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Biorefinery concepts that combine several valorization pathways, including both bioproducts and bioenergy/biofuels, are of high interest in order to maximize the economic value of microalgal cultivation systems. In this study, the residual biomass obtained from the microalga Chlorella vulgaris following the chemical extraction of a valuable fraction consisting of either chlorophyll, proteins, or lipids was used as feedstock for biogas production in a laboratory-scale biochemical methane potential (BMP) assay performed in batch mode. The main aim of this study is to assess the effect of chemical extraction and removal of valuable fractions on the methane yield of residual microalgal biomass. The dry weight ratio of residual biomass to total biomass was 0.54 g/g after lipid extraction, 0.62 g/g after chlorophyll extraction, and 0.42 g/g after protein extraction. The methane yields of raw microalgae as well as residual microalgal biomass after lipid, chlorophyll, or protein extraction and removal were in the range 207–237 mL CH4/g volatile solids. Differences in the methane yields were not statistically significant. Hence, the recovery and removal of valuable compounds did not have a significant impact on the final methane yields of microalgal biomass by anaerobic digestion. Nevertheless, the methane production kinetics of residual microalgal biomass after lipid extraction displayed a diauxic pattern, possibly due to the inhibition of methanogenic bacteria by chloroform used as extraction solvent. This suggests that solvent toxicity must be considered when applying anaerobic digestion as a downstream process in biomass-based biorefineries.

The interactive effects of light intensity, NaCl, nitrogen, and phosphorus on intracellular biomass content and extracellular polymeric substance production were assessed for Arthrospira sp. (Spirulina) in a two-phase culture process using principal component analysis and central composite face design. Under high light intensity (120 μmol photons m−2 s−1) and low NaCl (1 gL−1), NaNO3, and K2HPO4 (0.5 g L−1), the carbohydrate content was maximized to 26.61%. Interaction of both K2HPO4 (1.6 gL−1) and NaCl (1.19 gL−1) with low NaNO3 (0.5 gL−1) achieved the maximum content of lipids (15.62%), while high NaCl (40 gL−1), K2HPO4, and NaNO3 (4.5 gL−1) enhanced mainly total carotenoids (0.85%). Conversely, under low light intensity of 10 μmol photons m−2 s−1 combined with 11.76 gL−1 of NaCl, 0.5 gL−1 of NaNO3, and 2.68 gL−1 of K2HPO4, the phycobiliprotein content reached its highest level (16.09%). The maximum extracellular polymeric substance (EPS) production (0.902 gg−1 DW) was triggered under moderate light of 57.25 μmol photons m−2 s−1 and interaction of high NaCl (40 gL−1) and K2HPO4 (4.5 gL−1) with low NaNO3 (0.5 gL−1). The maximization ratios of intracellular biomass content in terms of carbohydrate, lipid, total carotenoid, phycobiliprotein, and EPS production were 3.55-, 1.73-, 9.55-, 2.92-, and 1.46-fold, respectively, greater than those obtained at optimal growth conditions. This study demonstrated that the multiple stress factors applied to the adopted two-phase culture process could be a promising strategy to produce biomass enriched in various high-value compound.

Herein a techno-economic assessment was performed on an energy-crop-based biogas plant coupled with a greenhouse for utilizing thermal energy produced by cogeneration. Seven energy crops were evaluated: triticale, maize, alfalfa, sunflower, clover, barley and wheat. According to the evaluation, triticale was the most competitive energy crop under selected climate conditions for northern Greece. Although maize displays higher biomass yield and biogas potential than the drought-resistant crop triticale, it has high irrigation demand that contributes significantly to total production costs. For a triticale-based biogas production to become economically feasible, agricultural arable area larger than 500 ha, or biogas plant size larger than 1000 kWel, is required. However, with public funding, biogas production becomes feasible at smaller area (>250 ha) or biogas plant size (>500 kWel). The inclusion of a greenhouse into the design of the biogas plant contributes positively to the economic viability of the entire system. Under this scenario, greenhouse financial income accounts for about 17–18% of total income. Results of a sensitivity analysis suggest that the selection of an appropriate energy crop for biogas production should be based principally on both digestibility (specific methane yield) and biomass yield per hectare, these factors being more critical than biomass production costs.

Flocculation of microalgae is a promising low-cost strategy to harvest microalgae for bulk biomass production. However, residual flocculants can interfere in further downstream processes or influence biomass quality. In this study, a new concept is demonstrated based on reversible magnesium hydroxide flocculation, using Chlorella vulgaris and Phaeodactylum tricornutum as respectively a freshwater and a marine model species. We show that flocculation was induced by precipitation of magnesium hydroxide at high pH (10 to 10.5). This resulted in a magnesium content of the microalgal biomass of 5% for Chlorella and 18% for Phaeodactylum. After pre-concentration of the microalgal biomass by gravity sedimentation, 95% of the precipitated magnesium hydroxide could be removed from the biomass by mild acidification (pH 7 to 8). The pH fluctuations experienced by the microalgae during flocculation/de-flocculation had no influence on biomass composition (FAME, total N and P, carbohydrates, proteins, mineral content) and on the viability of microalgal cells. Magnesium can thus be used as pH-dependent reversible flocculant for harvesting microalgae in both marine and freshwater medium. ispartof: BioEnergy Research vol:8 issue:2 pages:716-725 status: published

Microalgal biomass seems to be a promising feedstock for biofuel generation. Microalgae have relative high photosynthetic efficiencies, high growth rates, and some species can thrive in brackish water or seawater and wastewater from the food- and agro-industrial sector. Today, the main interest in research is the cultivation of microalgae for lipids production to generate biodiesel. However, there are several other biological or thermochemical conversion technologies, in which microalgal biomass could be used as substrate. However, the high protein content or the low carbohydrate content of the majority of the microalgal species might be a constraint for their possible use in these technologies. Moreover, in the majority of biomass conversion technologies, carbohydrates are the main substrate for production of biofuels. Nevertheless, microalgae biomass composition could be manipulated by several cultivation techniques, such as nutrient starvation or other stressed environmental conditions, which cause the microalgae to accumulate carbohydrates. This paper attempts to give a general overview of techniques that can be used for increasing the microalgal biomass carbohydrate content. In addition, biomass conversion technologies, related to the conversion of carbohydrates into biofuels are discussed.

This paper presents the effects of various phosphorus concentrations (10, 50, 250 and 500 mg l(-1) K(2)HPO(4)) on the biomass production and composition of Arthrospira (Spirulina) platensis in relation to light intensity (24, 42 and 60 μE m(-2) s(-1)). The maximum biomass production was 3,592 ± 392 mg l(-1) and this was observed in 250 mg l(-1) K(2)HPO(4) at 60 μE m(-2) s(-1) light intensity after 32 days of cultivation. A maximum specific growth rate (μ(max)) of 0.55 d(-1) was obtained in 500 mg l(-1) K(2)HPO(4) at 60 μE m(-2) s(-1). The protein, lipid and chlorophyll contents of the biomass varied from 33.59 to 60.57 %, 5.34 to 13.33 % and 0.78 to 2.00 %, respectively. The most significant finding was that phosphorus limitation (10 mg l(-1) K(2)HPO(4)) caused a drastic increase of the carbohydrate content (59.64 %). The effect of phosphorus limitation on the carbohydrate content was independent of the light intensity. The accumulated carbohydrates are proposed to be used as substrate for biofuel generation via one of the appropriate biomass energy conversion technologies. Also, it was observed that phosphorus removal is a function of biomass density, phosphorus concentration and light intensity.

Carbohydrates are a desirable biomass compound for the generation of several biofuels. Phosphorus nutrient limitation causes a significant increase in the carbohydrate content of the cyanobacterium Arthrospira (Spirulina) platensis. Carbohydrates accumulated up to a content of 63.09 ± 3.43 % (±SD) in both batch and semi-continuous cultures. In order the production of carbohydrate-rich biomass through nutrient limitation to be maximized, it is suggested that the limited nutrients have to be supplied in amounts that they on one hand can support the biomass production while on the other hand they alter the composition of the biomass. In this study, phosphorus of 1.82 ± 0.16 mg g−1 of dry biomass was the optimized amount for the maximization of carbohydrates production by A. platensis. Regarding the need to decrease the application amounts of nutrients for biomass production, this study demonstrates that the phosphorus supply could be decreased an order of magnitude with no significant decrease in biomass production. In addition, it was observed that biomass rich in carbohydrates bio-flocculates, during settling without the addition of any flocculation agent or any other intervention. The bio-flocculation efficiency appears to be related with the carbohydrate content of the biomass. In maximum carbohydrate content (60 %), the biomass bio-flocculated at 68.49 ± 7.73 % the first 15 min and reached 80.25 ± 5.58 % 60 min after settling. The produced carbohydrates might be used as feedstock for biofuel generation, while the bio-flocculation and the overall settling characteristics of the carbohydrate-rich biomass could make its harvesting process much easier.