Development of a methanogenic process to degrade exhaustively the organic fraction of municipal “grey waste” under thermophilic and hyperthermophilic conditions

Different laboratory-scale, continuously driven reactor concepts (up to 3 reactors in series, max. 70°C) for anaerobic digestion of the organic fraction of municipal grey waste were investigated. Over a period of 2½ 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 1st 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%. Removal efficiencies determined indirectly by the combined biogas yield of the 1st and 2nd reactor stage revealed even up to 91.5% of the theoretical possible yield of 807 l/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 conditions (60–70°C) or by a HRT of 1.25d. It also demonstrated that a stable methanogenesis was not possible at temperatures of 60–70°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/l/d. In contrast to this the VS-removal of the first hydrolysis reactor 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 cellulose degradation with conversion rates of 0.1–3x109 g cellulose/single bacterium (10^–12 g)/d. This was an indication for the cellulose degradation as a rate limiting step. Both reactor stages combined allowed an optimal VS removal efficiency at OLR of 10 g VS/l/d.

Analysis of bacterial populations of 28 reactors were referred either to eubacteria utilizing different sugars or cellulose or acetate or H₂–CO₂ or archaea (plus antibiotics) with acetate or H₂–CO₂ as substrate. H₂–CO₂ utilizers with numbers of 10⁸–10¹⁰/g TS dominated obviously the acetotrophic methanogens by the factor 10–10,000. This explained the observed short HRTs being possible.