Anaerobic digestion of livestock wastes. Vegetable residues as co-substrate and digestate post-treatment

Resumen

The main reason why this study was carried out was to give support to livestock sector with regard to wastes treatment. Among the diverse treatment systems, anaerobic digestion was chosen as a proper biological treatment since besides organic material reduction it provides methane, which can be transformed into renewable energy. Vegetable waste addition as co-substrate in the anaerobic digestion of two livestock wastes (swine manure (SM) and poultry litter (PL)) was analysed. Those processes were studied in terms of organic matter reduction and methane production following a central composite design and response surface methodology. It was concluded that in the case of SM co-digestion the vegetable addition resulted in an increase in organic matter reduction while in the case of PL co-digestion substrate concentration determined organic matter reduction registering ammonia-mediated inhibitions with volatile solids concentration above 80 g VS L-1. Organic matter degradation and more specifically lignocellulosic complex degradation during anaerobic digestion were investigated using SEM techniques and thermal analyses. Complete depletion and 50% reduction were obtained in the case of hemicelluloses and cellulose, respectively, while lignin was not degraded under anaerobic conditions. In the case of SM co-digestion, semi-continuous conditions were also investigated demonstrating the positive effect of vegetable addition on methane production. Changes in microorganisms population were studied with scanning electronic microscopy (SEM) finding that initial long rod-shaped bacteria changed to cocci and bacilli morphotypes. On the other hand, two different systems for treating nutrients in the anaerobically degraded SM were studied. First, anammox process was examined treating two different wastes, namely anaerobically degraded SM and partially oxidised anaerobically degraded SM, obtaining ammonium removal rates around 95%. It was found that organic matter concentration determined anammox process eficiencie. The other studied process was a microalgae-bacteria consortium system. Nutrients removal and nutrients biomass uptake as well as two different photobioreactor configurations were investigated. Ammonium, phosphorus and organic matter were removed up to 100, 80 and 60%, respectively. Nitrogen, phosphorus and carbon content accounted for 10, 2 and 48% of the dried biomass, respectively. Biofilm reactor was more effective in terms of biomass harvesting since 96% of the total biomass produced was retained.