Thermal pre-treatment of solid fraction from mechanically-separated raw and digested slurry to increase methane yield

Anaerobic digestion plants rely on large-capacity storage tanks to manage the agronomic utilisation of the digestate. As a consequence, many Italian AD. plants have introduced mechanical separation of the digested slurry to simplify process requirements. This study evaluated the possibility of reusing mechanically-separated solid fraction as a further biomass input anaerobic digestion plants. The effects of storage and thermal pre-treatment on digested solid fraction were assessed through biogas and methane yield measures, and then compared to the yields associated with undigested solid fraction of raw pig slurry. The specific CH4 yields of digested solid fractions ranged between 71.4 and 156.9 l(N)/kg VS, whereas the biogas yield from undigested solid fractions was 78.7 l(N)/kg VS. Solid fraction storage showed no significant effect on specific CH4 yields in any of the examined samples. However, in the case of the undigested solid fraction, thermal pre-treatment proved to be an effective method to increase CH4. (C) 2010 Elsevier Ltd. All rights reserved.