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

Abstract

Two-stage anaerobic digestion (AD) for integrated biohydrogen and biomethane 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 undegraded 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 able to degrade such a wide variability of intermediate metabolites while limiting other strains. Therefore, if two-stage AD may potentially lead to higher energy recoveries, further efforts should be directed to ensure process efficiency and stability.

Importance of storage time in mesophilic anaerobic digestion of food waste

“…In the literature, two-stage digestion is usually reported to have a methanogenesis efficiency of 7%-23% (Shen et al, 2013;Liu et al, 2006;Lim et al, 2013) higher than one-stage digestion, while some studies found that the methane production in two-stage systems might be comparable or even inferior to single stage systems due to lower bacteria diversity and negative effects on the syntrophic association between acidogenens/acetogens and methanogens (Schievano et al, 2012;Merlino et al, 2013). Compared to two-stage digestion, storage treatment together with methane fermentation can already increase methane production by 40% in 21 and by 100% in 60 days as demonstrated in this study, and has great advantages in terms of economy and manipulation.…”

Effect of different pH-values on process parameters in two-phase anaerobic digestion of high-solid substrates

“…[15] The specific methane yield in two-phase systems during the fermentation of glucose solutions, organic waste and household waste was up to 24% higher than in single-phase systems. [16][17][18][19] A further experiment of Lehtomäki and Björnsson [20] showed a 5% higher methane yield in a two-stage pilot-scale reactor compared with a laboratory batch system digesting grass. Another major advantage of the two-phase process is the fractionation of the gases during anaerobic digestion.…”

Process enhancement of hydrogen and methane production from palm oil mill effluent using two-stage thermophilic and mesophilic fermentation

“…Hence, it is significant to develop the high-rate methane production system using POME. Normally, the optimum pH for hydrogen production is 5.5e6.0 and a large amount of sodium hydroxide, which is used for pH adjustment can increase the production cost as well [14]. The POME generated at a high temperature of around 80e90 C from its source.…”