One-step and two-step anaerobic-digestion of solid agroindustrial residues

Large quantities of solid organic residues are generated during the processing of vegetable matters in agro-industry. In general, these residues are structureless and contain more than 80 % water, so that composting is hindered. A possible alternative for the stabilization of these residues may be the anaerobic methanogenic degradation, which has the additional advantage of biogas formation. For the development of an efficient and reliable anaerobic treatment process, two novel digestion systems with one- and two-step operation have been built in pilot-scale and tested with different agro-industrial residues. The results have shown that for residues with a C:N-ratio above 15 the one-step process should be used preferably, whereas protein-rich residues with a C:N-ratio below 10 can be treated only in the two-step process. For the different agro-industrial residues it was found that about 50-70 % of the organic matter can be degraded within retention times of 10-20 days. The biogas production was typically 300-500 m super(3) per ton of dry organic matter.