Bruno Sialve

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.

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.

Characterisation of the microbial 16S rDNA diversity of an aerobic phosphorus-removal ecosystem and monitoring of its transition to nitrate respiration

Abstract. The microbial community of a conventional anaerobic–aerobic sequencing batch reactor was investigated by cloning and sequencing bacterial 16S rDNA. The 92 16S rDNA sequences analysed ranged across 50 different operational taxonomic units (OTU). The majority of these sequences were not closely related to known species. They belonged to 12 different groups, but essentially to the Cytophagales and the Proteobacteria beta, which represented 38% and 17% of the retrieved sequences respectively. No OTU numerically outnumbered the others. However, similarities were observed with previous reports on molecular characterisation of phosphorus-accumulating ecosystems, suggesting an enrichment in microorganisms belonging to the Rhodocyclus group. Thereafter, the ability of this anaerobic–aerobic microbial community to accumulate phosphorus with nitrate as its energy source was investigated. The reactor was shifted from anaerobic–aerobic running conditions to anaerobic–anoxic conditions by injection of nitrate; and its microbial community was monitored by PCR-single strand conformation polymorphism (SSCP). The reactor maintained a good phosphorus accumulation and similar SSCP microbial community patterns for a period of 17 days, suggesting that the same microbial community was able to respire both oxygen and nitrate. However, this situation was unstable, since a breakdown in phosphorus accumulation occurred thereafter.

Anaerobic digestate as substrate for microalgae culture: The role of ammonium concentration on the microalgae productivity

In spite of the increasing interest received by microalgae as potential alternatives for biofuel production, the technology is still not industrially viable. The utilization of digestate as carbon and nutrients source can enhance microalgal growth reducing costs and environmental impacts. This work assesses microalgal growth utilizing the liquid phase of anaerobic digestate effluent as substrate. The effect of inoculum/substrate ratio on microalgal growth was studied in a laboratory batch experiment conduced in 0.5L flasks. Results suggested that digestate may be an effective substrate for microalgal growth promoting biomass production up to 2.6 gTSS/L. Microalgal growth rate was negatively affected by a self-shading phenomenon, while biomass production was positively correlated with the inoculum and substrate concentrations. Thus, the increasing of both digestate and microalgal initial concentration may reduce the initial growth rate (μ from 0.9 to 0.04 d(-1)) but significantly enhances biomass production (from 0.1 to 2.6 gTSS/L).

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.

Effect of organic loading rate on anaerobic digestion of thermally pretreated Scenedesmus sp. biomass

Biogas production is one of the means to produce a biofuel from microalgae. Biomass consisting mainly of Scenedesmus sp. was thermally pretreated and optimum pretreatment length (1 h) and temperature (90 °C) was selected. Different chemical composition among batches stored at 4 °C for different lengths of time resulted in organic matter hydrolysis percentages ranging from 3% to 7%. The lower percentages were attributed to cell wall thickening observed during storage for 45 days. The different hydrolysis percentages did not cause differences in anaerobic digestion. Pretreatment of Scenedesmus sp. at 90 °C for 1h increased methane production 2.9 and 3.4-fold at organic loading rates (OLR) of 1 and 2.5 kg COD m(-3) day(-1), respectively. Regardless the OLR, inhibition caused by organic overloading or ammonia toxicity were not detected. Despite enhanced methane production, anaerobic biodegradability of this biomass remained low (32%). Therefore, this microalga is not a suitable feedstock for biogas production unless a more suitable pretreatment can be found.

Anaerobic digestion of microalgal biomass: Challenges, opportunities and research needs

Integration of anaerobic digestion (AD) with microalgae processes has become a key topic to support economic and environmental development of this resource. Compared with other substrates, microalgae can be produced close to the plant without the need for arable lands and be fully integrated within a biorefinery. As a limiting step, anaerobic hydrolysis appears to be one of the most challenging steps to reach a positive economic balance and to completely exploit the potential of microalgae for biogas and fertilizers production. This review covers recent investigations dealing with microalgae AD and highlights research opportunities and needs to support the development of this resource. Novel approaches to increase hydrolysis rate, the importance of the reactor design and the noteworthiness of the microbial anaerobic community are addressed. Finally, the integration of AD with microalgae processes and the potential of the carboxylate platform for chemicals and biofuels production are reviewed.

Carbon conversion efficiency and population dynamics of a marine algae–bacteria consortium growing on simplified synthetic digestate: First step in a bioprocess coupling algal production and anaerobic digestion

Association of microalgae culture and anaerobic digestion seems a promising technology for sustainable algal biomass and biogas production. The use of digestates for sustaining the growth of microalgae reduces the costs and the environmental impacts associated with the substantial algal nutrient requirements. A natural marine algae-bacteria consortium was selected by growing on a medium containing macro nutrients (ammonia, phosphate and acetate) specific of a digestate, and was submitted to a factorial experimental design with different levels of temperature, light and pH. The microalgal consortium reached a maximum C conversion efficiency (i.e. ratio between carbon content produced and carbon supplied through light photosynthetic C conversion and acetate) of 3.6%. The presence of bacteria increased this maximum C conversion efficiency up to 6.3%. The associated bacterial community was considered beneficial to the total biomass production by recycling the carbon lost during photosynthesis and assimilating organic by-products from anaerobic digestion.

Control of nitrogen behaviour by phosphate concentration during microalgal-bacterial cultivation using digestate.

The cultivation of microalgae with digestate supernatant is a promising process for the recovery of mineralized nutrients (P, N) from anaerobic digestion. Nevertheless, the variability of phosphorus concentration in the influent could limit this process. The impact of initial N:P ratios between 3 and 76gNgP(-1) was studied and proved no growth limitation over 14-day batch experiments even when P was depleted. Nitrogen assimilation was not affected by phosphorus concentrations and reached 10.1mgNL(-1)d(-1) whereas phosphorus removal ranged from 0.6 to 2.0mgPL(-1)d(-1). The biomass N:P ratio was found to be a function of the influent N:P ratio. Phosphorus storage by microalgae was thus confirmed. Nitrification was found to be highly dependent on the initial phosphorus concentration. The evolution of microalgae communities was also monitored and revealed the advantage of Scenedesmus over Chlorella when the media was phosphorus-depleted.

Bioaerosol emissions from open microalgal processes and their potential environmental impacts: what can be learned from natural and anthropogenic aquatic environments?

Open processes for microalgae mass cultivation and/or wastewater treatment present an air-water interface. Similarly to other open air-aquatic environments, they are subject to contamination, but as such, they also represent a source of bioaerosols. Indeed, meteorological, physico-chemical and biological factors cause aerial dispersion of the planktonic community. Operating conditions like liquid mixing or gas injection tend to both enhance microbial activity, as well as intensify aerosolization. Bacteria, virus particles, fungi and protozoa, in addition to microalgae, are all transient or permanent members of the planktonic community and can thus be emitted as aerosols. If they should remain viable, subsequent deposition on various habitats could instigate their colonization of other environments and the potential expression of their ecological function.