One and two-stage digestion of solid organic waste

Digestion of municipal solid organic waste (MSW), particularly of kitchen refuse with a total solid concentration of 7–8% VS (volatile solids) was investigated. Experiments at mesophilic (37°C) and thermophilic (55°C) conditions were carried out comparatively in continuously stirred digesters with 30 l volume. Increasing the organic loading rate (OLR) by reducing the hydraulic retention time (HRT), the maximum capacity of the biological process was assessed. With OLR=6.0 g VS l−1 d−1 and HRT=11 d, the maximum VS reduction under steady state conditions reached 72% at mesophilic and 80% at thermophilic conditions. At 55°C, the yield of gas, Ygas, reached about 830 l gas kg VS−1, whereas at mesophilic conditions only about 800 l gas kg VS−1 were generated. A simple mathematical model was set up to describe digestion performance adequately designating the process conditions for most effective turnover. This model was based on kinetic data of a one stage digester. With the assistance of the model and the data of digestion experiments the process conditions could be optimized. The main result was that the digestion of MSW should be performed in a two stage plant with a concentration unit between the two stages. Under these optimized process conditions a turnover of the organic matter of 90% with low retention time could be realized.