Two-Stage vs Single-Stage Thermophilic Anaerobic Digestion : Comparison of Energy Production and Biodegradation Efficiencies

Two-stage anaerobic digestion (AD) for integrated bio-hydrogen and bio-methane production from organic materials has been reported to promise higher process efficiency and energy recoveries, as compared to traditional one-stage AD. This work presents a comparison between two-stage (reactors R1 and R2) and one-stage (reactor R3) AD systems, fed with identical organic substrates and loading rates, focusing the attention on chemical and microbiological aspects. Contrary to previous experiences, no significant differences in overall energy recovery were found for the two-stage and one-stage AD systems. However, an accumulation in R2 of un-degraded intermediate metabolites (volatile fatty acids, ketones, amines, amino-acids and phenols) was observed by GC-MS. These compounds were thought to be both cause and effect of this partial inefficiency of the two-stage system, as confirmed also by the less diverse, and thereby less efficient, population of fermentative bacteria observed (by PCR-DGGE) in R2. The extreme environment of R1 (low pH and high metabolites concentrations) probably acted as selector of metabolic pathways, favoring H2-producing bacteria, while limiting other strains, able degrade such a wide variability of intermediate metabolites. Therefore, if two-stage AD may potentially lead to higher energy recoveries, further efforts should be directed to ensure process efficiency and stability.