Nicolas Bernet

Anaerobic digestion of microalgae as a necessary step to make microalgal biodiesel sustainable

The potential of microalgae as a source of biofuels and as a technological solution for CO2 fixation is subject to intense academic and industrial research. In the perspective of setting up massive cultures, the management of large quantities of residual biomass and the high amounts of fertilizers must be considered. Anaerobic digestion is a key process that can solve this waste issue as well as the economical and energetic balance of such a promising technology. Indeed, the conversion of algal biomass after lipid extraction into methane is a process that can recover more energy than the energy from the cell lipids. Three main bottlenecks are identified to digest microalgae. First, the biodegradability of microalgae can be low depending on both the biochemical composition and the nature of the cell wall. Then, the high cellular protein content results in ammonia release which can lead to potential toxicity. Finally, the presence of sodium for marine species can also affect the digester performance. Physico-chemical pretreatment, co-digestion, or control of gross composition are strategies that can significantly and efficiently increase the conversion yield of the algal organic matter into methane. When the cell lipid content does not exceed 40%, anaerobic digestion of the whole biomass appears to be the optimal strategy on an energy balance basis, for the energetic recovery of cell biomass. Lastly, the ability of these CO2 consuming microalgae to purify biogas and concentrate methane is discussed.

Experimental study on a coupled process of production and anaerobic digestion of Chlorella vulgaris

The main goal of this present study is to investigate the feasibility of coupling algae production (Chlorella vulgaris) to an anaerobic digestion unit. An intermediate settling device was integrated in order to adapt the feed-flow concentration and the flow rate. Digestion of C. vulgaris was studied under 16 and 28 days hydraulic retention times (HRT), with a corresponding organic loading rate of 1g(COD)L(-1). Increasing the HRT achieved 51% COD removal with a methane production measured at 240 mL g(VSS)(-1). Performing different HRTs and dynamic monitoring during degradation highlighted differential hydrolysis of microalgae compartments. However, 50% of the biomass did not undergo anaerobic digestion, even under long retention times. This points out the interest for further studies on pre-treatment performances and more generally speaking on the need for intensifying microalgae biomass digestion.

Comparison of ultrasound and thermal pretreatment of Scenedesmus biomass on methane production.

Ultrasound at 20Hz was applied at different energy levels (Es) to treat Scenedesmus biomass, and organic matter solubilization, particle size distribution, cell disruption and biochemical methane potential were evaluated. An Es of 35.5 and 47.2MJ/kg resulted in floc deagglomeration but no improvement in methane production compared to untreated biomass. At an Es of 128.9, cell wall disruption was observed together with a 3.1-fold organic matter solubilization and an approximately 2-fold methane production in comparison with untreated biomass. Thermal pretreatment at 80°C caused cell wall disruption and improved anaerobic biodegradability 1.6-fold compared to untreated biomass. Since sonication caused a temperature increase in samples to as high as 85°C, it is likely that thermal effects accounted for much of the observed changes in the biomass. Given that ultrasound treatment at the highest Es studied only increased methane production by 1.2-fold over thermal treatment at 80°C, the higher energy requirement of sonication might not justify the use of this approach over thermal treatment.

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.

Challenges and innovations on biological treatment of livestock effluents

Intensification of animal production led to high amounts of manure to be managed. Biological processes can contribute to a sustainable manure management. This paper presents the biological treatments available for the treatment of animal manure, mainly focusing on swine manure, including aerobic processes (nitrification, denitrification, enhanced biological phosphorus removal) and anaerobic digestion. These processes are discussed in terms of pollution removal, ammonia and greenhouse gas emissions (methane and nitrous oxide) and pathogen removal. Application of emerging processes such as partial nitrification and anaerobic ammonium oxidation (anammox) applied to animal manure is also considered. Finally, perspectives and future challenges for the research concerning biological treatments are highlighted in this paper.

Methane yield as a monitoring parameter for the start-up of anaerobic fixed film reactors.

This paper describes the variation of the methane yield during the start-up period of an anaerobic fluidized bed reactor. After a lag phase, with acclimatized sludge, the methane yield increased with time during biofilm development up to the theoretical steady yield value, reported to be around 0.351 CH4/g CODdeg. The establishment of the biofilm required a high consumption of organic material through the microbial synthesis (anabolism), thereby reducing the proportion of substrate converted to methane. As a result, this yield could be an indirect metabolic parameter for evaluating a start-up operation. It could provide vital information about bacterial fixation processes and is easy to be applied to any biofilm reactor, such as anaerobic filters, where biomass sampling is impracticable. Monitoring this parameter could also give useful dynamic information about the different steps of colonization and biomass attachment, which could be used to improve the start-up performance.

Improving pig manure conversion into biogas by thermal and thermo-chemical pretreatments

Thermal (70-190 degrees C) and thermo-chemical (pH=10 and 12, 25 degrees C and 90-190 degrees C) treatments were investigated in order to maximise the production of methane from pig manure. Methane production from treated and raw manure was assessed from batch mesophilic biochemical methane potential tests. Methane potential of manure soluble fraction increased with the temperature of thermal treatments whereas temperatures higher than 135 degrees C were necessary to improve the methane potential of the total fraction. The best results were obtained with the highest temperature (190 degrees C). When thermo-chemical treatments were carried out at pH=12, both liquid phase and total fraction manure biodegradabilities were significantly decreased. Methane potential of manure total fraction was improved by treatments at pH=10 and temperatures ranging from 150 to 190 degrees C but biodegradability of liquid fraction was highly degraded, except for treatment at 190 degrees C. In both cases of thermal and thermo-chemical treatments at pH=10, the increase in manure biodegradability seemed to be linked to the reduction of the hemicellulosic like fraction.

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.

Effect of dissolved oxygen and carbon-nitrogen loads on denitrification by an aerobic consortium.

Abstract Four samples of natural ecosystems and one sample from an activated sludge treatment plant were mixed together and progressively adapted to alternating aerobic/anoxic phases in the presence of nitrate in order to enrich the microflora in aerobic denitrifiers. Aerobic denitrifying performances of this mixed ecosystem at various dissolved oxygen concentrations and various carbon–nitrogen loads were evaluated and compared to those obtained with the aerobic denitrifier Microvirgula aerodenitrificans. The consortium and the pure strain exhibited an aerobic denitrifying activity at air saturation conditions (7 mg dissolved oxygen l–1), i.e. there was co-respiration of the two electron acceptors with significant specific nitrate reduction rates. Dissolved oxygen concentrations had no influence on denitrifying performances above a defined threshold: 0.35 mg l–1 for the consortium and 4.5 mg l–1 for M. aerodenitrificans respectively. Under these thresholds, decreasing the dissolved oxygen concentrations enhanced the denitrifying activity of each culture. The higher the carbon and nitrogen loads, the higher the performance of the aerobic denitrifying ecosystem. However, for M. aerodenitrificans, the nitrate reduction percentage was affected more by variations in nitrogen load than in carbon load.