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Article

Comparison of Light Intensity Effect on Microalgal Growth in Cactus-like and Cylindrical Photo Bioreactors

by
Rayane Mustafa Hijazi
*,
Jihane Rahbani Mounsef
and
Hadi Youssef Kanaan
Faculty of Engineering and Architecture, Saint Joseph University of Beirut, Mar Roukoz, Dekwaneh, Beirut 1104 2020, Lebanon
*
Author to whom correspondence should be addressed.
Processes 2024, 12(8), 1664; https://doi.org/10.3390/pr12081664
Submission received: 29 June 2024 / Revised: 20 July 2024 / Accepted: 24 July 2024 / Published: 8 August 2024
(This article belongs to the Section Catalysis Enhanced Processes)
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Abstract

:
Improving photobioreactor performance for microalgae cultivation has been the aim of many researchers over the past few years. One of the primary challenges associated with existing photobioreactors is light penetration. An effective photobioreactor design should maximize light penetration, ensuring uniform illumination throughout the reactor. This study aims to assess the impact of light intensity on microalgal growth from the perspective of energy efficiency and productivity in two photobioreactors. A novel cactus-like and a cylindrical photobioreactor were designed and fabricated using three-dimensional printing technology. These two photobioreactors were used to cultivate two strains of microalgae. The novel photobioreactor achieved a maximum biomass productivity of 1 g/L/d and a maximum energy efficiency of 0.31 g/d/kWh. The cylindrical photobioreactor reached a maximum biomass productivity of 0.74 g/L/d and energy efficiency of 0.22 g/d/kWh. The increase in biomass productivity can be linked to enhancements in the photobioreactor’s surface-to-volume ratio and better light utilization.

1. Introduction

With the increase in population and economic and technological development, energy consumption has rapidly increased, resulting in high greenhouse gas emissions and global warming [1,2,3]. Therefore, the interest in alternative renewable energy sources has received international attention. Biofuels are renewable energy sources from biomass such as animal fats and vegetable oils. Most biofuel production heavily relies on conventional feedstock like sugar cane, corn, and soybeans [2]. However, to achieve a significant increase in biofuel production while minimising negative impacts on land use, food and feed prices, and the environment, it is essential to expand the utilization of advanced feedstocks. These advanced feedstocks consist of biofuels derived from waste materials, residues, and dedicated crops that do not compete with food crops, such as crops grown on marginal land [2]. Microalgae have emerged as up-and-coming contenders for fuel production. The fatty acids collected from microalgae can be converted into biodiesel, which makes microalgae a substantial alternative energy source [4]. Microalgae biomass holds great promise, as it can produce various organic compounds, such as proteins, carotenoids, vitamins, and fatty acids. Microalgae are used to create animal feed, nutraceuticals, cosmetics, and pharmaceutical end products [5]. Two primary cultivation systems are available industrially: open pond systems and closed photobioreactors (PBRs). The selection of the microalgae cultivation system depends on the end product, microalgae species, the total cost, and many other factors. Open pond systems are more commonly preferred because of their ease of operation, scalability, and lower energy and production costs; however, they suffer mainly from contamination and evaporation losses [6].
PBR systems overcome these limitations, prevent contamination, minimize water loss, and enhance biomass productivity. Many PBRs are present industrially; these systems include flat-plate PBRs, tubular PBRs (TPBRs), helical PBRs, airlift PBRs, bubble column PBRs, and cylindrical PBRs, the latter of which are the most frequently utilized [7,8]. Cylindrical PBRs are recognized for their mixing efficiency, which provides homogenous nutrient dispersion inside the reactor, homogenizing single or multiple phases in terms of the concentration of components, physical properties, and temperature. Cylindrical PBRs ensure effective absorption of carbon dioxide (CO2) and prevent dioxygen (O2) accumulation. However, cylindrical PBRs present a low surface area-to-volume ratio (SVR) and insufficient light-capturing capacity [9,10].
Still, light path and flow regimes should be considered for scale-up and design considerations, such as the baffle structure selection and height-to-diameter ratio. The topic of the effect of PBR geometry on light penetration inside reactors has attracted researchers. Therefore, many studies and experiments have designed PBRs with internal illumination [11,12,13,14]. However, this technology suffers from algae adhesion and difficulty in operation and maintenance. Posten [15] pointed out that the SVR was the principal trait in the design of a PBR. It determines the light intensity received by the reactor’s volume, thus playing an essential function in the photosynthetic growth of microalgae. Another important factor to consider in PBR design is the choice of the fabrication material. A wide variety of materials have been used for PBRs’ construction, including glass, low-density polyethylene (LDPE) film, poly(methyl methacrylate) (PMMA) (also known by the trade names Plexiglas® and Perspex®), and polylactic acid (PLA). Regarding microalgae cultivation, PLA offers several advantages [16,17,18]. Firstly, its non-toxic and biocompatible nature ensures safety in cultivation environments, aligning with environmentally friendly practices owing to its biodegradability. PLA’s transparency optimizes light utilization, enhancing the cultivation process, while its commendable thermal stability enables it to withstand moderate temperatures without deformation. Additionally, PLA offers cost-effectiveness, making it a favorable choice for PBR construction.
This paper introduces a new design inspired by cactus plants that could enhance light penetration and photosynthetic efficiency. The study compares the effects of the new geometry that has a higher surface-to-volume ratio on light penetration, biomass growth productivity, and energy efficiency against traditional cylindrical PBRs. For this comparative study, the reactors were fabricated using three-dimensional printing technology with PLA as the source material.

2. Materials and Methods

2.1. PBR Geometry Description

A unique PBR, resembling a cactus in shape, was created to optimize light utilization and improve the SVR compared to traditional cylindrical PBRs. Taking inspiration from the conical shape of a cactus plant and featuring a cross-section in the form of a wavy circle (see Figure 1), the dimensions of this reactor were thoughtfully selected. The column’s diameter was restricted to 0.2 m to ensure sufficient light penetration [19,20,21], and the height-to-diameter ratio was set at a minimum of two [19,20,21].
Consequently, the cactus PBR has a bottom inner diameter of 0.1 m and an outer diameter of 0.2 m. In contrast, the top inner diameter is 0.08 m, with an outer diameter of 0.14 m and a height of 0.3 m, as illustrated in Figure 1.
This novel PBR design was executed using Autodesk Revit 2020, a software platform well known for enabling users to craft detailed three-dimensional models. A notable tool employed in this process is the Solid Blend Tool. This tool allows for the creation of intricate geometric forms by blending two distinct profiles, resulting in a continuous, solid 3D shape that smoothly transitions from the characteristics of the first profile to the second profile. Therefore, the reactor’s wall was constructed by employing the blend tool to create two distinct profiles for the upper and lower sections of the structure. The detailed characteristics of these profiles, encompassing their specific dimensions, are illustrated in Figure 1. The PBR incorporates a ring sparger with 24 holes positioned 0.01 m above the bottom to minimize the formation of dead zones. For comparative analysis, a cylindrical PBR with height and volume identical to those of the novel PBR but with a 0.14 m diameter was also designed, as depicted in Figure 2.
The cactus PBR, which was 4.38 L in volume and featured an illuminated surface area of 0.2345 m2, was determined using Autodesk Revit 2020. It exhibits an SVR of 53.53 m−1, which is twice that of a standard column PBR of equivalent height and volume (26.6 m−1).
For the comparison of algal growth inside the novel PBR and the cylindrical PBR, the reactors were fabricated by 3D printing technology using fused deposition modelling technology (FDM) and PLA as the source material, as shown in Figure 3.

2.2. Cultivation Experiment

The two algal strains identified for the present study were Ankistrodesmus sp. and Chlorella sp., provided by the Culture Collection of Microalgae at the Saint-Joseph University of Beirut, Lebanon. These two strains were selected due to their high lipid content, making them ideal for biodiesel production. Pre-cultures were prepared by inoculating microalgal colonies into 1000 mL Erlenmeyer flasks containing 300 mL of a modified synthetic BG11 medium [22]. The BG11 medium has the following composition: 20 mg Na2CO3, 1500 mg NaNO3, 40 mg K2HPO4, 200 mg KH2PO4, 1 mg Fe citrate, 6 mg citric acid, 1 mg EDTA per liter of solution, and an aliquot of 1 mL obtained from 1 L of a metallic stock solution containing 80 mg of CuSO4, 20 mg of ZnSO4, 80 mg of CO(NO3)2, 1800 mg of MnSO4, and H2O. After seven days of growth, the cultures were used as inoculum for the PBRs. Experiments in the two PBRs were performed under the following conditions. The microalgae strains were cultivated in a 2.7 L BG11 medium. A volume of 0.3 L of pre-culture was inoculated into the PBR [23]. The temperature of the room where the microalgae were grown was kept at 25 °C. Air was fed into the PBR at a flow rate of 2 L/min using an aquarium air pump (HAILEA, ACO-5501, Havelock Rd, Singapore). The culture was agitated at 100 rpm with an anchor stirrer. The cultures were carried out for 14 days under five different light conditions as follows: Condition 1: light intensity at 43 µmol photons/m2/s, Condition 2: light intensity at 57 µmol photons/m2/s, Condition 3: light intensity at 70 µmol photons/m2/s, Condition 4: light intensity at 100 µmol photons/m2/s, and Condition 5: light intensity at 115 µmol photons/m2/s. GWW SMD LED Flexible Strips were used at a power of 9 W/m. It is important to note that these light intensities were measured with a lux meter in the absence of the PBRs. In all five conditions, the photoperiod was established as 8 h of light followed by 16 h of darkness, indicated as an 8:16 (h:h) light-to-dark cycle.

2.3. Light Intensity Measurement

A digital lux meter (model lx-1010B, MEXTECH, Kalbadevi, Mumbai, Maharashtra) was employed to measure light intensity at four distinct angles (x, −x, y, −y) at the center of the PBRs, positioned 15 cm above the bottom of the reactor. The light intensity was measured in the absence of the microalgal culture.
An average value was then calculated from these measurements. Subsequently, the conversion from lux units to photosynthetic photon flux density (PPFD) was carried out using an online converter. This conversion considered the light spectrum of LED with a high color rendering index (CRI) at 3000 K [24].
P P F D µ mol   photons / m 2 / s = 0.019 ∗ L u x

2.4. Biomass Measurement

The progression of microalgae growth was systematically observed weekly. To quantify the biomass, the dry weight of the microalgae was determined through a meticulous process. Three 20 mL samples of the biomass were taken, and these samples underwent a drying procedure in an oven set at a constant temperature of 80 °C. This drying process continued until a stable and consistent biomass weight was achieved, providing a reliable measure of its dry weight.

2.4.1. Specific Growth Rate

The specific growth rate (μ, d−1) was calculated using Equation (2), [25,26] where DW1 (g/L) and DW2 (g/L) are dry biomass weights at t 1 and t 2 , respectively. The duration between t1 and t2, which spans 14 days, represents the cultivation period.
μ = ln D w 2 / D w 1 / ( t 2 − t 1 )

2.4.2. Productivity

The productivity (P, g/L/d) was calculated using Equation (3), where DW1 (g/L) and DW2 (g/L) are dry biomass weights at t 2 and t 1 , respectively. The duration between t1 and t2, which spans 14 days, represents the cultivation period.
P = ( D W 2 − D W 1 ) / t 2 − t 1

2.4.3. Energy Efficiency

The energy efficiency (ɳ, g/d/kWh) of electrical energy to biomass production was calculated using Equation (4), where DW1 (g/L) and DW2 (g/L) are dry biomass weight on the initial and the last day of cultivation, respectively; V (L) is the volume of culture; and Ee (kWh) is electrical energy consumed by LEDs during cultivation time t (days).
ɳ = ( D W 2 − D W 1 ) · V / t · E e

2.5. Statistical Analysis

The experiments were conducted in triplicate, and the results are presented as mean values with standard deviations (mean ± standard deviation). A two-way ANOVA was performed to analyze the statistical differences at p ≤ 0.05 between the obtained mean values of productivity, specific growth rates, and energy efficiency at different light conditions and for the two types of reactors, using Microsoft Excel 2016.

3. Results and Discussion

3.1. Irradiance

The design of a PBR must allow for the maximum possible amount of light penetration with few unilluminated areas and a uniform illumination over the reactor. The wavy shape of the new PBR’s wall doubled the SVR compared to that of the cylindrical PBR, as discussed earlier [27]. This design’s higher SVR can mitigate the shading effect typically observed with high algal concentrations. The light intensity inside the two PBRs was measured for each experimental condition using a lux meter. The lux level and the ratio between the values found inside the cactus-like PBR and the cylindrical PBR are shown in Figure 4. The cactus-like PBR exhibited a 29% higher average light intensity inside its walls. The higher light intensity in the middle of the reactor could be linked to the shape of the reactor, which enhanced its light penetration due to the reduction in its light path compared to that of the cylindrical PBR (with an inner diameter of 8 and 11 cm versus 14 cm for the cylindrical PBR).

3.2. Biomass Production

3.2.1. Specific Growth Rate

Light serves as the primary energy source on Earth and plays a crucial role in the process of photosynthesis, which in turn facilitates the multiplication of microalgae cells. This study calculated the specific growth rates, productivity, and energy efficiency for two microalgae strains, Ankistrodesmus sp. and Chlorella sp., in a novel cactus-shaped PBR and a conventional cylindrical PBR under varying light intensities.
Figure 5 illustrates the specific growth rates of these two algal strains under five different light intensities. The lowest specific growth rate was observed under Condition 1 (with a light intensity of 43 µmol photons/m2/s) and Condition 5 (with a light intensity of 115 µmol photons/m2/s) for Ankistrodesmus sp. and Chlorella sp., respectively, in both PBRs. Conversely, the highest specific growth rates were attained under conditions 3 and 4 for Ankistrodesmus sp. and Chlorella sp., respectively (with light intensities of 70 and 100 µmol photons/m2/s).
Insufficient irradiance can retard photosynthesis and diminish biomass yield, a condition known as light limitation [28]. Our observations in Figure 5 reveal a significant surge in the specific growth rate when transitioning from a light intensity of 43 to 57 µmoles photons/m2/s. This increase amounts to approximately 2.5 times for the cactus-like PBR and 3.7 times for the cylindrical PBR with a 30% increase in light intensity. Thus, it can be inferred that a light intensity of 43 µmoles photons/m2/s falls below the lower threshold to sustain a high growth rate of Ankistrodesmus sp., while 57 µmoles photons/m2/s falls in the optimal threshold for the growth of this microalgae. This differentiation elucidates the notable surge in the specific growth rate observed between light intensities of 43 and 57 µmoles photons/m2/s.
For the Chlorella sp., the evolution of its specific growth rate differs from that of Ankistrodesmus sp. This could be explained by the fact that microalgae species exhibit varying light requirements, with the optimal light intensity varying from one strain to another [29]. Consequently, the optimal light conditions and light saturation point depend on the specific algal strain utilized.
It is important to note that higher light intensities (conditions 4 and 5 for Ankistrodesmus sp. and Chlorella sp., respectively) resulted in light-induced photoinhibition. This aligns with many studies [30,31,32,33,34,35,36,37], indicating that the specific growth rate of microalgae correlates positively with light intensity, up to a saturation point.
Another notable observation is the significant decline in the specific growth rate observed in the cylindrical PBR when subjected to light intensities of 100 and 115 µmoles photons/m2/s for both strains. Photoinhibition can be reduced by enhancing the light/dark frequency [38]. As for microalgae cultivation, it is recognized that swirling conditions foster microalgae growth by enhancing the light/dark frequency, prolonging the air bubble residence time, facilitating mass transfer between phases, and mitigating thermal stratification [39,40,41]. Therefore, the notable decline observed in the cylindrical PBR may be attributed to the superior mixing efficiency demonstrated by the cactus PBR, as evidenced by simulations anticipated to be detailed in a forthcoming publication.
Additionally, in all light conditions, the cactus PBR exhibited a higher specific growth rate than the cylindrical PBR. However, the increase in specific growth rate was particularly pronounced under Condition 1 (with a light intensity of 43 µmoles photons/m2/s), representing low irradiance, and conditions 4 and 5 (with a light intensity of 100 and 115 µmoles photons/m2/s) for Ankistrodesmus sp. and Chlorella sp., respectively, corresponding to photoinhibition. These findings validate the superior light distribution and improved frequency of light/dark cycles in the cactus PBR, attributed to its enhanced mixing efficiency.
The highest values attained for the specific growth rate are 0.17 d−1 in the cactus PBR compared to 0.14 d−1 in the cylindrical PBR for Ankistrodesmus sp. and 0.23 d−1 in the cactus PBR compared to 0.21 d−1 in the cylindrical PBR for Chlorella sp.; these values are comparable to those reported in the literature. Mohammed et al. [42,43] obtained a specific growth rate of 0.11 d−1 of a mixed microalgae culture grown in a pilot-scale red-LED-illuminated stirred PBR. Also, Cerón-García et al. [44] produced oil-rich biomass from Chlorella protothecoides in a conventional 2 L stirred-tank bioreactor and reported a specific growth rate of 0.15 d−1.

3.2.2. Microalgae Productivity

The results in Table 1 and Table 2 demonstrate that varying the light intensity from 43 µmol photons/m2/s to 100 µmol photons/m2/s resulted in a 2.3-fold increase Chlorella sp. productivity, rising from 0.43 g/L/d to 1 g/L/d in the cactus-like PBR and from 0.32 g/L/d to 0.74 g/L/d in the cylindrical PBR. When exposed to 115 µmol photons/m2/s, Chlorella sp. productivity decreased to 0.19 g/L/d in the cylindrical PBR and 0.24 g/L/d in the cactus PBR. Meanwhile, Ankistrodesmus sp. productivity showed a 1.6-fold increase, going from 0.59 g/L/d to 0.97 g/L/d in the cactus-like PBR and from 0.41 g/L/d to 0.48 g/L/d in the cylindrical PBR when increasing the light intensity from 43 µmol photons/m2/s to 70 µmol photons/m2/s. Moreover, it decreased to 0.23 g/L/d in the cylindrical PBR and 0.66 g/L/d in the cactus-like PBR when the light intensity was increased to 100 µmol photons/m2/s.
These findings are in line with previous studies, such as that of Sankar et al. [32], who observed the effect of three light intensities: 2000, 6000, and 10,000 lux on C. minutissima growth and found that 6000 lux is optimal for microalgal growth. Light intensities exceeding or falling below this value were found to be detrimental to the culture. Similarly, Khoeyi et al. [34] studied the effects of the light intensity and photoperiod on Chlorella vulgaris growth, finding that its productivity increased with moderate light intensities from 37.5 to 62.5 µmol photons/m2/s but decreased at a higher level of 100 µmol photons/m2/s. Elevated light intensity levels can lead to the photobleaching of photosynthetic pigments, peroxidation of lipid membranes, and DNA damage [32]. Conversely, lower light intensities tend to increase chlorophyll and carotenoid contents but reduce growth rates and cell density [33]. Lee and Palsson [37] documented that C. vulgaris could be cultured under LED illumination at significantly greater light intensities (exceeding 400 W/m2) without experiencing photoinhibition. Variations in results concerning the impact of light intensity on microalgal growth may stem from differences in light source type, PBR design, and microalgal strains.
This study also found that Chlorella sp. could withstand higher light intensities compared to Ankistrodesmus sp. In conformity with several studies [45,46,47], Chlorella sp. is generally more tolerant of higher light intensities than Ankistrodesmus sp. Chlorella sp. is a versatile and adaptable microalgae species that can thrive under a wide range of light conditions, including high light intensities. In contrast, Ankistrodesmus sp. is generally considered less light-tolerant and may be more sensitive to high light intensities [45].
Microalgae biomass absorbs the light photons supplied to a culture, reducing light availability in the inner parts of PBRs. This phenomenon, known as self-shading or the shadow effect [48], leads to a heterogeneous distribution of light within the PBR, which is unfavorable for algal growth [49]. Understanding light distribution is therefore crucial for the design of effective PBRs and for optimizing microalgae cultivation. As the width of the PBRs increases, light-limited zones become more prevalent due to the well-known light attenuation phenomenon in the microalgae culture broth. These light-limited zones result in lower growth rates of algal cells [50,51,52]. Consequently, any changes in PBR width significantly impact the light environment within the reactor [53]. In our studies, the cactus-shaped PBR consistently outperformed its cylindrical counterpart, showcasing a remarkable two-fold increase in productivity for both of the tested strains. The effectiveness of the cactus-like PBR can be attributed to its more efficient light distribution and reduced light attenuation within its volume due to shorter light paths compared to those of the cylindrical design. These features highlight the robustness and efficiency inherent in the cactus-shaped PBR design.

3.2.3. Energy Efficiency

In the context of energy efficiency, as indicated in Table 1 and Table 2, the energy efficiency in the Ankistrodesmus sp. culture decreases with increasing light intensity. It is noteworthy that while its productivity undergoes a 1.6-fold increase between Conditions 1 and 3, its energy efficiency in contrast decreases by a factor of 2.4. Conversely, for Chlorella sp., its energy efficiency is reduced by a factor of 3.4, accompanied by a 2.3-fold increase in productivity under Condition 4. The findings underscore that the optimal balance between biomass productivity and energy efficiency differs for Ankistrodesmus sp. and Chlorella sp., being achieved under Conditions 3 and 4, respectively. Additionally, it is worth emphasizing that the energy efficiency in the cactus-shaped PBR consistently exceeds that of the traditional cylindrical PBR by a factor of two.
Finally, while productivity varies with species, light source, and PBR configuration, the productivity in the new PBR was comparable to that reported in the literature [48,49,50,51,52,53,54,55,56,57]. Nevertheless, the novel PBR, with twice the SVR of the conventional cylindrical PBR, exhibited a superior performance for both microalgae strains. The productivity and energy efficiency obtained from the novel PBR were twice those of the cylindrical PBR.

4. Conclusions

This research aimed to investigate the impact of light intensity on microalgae growth, specifically focusing on productivity and energy efficiency. The study involved a comparative analysis between a newly designed PBR inspired by cactus plants and a traditional cylindrical PBR. Both PBRs were meticulously designed and were carefully examined using lux meters to evaluate the extent of light penetration into their structures.
The experimental results revealed that the innovative cactus-inspired PBR surpassed the conventional cylindrical PBR in light penetration and biomass productivity. The cactus PBR demonstrated a remarkable 29% improvement in light penetration compared to the cylindrical PBR. Furthermore, it exhibited twice the productivity of the other, highlighting its superior efficiency in cultivating microalgae.
However, it is important to note that while light penetration is a crucial factor influencing microalgae growth, other variables such as mixing and the hydrodynamic behavior of the innovative PBR may also play a role in fostering biomass productivity. These additional factors will be investigated in a future study using computational fluid dynamics to provide a comprehensive understanding of the cactus-inspired PBR’s performance.

Author Contributions

Conceptualization, R.M.H.; methodology, R.M.H.; writing—original draft preparation, R.M.H.; writing—review and editing, J.R.M.; supervision, J.R.M. and H.Y.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Lebanese National Council for Scientific Research (CNRS-L) and the Research Council at the Saint Joseph University of Beirut (PROJECT ESIB 52).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

The authors would like to thank the Lebanese National Council for Scientific Research (CNRS-L) and the Research Council at the Saint Joseph University of Beirut for their financial support.

Conflicts of Interest

The funders had no role in the design of the study; in the collection, analysis, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in the manuscript text:
PBRPhotobioreactors
TPBRTubular Photobioreactor
SVRSurface Area-to-Volume Ratio
LDPELow-Density Polyethylene
PMMAPoly (Methyl Methacrylate)
PLAPolylactic Acid
FDMFused Deposition Modelling Technology
PPFDPhotosynthetic Photon Flux Density
CRIColor Rendering Index

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Figure 1. (a) Cactus-like PBR; (b) Schematic of the circular sparger; (c) Elevation view of the cactus-like PBR.
Figure 1. (a) Cactus-like PBR; (b) Schematic of the circular sparger; (c) Elevation view of the cactus-like PBR.
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Figure 2. Elevation view of the cylindrical PBR.
Figure 2. Elevation view of the cylindrical PBR.
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Figure 3. The setup of the cactus-like and the cylindrical PBRs.
Figure 3. The setup of the cactus-like and the cylindrical PBRs.
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Figure 4. Lux level inside the two PBRs in five light conditions.
Figure 4. Lux level inside the two PBRs in five light conditions.
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Figure 5. The specific growth rate of two microalgae strains inside the two PBRs with different light conditions.
Figure 5. The specific growth rate of two microalgae strains inside the two PBRs with different light conditions.
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Table 1. Results of two-way ANOVA based on productivity, specific growth rate, and energy efficiency of Ankistrodesmus sp. Capital letters after mean values in rows indicate statistical differences at p < 0.05 between light conditions; lowercase letters after mean values in columns indicate statistical differences at p < 0.05 between the two types of reactors.
Table 1. Results of two-way ANOVA based on productivity, specific growth rate, and energy efficiency of Ankistrodesmus sp. Capital letters after mean values in rows indicate statistical differences at p < 0.05 between light conditions; lowercase letters after mean values in columns indicate statistical differences at p < 0.05 between the two types of reactors.
Condition 1Condition 2Condition 3Condition 4
AnkistrodesmusProductivity (g/L/d)Cactus0.59 ± 0.062 Aa0.69 ± 0.045 Ba0.97 ± 0.081 Ca0.66 ± 0.063 Da
Cylindrical0.41 ± 0.019 Ab0.42 ± 0.060 Bb0.49 ± 0.004 Cb0.23 ± 0.005 Db
Specific growth rate (d−1)Cactus0.07 ± 0.009 Aa0.17 ±0.009 Ba0.17 ± 0.010 Ca0.15 ± 0.010 Da
Cylindrical0.03 ± 0.002 Ab0.13 ± 0.017 Bb0.14 ± 0.013 Cb0.05 ± 0.003 Db
Energy efficiency (g/d/kWh)Cactus0.31 ± 0.033 Aa0.18 ± 0.012 Ba0.13 ± 0.010 Ca0.04 ± 0.004 Da
Cylindrical0.22 ± 0.010 Ab0.11 ± 0.016 Bb0.06 ± 0.003 Cb0.02 ± 0.000 Db
Table 2. Results of two-way ANOVA based on productivity, specific growth rate, and energy efficiency of Chlorella sp. Capital letters after mean values in rows indicate statistical differences at p < 0.05 between light conditions; lowercase letters after mean values in columns indicate statistical differences at p < 0.05 between the two types of reactors.
Table 2. Results of two-way ANOVA based on productivity, specific growth rate, and energy efficiency of Chlorella sp. Capital letters after mean values in rows indicate statistical differences at p < 0.05 between light conditions; lowercase letters after mean values in columns indicate statistical differences at p < 0.05 between the two types of reactors.
Condition 1Condition 2Condition 3Condition 4Condition 5
ChlorellaProductivity (g/L/d)Cactus0.43 ± 0.007 Aa0.49 ± 0.018 Ba0.52 ± 0.008 Ca1 ± 0.032 Da0.24 ± 0.002 Ea
Cylindrical0.32 ± 0.007 Ab0.29 ± 0.007 Bb0.28 ± 0.003 Cb0.74 ± 0.043 Db0.19 ± 0.012 Eb
Specific growth rate (d−1)Cactus0.09 ± 0.007 Aa0.14 ± 0.002 Ba0.14 ± 0.005 Ca0.23 ± 0.008 Da0.08 ±0.016 Ea
Cylindrical0.05 ± 0.013 Ab0.11 ± 0.002 Bb0.11 ± 0.001 Cb0.21 ± 0.004 Db0.03 ± 0.004 Eb
Energy efficiency (g/d/kWh)Cactus0.23 ± 0.003 Aa0.13 ± 0.004 Ba0.07 ± 0.001 Ca0.07 ± 0.002 Da0.01 ± 0.000 Ea
Cylindrical0.17 ± 0.003 Ab0.08 ± 0.002 Bb0.04 ± 0.000 Cb0.05 ± 0.002 Db0.01 ± 0.000 Eb
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Hijazi, R.M.; Mounsef, J.R.; Kanaan, H.Y. Comparison of Light Intensity Effect on Microalgal Growth in Cactus-like and Cylindrical Photo Bioreactors. Processes 2024, 12, 1664. https://doi.org/10.3390/pr12081664

AMA Style

Hijazi RM, Mounsef JR, Kanaan HY. Comparison of Light Intensity Effect on Microalgal Growth in Cactus-like and Cylindrical Photo Bioreactors. Processes. 2024; 12(8):1664. https://doi.org/10.3390/pr12081664

Chicago/Turabian Style

Hijazi, Rayane Mustafa, Jihane Rahbani Mounsef, and Hadi Youssef Kanaan. 2024. "Comparison of Light Intensity Effect on Microalgal Growth in Cactus-like and Cylindrical Photo Bioreactors" Processes 12, no. 8: 1664. https://doi.org/10.3390/pr12081664

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