Horizontal Tubular Photobioreactor Research Articles

Spatio-temporal techno-economic assessment of the algae-based supply chain: A proof-of-concept for North-West Europe

The growing world population is in need for new sustainable resources for healthy food, animal feed and chemicals. Algae produce a variety of compounds via conversion of CO2, nutrients, and light, and offer potential as renewable feedstock. However, producing microalgae is costly. Therefore, new economically viable supply chains must be developed. A variety of techno-economic assessments (TEA) has been performed to gain insight in the economic feasibility of algae production systems and refineries. While a TEA provides insights relevant to the technical and economic feasibility of a process, geographical conditions must be considered since the regional characteristics influence process feasibility and supply chain design.This paper presents a spatio-temporal TEA by combining a dynamic TEA model and a supply chain design optimisation model, MooV. The dynamic TEA outlines the conditions of the processes to reach a techno-economic feasible case as well as mass balances and OPEX and CAPEX of the different potential processes in the chain. MooV combines these parameters with geographic specific parameters such as the locations of activities and the characteristics of the transport network to define the optimal supply chain configuration (e.g., location and process type and capacity) based on minimal cost.The approach is demonstrated by year-round cultivation of Nannochloropsis gaditana in a horizontal tubular photobioreactor in 3 regions in North-West Europe. The analysis indicates that the cost per kg DM algae output can vary considerably (±40 %) depending on the region and the set-up of the supply chain. Including geographic characteristics as well as comparing different supply chain design options are essential to come to a viable business case. The approach can also be applied to different algae strains, different regions and different process options.

A novel stepwise dilution strategy to improve astaxanthin production of Haematococcus pluvialis in outdoor horizontal tubular photobioreactors

The volume average light intensity in the photobioreactor (PBR) used for astaxanthin accumulation of Haematococcus pluvialis gradually decreased due to light attenuation caused by the accumulated biomass and astaxanthin. Excessive light intensity in the early stage of photoinduction resulted in photoinhibition while insufficient light intensity in the late stage limited the astaxanthin accumulation. In this study, the effect of dry cell weight on astaxanthin production was first investigated and the astaxanthin accumulation was improved by dilution. Based on this, a stepwise dilution strategy in the horizontal tubular photobioreactor (HTPBR) was developed to promote astaxanthin production and afterward verified in outdoor 16.5 m3 HTPBRs with 5-row parallel tubes. It was proved that twice diluting with an initial dry cell weight of 1 g L−1 in 3-row parallel tubes was the optima. By the stepwise dilution strategy, the astaxanthin content in algal cells and the astaxanthin productivity per dry heterotrophic cell weight increased from 2.56 % to 3.40 % and from 41.0 mg g−1 to 55.0 mg g−1, respectively. The gross profit was improved by 51.32 % by the easily operated stepwise dilution strategy.

Sustainable production of biofuels from the algae-derived biomass

The worldwide fossil fuel reserves are rapidly and continually being depleted as a result of the rapid increase in global population and rising energy sector needs. Fossil fuels should not be used carelessly since they produce greenhouse gases, air pollution, and global warming, which leads to ecological imbalance and health risks. The study aims to discuss the alternative renewable energy source that is necessary to meet the needs of the global energy industry in the future. Both microalgae and macroalgae have great potential for several industrial applications. Algae-based biofuels can surmount the inadequacies presented by conventional fuels, thereby reducing the ‘food versus fuel’ debate. Cultivation of algae can be performed in all three systems; closed, open, and hybrid frameworks from which algal biomass is harvested, treated and converted into the desired biofuels. Among these, closed photobioreactors are considered the most efficient system for the cultivation of algae. Different types of closed systems can be employed for the cultivation of algae such as stirred tank photobioreactor, flat panel photobioreactor, vertical column photobioreactor, bubble column photobioreactor, and horizontal tubular photobioreactor. The type of cultivation system along with various factors, such as light, temperature, nutrients, carbon dioxide, and pH affect the yield of algal biomass and hence the biofuel production. Algae-based biofuels present numerous benefits in terms of economic growth. Developing a biofuel industry based on algal cultivation can provide us with a lot of socio-economic advantages contributing to a publicly maintainable result. This article outlines the third-generation biofuels, how they are cultivated in different systems, different influencing factors, and the technologies for the conversion of biomass. The benefits provided by these new generation biofuels are also discussed. The development of algae-based biofuel would not only change environmental pollution control but also benefit producers' economic and social advancement.Graphical abstract

The optimization of centrifugal pump driving horizontal tubular photobioreactor for enhancing astaxanthin production using heterotrophic Haematococcus pluvialis

Haematococcus pluvialis is the prime source of natural astaxanthin for commercial exploitation. The large-scale cultivation of H. pluvialis is one of the key technologies for the development of natural astaxanthin industry. So far, horizontal tubular photobioreactor (HTPBR) circulated by a centrifugal pump has been the main PBR for the large-scale cultivation of H. pluvialis. Shear stress is a negative factor in microalgal cultivation at different scales, particularly for large-scale cultivation. To reduce the adverse impact of shear stress, the tolerance of H. pluvialis to the shear stress during the induction stage was first investigated in this study. H. pluvialis aplanospore was not sensitive to stresses between 19.18 and 27.32 Pa, but was resulted in about 30% cell death under shear stress between 27.32 and 63.84 Pa. Accordingly, two centrifugal pumps with different impellers was selected in 400 L HTPBRs to study the outdoor photoinduction for astaxanthin accumulation. The highest astaxanthin productivity and astaxanthin concentration were obtained in HTPBRs using a centrifugal pump equipped with three unshrouded backward-bladed impellers. The HTPBR was then successfully scaled up to 800 L with a similar performance, showing good scalability.

Bioremediation of emerging micropollutants in irrigation water. The alternative of microalgae-based treatments

The present study evaluated the efficiency of a semi-closed horizontal tubular photobioreactor (PBR) at demonstrative scale to remove a total of 35 target compounds, including benzotriazoles, benzophenones, antibiotics and different pharmaceuticals present in irrigation water in a peri-urban rural area. This water run through an open channel and was a mixture of reclaimed wastewater from a nearby wastewater treatment plant (WWTP) and run-off from the different agricultural fields in the area. Most of the compounds studied are usually not fully eliminated during conventional wastewater treatment, which justifies the need to investigate alternative treatment strategies. A total of 21 of these compounds were detected in the irrigation water. Benzotriazoles were only partially removed after the microalgae treatment, with elimination rates similar to those of conventional WWTPs. The UV filter benzophenone-3 (BP3) showed variable removals, ranging from no elimination to 51%, whereas 4-methylbenzilidenecamphor (4MBC) was completely eliminated. Regarding pharmaceuticals, average removals were higher, in the range of 60–100%, with the exception of the antibiotics sulfamethoxazole (46%) and sulfapyridine, which was not removed. Despite the low biomass productivity of the PBR, parameters such as the size of the reactors, the specific mixed cultures developed and the high temperatures and pH in the closed system may account for the overall good results, The efficiency and sustainability of these systems make them a solid, feasible treatment choice.

Strengthening CO2 dissolution with zeolitic imidazolate framework-8 nanoparticles to improve microalgal growth in a horizontal tubular photobioreactor

Low dissolution of CO2 in closed tubular photobioreactors results in insufficient carbon source supply, and often limits microalgae growth. An optimized ZIF8-SE medium containing zeolitic imidazolate framework-8 (ZIF-8) nanoparticles was developed for strengthening CO2 mass transfer in the photobioreactor, which improved CO2 dissolution to increase microalgal biomass yield under 15% CO2. The CO2 mass transfer coefficient markedly increased from 1.82 to 3.85, leading to an increased dissolved inorganic carbon concentration. Meanwhile, microalgae biomass accumulation with 0.01 mmol/L ZIF-8 nanoparticles was 25.5% higher than that without ZIF-8 nanoparticles. Furthermore, the average cell size decreased from 2.61 to 2.38 μm and the cell fractal dimension increased from 1.38 to 1.56 when ZIF-8 nanoparticle concentration increased from 0 to 0.2 mmol/L, indicating that potential toxicity might occurred with excess ZIF-8 nanoparticles. Reutilization of ZIF-8 nanoparticles in the recycled culture medium improved microalgal growth by 23%, which was similar to the promotion of fresh ZIF8-SE medium. This new finding provides a new strategy to improve CO2 mass transfer in the horizontal tubular photobioreactor, which makes it a feasible platform for CO2 capture and utilization.

Influence of liquid-to-biogas ratio and alkalinity on the biogas upgrading performance in a demo scale algal-bacterial photobioreactor

The influence of the liquid-to-biogas ratio (L/G) and alkalinity on methane quality was evaluated in a 11.7 m3 outdoors horizontal semi-closed tubular photobioreactor interconnected to a 45-L absorption column (AC). CO2 concentrations in the upgraded methane ranged from <0.1 to 9.6% at L/G of 2.0 and 0.5, respectively, with maximum CH4 concentrations of 89.7% at a L/G of 1.0. Moreover, an enhanced CO2 removal (mediating a decrease in CO2 concentration from 9.6 to 1.2%) and therefore higher CH4 contents (increasing from 88.0 to 93.2%) were observed when increasing the alkalinity of the AC cultivation broth from 42 ± 1 mg L−1 to 996 ± 42 mg L−1. H2S was completely removed regardless of the L/G or the alkalinity in AC. The continuous operation of the photobioreactor with optimized operating parameters resulted in contents of CO2 (<0.1%–1.4%), H2S (<0.7 mg m−3) and CH4 (94.1%–98.8%) complying with international regulations for methane injection into natural gas grids.

Strengthening mass transfer of carbon dioxide microbubbles dissolver in a horizontal tubular photo-bioreactor for improving microalgae growth

A CO2 microbubbles dissolver (CMD) was developed to facilitate dissolving inorganic carbon and strengthening mass transfer in a horizontal tubular photo-bioreactor system (HTPBRS), which enhanced microalgae biomass productivity with flue gas containing 15% CO2. The influence of pump power on the bubble formation and mixing effect was found to be more obvious than that of gas flow rate. Ceramic shell aerator was more favorable for reducing bubble diameter and enhancing mass transfer than traditional rubber strip aerator. Bubble formation time decreased by 53.4% and mixing time decreased by 68.9% in response to the increased pump power. When the base area ratio of ceramic shell aerator to dissolver in the HTPBRS increased, bubble formation time decreased by 19.6% and mass transfer coefficient increased by 80.9%. The biomass yield of microalgae Chlorella PY-ZU1 with ceramic shell aerator was 30% higher than that with rubber strip aerator in the HTPBRS.

Nutrient removal from agricultural run-off in demonstrative full scale tubular photobioreactors for microalgae growth

The objective of this paper is to present the design, construction and operation (during one year) of 3 full scale semi-closed, horizontal tubular photobioreactors (PBR, 11.7 m3 of volume each) used to remove nutrients of a mixture of agricultural run-off (90%) and treated domestic wastewater (10%). PBRs were located outdoor and have 2 paddlewheels (engines of 0.25 kW) to ensure the movement of the mixed liquor. The microalgal biomass produced in the PBRs was harvested in a static lamella settling tank in which a polyaluminium chloride coagulant is applied. Each PBR treated in average 2.3 m3/d, being the actual mean hydraulic retention time 5 d. PBRs were submitted to strong seasonal changes regarding solar radiation and temperature, which had a direct impact in the activity of microalgae and the efficiency of the system. Higher mixed liquor pH values were registered in summer (daily average > 10). These high values were not observed in the effluents because the system was designed to discharge the mixed liquor (effluent) only at the end of night, when pH reached the lowest daily values (around 8.5). Most of the influent and effluent nitrogen content was inorganic (average of 9.0 mg N/L and 3.17 mg N/L, respectively), and in the form of nitrate (62% and 50%, respectively). Average nitrogen removal efficiency was 65%, with values of around 90% in summer, 80% in autumn, 50% in winter and 60% in spring. Most of the influent and effluent phosphorus content was in the form of orthophosphate. Influent average was 0.62 mg P/L, but with great variations and in a considerable number of samples not detected. Removal efficiency (when influent values were detected) was very high during all the study, usually greater than 95%, and there were not clear seasonal trends for efficiency as observed for TIN. Volumetric biomass production greatly changed between seasons with much lower values in winter (7 g VSS (volatile suspended solids)/m3·d) than in summer (43 g VSS/m3·d). Biomass separation efficiency of the settler was very good in either terms of turbidity and total suspended solids, being most of the time lower than 5 UNT and 25 mg/L, respectively. Overall this study demonstrated the reliable and good effectiveness of microalgae based technologies such as the PBR to remove nutrients at a full scale size.

Nutrient removal from agricultural run-off in demonstrative full scale tubular photobioreactors for microalgae growth

The objective of this paper is to present the design, construction and operation of 3 full scale semi-closed, horizontal tubular photobioreactors (PBR) used to remove nutrients of a mixture of agricultural run-off (90%) and treated domestic wastewater (10%). The microalgal biomass produced in the PBRs was harvested in a static lamella settling tank. Each PBR treated in average 2.3 m3/d. PBRs were submitted to strong seasonal changes regarding solar radiation and temperature, which had a direct impact in the activity of microalgae and the efficiency of the system. Higher mixed liquor pH values were registered in summer (daily average> 10). Most of the influent and effluent nitrogen content was inorganic (average of 9.0 mg N/L and 3.17 mg N/L, respectively), and in the form of nitrate (62% and 50%, respectively). Average nitrogen removal efficiency was 65%, with values of around 90% in summer, 80% in autumn, 50 % in winter and 60% in spring. Most of the influent and effluent phosphorus content was in the form of ortophosphate. Influent average was 0.62 mg P/L, but with great variations and in a considerable number of samples not detected. Removal efficiency (when influent values were detected) was very high during all the study, usually greater than 95%, and there were not clear seasonal trends for efficiency as observed for TIN. Volumetric biomass production greatly changed between seasons with much lower values in winter (7 g VSS/m3d) than in summer (43 g VSS/m3d). Biomass separation efficiency of the settler was very good in either terms of turbidity and total suspended solids, being most of the time lower than 5 UNT and 15 mg/L, respectively. Overall this study demonstrated the reliable and good effectiveness of microalgae based technologies such as the PBR to remove nutrients at a full scale size.

Use of full-scale hybrid horizontal tubular photobioreactors to process agricultural runoff

Diffuse pollution in rural areas due to agricultural run-off is a widespread and difficult problem to address due to the vast areas affected. Drainage channels do receive these polluted waters, but its introduction in the conventional treatment network is unfeasible. Within this context, microalgae-based treatment systems could be used as alternative treatment plants. In this study, a new design of semi-closed (hybrid) tubular horizontal photobioreactor (HTH-PBR) with low energy requirements has been evaluated for microalgae cultivation at full-scale (8.5 m3), using agricultural runoff as feedstock. This novel system was tested in batch and continuous mode during a total of 4 and 135 days. Considering a full-scale application in an agricultural context, a batch test was carried out to evaluate the performance of the system. An increase of 22% in the biomass concentration in 4 days was registered, and all nutrients were consumed during the first two days. In the continuous experiment (December- April), a productivity between 2-14 g TSS/m3d was reached in winter, whereas values up to 76.4 g TSS/m3d were reached at the end of the study in spring, despite the low nutrients concentration in the feedstock. The elimination of emerging contaminants was also evaluated, obtaining the highest removals for the fragrances tonalide and galaxolide (73% and 68%), and the antiinflamatory diclofenac (61%).

Use of full-scale hybrid horizontal tubular photobioreactors to process agricultural runoff

Diffuse pollution in rural areas due to agricultural runoff is a widespread and difficult problem to address due to the large areas affected. Drainage channels receive polluted water, but its introduction into conventional treatment network is often unfeasible. Within this context, microalgae-based treatment systems could be used as alternative treatment plants. A new design of semi-closed (hybrid) tubular horizontal photobioreactor (HTH-PBR) with low energy requirements has been evaluated for microalgae cultivation at full-scale (8.5 m3), using agricultural runoff as feedstock. This novel system was tested in batch and continuous mode over 4 and 135 d. Considering a full-scale application in an agricultural context, a batch test was carried out to evaluate the performance of the system. An increase of 22% in the biomass concentration in 4 d was registered, and all nutrients were consumed during the first two days. In the continuous experiment carried out over winter (December–April), productivity was between 2 and 14 g g [TSS] m−3 d, but values up to 76.4 g [TSS] m−3 d were reached at the end of the study in spring, despite the low nutrients concentration in the feedstock. Elimination of emerging contaminants was also evaluated, obtaining the highest removals for the synthetic musk fragrances tonalide and galaxolide (73% and 68%), and the anti-inflammatory drug diclofenac (61%).

US: Techmer – plastic compounds

The objective of this paper is to present the design, construction and operation (during one year) of 3 full scale semi-closed, horizontal tubular photobioreactors (PBR, 11.7 m3 of volume each) used to remove nutrients of a mixture of agricultural run-off (90%) and treated domestic wastewater (10%). PBRs were located outdoor and have 2 paddlewheels (engines of 0.25 kW) to ensure the movement of the mixed liquor. The microalgal biomass produced in the PBRs was harvested in a static lamella settling tank in which a polyaluminium chloride coagulant is applied. Each PBR treated in average 2.3 m3/d, being the actual mean hydraulic retention time 5 d. PBRs were submitted to strong seasonal changes regarding solar radiation and temperature, which had a direct impact in the activity of microalgae and the efficiency of the system. Higher mixed liquor pH values were registered in summer (daily average > 10). These high values were not observed in the effluents because the system was designed to discharge the mixed liquor (effluent) only at the end of night, when pH reached the lowest daily values (around 8.5). Most of the influent and effluent nitrogen content was inorganic (average of 9.0 mg N/L and 3.17 mg N/L, respectively), and in the form of nitrate (62% and 50%, respectively). Average nitrogen removal efficiency was 65%, with values of around 90% in summer, 80% in autumn, 50% in winter and 60% in spring. Most of the influent and effluent phosphorus content was in the form of orthophosphate. Influent average was 0.62 mg P/L, but with great variations and in a considerable number of samples not detected. Removal efficiency (when influent values were detected) was very high during all the study, usually greater than 95%, and there were not clear seasonal trends for efficiency as observed for TIN. Volumetric biomass production greatly changed between seasons with much lower values in winter (7 g VSS (volatile suspended solids)/m3·d) than in summer (43 g VSS/m3·d). Biomass separation efficiency of the settler was very good in either terms of turbidity and total suspended solids, being most of the time lower than 5 UNT and 25 mg/L, respectively. Overall this study demonstrated the reliable and good effectiveness of microalgae based technologies such as the PBR to remove nutrients at a full scale size.

Turbidostat operation of outdoor pilot-scale photobioreactors

The effect of biomass concentration on areal productivity and photosynthetic efficiency of Nannochloropsis sp. CCAP211/78 was studied in three outdoor pilot-scale photobioreactors: an open raceway pond (OPR), a horizontal tubular (HT) photobioreactor and a vertically stacked horizontal tubular (VT) photobioreactor. The reactors were operated continuously as turbidostat at different biomass concentrations. For all systems highest areal productivities were obtained on days with a high light intensity, while the highest photosynthetic efficiencies were obtained on days with a low light intensity. Ground areal biomass concentration exceeding 51gm−2 had a negative effect on the areal productivity and photosynthetic efficiency. No significant effect of biomass concentration on the productivity was found for the HT at ground areal biomass concentration lower than 51gm−2. Also for the VT, no significant effect of biomass concentration was found with the exception of the highest biomass concentration of 2.0gL−1 (68gm−2) resulting in decreased productivity. For the open raceway pond the highest biomass concentration (0.5gL−1 or 94gm−2) resulted in significantly lower areal productivity, compared to the lower biomass concentration (0.25gL 47gm−2). Highest areal productivities were obtained for OPR and VT, most likely due to more efficient light interception. In this study we observed that night biomass loss was coupled to net growth. At lower biomass concentrations and concomitant higher growth rates the specific biomass loss rate was higher. Microalgal specific light absorption coefficient was correlated to biomass concentration; higher biomass concentrations resulted in higher specific absorption coefficients, resulting in a steeper light gradient in the microalgal cultures.

CO2 mass transfer and conversion to biomass in a horizontal gas–liquid photobioreactor

This study deals with CO2 mass transfers and biomass conversion in an industrial horizontal tubular photobioreactor. An analytical approach is used to determine an expression modeling the influence of CO2 mass transfers on the overall biomass conversion efficiency for a given culture broth, heat and light conditions. Fluid mechanics and mass transfer are predicted with a classical two-phase flow approach (Taitel and Dukler, 1976) combined with a dissolution correlation developed and tested in the laboratory (Valiorgue et al., 2011). The influence of the stripping gas, removing the excess of oxygen in the liquid, on the conversion to biomass efficiency is shown to be not negligible. The expression is used to evaluate how the photobioreactor's design and process parameters can be tuned in order to improve biomass conversion efficiency. The biomass conversion efficiency evolution with the photobioreactor's length was found to behave asymptotically and it was explained by the relative orders of magnitude of gas dissolution and gas stripping. It has been shown that the gas flow rate for stripping and therefore the oxygen removal will be limited when further increasing the industrial photobioreactor's length for a given objective of CO2 conversion to biomass efficiency.

Multi-criterion Evaluation of Cultivation Systems for Sustainable Algal Biofuel Production Using Analytic Hierarchy Process and Monte Carlo Simulation

The potential benefits from algal biofuels, such as reduced greenhouse gas emissions, come with natural resource consumption and potentially negative environmental and social impact. A multi-criterion approach based on analytic hierarchy process (AHP) and Monte Carlo simulation is proposed to holistically evaluate the suitable cultivation system for sustainable production of algal biofuels. The multiple criteria identified to evaluate the alternatives are: environmental impact, energy consumption, economic viability, technical considerations, and social acceptability. Sub-criteria are defined from these main criteria to further enhance the evaluation of each cultivation system. A case study using Nannochloropsis sp. is then solved to demonstrate the decision model for the following cultivation systems: open pond, flat-panel photobioreactor, and horizontal tubular photobioreactor. The results show that flat panel photobioreactor is the preferred cultivation system to sustainably produce algal biofuels based on the established multi-criterion evaluation using AHP.

Comparison of algae cultivation methods for bioenergy production using a combined life cycle assessment and life cycle costing approach

Algae are an attractive energy source, but important questions still exist about the sustainability of this technology on a large scale. Two particularly important questions concern the method of cultivation and the type of algae to be used. This present study combines elements of life cycle analysis (LCA) and life cycle costing (LCC) to evaluate open pond (OP) systems and horizontal tubular photobioreactors (PBRs) for the cultivation of freshwater (FW) or brackish-to-saline water (BSW) algae. Based on the LCA, OPs have lower energy consumption and greenhouse gas emissions than PBRs; e.g., 32% less energy use for construction and operation. According to the LCC, all four systems are currently financially unattractive investments, though OPs are less so than PBRs. BSW species deliver better energy and GHG performance and higher profitability than FW species in both OPs and PBRs. Sensitivity analyses suggest that improvements in critical cultivation parameters (e.g., CO2 utilization efficiency or algae lipid content), conversion parameters (e.g., anaerobic digestion efficiency), and market factors (e.g., costs of CO2 and electricity, or sale prices for algae biodiesel) could alter these results.

Investigation of the Effect of Growth From Low to High Biomass Concentration Inside a Photobioreactor on Hydrodynamic Properties of Scenedesmus obliquus

Most of the current production cost in algae biodiesel plants utilizing photobioreactors comes from the high energy required for pumping, CO2 transfer, mixing, and harvesting. Since pumping affects the mixing and CO2 transfer, which are the main factors in algae productivities, solutions to reduce the required energy for pumps can significantly make algae biodiesel production more economically feasible. An investigation on the effect of Scenedesmus obliquus’s growth from low to high biomass concentration inside a horizontal tubular photobioreactor to determine the impact that it has on hydrodynamic performances, which will affect cost and production efficiency, was performed. As the biomass concentration increased, the algal culture was found to remain Newtonian. Additionally, the biomass concentration (expressed in cell density) was found to have lower viscosity even at the highest concentrations evaluated at 2.48 × 108 cell/ml (1.372 × 10−3 ± 1.32 × 10−4 Pa s) compared to the Modified Bold’s 3N medium (1.408 × 10−3 ± 9.41 × 10−5 Pa s). Furthermore, the total energy consumption does not appear to depend on the S. obliquus biomass concentrations, but rather on the medium the algae grows in. The rheological properties of autotrophic algae will not have significant impact on energy requirements until technology improves so that the concentrations reach those of heterotrophic algae.

Outdoor H2 production in a 50-L tubular photobioreactor by means of a sulfur-deprived culture of the microalga Chlamydomonas reinhardtii

In the past decade, H₂ production using the green microalga Chlamydomonas reinhardtii has been extensively studied under laboratory-scale photobioreactors, while information on outdoor cultures is still lacking. In this paper, the results of experiments conducted with sulfur-deprived cultures of C. reinhardtii carried out in a 50-L horizontal tubular photobioreactor are presented. Hydrogen production experiments were carried out under both artificial and direct solar light. In both cases, the H₂ output attained was 18-20% of what obtained in the laboratory. However, no significant changes in the H₂ production were observed when cells grown outdoors were tested under laboratory conditions. Chlorophyll fluorescence measurements showed that outdoor cultures were subjected to strong photo-inhibition, due to the combination of high solar light intensity and sulfur-deprivation. Indeed, H₂ production was only achieved outdoors when cultures were previously acclimated to sunlight, a condition that caused a number of physiological changes, namely: (i) a decrease in the chlorophyll content per unit of dry weight; (ii) an increase in the photosynthesis and respiration rates, and (iii) a higher induction of the xanthophyll cycle pigments as compared to non-acclimated cultures. It was concluded that the reduced H₂ output achieved in the 50-L photobioreactor was due to the different illumination pattern to which the cultures were exposed (one-sided vs. two-sided illumination provided in the laboratory), as well as to the great difference in the mixing times (60 min vs. 15.5s achieved in the lab-scale photobioreactor). To the very best of our knowledge this is the first time that H₂ production with green algae has been achieved by means of solar light.

Low and variable productivity and low efficiency of mass cultures of the haptophyte Isochrysis aff. galbana (T-iso) in outdoor tubular photobioreactors

The haptophyte Isochrysis aff. galbana (T-iso) was cultured during 2 consecutive years (from spring to autumn) in outdoor horizontal tubular photobioreactors (PBRs) that had a size compatible with that recommended for hatchery service. Around one-third of the cultures carried out in these PBRs showed immediate growth after inoculation up to a maximum biomass. In all other instances a lag phase or a culture collapse were found. In the cultures that showed productivity, average growth rate, μ (d −1), productivity (g L −1 d −1) and photosynthetic efficiency, PE (%), were 0.39 ± 0.085 d −1, 0.075 ± 0.038 g L −1 d −1 and 2.51 ± 0.85%, respectively. These values were low compared to values for cultures routinely incubated indoors, which were used as inocula for the outdoor cultures, and were studied as a reference. In these 50-L acrylic columns (19.4 cm internal diameter) growth, productivity and PE were 0.91 ± 0.087 d −1, 0.076 ± 0.012 g L −1 d −1 and 13.72 ± 2.23%, respectively. Outdoor productivity of T-iso was limited to average temperatures above 15 °C, while below an average temperature of 21 °C growth rate and productivity were lower than above 21 °C. Above this temperature, solar irradiance and biomass density had a more important effect on growth rate and productivity than temperature. Analysis of our data revealed a critical value of the ratio between incident solar irradiance (MJ m −2 d −1) and cell density (10 6 cells ml −1) at the moment when the protective black netting was removed from the cultures. Cultures that had a ratio above 1.5 suffered an initial lag phase or collapsed, while cultures that showed direct growth had a value below 1.5. This seemed to be independent of absolute initial cell density. However, we recommend using cell densities higher than the ca. 5 × 10 6 cells ml −1 used in the present study. In the South of Spain this seems to be especially relevant in spring, when temperatures are relatively low in relation to incident solar irradiance. T-iso seems to be particularly sensitive to the variable conditions outdoors and we suggest that new designs of closed PBRs, specifically developed for outdoor cultivation of T-iso, try to achieve more stable and homogeneous conditions throughout the system and over the daily cycle.