Co-digestión de purines, residuos urbanos y de la industria del biodiésel

Resumen

Livestock wastes are the major source of organic wastes in the European Union. The increasing amount and concentration of these wastes, as a consequence of intensive farming, and the more strict environmental regulations poses restrictions to land application. It is necessary to develop new management practices to reduce the environmental impact. Among the different sectors, swine industry raises great concerns due to the potential of surface and groundwater contamination by nitrates and phosphates. In this same line, the waste management policy of the EU is focused in reducing, reuse and recycling, encouraging source separation of the organic fraction of municipal solid wastes (OFMSW), and prompting a significant reduction of the amount of organic material that ends in landfills. In either case, anaerobic digestion (AD) allows the treatment and valorisation of these wastes, obtaining a biofuel which is classified by the European Commission as one of the most sustainable fuels, along with a digested material that can be further composted to be used as biofertiliser. Nevertheless, AD can be problematic when treating some residues, due to their low biogas potential or to the presence of some compounds acting as inhibitor based on their concentration. Codigestion of wastes with complementary characteristics can solve these problems. This work assesses the co-digestion of swine manure (SM) and OFMSW using wastes obtained in the production of biodiesel to achieve a greater stability and methane production of the system. In addition, the use of by-products and wastes from biodiesel plants is a key factor in the application of the biorefinery concept, where several products are obtained from a raw material, as it occurs in a conventional petroleum refinery, maximising the use of biomass and increasing sustainability of biofuel production. Co-digestion tests were performed using two types of glycerines and evaluating different glycerine-SM proportions to determine their effect over biogas production and obtain the best glycerine-manure ratio. Posteriorly, the digestion process was carried out under semi-continuous conditions to evaluate the stability of the system. Tests showed an increase in biogas production as a consequence of the addition of glycerine, showing also differences in behaviour linked to glycerine types. XVIIResumen - Abstract Following a similar methodology, the co-digestion of OFMSW and glycerine was assessed, obtaining as a result a significant increase in biogas production as well as a stable operation during the period under study. Co-digestion of SM and waste oil sludge (WOS) was also evaluated. WOS used as co-substrate were obtained at the pre-treatment stage of the biodiesel production process. Results showed an increase in biogas production of 1.5 - 2 fold. Unlike glycerine tests, in this case a synergistic effect was observed. Due to the complex composition of this waste, problems related with the accumulation of long chain fatty acids (LCFA) were experienced when increasing organic loading. Stability tests performed over digested samples showed the need of a post-treatment stage if its use as organic amendment is intended. Finally, the theoretical assessment of the biogas potential of these wastes at a national level was performed, evaluating the potential applications of biogas as substitute of natural gas in transport and homes. In accordance with results obtained, codigestion of these wastes could significantly help to fulfill renewable energy targets in transport sector for 2020, with a reduction of 1 - 1.2 Mt CO2 eq.