Comparison of two‐stage thermophilic (68°C/55°C) anaerobic digestion with one‐stage thermophilic (55°C) digestion of cattle manure

A two‐stage 68°C/55°C anaerobic degradation process for treatment of cattle manure was studied. In batch experiments, an increase of the specific methane yield, ranging from 24% to 56%, was obtained when cattle manure and its fractions (fibers and liquid) were pretreated at 68°C for periods of 36, 108, and 168 h, and subsequently digested at 55°C. In a lab‐scale experiment, the performance of a two‐stage reactor system, consisting of a digester operating at 68°C with a hydraulic retention time (HRT) of 3 days, connected to a 55°C reactor with 12‐day HRT, was compared with a conventional single‐stage reactor running at 55°C with 15‐days HRT. When an organic loading of 3 g volatile solids (VS) per liter per day was applied, the two‐stage setup had a 6% to 8% higher specific methane yield and a 9% more effective VS‐removal than the conventional single‐stage reactor. The 68°C reactor generated 7% to 9% of the total amount of methane of the two‐stage system and maintained a volatile fatty acids (VFA) concentration of 4.0 to 4.4 g acetate per liter. Population size and activity of aceticlastic methanogens, syntrophic bacteria, and hydrolytic/fermentative bacteria were significantly lower in the 68°C reactor than in the 55°C reactors. The density levels of methanogens utilizing H2/CO2 or formate were, however, in the same range for all reactors, although the degradation of these substrates was significantly lower in the 68°C reactor than in the 55°C reactors. Temporal temperature gradient electrophoresis profiles (TTGE) of the 68°C reactor demonstrated a stable bacterial community along with a less divergent community of archaeal species. © 2004 Wiley Periodicals, Inc.