Valorization of waste from a vegetable processing plant by anaerobic digestion: scale effect

This work is aimed to assess the methane potential production of a mixture of three waste that come from a food industry plant with a capacity of treating vegetables of about 12,000 t/y and an annual waste production of 400 t. For the above-mentioned purpose, tests in semi-pilot (batch, 6L-reactors) and pilot (semi-continuous, 300L-reactor, hydraulic retention time, HRT = 30 d) scale in mesophilic conditions were carried out. The mixture of organic waste subjected to tests had a total solid (TS) content of about 10% b.w. (by weight), a VS/TS ratio of 0.9, a C/N ratio of 15 and a disintegration rate constant of 0.382 d-1. Semi-pilot scale tests returned a specific biogas production of 0.55 Nm3/kg VS (VS, volatile solid), with an average methane content of 53% b.v. (by volume). On the other hand, the specific biogas production from the pilot scale test set up at 0.503 Nm3/kg VS with a methane content of only 44% b.v. The high residual VS content detected in the digestate coming from the test in semi-continuous mode (about 35%) leads to assume that better results could have been obtained using longer HRTs or a more efficient mixing system.