Promotion of Microalgal Growth, CO₂ Fixation, and Pollutant Removal in Piggery Effluent by a Column Photobioreactor with Funnel-Shaped Spoilers

Abstract

To enhance contact time between microalgae and nutrients in reactors, thereby improving the growth rate of microalgae and increasing pollutant removal efficiency, two funnel-shaped spoilers were added inside a traditional column photobioreactor. Compared to conventional column photobioreactors, the addition of these spoilers resulted in increased updraft, which improved horizontal flow. This change led to a greater shear force near the spoilers and a reduction in bubble diameter. As a result, the mass transfer coefficient and gas content increased by 12.17% and 7.71%, respectively, while the mixing time decreased by 30.57%. These improvements resulted in an 18.18% increase in microalgal biomass, a 13.95% increase in the CO₂ fixation rate, and increases of 4.48%, 7.5%, and 4.7% in the removal of COD, TP, and NH₄⁺-N, respectively, in the column photobioreactor with funnel-shaped spoilers. This was achieved when CO₂ was introduced at a concentration of 10%, compared to a conventional column photobioreactor. This innovative design enhances the growth efficiency of microalgae, offering a new solution for reducing carbon emissions, promoting recycling of water resources, and advancing sustainable development.

1. Introduction

As industries develop and populations grow, greenhouse gas emissions have soared, leading to a series of natural disasters. CO₂ accounts for more than 68% of these greenhouse gas emissions, making CO₂ reduction technologies a major focus of research interest [1]. Various methods for capturing CO₂ have been developed, including absorption, membrane treatment, geological cycling, deep-sea cycling, and cryogenic capture [2]. Among these methods, carbon cycling using microalgae offers several advantages over other technologies. It is highly efficient [3], requires a smaller footprint [4], and can produce a variety of valuable products from the processed microalgae [5,6]. Additionally, microalgae can be cultivated in aquaculture wastewater, where they help remove pollutants such as nitrogen, phosphorus, and chemical oxygen demand (COD) [7]. Thus, integrating microalgae-based carbon cycling with wastewater treatment holds significant promise. This technology not only effectively reduces greenhouse gas emissions but also promotes the recycling of resources, offering a green and efficient solution for achieving carbon neutrality goals and sustainable development.

Photobioreactors play a crucial role in the cultivation of microalgae and are essential for the successful implementation of various related technologies. The primary types of photobioreactors used for microalgae cultivation include raceway pond photobioreactors, flat panel photobioreactors, tubular photobioreactors, air-lift photobioreactors, and column photobioreactors [8]. Column photobioreactors are renowned for their excellent mixing efficiency, which enables the uniform dispersion of nutrients within the reactor, thereby effectively promoting CO₂ absorption and preventing O₂ accumulation. However, traditional column photobioreactors suffer from inherent limitations such as a low surface area-to-volume ratio and insufficient light capture capability [9]. To address these issues, researchers have conducted numerous innovative studies to enhance their performance. In terms of structural optimization, Hijazi [10] designed a biomimetic cactus-shaped column photobioreactor. This design significantly increased the reactor’s surface area, and experimental data showed that, compared to traditional column photobioreactors, its light transmittance improved by 29%, and biomass productivity achieved a remarkable increase of 200%. This breakthrough demonstrates the importance of structural optimization in improving photobioreactor performance. Regarding internal component improvements, Kubar [11] enhanced the mixing and mass transfer effects within column photobioreactors by installing serrated baffles. This design increased the frequency of cell movement between the light and dark zones, thereby improving photochemical efficiency and ultimately boosting biomass yield by 28.8%. This research provides new insights for optimizing internal components of reactors. In summary, by modifying reactors’ external shape and adding internal turbulence-inducing components, the overall performance of column photobioreactors can be significantly enhanced. These improvements not only address the inherent limitations of traditional reactors but also point the way for the future development of photobioreactors. Currently, experimental approaches to investigating the complex flow field characteristics inside bioreactors face significant challenges. In contrast, computational fluid dynamics (CFD) technology provides an efficient and reliable solution for simulating and optimizing flow fields in microalgae cultivation systems. Through CFD numerical simulation, key parameters such as gas flow rate, light distribution, and nutrient mixing can be systematically studied to evaluate their effects on the cultivation process. More importantly, CFD-based virtual design and geometric optimization methods not only significantly shorten bioreactor development cycles but also greatly reduce resource consumption associated with traditional trial-and-error approaches, offering robust theoretical support and technical assurance for performance enhancement and structural innovation of bioreactors [12]. Bitog [13] conducted simulations of various column photobioreactors using computational fluid dynamics. They assessed factors such as the time microalgae spent in light and dark zones, the frequency of light–dark cycles, and the turbulent kinetic energy within the photobioreactor. The study concluded that the most effective design for microalgae cultivation is a column photobioreactor featuring an internal spoiler and a conical extension at the bottom.

This paper presents the design of a novel column photobioreactor incorporating funnel-shaped baffles. By installing two funnel-shaped baffles inside the reactor, the internal space is divided into three distinct zones, effectively enhancing turbulent kinetic energy within the reactor and prolonging the fluid circulation cycle. The study employed CFD numerical simulation to systematically analyze the flow field characteristics inside the reactor, investigating the velocity distribution and turbulent kinetic energy variation of algal suspensions along the characteristic line at x = 30 mm in different directions. Furthermore, a high-speed camera system was utilized to study bubble dynamics near the baffle region, focusing on three key parameters: bubble velocity, bubble diameter, and overall gas holdup in the reactor. The experimental results demonstrate that, without altering the main structure of the reactor, simple structural optimization by adding two funnel-shaped baffles significantly improves turbulent kinetic energy near the baffles, reduces bubble diameter and velocity, promotes the growth of Scenedesmus cells, and enhances both CO₂ fixation rate and wastewater treatment efficiency compared to the pre-modified reactor.

2. Materials and Methods

2.1. Reactor Structure Parameters

Figure 1 present a schematic diagram of a column photobioreactor featuring funnel-shaped spoilers. This reactor has a closed column design with a height of 600 mm and a diameter of 150 mm. During operation, the height of the algal liquid within the reactor is maintained at 500 mm. Two funnel-shaped spoilers are installed at heights of 200 mm and 350 mm, respectively. Each spoiler has a height of 50 mm and is directly connected to the inner wall of the column reactor. Both the reactor wall and the spoilers are made of transparent acrylic material. CO₂ is supplied to the reactor through a circular aeration disk located at the bottom. The opening diameter (di) of the spoilers is adjustable and was modified to 20 mm, 30 mm, 40 mm, 50 mm, and 60 mm, depending on the experimental conditions. The relative diameter (d/D) is defined as the ratio of the spoiler’s opening diameter to the diameter of the column reactor. In this experiment, the values for d/D were 0.13, 0.20, 0.27, 0.33, and 0.40.

2.2. Model Selection and Parameter Setting for Numerical Calculation of Reactor Flow Field

In this study, the Fluent module in Ansys Workbench 2024 R1 was utilized for three-dimensional mesh creation and numerical simulation of the photobioreactor. The number of meshes for the column photobioreactor with funnel-shaped spoilers is 744,476, while the number for the column photobioreactor with the same external structure but without spoilers is 717,539. The mesh quality for both configurations is more than 0.4. The solver is set to transient, with a time step of 0.1 s. The Euler–Euler model was employed, and the standard k-ε turbulence model was selected. The effects of gas–liquid interphase resistance and turbulent dispersion force were also taken into account to simulate turbulent flow in the liquid phase. The vent area at the top of the reactor is designated as a degassing boundary, which allows only the gas phase to pass through [14]. The gas inlet is set as a velocity inlet, with the volume fraction of air set to 1, meaning only gas is introduced. The funnel-shaped spoilers are extracted using Boolean operations, and the computed surface is defined as a wall. The outer wall of the reactor and the area where the spoilers were located are both treated as no-slip wall boundaries [15].

The fluid radial flow velocity (Vx) and axial flow velocity (Vy) were calculated using the magnitude and direction of the velocity at each point along the characteristic line x = 30 mm. This line segment extends from point (30, 0, 0) to point (30, 0, 500). Additionally, turbulent kinetic energy was calculated based on this characteristic line. To analyze the flow characteristics of the Desmodesmus sp. CHX1 (phylum Chlorophyta) inside the reactor, the position and trajectory of the cells were tracked using a Discrete Particle Model (DPM) simulation. The positions of the particles were recorded every 0.1 s. After the flow field inside the reactor stabilized (with a calculation time of at least 20 s), 400 spherical particles were introduced simultaneously through two ports located at points (25, 0, 500) and (−25, 0, 500). The particle diameter was set to 5 μm with a density of 1000 kg/m³. The movement of these particles was tracked for 40 s, recording their positions every 0.1 s [4]. The moments when the microalgal cells entered and exited the light and dark zones were determined by tracking the movement of the particles within the reactor [16]. To enhance the accuracy of the calculations, a Discrete Random Walk (DRW) model was employed in the simulations, taking into account both pressure gradient forces and drag forces, while neglecting lift and volume forces [17].

2.3. Measurement of Bubble Movement in the Reactor

The movement of the bubbles in the reactor was captured using a high-speed camera system (Osaka, KEYENCE/Japan). Before the experiment, a ruler was placed in the designated shooting area to calibrate the bubble parameters. A movable high-speed camera was then set up parallel to the reactor’s axis to assess these parameters. After initial processing with the camera’s accompanying software VW-AX1 V2.00, the images were further refined using ImageJ software version 1.54f. A ruler positioned in the shooting area helped determine the size of the area, facilitating calculation of various bubble parameters.

An ultra-high-speed imaging system was used to capture images within a 40 mm (length) × 30 mm (height) observation region along horizontal axes located 150 mm and 300 mm from the central vertical axis of the column photobioreactor. The area occupied by each bubble was then calculated using ImageJ software after converting the images to grayscale and applying coloring techniques (Figure S1). This allowed for determination of the average diameter of the bubbles using Formula (1).

$$\mathrm{D}={\displaystyle \frac{2{{\displaystyle \sum }}_{i=1}^n\sqrt{\frac{S_i}{\pi }}}{n}}$$

(1)

In this equation, $s_i$ represents the area occupied by a single bubble, and n is the total number of bubbles.

An ultra-high-speed camera was utilized to record the duration from entry ($t_{i_1}$) to exit ($t_{i_2}$) of five bubbles by continuously filming in the axial position of the column reactor (Figure S2). This was done in two specific ranges: 145–155 mm (Y1) and 295–305 mm (Y2). The purpose of this recording was to determine the bubble flow rate, which is calculated using Formula (2).

$$v={{\displaystyle \sum }}_{i=1}^{n}0.01/\left(t_{i_2}-t_{i_1}\right)$$

(2)

The gas content rate was measured using the volume expansion method [18]. First, the initial liquid level height (h₀) of the reactor was recorded before aeration began. Next, the aerator was activated, and the aeration rate was controlled by a rotor flow meter. Once the liquid level in the reactor stabilized, the increase in liquid level height (h₁) was recorded. The gas content rate was then calculated using Formula (3).

ε = (h₁/h₀) × 100%

(3)

2.4. Measurement of Mixing Time and Mass Transfer Coefficient

Before measuring the mixing time, the deionized water in the photobioreactor was acidified by adding 35% (v/v) H₂SO₄ solution until the pH of the liquid in the reactor dropped to 3.1~3.3. Subsequently, 10 mL of 12 mol/L NaOH solution was added to the reactor. The pH change pulse signal was monitored in real time using a pH probe V1.5.16 (PHSJ-3F, Shanghai, China, Rex) inserted into the reactor and displayed in the data acquisition software. The mixing time was calculated as the time required for the pH of the liquid in the photobioreactor to reach 95% of its final state after addition of the alkaline solution [19].

Before testing the mass transfer coefficient, the pH probe was replaced with a dissolved oxygen probe (SD02, Hangzhou, China, Lohand). Subsequently, nitrogen gas was introduced into the reactor until the dissolved oxygen concentration in the water dropped below 0.8 mg/L. After stopping nitrogen aeration, air was introduced into the reactor instead. The region where the dissolved oxygen concentration ranged from 2 mg/L to 6 mg/L was selected to calculate the mass transfer coefficient using Formula (4).

$$k_L{a}_L=-\left(1/t\right)·\ln \left(\left(C^{*}-C_L\right)/(C^{*}-C_0)\right)$$

(4)

In this formula, C* represents the saturated concentration of dissolved oxygen [20].

2.5. Microalgal Growth and Contaminant Measurements

The microalga used in this study was Desmodesmus sp. CHX1 (phylum Chlorophyta) [21], which was cultured in BG11 medium until it reached the logarithmic phase for inoculation for subsequent experiments. The experiment was conducted over a 10-day period. Both reactors were placed simultaneously in an artificial climate chamber (MGC-300, Changzhou, China, Jintan friend), with experimental conditions set to a constant temperature of 25 °C and a light intensity of 2000 lux. Continuous illumination was maintained for 24 h per day. The artificial climate chamber was equipped with six parallel LED light strips on each side and featured an intelligent program control system for precise light intensity regulation. To ensure uniformity and accuracy of illumination conditions during the experiment, a digital illuminometer (TA631A, Suzhou, China, TASI) was used to monitor and calibrate the light intensity on both sides of the chamber, maintaining it strictly within the set range of 2000 ± 50 Lux. A total of 9 L of aquaculture wastewater, with a concentration of 40% (v/v), was added to the reactors. The aquaculture wastewater was filtered through a 0.45 μm microporous membrane before the experiment to eliminate impurities that could interfere with measurement of algal cell dry weight. Subsequently, 0.1 g/L of microalgae in the logarithmic growth phase was added to the reactors for the experiment. Additionally, a gas mixture containing 10% carbon dioxide was introduced at a flow rate of 0.02 vvm.

The optimal optical density refers to the characteristic absorption peak of microalgae within a specific wavelength range, which can be used to characterize the relative growth of microalgae [22]. Using a UV spectrophotometer (DR2000, Loveland, USA, HACH), it was determined that this strain exhibits an optimal absorption peak at a wavelength of 620 nm.

The measurement method for microalgal dry weight is as follows. First, a 0.45 μm microporous filter membrane is placed in an oven and dried at 120 °C for 4 h until its weight reaches a constant value (W₁). Subsequently, a 10 mL algal sample (V₁) is filtered using the pre-dried and pre-weighed filter membrane. After filtration, the membrane, along with the algal sample, is placed back into the oven and dried at 120 °C until its weight reaches a constant value (W₂). The microalgal dry weight (MDW, g/L) is calculated using Formula (5):

$$MDW\left(g/L\right)=\left(W_2-W_1\right)/V_1\times 1000$$

(5)

The rate of carbon dioxide fixation was calculated with the following Formula (6):

$$R_{C{O}_2}=\left(\left(C_{C2}\times m_2-C_{C1}\times m_1\right)-\left(C_{water,1}-C_{water,2}\right)\right)\times M_{C{O}_2}/\left(\left(t_2-t_1\right)\times M_C\right)$$

(6)

where C_C2 is the carbon content (%) of microalgal dry weight m₂ at time t₂, C_C1 is the carbon content (%) of microalgal dry weight m1 at time t₁, C_water,1 is the carbon content (g/L) in the liquid phase of the culture medium at time t₁, and C_water,2 is the carbon content (g/L) in the liquid phase of the culture medium at time t₂. In addition, $M_{C{O}_2}$ is the molecular weight of CO₂, whereas M_C is the molecular weight of carbon [23]. The carbon content in both algal cells and the liquid phase was measured using a total organic carbon/total nitrogen (multi N/C 2100s, Jena, Germany, Analytikjena) analyzer.

Chlorophyll is a ubiquitous photosynthetic pigment in microalgae, widely present in all oxygenic photoautotrophic algal cells. Among these, chlorophyll a is the most predominant type, accounting for approximately 0.1% to 9.7% of the dry weight of algal cells. As the core component of the pigment–protein complex in the photosynthetic reaction center, chlorophyll a plays a pivotal role in the conversion of light energy into chemical energy. The chlorophyll content not only serves as a crucial indicator for assessing the ability of microalgae to convert inorganic nutrients into organic matter, but also reflects the efficiency of microalgae in utilizing organic nutrients and other nutritional elements in the culture medium [24]. In this study, the chlorophyll a content was determined using the hot ethanol method [25]. Determination of COD, total TP, and NH₄⁺-N was carried out according to standard methods [26].

3. Result and Discussion

3.1. Numerical Calculation of the Flow Field Inside the Reactor

Figure 2a,b display the numerical calculation results for the flow field in two types of column photobioreactors: one with two funnel-shaped spoilers and the other a standard column photobioreactor. Both reactors share the same overall structure, differing only in their internal designs. In the standard column reactor, bubbles primarily ascend vertically, while the liquid moves horizontally at both the bottom and top of the reactor, creating a circulating flow. However, this flow pattern leads to poor mass transfer of microalgae, which is not conducive to their growth. Conversely, introduction of the funnel-shaped spoiler plates significantly alters the internal flow dynamics of the reactor. The spoiler plates divide the reactor into three sections, enabling the fluid to move horizontally and generating three distinct circulating flow areas. This modification increases horizontal flow of the liquid phase, thereby enhancing mass transfer of the algal liquid. Additionally, the funnel-shaped design allows liquid at the upper end to flow rapidly toward the opening at the bottom, where it meets the updraft. As a result, some of the liquid follows the upward movement, creating a whirlpool effect at the upper end of the spoilers. Due to the influence of gravity, a portion of the liquid flows to the lower end of the spoilers. As the airflow rises, it creates a whirlpool at this lower end. The presence of the spoilers disrupts the updraft; some of the airflow continues upward, while another part flows horizontally at the lower end, forming a cyclone. This cyclone enhances mixing of the liquid that falls from above with the liquid below. The formation of these cyclones divides the reactor into three distinct regions. These regions interact with one another, further improving the mass transfer coefficient in the liquid phase. This setup increases the residence time of algal cells in the light zone, which helps to enhance the dissolution rate of CO₂.

To analyze the effect of the spoilers on the flow field, the radial velocity, axial velocity, and turbulent kinetic energy of the algal liquid at the characteristic line x = 30 mm are illustrated in Figure 2c for two different reactors. The presence of the spoilers significantly increases the radial flow velocity of the algal liquid near z = 150 mm and z = 300 mm, also altering its direction. For instance, at z = 150 mm, when the axial position is adjusted from z = 130 mm to z = 160 mm, the radial flow velocity rises from 0.0145 m/s to 0.13 m/s. This velocity is notably higher than the −0.04 m/s observed without the spoiler. Subsequently, the radial flow velocity drops rapidly, reaching −0.009 m/s at z = 166 mm, indicating a change in direction from right to left. The trend of turbulent kinetic energy along the characteristic line of the reactor with spoilers closely follows that of the radial flow velocity. Notable peaks occur at z = 157 mm and z = 308 mm, where the turbulent kinetic energies are 0.0149 m²/s² and 0.01326 m²/s², respectively. The changes in radial flow velocity and turbulent kinetic energy in the reactor with the spoilers are also more pronounced compared to the reactor without them, which has values of 0.0029 m²/s² and 0.00673 m²/s², respectively. These data suggest that the presence of the spoilers effectively influences the flow field and enhances the mixing performance of the reactor [27]. The variation trend of axial flow velocity significantly differs from the other two. When the radial flow velocity reaches its peak, the axial flow velocity near the corresponding position drops to its minimum value. This phenomenon is primarily attributed to the hydrodynamic effects of the baffle. Specifically, the baffle obstructs the upward flow of algal suspension, causing the upward-moving fluid to be divided and resulting in conversion of part of the axial flow into radial flow. This mechanism can be verified through comparative experiments: under the same positional conditions, the axial flow velocity in a device without a baffle is significantly higher than that in a device with a baffle, further confirming the baffle’s partitioning effect on the upward fluid. From a photobiological perspective, the reactor can be divided into two characteristic regions: the light zone (where light intensity ≥ light saturation point, i.e., the light intensity at which the photosynthetic rate reaches its maximum) and the dark zone (where light intensity < light saturation point). The use of baffles significantly enhances radial flow, which alters the direction of fluid velocity and influences the trajectory of bubbles. As a result, bubbles follow more complex paths within the algal suspension, increasing opportunities for contact between the bubbles and microalgal cells. This process improves mass transfer efficiency and extends the residence time of light in the zone. Additionally, the dynamic flow helps prevent local stagnation of microalgae, ensuring uniform distribution of nutrients and CO₂ throughout the reactor while avoiding the formation of concentration gradients. Furthermore, the variation in velocity direction increases turbulent kinetic energy. This rise in turbulent kinetic energy facilitates rapid renewal of the gas–liquid interface, reduces the boundary layer resistance to CO₂ diffusion, and further optimizes gas transfer efficiency. Ultimately, these effects enhance the CO₂ fixation rate of microalgae.

3.2. Influence of Funnel-Shaped Spoilers on Microalgal Flash Frequency and Light Zone Time Ratio

The flash frequency is defined as the number of times algal cells move between light and dark zones per unit time, while the light zone time ratio refers to the proportion of time spent in illuminated areas during a specified period. Both parameters directly reflect the photosynthetic efficiency of microalgae. As illustrated in Figure 3, when the gas flow rate increases from 0.02 vvm to 0.1 vvm, both the flash frequency and the light zone time ratio show an upward trend in both reactor configurations. This suggests that changes in the internal flow field enhance the light/dark cycling for microalgae. The presence of spoilers significantly modifies the hydrodynamic regime by generating multiple vortices near the baffles, which intensifies horizontal fluid motion. As a result, the reactor is effectively divided into three interconnected zones, transforming the single-circulation pattern observed in the conventional reactor into a multi-circulation mode. This modification greatly increases the cycling frequency of algal cells between light and dark regions. At a gas flow rate of 0.1 vvm, the reactor equipped with spoilers achieved superior performance, reaching a flash frequency of 0.474 Hz and a light zone time ratio of 34.8%. In contrast, the baffle-free reactor showed a flash frequency of 0.396 Hz and a light zone time ratio of 30.2%. The conventional column reactor only allows for a simple bottom-to-top-to-bottom circulation pattern, leading to consistently lower light/dark transition frequencies and illumination exposure compared to the spoiler-modified design.

3.3. Bubble Movement Captured by High-Speed Camera System

3.3.1. Effect of a Funnel-Shaped Spoiler on Air Bubbles

The effect of the spoiler on bubble diameter is illustrated in Figure 4a. Under identical conditions, the bubble diameter in the reactor with the spoiler is consistently smaller than that in the reactor without the spoiler. This is due to the shearing effect of the spoiler on radial fluid flow. As the bubbles rise, the liquid pressure exerted on them decreases, causing the bubbles to expand during their ascent. Therefore, the bubble diameter at location Y2 is always larger than that at location Y1 for the same gas flow rate in both reactors. Turbulent stress generated by fluid turbulence is the primary cause of bubble breakage. As the gas flow increases, turbulent kinetic energy also rises in both reactors. This increase in turbulence leads to higher shear stress on the bubbles, exacerbating collisions and breakage, which ultimately reduces the bubble diameter. As the gas flow increases, the presence of a spoiler intensifies turbulence within the flow field. At a gas flow rate of 0.02 vvm at location Y1, the bubble diameters are 1.216 mm with the spoiler and 1.241 mm without it. The use of the spoiler results in a decrease in bubble diameter by 2.01%. When the gas flow rate increases to 0.1 vvm at Y1, the bubble diameter without the spoiler is 0.923 mm, and this diameter decreases by 2.01% with the spoiler. At the same gas flow rate of 0.1 vvm at Y1, the bubble diameter drops by 7.37% to 0.855 mm with the spoiler, compared to 0.923 mm without it. The reduction in bubble diameter enhances their susceptibility to turbulent kinetic energy fluctuations, making them more easily entrained and dispersed by the fluid flow. This phenomenon inhibits rapid bubble coalescence and upward migration, thereby promoting gas retention within the liquid phase and ultimately increasing the gas content. Thus, the trend of increasing gas content corresponds to the trend of decreasing bubble diameter. As illustrated in Figure 4b, the difference in gas content between scenarios with and without the spoiler increases from 0.0088% at a gas flow rate of 0.02 vvm to 0.0314% at a gas flow rate of 0.1 vvm.

Figure 5 illustrates the effect of spoilers on bubble velocity. The experimental results indicate that, at both monitoring locations, Y1 and Y2, the bubble velocity is significantly higher without spoilers than with them. This phenomenon can be attributed to the spoilers converting part of the axial flow of gas into radial flow, thereby reducing the kinetic energy of the rising bubbles and causing a decrease in velocity. Additionally, due to the continuous action of liquid resistance, the kinetic energy of the bubbles gradually dissipates as they rise, resulting in the bubble velocity at position Y2 always being lower than that at position Y1. As the gas flow rate increases, the velocity of the ascending gas stream correspondingly rises, leading to an increase in the gas flux passing through the spoilers, which enhances their regulatory effect on the gas flow. Specifically, at position Y1, when the gas flow rate is 0.02 vvm, the bubble velocity with spoilers installed is 0.1136 m/s, which is 0.0027 m/s lower than the 0.1163 m/s observed without spoilers. When the gas flow rate is increased to 0.1 vvm, the difference in velocity becomes more pronounced, with the bubble velocity with spoilers (0.2173 m/s) being 0.0112 m/s lower than without spoilers (0.2285 m/s). This reduction in velocity not only effectively extends the residence time of CO₂ within the reactor, increasing its contact time with the microalgae [28], but also, by decreasing the bubble diameter and increasing the number of bubbles, enhances the mixing effect of the algal suspension in the reactor [29].

3.3.2. Influence of Spoiler Structural Parameters on Air Bubbles

Figure 6 illustrates the influence of the spoiler’s relative diameter (d/D) on bubble velocity, diameter, and gas holdup under a gas flow rate of 0.08 vvm. As the relative diameter increases, the bubble diameter at position Y1 rises from 0.9353 mm to 1.0121 mm. This phenomenon can be explained as follows: an increase in the relative diameter enhances the resistance to the ascending gas flow, which leads to a greater amount of gas flow being intercepted by the spoiler. Consequently, this raises the turbulent kinetic energy near the spoiler. The heightened turbulent kinetic energy intensifies the shear-induced breakup of bubbles, resulting in a reduction in bubble diameter, which in turn increases the gas holdup. When the relative diameter (d/D) decreases from 0.4 to 0.13, the bubble velocity at position Y1 drops from 0.1863 m/s to 0.1712 m/s. This reduction in relative diameter causes more gas flow to shift from axial motion to radial motion, thus decreasing the kinetic energy of the ascending gas flow. Comparative analysis reveals that, in relation to position Y1, the bubble diameter at position Y2 increases by 4.21% and 6.64% at relative diameters of 0.4 and 0.13, respectively. At the same time, the bubble velocity decreases by 4.16% and 15.5%, respectively. This trend is attributed to the reduced ascending gas flow caused by the decrease in relative diameter, which weakens the shear forces acting on the bubbles, thereby lowering the axial velocity of the gas flow as it reaches the upper spoiler.

3.4. Funnel-Shaped Spoiler to Reduce Mixing Time and Improve Mass Transfer Coefficient

The mixing time is defined as the duration required for the liquid phase to return to a homogenized state following an external disturbance. This metric reflects the degree of homogenization of the liquid phase and its dissolved bubbles with the material components [30]. Figure 7a illustrates the effect of the spoilers on mixing time and mass transfer coefficient at different gas flow rates, using a relative diameter of 0.2. At a gas flow rate of 0.02 vvm, the mixing time decreases by 5.7%, while the mass transfer coefficient increases by 10.4% compared to conditions without spoilers. With the spoilers, the mixing time and mass transfer coefficient are 73.24 s and 25.39 h⁻¹, respectively. In contrast, without the spoilers, the mixing time is 77.67 s, and the mass transfer coefficient is 22.75 h⁻¹. Conventional column photobioreactors typically generate swirl flow only after the gas reaches the top of the reactor, which alters the direction of fluid flow. In contrast, the presence of baffles in the reactor facilitates swirl formation in the rising gas stream. This enhancement promotes horizontal fluid flow within the reactor and increases turbulent kinetic energy. For example, at an axial position of z = 150 mm, the baffles increase the horizontal flow velocity from −0.04 m/s (without baffles) to 0.13 m/s. Additionally, the turbulent kinetic energy rises from 0.0029 m²/s² to 0.0149 m²/s². The improved horizontal velocity and turbulent kinetic energy lead to more frequent gas-liquid exchanges near the baffles, thereby reducing the gas-liquid mixing time and enhancing the efficiency of gas-liquid mass transfer. As the gas flow rate increases, the mixing time for the unscrambled plate decreases from 27.41 s to 19.03 s when the gas flow rate reaches 0.1 vvm. This represents a 30.57% reduction in mixing time. Additionally, the mass transfer coefficient for the unscrambled plate rises from 51.55 h⁻¹ to 58.69 h⁻¹, indicating a 12.17% increase in the mass transfer coefficient. This improvement is attributed to the increased gas flow rate, which enhances both the horizontal flow rate and the gas horizontal flow rate. The turbulent kinetic energy near the spoilers also increases, leading to greater horizontal flow rates and a higher frequency of gas-liquid exchanges. Consequently, the mixing time shortens significantly for the unscrambled plate.

When the gas flow rate is set at 0.08 vvm, the impact of the relative diameter of the spoilers (d/D) on the mixing time and mass transfer coefficient is illustrated in Figure 7b. As the relative diameter decreases from 0.4 to 0.2, the mixing time in the reactor reduces from 34.16 s to 27.21 s, while the mass transfer coefficient increases from 47.37 h⁻¹ to 53.61 h⁻¹. This reduction in the relative diameter of the spoilers enhances their interception effect at the same gas flow rate, resulting in increased turbulence energy of the fluid near the spoilers. Consequently, the radial flow velocity around the spoilers rises, which accelerates mixing of microalgae and nutrients within the section subdivided by the spoilers. This acceleration leads to a decrease in mixing time and an increase in the mass transfer coefficient. Continuous reduction in the relative diameter did not lead to shorter mixing times or improved mass transfer coefficient. When the relative diameter is further decreased to 0.13, the mixing time increases to 30.32 s, and the mass transfer coefficient drops to 51.76 h⁻¹, which is less effective compared to a relative diameter of 0.2. This outcome is attributed to excessive interception caused by the spoilers, which retains too much of the gas flow in the lower region of the reactor. As a result, turbulence kinetic energy near the lower spoilers increases, while the turbulence kinetic energy near the upper spoilers and the top of the reactor decreases due to insufficient gas flow. Consequently, the mixing of algal blooms and nutrients lacks the necessary kinetic energy, slowing down the mixing times of fluids in the middle and upper sections of the reactor. Although there is an effective increase in turbulent kinetic energy in the lower region, the overall mixing time remains unchanged, and the mass transfer coefficient does not improve. Reducing mixing time is crucial for allowing nutrients to quickly reach all parts of the reactor, providing sufficient inorganic carbon and a conducive mixing environment for microalgae [31]. Additionally, bubbles in the reactor with a high mass transfer coefficient can efficiently remove the oxygen produced by photosynthesis, preventing the accumulation of dissolved oxygen that can inhibit microalgal growth [3]. Reducing the relative diameter does not sustainably increase the growth rate of microalgae. Additionally, adequate mixing of nutrients is crucial, as a diameter that is too small can cause excessive shear at the bottom spoilers, leading to the death of microalgae, which is detrimental to their growth [32]. Therefore, choosing the appropriate relative diameter is essential, and it can be concluded that the optimal relative diameter is 0.2.

3.5. Funnel-Shaped Spoilers Promote Microalgae Growth and Enhance Pollutant Degradation Efficiency

Figure 8 displays physical images of the algal culture in the two reactors on the first day and the tenth day. At the initial stage of the experiment, the algal culture appeared brown due to the low concentration of microalgae cells, reflecting the inherent color of the aquaculture wastewater. As cultivation progressed, the concentration of microalgae cells increased, and the culture gradually exhibited the characteristic green color of the algal cells. The changes in OD620, chlorophyll a, dry weight, and algal pH in the two photobioreactors are shown in Figure 9. The experimental results indicate that, starting from the fourth day, the column-type photobioreactor equipped with a funnel-shaped spoiler consistently outperformed the conventional column-type photobioreactor without a spoiler in all three key metrics. During the initial cultivation stage (day 2), the reactor with the funnel-shaped spoiler exhibited a higher OD620 value, and this advantage persisted until the end of the experiment. Specifically, the absorbance increased by 0.55, which was 20.6% higher than that of the non-spoiler reactor (0.456). Additionally, differences in chlorophyll a content began to emerge on the third day of cultivation and became more pronounced by the fourth day. By the end of the experiment, the chlorophyll a increment in the reactor with the funnel-shaped spoiler reached 4.58 mg/L, which was 13.9% higher than that of the conventional reactor (4.02 mg/L). The design of the funnel-shaped spoiler promoted the absorption and utilization of nutrients by algal cells, thereby enhancing the growth rate of the algae. This increase in growth rate led to a higher nutrient demand by the microalgae. As shown in Figure 10, the CO₂ fixation rate and pollutant removal rates in the two types of reactors demonstrated that the column photobioreactor with the spoilers achieved removal efficiencies of 46% for COD, 71% for TP, and 64.9% for NH₄⁺-N, with an overall removal efficiency of 76% at the end of the experiment. In contrast, the removal efficiencies in the column photobioreactor without the spoilers were 43.75% for COD, 60.37% for TP, and 72.78% for NH₄⁺-N.

Due to the continuous passage of CO₂, CO₂ was rapidly dissolved into the algal bloom, leading to a continuous decrease in the pH of the algal bloom during the first two days, and the pH in the reactor with and without the spoilers decreased from 8.96 and 8.94 to 6.46 and 6.39, respectively. Subsequently, as the microalgae continued to grow, the microalgae required an increasing carbon source and absorbed more and more CO₂, leading to a continuous increase in the pH of the algal bloom after the second day. The faster the growth rate of the microalgae, the higher the consumption of CO₂, so the increase in dry weight of the microalgae will inevitably lead to an increase in algal pH and carbon sequestration rate [33]. The increase in dry weight of algal cells in the reactor with and without spoilers was 0.26 g/L and 0.22 g/L, respectively, resulting in PH and CO₂ fixation rates of 6.94 and 150 mg/L/d in the new photobioreactor, which were 2.66% and 13.64% higher than that of the photobioreactor without spoilers, respectively.

4. Conclusions

This study aimed to enhance the contact duration between microalgae and nutrients in a reactor, thereby improving the growth rate of microalgae and the efficiency of pollutant removal. To achieve this, two funnel-shaped spoilers were incorporated into a conventional column photobioreactor. In the modified reactor, the rising gas stream was bifurcated by the spoilers, significantly enhancing horizontal fluid flow within the system. At a gas flow rate of 0.02 volume per volume per minute (vvm), the turbulent kinetic energy near the upper and lower spoilers increased by 88.66% and 424.56%, respectively, compared to the control reactor that did not have spoilers. This increased horizontal flow velocity and turbulent kinetic energy enhanced the frequency of gas–liquid exchange near the spoilers, leading to a 5.7% reduction in mixing time and a 10.4% increase in the mass transfer coefficient. The spoilers also affected the rising gas streams by reducing bubble diameter from 1.241 mm to 1.216 mm and decreasing the rising velocity (at position Y1) from 0.1163 m/s to 0.1136 m/s. These changes prolonged bubble residence time and increased gas holdup by 0.0088%. Comparative cultivation experiments demonstrated that the novel reactor increased biomass dry weight by 18.18%, improved the CO₂ fixation rate by 13.95%, and enhanced the removal efficiencies of COD, TP, and NH₄⁺-N by 4.48%, 7.5%, and 4.7%, respectively, compared to the conventional design. This innovative configuration effectively optimizes microalgae cultivation efficiency and offers a practical solution for reducing carbon emissions, recovering wastewater resources, and advancing green low-carbon development. As such, it significantly contributes to the achievement of sustainable development goals. However, scaling up this technology will require further investigation into the synergistic effects of wastewater treatment and carbon sequestration in photobioreactors. Future research should include life cycle assessment methodologies to systematically evaluate the economic viability and environmental benefits of this technical approach. A quantitative analysis of input–output ratios at different operational stages will provide crucial insights for commercial-scale implementation.