One-stage Anaerobic Digestion Research Articles

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

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 H(2)-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.

Influence of organic loading rate and hydraulic retention time on the performance, stability and microbial communities of one-stage anaerobic digestion of two-phase olive mill solid residue

A study of the effect of organic loading rate (OLR) and hydraulic retention time (HRT) on the performance, stability and microbial communities of a laboratory-scale completely stirred tank anaerobic reactor treating two-phase olive mill solid residue (OMSR) was carried out at mesophilic temperature (35 °C). The reactor operated at a fixed influent substrate concentration of 162 g total chemical oxygen demand (COD)/L and 126 g volatile solids (VS)/L. The OLR and HRT varied in the ranges of 0.8–11.0 g COD/L day and 108–15 days, respectively. COD removal efficiencies in the range of 97–77% were achieved for OLRs and HRTs in the ranges of 1.5–9.2 g COD/L day and 108–17 days, respectively. The maximum methane production rate was found to be 1.7 L CH 4 STP/L day and it was achieved for an OLR of 9.2 g COD/L day and HRT of 17 days. The methane yield coefficient was 0.244 ± 0.005 L methane at STP conditions/g COD removed. The results obtained demonstrated that an OLR of 11.0 g COD/L day and a HRT of 15 days brought about a decrease in the pH and total volatile fatty acids (TVFA)/alkalinity ratios up to values of 5.3 and 1.5 (mequiv. acetic acid/mequiv. CaCO 3), respectively, causing the destabilization of the reactor and process failure. Microbial communities, both Bacteria and Archaea, were studied by molecular fingerprinting methods, cloning and sequencing. Molecular fingerprints of bacterial communities showed higher number of major bands at increasing OLR. Firmicutes, mostly represented by the genus Clostridium, were the predominant bacteria at low OLR. Other bacterial communities such as Gammaproteobacteria, Actinobacteria, Bacteroidetes and Deferribacteres were the most abundant at high OLR. The Archaea were mainly represented by four phylotypes belonging to the genus Methanosaeta independently of the OLR. This study remarks the interest of relating OMSR decomposing bioreactor performance with the microbial communities carrying out the process in order to better understand and monitor this anaerobic digestion.