One and two-stage digestion of solid organic waste

Digestion of municipal solid organic waste (MSW). particularly of kitchen r efuse with a total solid concentration of 7-8% VS (volatile solids) was inv estigated. Experiments at mesophilic (37 degrees C) and thermophilic (55 de grees C) conditions were carried out comparatively in continuously stirred digesters with 30 1 volume, Increasing the organic loading rate (OLR) by re ducing the hydraulic retention time (HRT). the maximum capacity of the biol ogical 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 mes ophilic and 80% at thermophilic conditions. At 55 degrees C, the yield Y-ga s, reached about 830 1 gas kg VS-1 whereas at mesophilic conditions only ab out 800 I gas kg VS-1 were generated. A simple mathematical model was set u p to describe digestion performance adequately designating the process cond itions 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 dig estion experiments the process conditions could be optimized. The main resu lt was that the digestion of MSW should be performed in a two stage plant w ith a concentration unit between the two stages, Under these optimized proc ess conditions a turnover of the organic mutter of 90% with low retention t ime could be realized. (C) 1998 Elsevier Science Ltd. All rights reserved.