected to biogas upgrading through H2 addition. These allowed confirming that biodiversity within reactors is enriched as the process progresses. Finally, this dissertation has also addressed potential inhibitors arising from the substrate. Two commonly used pesticides were added in anaerobic reactors. Scale-up has also been carried out with pesticides and further studies are recommended to be pursued. Overall results showed that mesophilic range provides the most stable operation but thermophilic range yields more efficacious removals and shorter processes. Also, AcoD leads to higher improvements than TPAD on specific substrates. Lastly, the presence of pesticides has positive effects on general performance.