Effect of substrate-seed mixing and leachate recirculation on solid state digestion of biowaste.

Lab-scale experiments were performed and a mechanistic model was developed to simulate the solid state digestion of biowaste in a batch reactor. Both experiments and model showed that the substrate-seed mixing degree and leachate recirculation rate have a strong effect on the reactor performance. This is due to mass transport limitations of volatile fatty acids (VFA) within the biowaste-seed bed. In that case two regions are developed in the digester, so-called acidogenic and methanogenic pockets. Limitations in mass transport will prevent irreversible acidification during start-up of the reactor because whereas high VFA concentration is met in the fresh waste pockets, the VFA concentration in the methanogenic pockets will remain low. However, accumulation of VFA in the acidogenic pockets will reduce the hydrolysis rate of biowaste due to inhibition by VFA. Moreover, experiments and simulations showed that the reactor performance can be improved by varying the leachate recirculation rate or applying sequential batch operation.