Development of a methanogenic process to degrade exhaustively the organic fraction of municipal "grey waste" under thermophilic and hyperthermophilicconditions

Different laboratory-scale, continuously driven reactor concepts (up to 3 r eactors in series, max. 70 degrees C) for anaerobic digestion of the organi c fraction of municipal grey waste were investigated. Over a period of 2 1/ 2 years several setups of reactors being daily fed and held in steady state balance were investigated. The preferred variant was a 2-stage setup with a HRT of 4.3d for the ist and 14.2d for the 2nd reactor. Removal efficiencies of VS obtained by comparing the organic loading rate ( OLR, g VS/l/d) of the effluent with the OLR of the feed could reach 80%. Re moval efficiencies determined indirectly by the combined biogas yield of th e 1st and 2nd reactor stage revealed even up to 91.5% of the theoretical po ssible yield of 807 I/kgVS. The produced gas had a methane content of 60-65 %. A completely distinct hydrolysis stage with a gas production of only 1. 6-5.5% of the theoretical yield could be reached by hyperthermophilic condi tions (60-70 degrees C) or by a HRT of 1.25d. It also demonstrated that a s table methanogenesis,was not possible at temperatures of 60-70 degrees C. Kinetic analyses of the 2nd reactor stage revealed that the degradation of VS fell from 80 to 40% with raising organic loading rate (OLR) from 3 to 11 g VS/I/d. In contrast to this the VS-removal of the first hydrolysis react or stage increased linearily from 5 to 20% at raising OLR's from 12 to 26 g VS/l/d. The same kinetics with linear increase exhibited the specific cellu lose degradation with conversion rates of 0.1-3x109 g cellulose/single bact erium (10(-12) g)/d. This was an indication for the cellulose degradation a s a rate limiting step. Both reactor stages combined allowed an optimal VS removal efficiency at OLR of 10 g VS/I/d. Analysis of bacterial populations of 28 reactors were referred either to eu bacteria utilizing different sugars or cellulose or acetate or H-2-CO2 or a rchaea (plus antibiotics) with acetate or H-2-CO2 as substrate. H-2-CO2 uti lizers with numbers of 10(8)-10(10)/g TS dominated obviously the acetotroph ic methanogens by the factor 10-10,000. This explained the observed short H RTs being possible.