Renaud Escudié

Total solids content drives high solid anaerobic digestion via mass transfer limitation

The role of the total solids (TS) content on anaerobic digestion was investigated in batch reactors. A range of TS contents from 10% to 35% was evaluated, four replicates were performed. The total methane production slightly decreased with TS concentrations increasing from 10% to 25% TS. Two behaviors were observed at 30% TS: two replicates had similar performances to that at 25% TS; for the two other replicates, the methane production was inhibited as observed at 35% TS. This difference suggested that 30% TS content corresponded to a threshold of the solids content, above which methanogenesis was strongly inhibited. The Anaerobic Digestion Model No. 1 (ADM1) was used to describe the experimental data. The effects of hydrolysis step and liquid/gas mass transfer were particularly investigated. The simulations showed that mass transfer limitation could explain the low methane production at high TS, and that hydrolysis rate constants slightly decreased with increasing TS.

Role of shear stress on composition, diversity and dynamics of biofilm bacterial communities

This article evaluates the effect of shear stress on the composition of biofilm bacterial communities. For the first time, a Conical Couette-Taylor Reactor (CCTR) was used to develop biofilms at varying shear stresses (from 0.055 to 0.27 Pa) and provided a useful model for studying the effect of hydrodynamics on biofilms. The composition, diversity and dynamics of biofilm bacterial communities were analysed using the PCR-SSCP fingerprint method. Results clearly demonstrate a link between shear stress and composition of the microbial communities. High shear stresses decrease biofilm diversity and the analysis of biofilm community dynamics suggests that shear stress would slow down biofilm maturation and tend to maintain a young biofilm.

Dynamic effect of total solid content, low substrate/inoculum ratio and particle size on solid-state anaerobic digestion

Among all the process parameters of solid-state anaerobic digestion (SS-AD), total solid content (TS), inoculation (S/X ratio) and size of the organic solid particles can be optimized to improve methane yield and process stability. To evaluate the effects of each parameter and their interactions on methane production, a three level Box-Behnken experimental design was implemented in SS-AD batch tests degrading wheat straw by adjusting: TS content from 15% to 25%, S/X ratio (in volatile solids) between 28 and 47 and particle size with a mean diameter ranging from 0.1 to 1.4mm. A dynamic analysis of the methane production indicates that the S/X ratio has only an effect during the start-up phase of the SS-AD. During the growing phase, TS content becomes the main parameter governing the methane production and its strong interaction with the particle size suggests the important role of water compartmentation on SS-AD.

Control of start-up and operation of anaerobic biofilm reactors: an overview of 15 years of research.

Anaerobic biofilm reactors have to be operated in a way that optimizes on one hand the start-up period by a quick growth of an active biofilm, on the other hand the regular operation by an active control of the biofilm to avoid diffusion limitations and clogging. This article is an overview of the research carried out at INRA-LBE for the last 15 years. The start-up of anaerobic biofilm reactors may be considerably shortened by applying a short inoculation period (i.e. contact between the inoculum and the support media). Then, the increase of the organic loading rate should be operated at a short hydraulic retention time and low hydrodynamic constraints in order to favor biofilm growth. After the start-up period, biofilm growth should be controlled to maintain a high specific activity and prevent clogging. This can be done in particulate biofilm systems by using hydrodynamics to increase or decrease shear forces and attrition but is much more difficult in anaerobic fixed bed reactors.

Total solids content: a key parameter of metabolic pathways in dry anaerobic digestion.

BackgroundIn solid-state anaerobic digestion (AD) bioprocesses, hydrolytic and acidogenic microbial metabolisms have not yet been clarified. Since these stages are particularly important for the establishment of the biological reaction, better knowledge could optimize the process performances by process parameters adjustment.ResultsThis study demonstrated the effect of total solids (TS) content on microbial fermentation of wheat straw with six different TS contents ranging from wet to dry conditions (10 to 33% TS). Three groups of metabolic behaviors were distinguished based on wheat straw conversion rates with 2,200, 1,600, and 1,400 mmol.kgVS-1 of fermentative products under wet (10 and 14% TS), dry (19 to 28% TS), and highly dry (28 to 33% TS) conditions, respectively. Furthermore, both wet and dry fermentations showed acetic and butyric acid metabolisms, whereas a mainly butyric acid metabolism occurred in highly dry fermentation.ConclusionSubstrate conversion was reduced with no changes of the metabolic pathways until a clear limit at 28% TS content, which corresponded to the threshold value of free water content of wheat straw. This study suggested that metabolic pathways present a limit of TS content for high-solid AD.

Influence of substrate concentration and moisture content on the specific methanogenic activity of dry mesophilic municipal solid waste digestate spiked with propionate

The objective of this study was to evaluate the influence of substrate concentration and moisture content on the specific methanogenic activity (SMA) of a fresh dry mesophilic digestate from a municipal solid waste digester plant. For this purpose, SMA tests were performed under mesophilic conditions into glass bottles of 500 mL volume used as batch reactors, during a period of 20-25 days. Propionate was used as substrate at concentrations ranging from 1 to 10 gCOD/kg. Four moisture contents were studied: 65%, 75%, 80% and 82%. Experimental results showed that propionate concentration and moisture content strongly influenced the SMA. The highest SMA was observed at a substrate concentration of 10 gCOD/kg (11.3 mgCOD gVS(-1) d(-1) for the second dose of propionate) and at a moisture content of 82% (7.8 mgCOD gVS(-1) d(-1) for the second dose of propionate, at a concentration of 5 gCOD/kg). SMA was found to decrease linearly when decreasing the moisture content.

Food waste valorization via anaerobic processes: a review

The increasing production of food waste worldwide and new international regulations call for the development of new technologies to treat this biowaste. Anaerobic processes are able to treat efficiently organic wastes, producing at the same time different value-added compounds. In addition, due to the lower costs and environmental impacts associated with these processes when compared to other options, they are among the most promising technologies for food waste treatment. This article reviews the state-of-the-art dealing with treatment of food waste by anaerobic processes, with emphasis on the most recent research carried out. The different processes that are assessed are anaerobic digestion for methane production, anaerobic fermentation for hydrogen and/or volatile fatty acids production and 2-stage systems. The primary issues associated with each alternative are presented, paying special attention to accumulation of ammonia and volatile fatty acids in the reactor. In addition, the latest developments to overcome the complications of each system are also described, focusing on how they improve its stability and performance. Moreover, the relevant economic and environmental research has also been reviewed, including several life cycle analyses that compare anaerobic processes with other technologies used for food waste treatment. Different case studies are also presented. Finally, recommendations for future research for the anaerobic processes studied and options for process integration are discussed. Moving towards the idea of a circular economy, a potential biorefinery for food waste valorization is also proposed.

Biomass hydrolysis inhibition at high hydrogen partial pressure in solid-state anaerobic digestion

In solid-state anaerobic digestion, so-called ss-AD, biogas production is inhibited at high total solids contents. Such inhibition is likely caused by a slow diffusion of dissolved reaction intermediates that locally accumulate. In this study, we investigated the effect of H2 and CO2 partial pressure on ss-AD. Partial pressure of H2 and/or CO2 was artificially fixed, from 0 to 1 557mbars for H2 and from 0 to 427mbars for CO2. High partial pressure of H2 showed a significant effect on methanogenesis, while CO2 had no impact. At high [Formula: see text] , the overall substrate degradation decreased with no accumulation of metabolites from acidogenic bacteria, indicating that the hydrolytic activity was specifically impacted. Interestingly, such inhibition did not occur when CO2 was added with H2. This result suggests that CO2 gas transfer is probably a key factor in ss-AD from biomass.

Microbial community signature of high-solid content methanogenic ecosystems

In this study, changes in bacterial and archaeal communities involved in anaerobic digestion processes operated with high solid contents were investigated. Batch tests were performed within a range of total solids (TS) of 10-35%. Between 10% and 25% TS, high methanogenic activity was observed and no overall specific structure of active bacterial communities was found. At 30% and 35%, methanogenesis was inhibited as a consequence of volatile fatty acids accumulation. Here, a specific bacterial signature was observed with three main dominant bacteria related to Clostridium sp., known for their ability to grow at low pH. Additionally, archaeal community was gradually impacted by TS content. Three archaeal community structures were observed with a gradual shift from Methanobacterium sp. to Methanosarcina sp., according to the TS content. Overall, several species were identified as biomarkers of methanogenesis inhibition, since bacterial and archaeal communities were highly specific at high TS contents.

Biohydrogen production by dark fermentation: scaling-up and technologies integration for a sustainable system

Currently, the use of alternative renewable energies is broadly supported in many countries, some of which are seriously evaluating the possibility of using hydrogen as an alternative fuel in their power systems. Hydrogen production by biological processes, such as dark fermentation, is a very promising alternative. However, this process has only been studied on the laboratory scale, and there is limited experience at the pilot scale. The main reasons of non-scaling hydrogen production by dark fermentation at large scale are unpurified hydrogen production, stability of the bioprocesses, as well as their low conversion yields joined at the formation of byproducts. Improvement of energetic yields of dark fermentation requires a better knowledge of the microorganisms involved in the mixed culture and their possible interactions, as well as the use of appropriate substrates and strategies, such as solid-state fermentation, the purification of hydrogen and the coupling of dark fermentation with other biological processes as anaerobic digestion. The present work offers an overview of the current knowledge dealing with H2-production by dark fermentation and its integration into a concept of an environmental biorefinery. Several key points are addressed, such as the benefits of using local waste as substrates, the new solid-state fermentation processes, the coupling of hydrogen purification with the production process, the association of the H2-producing process with other biological processes, such as anaerobic digestion towards biohythane production (H2/CH4). Information about pilot plant experiments was added to illustrate the feasibility of producing fermentative hydrogen and methane from organic waste at a pilot scale, as developed at Feng Chia University (Taiwan).