Horizontal Photobioreactor Research Articles

Phytoplankton as CO2 Sinks: Redirecting the Carbon Cycle

Since the Industrial Revolution, nearly 700 Gt of carbon (GtC) have been emitted into the atmosphere as CO2 derived from human activities, of which 292 GtC remain uncontrolled. By the end of this century, the atmospheric CO2 concentration is predicted to surpass 700 ppm. The effects of this sudden carbon release on the worldwide biogeochemical cycles and balances are not yet fully understood, but global warming and climate change are undeniable, with this gas playing a starring role. Governmental policies and international agreements on emission reduction are not producing results quickly enough, and the deadline to act is running out. Biological CO2 capture is a fast-acting carbon cycle component capable of sequestering over 115 GtC annually through photosynthesis. This study analyses a hypothetical scenario in which this biological CO2 capture is artificially enhanced through the large-scale cultivation of phytoplankton in partially natural photobioreactors (PBRs). To develop this approach, the current figures of the carbon cycle have been updated, and the key aspects of phytoplankton cultivation technology have been analysed. Our results show that a global increase of 6.5% in biological capture, along with the subsequent stabilization of the produced biomass, could counteract the current CO2 emission rate and maintain atmospheric levels of this gas at their current levels. Based on a review of the available literature, an average production rate of 17 g/m2·day has been proposed for phytoplankton cultivation in horizontal PBRs. Using this value as a key reference, it is estimated that implementing a large-scale production system would require approximately 2.1 × 106 km2 of the Earth’s surface. From this, a production system model is proposed, and the key technological and political challenges associated with establishing these extensive cultivation areas are discussed.

Microalgae production in an industrial-scale photobioreactors plant: A comprehensive Life Cycle assessment

Microalgae cultivation provides multiple opportunities to produce valuable bioproducts, but greater clarity must be achieved regarding the real sustainability of current technologies. Numerous life cycle assessment (LCA) studies have been conducted so far. However, most of them were based on literature data and/or extrapolations of lab-scale results, while only a few studies used primary data from pilot or full-scale microalgal plants. Moreover, the obtained results showed great variability, leaving the debate on microalgae sustainability fully open.This work presents a thorough LCA based on primary data from an industrial-scale microalgal facility located in Caltagirone, Italy. The plant is based on vertically-stacked horizontal photobioreactors (total volume of 40.4 m3) installed in a greenhouse and has a capacity of 1200 kgDW/y (Chlorella vulgaris). A cradle-to-gate assessment was performed with the functional unit of 1 kgDW biomass, including operational and infrastructural data. The results emphasized the key role in the generation of potential impacts played by cultivation among process stages and by chemicals (nutrients and cleaning agents) and electricity (mainly for agitation and thermoregulation) among flow types. In comparison with studies from the literature, the analysed microalgal plant has an intermediate environmental performance (e.g., global warming potential of 153 kg CO2,eq/kgDW). This result is encouraging, as it comes from a reliable assessment built on full-scale primary data. On the other hand, it highlights the need to explore alternative strategies (e.g., industrial symbiosis and circular bioeconomy) to reduce the environmental footprint of the process and enhance its economic attractiveness.

A novel V-shaped photobioreactor design for microalgae cultivation at low latitudes: Modelling biomass productivities of Chlorella sorokiniana on Bonaire

• A novel V-shaped photobioreactor is proposed for light capture and dilution. • Light dilution enables sunlight to be used more efficiently for biomass production. • Biomass productivities can be improved significantly in V-shaped photobioreactors. • Theoretical biomass productivities of 38.3–50.5 g m −2 day −1 were estimated on Bonaire. Microalgae are a promising renewable feedstock for a wide range of biobased products, such as food, feed, chemicals, and biofuels. To commercialize bulk products from microalgae, the production costs need to be reduced, for example, by improving biomass productivities in outdoor photobioreactors. Geographical locations near the equator are considered ideal for outdoor cultivation, due to the abundance of sunlight throughout the entire year. However, at high light intensities the photosystems of microalgae become oversaturated, which limits photosynthetic efficiencies and biomass productivities. Therefore, we propose a novel V-shaped photobioreactor to capture and dilute available sunlight at low latitudes. For different V-shaped designs, we modelled the sunlight entering the photobioreactor during several days of the year and theoretically estimated the maximal biomass productivity of Chlorella sorokiniana on the island Bonaire (12°N, 68°W) assuming clear-sky conditions and light-limited growth. Our results show that theoretical biomass productivities of 38.3–50.5 g m −2 day −1 can be achieved in V-shaped photobioreactors, corresponding to photosynthetic efficiencies of 2.5–3.3%. These productivities are up to 1.4 times higher than those estimated for a flat horizontal photobioreactor, primarily due to improved light dilution in V-shaped photobioreactors. Thus, V-shaped photobioreactors present opportunities for more efficient microalgae production.

Cultivation of Chlorella sorokiniana IPPAS C-1 in Flat-Panel Photobioreactors: From a Laboratory to a Pilot Scale.

Flat-panel photobioreactors are effective systems for microalgae cultivation. This paper presents the growth characteristics of the microalgae Chlorella sorokiniana IPPAS C-1 as a result of three-stage scale-up cultivation in a specially designed cultivation system. First, C. sorokiniana was grown aseptically in 250 mL glass vessels; then, it was diluted and inoculated into a 5-liter flat-panel horizontal photobioreactor; and, at the last stage, the culture was diluted and inoculated into a 70-liter flat-panel vertical photobioreactor. In the presented cycle, the cultured biomass increased by 326 times in 13 days (from 0.6 to 195.6 g dw), with a final biomass concentration of 2.8 g dw L−1. The modes of semi-continuous cultivation were considered. The biomass harvest and dilution of the suspension were carried out either every day or every 3–4 days. For C. sorokiniana IPPAS C-1, a conversion coefficient of optical density values to dry biomass (g L−1) was refined through a factor of 0.33. The key parameters of the photobioreactors tested in this work are discussed.

Use of the cyanobacterium Spirulina platensis in cattle wastewater bioremediation

In the present study, the microalga Spirulina platensis (Arthrospira) was grown in two horizontal photobioreactors (HPBR) under two different irradiances (150 and 270 μmol m-2 s-2). Anaerobically digested cattle wastewater (ACWW) was used as substrate. The experiment was carried out in batches for a period of 8 days. The maximum specific growth rate of 0.347 day-1 and the doubling time of 2.08 days were obtained under the highest illumination of the culture. Dry biomass production reached maximum values between 2.17 g L-1 and 6.52 g L-1, with volumetric biomass productivities between 0.0812 and 0.5578 g L-1 day-1. Productivity per area was equal to 47.97 g m-2 d-1, which is the highest value recorded compared to those found in the literature consulted. As for CO2 biofixation, relevant values for reducing this gas in the atmosphere were obtained, ranging from 128.52 to 882.36 mg L-1 day-1. In terms of organic matter, 16.3-77% of BOD5 and 12.6-61.6% of COD were reduced. In the reduction of TS, TSS and VSS, values of 71.3-78.5%, 79.5-84.4% and 87.0-88.3%, respectively, were reached. NH4+ reduction was 32.5-98.3%, organic nitrogen reduction was 20.3-95.9% and total phosphorus reduction was 33.5-89.9%. The reductions of thermotolerant coliforms were between 71.7% and 99.9%. In view of the results found, it can be considered that the bioremediation of the effluent reached promising efficiencies, with the advantage of producing biomass with potential to obtain bioproducts of relevant economic value

Enhanced Arthrospira platensis Biomass Production Combined with Anaerobic Cattle Wastewater Bioremediation.

Microalgae biomasses offer important benefits regarding macromolecules that serve as promising raw materials for sustainable production. In the present study, the microalgae Arthrospira platensis DHR 20 was cultivated in horizontal photobioreactors (HPBR), with and without temperature control, in batch mode (6 to 7 days), with anaerobically digested cattle wastewater (ACWW) as substrate. High dry biomass concentrations were observed (6.3–7.15 g L−1). Volumetric protein, carbohydrate, and lipid productivities were 0.299, 0.135, and 0.108 g L−1 day−1, respectively. Promising lipid productivities per area were estimated between 22.257 and 39.446 L ha−1 year−1. High CO2 bio-fixation rates were recorded (875.6–1051 mg L−1 day−1), indicating the relevant potential of the studied microalgae to mitigate atmospheric pollution. Carbon concentrations in biomass ranged between 41.8 and 43.6%. ACWW bioremediation was satisfactory, with BOD5 and COD removal efficiencies of 72.2–82.6% and 63.3–73.6%. Maximum values of 100, 95.5, 92.4, 80, 98, and 94% were achieved concerning the removal of NH4+, NO3−, Pt, SO42−, Zn, and Cu, respectively. Total and thermotolerant coliform removals reached 99–99.7% and 99.7–99.9%. This microalgae-mediated process is, thus, promising for ACWW bioremediation and valuation, producing a microalgae biomass rich in macromolecules that can be used to obtain friendly bio-based products and bioenergy.Supplementary InformationThe online version contains supplementary material available at 10.1007/s12155-021-10258-4.

Removal and environmental risk assessment of contaminants of emerging concern from irrigation waters in a semi-closed microalgae photobioreactor

The present study evaluated the efficiency of a semi-closed, tubular, horizontal photobioreactor (PBR) to treat a mixture of irrigation and rural drainage water, focusing in the removal of different contaminants of emerging concern (CECs), and evaluating the environmental impact of the resulting effluent. Target CECs included pharmaceuticals, personal care products and flame retardants. Of the 13 compounds evaluated, 11 were detected in the feed water entering the PBR, and diclofenac (DCF) (1107 ng L−1) and N,N-diethyl-toluamide (DEET) (699 ng L−1) were those present at the greatest concentrations. The best removal efficiencies were achieved for the pharmaceuticals diazepam (94%), lorazepam (LZP) (83%) and oxazepam (OXA) (71%), and also for ibuprofen (IBU) (70%). For the rest of the CECs evaluated, attenuation was similar to that obtained after conventional wastewater treatment, ranging from basically no elimination (carbamazepine (CBZ) and tris-(2-chloroethyl) phosphate (TCEP)) to medium efficiencies (DCF and tributyl phosphate (TBP) (50%)). Environmental risk assessment based on hazard quotients (HQs) resulted in HQ values < 0.1 (no risk associated) for most of the compounds and most of the trophic levels considered. Values between 1 and 10 (moderate risk) were obtained for tonalide (AHTN) (fish) and CBZ (invertebrates). The most sensitive trophic level was green algae, whereas fish and aquatic plants were the most resilient. Our results suggest that microalgae-based treatments could become a green, cost-effective alternative to conventional wastewater treatment regarding the efficient elimination of these contaminants.

Microalgae-based bioremediation of water contaminated by pesticides in peri-urban agricultural areas

The present study evaluated the capacity of a semi-closed, tubular horizontal photobioreactor (PBR) to remove pesticides from agricultural run-off. The study was carried out in summer (July) to study its efficiency under the best conditions (highest solar irradiation). A total of 51 pesticides, including 10 transformation products, were selected and investigated based on their consumption rate and environmental relevance. Sixteen of them were detected in the agricultural run-off, and the estimated removal efficiencies ranged from negative values, obtained for 3 compounds, namely terbutryn, diuron and imidacloprid, to 100%, achieved for 10 compounds. The acidic herbicide MCPA was removed by 88% in average, and the insecticides 2,4-D and diazinon showed variable removals, between 100% and negative values. The environmental risk associated to the compounds still present in the effluent of the PBR was evaluated using hazard quotients (HQs), calculated using the average and highest measured concentrations of the compounds. HQ values > 10 (meaning high risk) were obtained for imidacloprid (21), between 1 and 10 (meaning moderate risk) for 2,4-D (2.8), diazinon (4.6) and terbutryn (1.5), and <1 (meaning low risk) for the remaining compounds diuron, linuron and MCPA. The PBR treatment yielded variable removals depending on the compound, similarly to conventional wastewater treatment plants. This study provides new data on the capacity of microalgae-based treatment systems to eliminate a wide range of priority pesticides under real/environmental conditions.

Outdoor Closed System of Algal Mass Culture : In Sight of Comparison on Vertical and Horizontal Photobioreactor for Cultivating the Spirulina sp.

Spirulina are multiceluller and filamentous blue-green algae that has gained considerable popularity in the health food industry and increasingly as a protein and vitamin supplement to aquaculture diets. The challenge for economically and fulfill the requirement for food and medical purposes has create many ways for mass-growth production, that possibly cultivated in the open-system (such as a raceway pond) or closed-system photobioreactors (such as tubular, bubble-column, airlift, flat-panel, and vertical). the cultivation of teh Spirulina on the vertical and horizontal photobioreactor has been studied. The photobioreactor, namely BJVP and BJHP, has a design to be less energy consumption using the air bubbling or circular paddle. The observation was conducted in a whole year with parameters of rainfall, temperature, light intensity, pH, and salinity. Result showed that cultivation of Spirulina on the vertical photobioreactor growth faster than teh horizontal photobioreactor systems and the yield of biomass was about 0.94 gDW/L. Average of temperature ranges of BJHP were 31.0C-35.5C, salinities were 35 per mil level, pH were 8.55-10.86, and light intensity were 427-2001 umol photon s-1m-2. Whereas the BJVP has averages temperature range of 31.4C-33.9C, salinity 33-35 per mil level, pH 8.46-10.75, and light intensity 532-2062 umol photon s-1m-2. The proximate analyses of biomass from BJVP cultivation shows has tendency higher protein content compared to BJHP. The optimization of both reactors has continuing evaluated in order to get the optimum parameters required for economically Spirulina cultivation systems.Keywords: Spirulina, BJVP, BJHP, outdoor mass cultivating system, photobioreactor.

Large-scale cultivation of Spirulina in a floating horizontal photobioreactor without aeration or an agitation device.

A low-cost floating photobioreactor (PBR) without the use of aeration and/or an agitation device, in which carbon was supplied in the form of bicarbonate and only wave energy was utilized for mixing, was developed in our previous study. Scaling up is a common challenge in the practical application of PBRs and has not yet been demonstrated for this new design. To fill this gap, cultivation of Spirulina platensis was conducted in this study. The results demonstrated that S. platensis had the highest productivity at 0.3molL-1 sodium bicarbonate, but the highest carbon utilization (104 ± 2.6%) was obtained at 0.1molL-1. Culture of Spirulina aerated with pure oxygen resulted in only minor inhibition of growth, indicating that its productivity will not be significantly reduced even if dissolved oxygen is accumulated to a high level due to intermittent mixing resulting from the use of wave energy. In cultivation using a floating horizontal photobioreactor at the 1.0m2 scale, the highest biomass concentration of 2.24 ± 0.05gL-1 was obtained with a culture depth of 5.0cm and the highest biomass productivity of 18.9gm-2day-1 was obtained with a depth of 10.0cm. This PBR was scaled up to 10m2 (1000L) with few challenges; biomass concentration and productivity during ocean testing were little different than those at the 1.0m2 (100L) scale. However, the larger PBR had an apparent carbon utilization efficiency of 45.0 ± 2.8%, significantly higher than the 39.4±0.9% obtained at the 1m2 scale. These results verified the ease of scaling up floating horizontal photobioreactors and showed their great potential in commercial applications.

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%).

Start-up of a microalgae-based treatment system within the biorefinery concept: from wastewater to bioproducts.

Within the European project INCOVER, an experimental microalgae-based treatment system has been built for wastewater reuse and added-value products generation. This article describes this new experimental plant and the start-up stage, starting from the new design of three semi-closed horizontal photobioreactors with low energy requirements for microalgae cultivation (30 m3 total), using agricultural runoff and urban wastewater as feedstock. The inflow nutrients concentration is adjusted to select cyanobacteria, microalgae able to accumulate polyhydroxybutyrates, which can be used for bioplastics production. Part of the harvested biomass is used as substrate for anaerobic co-digestion (AcoD) with secondary sludge to obtain biogas. This biogas is then cleaned in an absorption column to reach methane concentration up to 99%. The digestate from the AcoD is further processed in sludge wetlands for stabilization and biofertilizer production. On the other hand, treated water undergoes ultrafiltration and disinfection through a solar-driven process, then it is pumped through absorption materials to recover nutrients, and eventually applied in an agricultural field to grow energy crops by means of a smart irrigation system. This plant presents a sustainable approach for wastewater management, which can be seen as a resource recovery process more than a waste treatment.

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%).

Plastic bag as horizontal photobioreactor on rocking platform driven by water power for culture of alkalihalophilic cyanobacterium

BackgroundMixing in traditional algae culture system consumes intensive electricity. This should be replaced by nature force to reduce energy cost and, more importantly, to realize positive energy balance of algal biofuel production. This study aims to develop a horizontal photobioreactor, in which mixing can be provided with rocking movement driven by nature force.ResultsSimple boxes were used as small-scale horizontal photobioreactors on a rocking platform for culture of alkalihalophilic Euhalothece sp. ZM001. There was no CO2 gas bubbling since 1.0 M NaHCO3 supplied sufficient inorganic carbon in it. Effect of culture depth, rocking cycle, and light intensity to algal biomass production, pH change, and DO accumulation were investigated in this system. Biomass concentration of 2.73 g/L was achieved in culture with 2.5 cm depth, and maximum productivity of 17.06 g/m2/day was obtained in culture with 10 cm depth. kLa in PBR with different culture depths and rocking cycles was measured, and it was from 0.57 to 33.49 h−1, showing great variation. To test this system at large scale, a plastic bag with a surface area of 1 m2 was placed on a rocking platform driven by water power, and it resulted in a biomass concentration of 1.88 g/L.ConclusionThese results proved feasibility of a novel photobioreactor system driven by nature force, as well as low cost of manufacturing, and easy scaling-up.

Conceptual design of sustainable integrated microalgae biorefineries: Parametric analysis of energy use, greenhouse gas emissions and techno-economics

This study covers four main aspects of the conceptual design of sustainable integrated microalgae-based biorefineries using flue gas from CO2-intensive industries (i.e. 100% CO2): i) screening of technologies (4 options for cultivation, 3 for culture dewatering, 3 for cell disruption, 4 for lipids extraction & purification, and 4 for fractions upgrading); ii) analysis of processing variables (parametric study of the main cultivation conditions affecting the global performance of the biorefinery systems); iii) combination of final products (10 biorefinery configurations are generated from the combination of 9 final products); and iv) assessment of sustainability criteria (i.e., non-renewable energy use (NREU), greenhouse gas (GHG) emissions and economics). The used approach compares processing options and operation conditions to identify those combinations of production processes that minimize NREU and GHG emissions. In a second step, 10 integrated biorefinery concepts are selected and compared with respect to their environmental and economic performances. The impacts of choosing different microalgae species, locations, and nutrient sources were also studied as part of the scenario analysis. The results showed that the biorefinery systems with the best economic and environmental performances are those where microalgae oil-free cake is used as nutrient for substitution of animal feed and where lipids are used for substitution of vegetable oils. The worst economic and environmental performances of biorefineries were obtained when microalgae oil-free cake is anaerobically digested to biogas and lipids are converted to either biodiesel or green diesel. Regarding the cultivation technologies for the biorefinery systems with the best performance, favorable environmental results were obtained for flat panel photobioreactors (FPPBRs), followed by open ponds (OPs), vertical photobioreactors (VPBRs) and horizontal photobioreactors (HPBRs). In contrast, the best economic results were found for FPPBRs followed by VPBRs, HPBRs and OPs.

A new bioenergetic and thermodynamic approach to batch photoautotrophic growth of Arthrospira (Spirulina) platensis in different photobioreactors and under different light conditions

Photobioreactor configuration, mode of operation and light intensity are known to strongly impact on cyanobacteria growth. To shed light on these issues, kinetic, bioenergetic and thermodynamic parameters of batch Arthrospira platensis cultures were estimated along the time at photosynthetic photon flux density (PPFD) of 70μmolm−2s−1 in different photobioreactors with different surface/volume ratio (S/V), namely open pond (0.25cm−1), shaken flask (0.48cm−1), horizontal photobioreactor (HoP) (1.94cm−1) and helicoidal photobioreactor (HeP) (3.88cm−1). Maximum biomass concentration and productivity remarkably increased with S/V up to 1.94cm−1. HoP was shown to be the best-performing system throughout the whole runs, while HeP behaved better only at the start. Runs carried out in HoP increasing PPFD from 40 to 100μmolm−2s−1 revealed a progressive enhancement of bioenergetics and thermodynamics likely because of favorable light distribution. HoP appeared to be a promising configuration to perform high-yield indoor cyanobacterial cultures.

Comparative Analysis of Hydrodynamics Behavior of Microalgae Suspension Flow in Circular, Square and Hexagonal Shape Photo Bioreactors

Microalgae based biofuel is an emerging natural source of energy alternative to the fossil fuel. As microalgae are photosynthetic microorganisms, light is one of the limiting factors for its culture. Though many researches have been carried out for findings behind suitable culture system for the proper growth of microalgae, those are confined only to tubular Photo-bioreactor (PBR). This paper aims to make comparison among the horizontal loop photo bioreactors with different cross sections based on the analysis of hydrodynamics behavior. Three different geometrical shapes having vertical cross sections of circular, square and hexagonal PBR, have been proposed taking into account light intensity for microalgae culture. In this study, we simulate the flow dynamics of three types of PBRs and discuss the velocity, pressure and shear stress properties as microalgae endurance capacity depends on them. For the dimension of the three PBRs we considered here, each of them have radius of about 0.05 m while the length together with bending portion is approximately 20.5 m for a single loop. From the study, the hydrodynamic behaviors are observed to be quite dissimilar in case of three PBR’s. In the straight portion the velocity profile is quite parabolic in tubular but distorted minimally in case of square and hexagonal PBRs. In the middle of the U-loop, a haphazard fluid distribution is noticed. The velocity magnitude and agitation of microalgae cells are higher in hexagonal than in square and tubular. The shear rate is less in case of tubular compared to square and hexagonal. A linear pressure drop is found from the inlet to the outlet for three PBR’s. From this comparison, it can be said that the tubular one would be the best option for microalgae culture in case of industrial purposes.

Comparison of four outdoor pilot-scale photobioreactors.

BackgroundMicroalgae are a potential source of sustainable commodities of fuels, chemicals and food and feed additives. The current high production costs, as a result of the low areal productivities, limit the application of microalgae in industry. A first step is determining how the different production system designs relate to each other under identical climate conditions. The productivity and photosynthetic efficiency of Nannochloropsis sp. CCAP 211/78 cultivated in four different outdoor continuously operated pilot-scale photobioreactors under the same climatological conditions were compared. The optimal dilution rate was determined for each photobioreactor by operation of the different photobioreactors at different dilution rates.ResultsIn vertical photobioreactors, higher areal productivities and photosynthetic efficiencies, 19–24 g m−2 day−1 and 2.4–4.2 %, respectively, were found in comparison to the horizontal systems; 12–15 g m−2 day−1 and 1.5–1.8 %. The higher ground areal productivity in the vertical systems could be explained by light dilution in combination with a higher light capture. In the raceway pond low productivities were obtained, due to the long optical path in this system. Areal productivities in all systems increased with increasing photon flux densities up to a photon flux density of 30 mol m−2 day−1. Photosynthetic efficiencies remained constant in all systems with increasing photon flux densities. The highest photosynthetic efficiencies obtained were; 4.2 % for the vertical tubular photobioreactor, 3.8 % for the flat panel reactor, 1.8 % for the horizontal tubular reactor, and 1.5 % for the open raceway pond.ConclusionsVertical photobioreactors resulted in higher areal productivities than horizontal photobioreactors because of the lower incident photon flux densities on the reactor surface. The flat panel photobioreactor resulted, among the vertical photobioreactors studied, in the highest average photosynthetic efficiency, areal and volumetric productivities due to the short optical path. Photobioreactor light interception should be further optimized to maximize ground areal productivity and photosynthetic efficiency.

A novel horizontal photobioreactor for high-density cultivation of microalgae

Microalgae are a promising source of biofuels and bioproducts, as they consume CO2 to grow, multiply quickly, and can be cultivated in wastewater and on marginal land. Development of low-cost and high-efficiency microalgal cultivation systems is important to the cost-competitiveness of algae. A floating horizontal photobioreactor (HBR) was developed that is inexpensive and scalable, as it is manufactured from inexpensive plastic film and is modular. Its performance was successfully tested using the marine microalgae Nannochloris atomus Butcher CCAP 251/4A in a 65-L prototype unit. High biomass concentration of 4.0gL−1 and productivity of 12.9gm−2d−1 was achieved indoors under artificial illumination of 31.3klux (435μmolm−2s−1). Outdoors, during semi-continuous operation in Florida, the HBR achieved over the course of 165days a maximum biomass concentration of 4.3gL−1 and an average biomass productivity of 18.2gm−2d−1 without any contamination issues.

Cultivation of Chlorella vulgaris in tubular photobioreactors: A lipid source for biodiesel production

Chlorella vulgaris was cultivated in two different 2.0L-helicoidal and horizontal photobioreactors at 5klux using the bicarbonate contained in the medium and ambient air as the main CO2 sources. The influence of bicarbonate concentration on biomass growth as well as lipid content and profile was first investigated in shake flasks, where the stationary phase was achieved in about one half the time required by the control. The best NaHCO3 concentration (0.2gL−1) was then used in both photobioreactors. While the fed-batch run performed in the helicoidal photobioreactor provided the best result in terms of biomass productivity, which was (84.8mgL−1d−1) about 2.5-fold that of the batch run, the horizontal configuration ensured the highest lipid productivity (10.3mgL−1d−1) because of a higher lipid content of biomass (22.8%). These preliminary results suggest that the photobioreactor configuration is a key factor either for the growth or the composition of this microalga. The lipid quality of C. vulgaris biomass grown in both photobioreactors is expected to meet the standards for biodiesel, especially in the case of the helicoidal configuration, provided that further efforts will be made to optimize the conditions for its production as a biodiesel source.