Archaeal community composition as key driver of H2 consumption rates at the start-up of the biomethanation process.

The performance of hydrogen consumption by various inocula derived from mesophilic anaerobic digestion plants was evaluated under ex situ biomethanation. A panel of 11 mesophilic inocula was operated at a concentration of 15 gVS.L-1 at a temperature of 35 °C in batch system with two successive injections of H2:CO2 (4:1 mol:mol). Hydrogen consumption and methane production rates were monitored from 44 h to 72 h. Hydrogen consumption kinetics varies significantly based on the inoculum origin, with no accumulation of volatile fatty acids. Microbial community analyses revealed that microbial indicators such as the increase in Methanosarcina sp. abundance and the increase of the Archaea/Bacteria ratio were associated to high initial hydrogen consumption rates. The improvement in the hydrogen consumption rate between the two injections was correlated with the enrichment in hydrogenotrophic methanogens. This work provides new insights into the early response of microbial communities to hydrogen injection and on the microbial structures that may favor their adaptation to the biomethanation process.

Microbial community redundance in biomethanation systems lead to faster recovery of methane production rates after starvation.

The Power-to-Gas concept corresponds to the use of the electric energy surplus to produce H2 by water electrolysis, that can be further converted to methane by biomethanation. However, the fluctuant production of renewable energy sources can lead to discontinuous H2 injections into the reactors, that may interfere with the adaptation of the microbial community to high H2 partial pressures. In this study, the response of the microbial community to H2 and organic feed starvation was evaluated in in-situ and ex-situ biomethanation. The fed-batch reactors were fed with acetate or glucose and H2, and one or four weeks of starvation periods were investigated. Methane productivity was mostly affected by the four-week starvation period. However, both in-situ and ex-situ biomethanation reactors recovered their methane production rate after starvation within approximately one-week of normal operation, while the anaerobic digestion (AD) reactors did not recover their performances even after 3 weeks of normal operation. The recovery failure of the AD reactors was probably related to a slow growth of the syntrophic and methanogen microorganisms, that led to a VFA accumulation. On the contrary, the faster recovery of both biomethanation reactors was related to the replacement of Methanoculleus sp. by Methanobacterium sp., restoring the methane production in the in-situ and ex-situ biomethanation reactors. This study has shown that biomethanation processes can respond favourably to the intermittent H2 addition without compromising their CH4 production performance.

Dynamic observation of the biodegradation of lignocellulosic tissue under solid-state anaerobic conditions.

The solid-state anaerobic digestion (SS-AD) of wheat straw was characterized under low inoculated batch tests during 244 days. High levels of degradation of the cellulose (52%±1) and hemicelluloses (55%±2) were observed at the final stages and associated to a methane yield of 204±16 NmL gTS(-1). Ultrastructural observations, using transmission electronic microscopy, indicated that microorganisms degraded wheat straw from the central to the outer tissue (i.e. parenchyma to epidermis), depending on cell chemical, physical accessibility and the degree of lignification. Furthermore, major degradation of sclerenchyma secondary walls was observed. The bioaccessibility of lignocellulosic structures of wheat straw is mainly limited by the external waxy layer (cuticle), tertiary cell walls, high silica content and access to the cell lumen.

Substrate milling pretreatment as a key parameter for Solid-State Anaerobic Digestion optimization.

The effect of milling pretreatment on performances of Solid-State Anaerobic Digestion (SS-AD) of raw lignocellulosic residue is still controverted. Three batch reactors treating different straw particle sizes (milled 0.25 mm, 1 mm and 10 mm) were followed during 62 days (6 sampling dates). Although a fine milling improves substrate accessibility and conversion rate (up to 30% compared to coarse milling), it also increases the risk of media acidification because of rapid and high acids production during fermentation of the substrate soluble fraction. Meanwhile, a gradual adaptation of microbial communities, were observed according to both reaction progress and methanogenic performances. The study concluded that particle size reduction affected strongly the performances of the reaction due to an increase of substrate bioaccessibility. An optimization of SS-AD processes thanks to particle size reduction could therefore be applied at farm or industrial scale only if a specific management of the soluble compounds is established.

French Brittany macroalgae screening: composition and methane potential for potential alternative sources of energy and products.

Macroalgae are biomass resources that represent a valuable feedstock to be used entirely for human consumption or for food additives after some extractions (mainly colloids) and/or for energy production. In order to better develop the algal sector, it is important to determine the capacity of macroalgae to produce these added-values molecules for food and/or for energy industries on the basis of their biochemical characteristics. In this study, ten macroalgae obtained from French Brittany coasts (France) were selected. The global biochemical composition (proteins, lipids, carbohydrates, fibers), the presence and characteristics of added-values molecules (alginates, polyphenols) and the biochemical methane potential of these algae were determined. Regarding its biochemical composition, Palmaria palmata is interesting for food (rich in nutrients) and for anaerobic digestion (0.279 LCH4/gVS). Saccharina latissima could be used for alginate extraction (242 g/kgTS, ratio between mannuronic and guluronic acid M/G=1.4) and Sargassum muticum for polyphenol extraction (19.8 g/kgTS).

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.

Pretreatment methods to improve sludge anaerobic degradability: a review.

This paper presents a review of the main sludge treatment techniques used as a pretreatment to anaerobic digestion. These processes include biological (largely thermal phased anaerobic), thermal hydrolysis, mechanical (such as ultrasound, high pressure and lysis), chemical with oxidation (mainly ozonation), and alkali treatments. The first three are the most widespread. Emphasis is put on their impact on the resulting sludge properties, on the potential biogas (renewable energy) production and on their application at industrial scale. Thermal biological provides a moderate performance increase over mesophilic digestion, with moderate energetic input. Mechanical treatment methods are comparable, and provide moderate performance improvements with moderate electrical input. Thermal hydrolysis provides substantial performance increases, with a substantial consumption of thermal energy. It is likely that low impact pretreatment methods such as mechanical and thermal phased improve speed of degradation, while high impact methods such as thermal hydrolysis or oxidation improve both speed and extent of degradation. While increased nutrient release can be a substantial cost in enhanced sludge destruction, it also offers opportunities to recover nutrients from a concentrated water stream as mineral fertiliser.

Dry anaerobic digestion in batch mode: design and operation of a laboratory-scale, completely mixed reactor.

A laboratory-scale (40 l) reactor was designed to investigate dry anaerobic digestion. The reactor is equipped with an intermittent paddle mixer, enabling complete mixing in the reactor. Three consecutive batch dry digestion tests of municipal solid waste were performed under mesophilic conditions and compared to operation results obtained on a pilot-scale (21 m(3)) with the same feedstock. Biogas and methane production at the end of the tests were similar (around 200 m(3) CH(4)STP/tVS), and the dynamics of methane production and VFA accumulation concurred. However, the maximal levels of VFA transitory accumulation varied between reactors and between runs in a same reactor. Ammonia levels were similar in both reactors. These results show that the new reactor accurately imitates the conditions found in larger ones. Adaptation of micro-organisms to the waste and operating conditions was also pointed out along the consecutive batches.

Slaughterhouse fatty waste saponification to increase biogas yield.

A thermochemical pretreatment, i.e. saponification, was optimised in order to improve anaerobic biodegradation of slaughterhouse wastes such as aeroflotation grease and flesh fats from cattle carcass. Anaerobic digestion of raw wastes, as well as of wastes saponified at different temperatures (60 degrees C, 120 degrees C and 150 degrees C) was conducted in fed-batch reactors under mesophilic condition and the effect of different saponification temperatures on anaerobic biodegradation and on the long-chain fatty acids (LCFAs) relative composition was assessed. Even after increasing loads over a long period of time, raw fatty wastes were biodegraded slowly and the biogas potentials were lower than those of theoretical estimations. In contrast, pretreated wastes exhibited improved batch biodegradation, indicating a better initial bio-availability, particularly obvious for carcass wastes. However, LCFA relative composition was not significantly altered by the pretreatment. Consequently, the enhanced biodegradation should be attributed to an increased initial bio-availability of fatty wastes without any modification of their long chain structure which remained slowly biodegradable. Finally, saponification at 120 degrees C achieved best performances during anaerobic digestion of slaughterhouse wastes.

Treatment of fatty solid waste from the meat industry in an anaerobic sequencing batch reactor: start-up period and establishment of the design criteria.

An anaerobic sequencing batch reactor (AnSBR) was used to treat the dissolved air flotation skimmings from a cooked pork meat plant. During the start-up period, the reactor was operated in fed-batch mode for 25 days and 7 batches were treated. The SBR was inoculated with sludge taken from a reactor treating distillery vinasse. The results showed that this kind of sludge is a very good source of inoculum for digesters treating residues with a high content in fats and long-chain fatty acids because it was able to adapt very rapidly to the new substrate and, from the second batch on, the sludge was already able to metabolize the fatty residue at quite high rates. The AnSBR was then operated with 5 batches per week for 110 days and the quantity of VS added per batch was regularly increased until the maximum treatment capacity of the reactor (i.e. maximum loading rate) was reached. The maximum organic loading rates were found to be 0.16 g VS/g VSS d, or 0.224 g VS/g VSS.batch when the reactor is fed 5 times a week. The biodegradability of the skimmings was very high, with more than 97% of TS removal, and the methane production was 880+/-90 mL of methane/g of VS(added).

Successful bacterial incorporation into activated sludge flocs using alginate.

Bioaugmentation experiments with the aerobic denitrifier Microvirgula aerodenitrificans were performed in an aerobic continuous stirred tank reactor (CSTR) treating urban wastewater. The fate of the added bacteria was monitored by a specific fluorescent oligonucleotide probe targeting 16S rRNA. The first addition of the strain led to its rapid disappearance because of grazing. Bacteria were then embedded within an alginate matrix before inoculation. Alginate fragments adhered to the existing flocs and were progressively colonized by the indigenous flora. Thereafter, microcolonies of the exogenous bacterium were found to be incorporated into existing flocs.

High-solids anaerobic digestion: comparison of three pilot scales.

Two experiments were undertaken in three different experimental set-ups in order to compare them: an industrial 21-m3 pilot reactor, a new 40-l laboratory pilot reactor and bmp type plasma bottles. Three consecutive batch dry digestion tests of municipal solid waste were performed under mesophilic conditions with the same feedstock in all vessels. Biogas and methane production at the end of the tests were similar (around 200 m3 CH4STP/tVS) for both pilot reactors and were different from the bottle tests. The dynamics of methane production and VFA accumulation concurred. However, the maximal levels of VFA transitory accumulation varied between reactors and between runs in a same reactor. Ammonia levels were similar in both reactors. These results show that the new reactor accurately imitates the conditions found in the larger one. Adaptation of microorganisms to the waste and operating conditions was also pointed out along the consecutive batches. Thermophilic semi-continuous tests were performed in both reactors with similar conditions. The methane production efficiencies were similar.

Anaerobic digestion of residues from production and refining of vegetable oils as an alternative to conventional solutions.

The purpose of this work was to study the anaerobic digestion of by-products generated during the production and refining of oil with the objective of proposing an alternative solution (methanisation) to the conventional solutions while reducing the energy consumption of fossil origin on refinery sites. The production of sunflower oil was taken as example. Glycerine from the production of biodiesel was also included in this study. The results show that glycerine has a high potential for methanisation because of its high methane potential (465 ml CH4/g VS) and high metabolization rates (0.42 g VS/g VSS.d). The use of oil cake as substrate for anaerobic digestion is not interesting because it has a low methane potential of 215 ml CH4/g VS only and because it is easily recovered in animal feed. Six residues have quite a high methane potential (465 to 850 ml CH4/g VS) indicating a good potential for anaerobic digestion. However, they contain a mixture of rapidly and slowly biodegradable organic matter and the loading rates must remain quite low (0.03 to 0.09 g VS/g VSS.d) to prevent any accumulation of slowly biodegradable solids in the digesters.

Relevant approach to assess performances of wastewater biosolids composting in terms of micropollutants removal.

The presence of organic pollutants in wastewater biosolids and their possible impact to the environment contribute to decrease interest for the agricultural spreading of biosolids. It is thus important to have a better overview of sewage sludge quality in terms of organic pollutant content and ecotoxicity assessment. It is also necessary to better understand the impact of biosolid composting processes on the pollutant and toxicity removal. Therefore, concentrations of oestrogens (E), nonyphenol ethoxylates (NPE), polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB) and linear alkyl benzene sulphonates (LAS) and some of their associated toxic effects were determined at different stages of a composting process using, respectively, chemical analysis and in vitro bioassays (estrogen receptor alpha, dioxin receptor and pregnan X receptor reporter cell lines). Pollutants concentrations were higher in the final compost than in biosolid due to dry matter reduction through composting. Mass balance calculation shows a positive impact of the aerobic treatment on the removal of the most degradable pollutants. The three toxicological activities were measured in both biosolids and in the initial and final compost: oestrogenic activity increased whereas dioxin-like and pregnan X activities decreased. The difficulty in correlating chemical and toxicological results underlines the importance of combining both approaches in order to improve the assessment of the compost quality.

Anaerobic digestion of gelatinous water at laboratory and pilot scale and nitrogen inhibition.

The anaerobic digestion of the liquid residue (gelatinous water) coming from the production of fat from animal residue, was studied at laboratory and pilot scale. Biodegradability (>98%) and biogas potential (675 mL of biogas/g of COD(applied)) of this wastewater are very high. However, due to the high content on nitrogen, an inhibition of the anaerobic activity was observed for quite low concentrations of N-NH(3). Dilution of the wastewater and pH regulation in the reactor around 7.3 are the 2 solutions which were investigated to overcome the nitrogen inhibition at industrial scale. These two solutions were validated at laboratory scale in an anaerobic SBR and then onsite at pilot scale in a continuous reactor. A stable anaerobic digestion was observed in both reactors showing that no nitrogen inhibition was obtained when N-NH(3) concentration in the reactor was kept low.

Characterization of the activated sludge and of the operating conditions of six SBR treating dairy effluent and operated at industrial scale.

The sludge from six SBRs treating dairy effluent and located at same geographical location, in North East of France, were collected to study their characteristic behavior. The six plants were designed and constructed by the same manufacturer and are working under quite similar operating conditions. The objective of the study was to observe if any similarity existed in the characteristics of the sludge collected from the SBRs. The sludge was characterized for morphological properties (filament index, floc size), settling, compressibility, suspended solids (SS) concentration. The sludge from each plant was different from the others in most of the characteristics. One sludge out of six (sludge G) was completely different from the others with a very degraded structure and low discrete settling and compression. This reactor was not working fully satisfactorily with a too high COD at outlet, probably because this SBR was undergoing repetitive overloading linked to a very bad recovery of the whey by the cheese maker. The five other SBRs were working fully satisfactorily but the characteristics of the five sludges were quite different from one sludge to another. The size of the flocs seemed to be the only parameter measured which could be correlated to the settling characteristics of the sludge. The sludge characteristics and the parameter correlations were also compared with that of municipal activated sludge and were found to be very different.

Impact of increasing NaCl concentrations on the performance and community composition of two anaerobic reactors.

The anaerobic treatment of saline effluents using halophilic and halotolerant microbial consortia is of major interest. Inhibition of anaerobic digestion is known to occur at high salt content. However, it seems that the suitable adaptation of an anaerobic sludge makes possible the treatment of saline wastewater. In this study, a non-saline anaerobic sludge was inoculated in two anaerobic batch reactors operating with a different substrate (distillery vinasse and ethanol) and subjected to increasing NaCl concentrations. The performance of the digesters appeared to be highly dependent on the nature of the substrate, and a similar level of inhibition (i.e. around 90% of the specific loading rate and specific methanogenic activity) was stated at 10 g l(-1) of NaCl with distillery vinasse and 60 g l(-1) of NaCl with ethanol. The characterization of the microflora and its adaptation to increasing NaCl conditions were also investigated using molecular tools based on the analysis of genomic 16S rDNA. The microbial communities revealed a high diversity that could be maintained in both reactors despite the increase in NaCl concentrations.

Survival of Listeria monocytogenes and Enterococcus faecium in sludge evaluated by real-time PCR and culture methods.

AIMS: This study evaluates the behaviour in spiked sludge of a pathogenic bacteria, Listeria monocytogenes, by cultural and molecular techniques, and compares its survival with the one of a faecal indicator, Enterococcus faecium. METHODS AND RESULTS: Listeria monocytogenes strain Scott A and E. faecium(T) were followed for 17 days after inoculation in sludge. Kinetics of survival depended on the bacteria and on the technique used [most probable number method, direct plate count or real-time quantitative PCR (qPCR)]. The concentration of L. monocytogenes decreased rapidly regardless of the technique, but the decrease was much more dramatic with culture techniques than with qPCR. On the contrary, the concentrations of culturable E. faecium(T) were stable. CONCLUSIONS: The results suggest that the cells of L. monocytogenes strain Scott A might have entered a viable, but nonculturable (VBNC) status, whereas cells of the indicator bacteria, E. faecium(T), maintained themselves better and stayed culturable. SIGNIFICANCE AND IMPACT OF THE STUDY: The difference of survival kinetics in the sludge of a faecal indicator (E. faecium) and a pathogenic bacterium (L. monocytogenes) may be linked to the fact that they either enter or do not enter into a VBNC status.

Towards new indicators for the prediction of solid waste anaerobic digestion properties.

The biochemical composition can be seen as a good indicator of both the biodegradability and the methane potential of a given waste. The work presented here is an attempt to elaborate a typology of wastes and to compare it to the anaerobic degradation characteristics. The first data indicate that there is a link between the ligno-cellulosic content of the waste and the biodegradability. When dealing with application to anaerobic digestion processes, having a tool to predict the ability of the waste to be degraded could be of the greatest interest for preventing failures, estimating biogas production, methane content, or for the management of co-digestion processes.

Influence of closed loop control on microbial diversity in a nitrification process.

This paper compares two control strategies for a nitrification process. The objective is to achieve partial nitrification and thus to accumulate nitrite instead of nitrate. To this end, change in temperature setpoint and active control of oxygen and ammonia concentrations are evaluated in the long term. Evaluation is made on the control performances that are obtained, but also--and more importantly--on the microbial diversity. In particular, it is shown that the combined oxygen and ammonia control strategy is more appropriate since shift in the temperature setpoint strongly affects the composition of the microbial ecosystem present in the reactor whereas active control of oxygen and ammonia does not.