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1 Overview
Our goal is to develop an efficient industrial device to recover rare earth elements from bioleaching solutions of rare earth ores or mining wastewater. Rare earth elements are highly valued for their critical role in electronic products, magnetic materials, and other high-tech applications. However, the recovery rates of rare earth elements from industrial wastewater are low, and efficient and economical treatment methods are lacking. Therefore, we have designed a set of modular equipment (Figure 1) to optimize the wastewater treatment process, maximizing the recovery of rare earth elements.
Figure 1. CaptuREE: Schematic Diagram of a Sustainable Biomining Process for Lanthanide Rare Earth Elements
In our sustainable biomining process flowchart for rare earth elements (Figure 1), we integrate a complete process involving bioleaching, biosorption, and biosecurity modules. However, during the actual hardware construction, we focus on the membrane bioreactor, as this is the key step in recovering rare earth elements through biosorption in the mining wastewater treatment process. The upstream and downstream modules, such as agitation leaching, membrane filtration, pH neutralization, and sterilization, are already relatively mature and can be achieved using existing process equipment.
Our device is designed as a modular system, divided into three core modules: Moving Bed Biofilm Reactor (MBBR) Module, Membrane Aerated Biofilm Reactor (MABR) Module, and a Water Environment Monitoring System based on STM32.
Figure 2. Hardware Model Diagram (Water Environment Monitoring System Based on STM32 is not Illustrated)
Module 1: Moving Bed Biofilm Reactor (MBBR) Module
In the MBBR module, engineered yeast modified via genetic engineering is immobilized on K1 carriers, forming a stable biofilm. The design of the K1 carrier not only provides excellent surface adhesion conditions and a high specific surface area, but also promotes microbial growth and the adsorption efficiency of rare earth elements through its hollow structure and unique physical properties. In the MBBR reaction tank, the biofilm remains suspended due to the action of an aeration pump, and the efficient material exchange in this dynamic environment greatly enhances the efficiency of wastewater treatment. Once the adsorption process is complete, the carrier can be easily retrieved and replaced with new carriers, enabling continuous wastewater treatment and rare earth element recovery.
Figure 3. Moving Bed Biofilm Reactor (MBBR) Module Design Diagram
Module 2: Membrane Aerated Biofilm Reactor (MABR) Module
In the MABR module, the engineered yeast is loaded onto hollow fiber membrane carriers in the MABR. This design further optimizes the recovery efficiency of rare earth elements. We have adopted a new type of combined aeration MABR membrane module, with the aeration pipe directly installed beneath the membrane module. This design not only solves the problem of difficulty in disassembly during maintenance but also utilizes the exhaust gas from the MABR for regeneration aeration, thereby reducing energy consumption and improving overall system efficiency.
Figure 4. Membrane Aerated Biofilm Reactor (MABR) Module Design Diagram
Module 3: Water Environment Monitoring System Based on STM32
In this module, we use the STM32 microcontroller to monitor water quality indicators, such as temperature, pH value, and turbidity. Through these data, we can accurately track changes in the wastewater environment, thereby providing optimal conditions for the growth and metabolic activities of the engineered yeast. The data collected by the sensors is uploaded to the cloud via the ESP8266 Wi-Fi module, enabling remote monitoring and data analysis. The system also supports remote monitoring and control through a mobile app, further enhancing convenience and flexibility. In addition, the system is equipped with a solar power supply module, which converts solar energy into electricity, reducing dependence on traditional power sources, lowering operational costs, and achieving green energy use.
Figure 5. STM32 Microcontroller Diagram
System Series Recirculation Design
Our system uses a series recirculation design, connecting Module 1 (MBBR) and Module 2 (MABR) through water pipes and valve controls, achieving precise water flow adjustment and convenient operation. The two modules have complementary treatment mechanisms: the MBBR module uses biofilms on suspended carriers for efficient degradation of organic matter and preliminary adsorption of rare earth elements, while the MABR module uses hollow fiber membranes to provide more efficient oxygen transfer, further optimizing the biofilm's treatment capacity and enhancing the recovery efficiency of rare earth elements. The series recirculation design ensures that wastewater passes through multiple different treatment stages, achieving more thorough pollutant removal and rare earth element recovery.
The modular design not only simplifies carrier replacement and system maintenance but also offers high flexibility and scalability, allowing for customization based on different wastewater components and treatment requirements. In addition, the water environment monitoring system installed between the two modules can detect changes in water quality in real-time, ensuring that each module always operates under optimal conditions.
Figure 6. System Series Recirculation Design
Through the collaboration of the above modules, our equipment can efficiently recover rare earth elements from industrial wastewater while ensuring low energy consumption and high operational flexibility. This system provides a sustainable solution for the wastewater treatment industry and opens new possibilities for the recovery and reuse of rare earth elements.
2 Moving Bed Biofilm Reactor (MBBR) Module
Figure 7. Moving Bed Biofilm Reactor module (MBBR) Module
2.1 Problems to Be Solved
The Moving Bed Biofilm Reactor (MBBR) system, as an innovative wastewater treatment technology, can efficiently remove pollutants from wastewater. However, in traditional MBBR systems, the formation and maintenance of biofilms pose certain challenges. When treating industrial wastewater containing rare earth elements, biofilm fouling can lead to a decline in carrier material efficiency, thus affecting the recovery of rare earth elements from the wastewater. Therefore, we need a more flexible and convenient MBBR system that can quickly and efficiently replace the carrier material when the biofilm becomes saturated or contaminated. Such a system will not only improve the recovery efficiency of rare earth elements but also reduce system downtime and maintenance costs, enhancing the overall economic efficiency and operational convenience of the wastewater treatment process.
2.2 Hardware Design
The FLO11 gene is responsible for surface adhesion in Saccharomyces cerevisiae and helps bind cells to substrates like plastic. By immobilizing engineered yeast that overexpresses FLO11 on suspended K1 carriers, a strong biofilm with enhanced adhesion properties is formed (Figure 8). This yeast biofilm increases the contact area between the yeast and rare earth elements in the wastewater, effectively adsorbing and removing rare earth elements while simultaneously treating the wastewater.
Figure 8. Saccharomyces Cerevisiae Strains Overexpressing FLO11 Gene Can Form Biofilms on Plastic Carrier Fillers
The system utilizes K1 carriers, commonly used in aerobic biofilm fluidized bed processes. These carriers feature a three-dimensional hollow structure. In the reactor, aerobic bacteria on the exterior of the K1 carriers are responsible for organic matter removal, while the K1 carriers provide strong biofilm support, enhancing the adsorption capacity for rare earth ions.
Figure 9. K1 Carriers in Different Sizes
The specific reasons for choosing K1 carriers are as follows:
1. Rapid biofilm formation: K1 carriers have a special surface treatment, with a rough internal surface that facilitates microbial adhesion and the rapid formation of biofilms.
2. High hydrophilicity and biocompatibility: The carriers are modified with hydrophilic groups and charge-modified to further improve biofilm formation and increase biomass.
3. High surface area and biomass: K1 carriers have a surface area of ≥600 m²/m³, with biomass ranging from 10-20 g/m², providing a large microbial attachment and growth surface within a relatively small volume, thereby increasing the volumetric load capacity of the reactor.
4. Superior physical properties: Made from HDPE (high-density polyethylene), K1 carriers have a density of 0.96 g/cm³, ensuring good suspension in the reactor.
We also use an STM32 microcontroller to perform real-time turbidity monitoring, which allows for the quick identification of membrane fouling and, when necessary, the rapid replacement of carriers to maintain system efficiency.
Figure 10. Turbidity Sensor
Additionally, we have equipped the system with an efficient aeration system, including aeration disks and water flow injection devices. The aeration disks generate bubbles to facilitate the suspension and mixing of the carriers, while the water flow helps maintain an even distribution of the carriers, ensuring that they can effectively perform biological treatment within the reactor.
Figure 11. Aeration System
2.3 System Features
1. Flexible carrier replacement: The system design allows for the quick replacement of K1 carriers when the biofilm becomes saturated or fouled, reducing downtime and maintaining high wastewater treatment efficiency.
2. Real-time turbidity monitoring: Using the STM32 microcontroller, the system can detect membrane fouling in real-time and adjust operations based on monitoring data, ensuring the stability and reliability of the wastewater treatment process.
3. High-performance carriers: The K1 carriers provide excellent biofilm support, significantly enhancing the adsorption efficiency of rare earth ions and improving overall wastewater treatment effectiveness.
4. Comprehensive aeration system: The system is equipped with high-efficiency aeration disks and water flow injection devices to ensure the optimal suspension and uniform distribution of the carriers in the reactor, optimizing the contact between wastewater and the biofilm and increasing treatment efficiency.
2.4 Components
1. STM32 microcontroller: For real-time turbidity monitoring and system control.
2. K1 carriers: Provide biofilm support in wastewater treatment and enhance rare earth ion adsorption capacity.
3. Aeration system: Includes aeration disks and water flow injection devices, responsible for carrier suspension and even distribution.
4. Turbidity sensor: Monitors changes in wastewater turbidity and provides data on membrane fouling.
5. Control module: Includes a display and control interface for system settings and data viewing.
6. Other accessories: Such as pipes, connectors, and brackets, ensuring system integrity and stability.

Table 1. Cost calculation for the MBBR module
3 Membrane Aerated Biofilm Reactor (MABR) Module
Figure 12. Membrane Aerated Biofilm Reactor (MABR) Module
3.1 Problems to Be Solved
To meet the need for more personalized wastewater treatment and rare earth element recovery, we decided to use a Membrane Aerated Biofilm Reactor (MABR) system. This system utilizes hollow fiber membrane materials to transfer oxygen from the aeration side to the wastewater, and engineered yeast accumulates on the membrane surface adjacent to the wastewater, forming a biofilm that adsorbs rare earth elements.
However, in traditional MABR systems, the aeration system cannot fully utilize the exhaust gas from the MABR, affecting the economic efficiency and effectiveness of wastewater treatment. Therefore, we need an improved MABR system that can make maintenance and replacement of the membrane module easier, reduce energy consumption, and improve the efficiency of rare earth element recovery during wastewater treatment while enhancing the overall operational convenience of the system.
3.2 Hardware Design
We use a selectively permeable membrane loaded with engineered yeast to improve gas-liquid mass transfer efficiency. Air enters the MABR membrane box, and oxygen permeates outward through the MABR membrane fibers, while organic matter and rare earth elements in the reactor transfer to the membrane. Through this synergistic action, engineered yeast forms a biofilm on the exterior of the MABR membrane, enabling rare earth adsorption (Figure 13).
Figure 13. Principle of MABR Membrane.[2]
We employed a curtain-type MABR membrane module, with the membrane sheets hanging vertically in the reactor. Wastewater flows over the membrane surface while gas passes through the hollow fibers within the membrane into the biofilm layer. This compact design also facilitates efficient oxygen transfer.
Furthermore, we designed a combined aeration MABR membrane module, where the aeration pipe is installed directly below the membrane module, allowing for easier cleaning or replacement of the membrane. The aeration pipe position is carefully designed to ensure that the rising bubbles effectively agitate the wastewater, thereby enhancing the contact efficiency between the wastewater and the biofilm.
Figure 14. Design of the Curtain-type Combined Aeration MABR Membrane Module
3.3 System Features
1. Efficient oxygen transfer: The MABR membrane has the advantage of very low resistance to gas transfer within the micro-pores, providing oxygen to aerobic microorganisms more effectively, reducing oxygen loss, and improving oxygen utilization efficiency.
2. Curtain-type membrane module: This design optimizes the gas-liquid contact area, improving the activity and stability of the biofilm. It also results in a more compact arrangement of membrane modules, effectively increasing the membrane surface area per unit volume, and further enhancing the recovery efficiency of rare earth elements.
3. Directly installed aeration pipe: By placing the aeration pipe directly under the membrane module, the rising bubbles enhance the mixing of wastewater, improving oxygen transfer efficiency and reducing energy consumption.
4. Detachable membrane module design: The modular design makes it easier to remove and replace the membrane module, effectively solving the maintenance difficulties that are often caused by the complex structure of traditional MABR systems.
3.4 Components
1. MABR hollow fiber membrane: Serves as the carrier for the biofilm, supporting efficient rare earth element adsorption.
2. Aeration pipe: Installed beneath the membrane module to provide necessary bubbles for aeration and wastewater mixing.
3. Modular membrane module: Easy to disassemble and replace, reducing maintenance time and costs.
Table 2. Cost calculation for the MABR module
4 Water Environment Monitoring System Based on STM32
Figure 15. Display Diagram of the Water Environment Monitoring System Based on STM32
4.1 Problems to Be Solved
To achieve the goal of recovering rare earth elements from wastewater, it is essential to precisely monitor various parameters of the microbial environment to optimize their metabolic activities and adsorption efficiency. Currently, most water quality monitoring equipment used in laboratories is expensive and difficult to customize, which limits the scalability and practicality of synthetic biology experiments.
We need a low-cost and flexible water quality monitoring solution that can accurately monitor environmental parameters related to microbial activities during wastewater treatment, such as temperature, pH, and turbidity, in real-time to improve the performance and recovery efficiency of engineered yeast.
4.2 Hardware Design
We developed a low-cost, modular water environment monitoring system that allows researchers to optimize the growth conditions of strains and enhance their rare earth element adsorption efficiency in wastewater by precisely monitoring water quality parameters.
This system uses an STM32 microcontroller as the core controller. STM32 is a 32-bit microcontroller (MCU) based on the ARM Cortex-M core, introduced by STMicroelectronics. We integrated various sensors on the microcontroller, including temperature, pH, and turbidity sensors, and used an ESP8266 Wi-Fi module to upload data to the cloud in real-time. Users can view water quality data remotely via a mobile app and make necessary adjustments. In addition, the system is powered by solar energy, enabling an environmentally friendly energy solution.
Figure 16. APP Control Interface Display
4.3 System Features
1. Real-time Data Monitoring: Capable of accurately monitoring key water quality parameters such as temperature, pH, and turbidity, providing data support to optimize engineered yeast performance.
2. Remote Access and Control: Users can view water quality data in real-time through a mobile app and perform remote operations, allowing for quick responses and adjustments during experiments.
3. Solar Power Supply: The system supports solar power, making it suitable for remote or electricity-scarce experimental scenarios, reducing energy consumption and increasing the system's mobility and adaptability.
4.4 Components
• STM32F103C8T6 Microcontroller
• Temperature Sensor
• pH Sensor
• Turbidity Sensor
• ESP8266 Wi-Fi Module
• OLED Display
• Solar Power Module
• Others: Cables, power modules, etc.

Table 3. Cost calculation for the water environment monitoring system
4.5 Feedback and Testing
In the laboratory, we tested the system under various water quality conditions. The results demonstrated that the system reliably provides real-time monitoring data, supporting microbial engineering applications in synthetic biology research.

The design and construction of the water environment monitoring system based on STM32 can be found below.

5 System Design Iteration and Feedback
5.1 Design Iteration Process
After referencing iGEM XJTLU-CHINA 2022、iGEM REC-CHENNAI 2022, we determined that MBBR(Moving Bed Biofilm Reactor) is a good choice as a carrier for engineered yeast to adsorb rare earth elements.
Dr. Chunyu Lai is a researcher at the College of Environment and Resources at Zhejiang University. His research interests include novel low-carbon sewage treatment technologies and processes, enrichment and regulation of functional microorganisms, and material-microbe coupling technologies. Under his guidance, we explored other advanced water treatment technologies such as MBR (Membrane Bioreactor) and MABR (Membrane Aerated Biofilm Reactor) .
Considering the adsorption characteristics of the engineered yeast and the need for efficient rare earth recovery, we decided to use a combination of MBBR and MABR to optimize the performance of the engineered yeast in adsorbing rare earth elements.
When integrating these two technologies, we weighed the pros and cons of placing MBBR and MABR in a single reactor or in two separate reactors.
Figure 17. Comparison of the Advantages and Disadvantages of Two Design Schemes
To provide more flexible control over the growth conditions of the engineered yeast and to optimize rare earth adsorption efficiency, we chose to use two separate reactors. This design allows for customization and easy replacement through modular configuration, adapting to different types of wastewater and different treatment stages. We proceeded with a preliminary model design using recycled plastic bottles to visualize our concept.
Figure 18. Preliminary MBBR-MABR Series Model
Next, we focused on testing the suspension and fluidity of the K1 carrier in the MBBR reactor. The test results showed that the K1 carrier performed well.
Subsequently, we designed two reaction tanks, selecting appropriate dimensions and configurations to ensure suitability for laboratory-scale operations.
Figure 19. Design of Reaction Tank Dimensions
Figure 20. Multidimensional View of Model Design
Prof. Baolan Hu , a professor at the College of Environment and Resources at Zhejiang University, pointed out that MBBR carriers are susceptible to membrane fouling, which could reduce the adsorption efficiency of engineered yeast. To address this, we added turbidity monitoring to the STM32 monitoring system, ensuring real-time detection and control of membrane fouling, thereby maintaining high efficiency in yeast operation.
During operation, we found it difficult to precisely control the flow rate of the water, so we added three-way valves to enhance water flow control.
Finally, we adopted a serial connection of the MBBR and MABR modules combined with the water environment monitoring system.
Figure 21. MBBR-MABR Series Connection Device Diagram
To further improve flow regulation and ensure optimal suspension of the carrier, we should examine the height settings of the pipes and the theoretically ideal flow rate from a fluid mechanics perspective. However, we have not yet made the necessary improvements to water flow control, nor have we introduced the engineered microorganisms into the equipment, due to time constraints.
5.2 Feedback
In this process, we actively gathered suggestions from various parties and engaged with companies in the wastewater treatment sector, such as Hangzhou Water Group Co., Ltd. , to obtain their experience and feedback on using this device. Based on the feedback from these relevant companies, our MBBR-MABR device has the following advantages compared to traditional devices:
1. Complementary Treatment Mechanisms: The MBBR module efficiently degrades organic matter through biofilm on suspended carriers, relying on biofilm formed by engineered bacteria to degrade organic pollutants in the wastewater while also adsorbing some rare earth elements. The MABR module uses the special structure of hollow fiber membranes and a combined aeration design to effectively transfer oxygen to the microorganisms inside the membrane, ensuring oxygen supply and enhancing biofilm activity, thus allowing engineered yeast to better adsorb and process rare earth elements in wastewater.
2. Flow Control and Adjustment: Module 1 (MBBR) and Module 2 (MABR) are connected by pipelines, which are equipped with adjustable three-way valves to precisely control the flow rate and flow between each module. In addition, a pump system is used to recirculate water treated by the MABR module back into the MBBR module, maximizing the use of each module's treatment capacity. This design not only improves flow control and operational convenience but also allows for flexible adjustments based on actual treatment needs, ensuring optimal operation of each module.
3. Flexible Operations in Time: After each module has operated for a period, the internal carrier (e.g., hollow fiber membrane in the MABR module or K1 carrier in the MBBR module) may reach adsorption saturation, affecting treatment efficiency. Our system features a rapid carrier replacement function. When a module's carrier approaches saturation, it can be quickly replaced to prevent efficiency loss, ensuring continuous high-performance operation. This modular approach over time not only simplifies maintenance but also extends the equipment's service life.
4. Module Replaceability: Our modular design also allows for interchangeability and upgrades between modules. When wastewater characteristics or treatment needs change, users can quickly replace or upgrade MBBR or MABR modules or add new treatment modules based on actual needs, adapting to different working conditions. This design greatly enhances the system's flexibility and scalability, allowing it to meet various wastewater treatment requirements. Additionally, depending on different wastewater compositions, replacing and adjusting modules can improve treatment effectiveness and resource recovery rates, providing a more efficient and sustainable solution for the wastewater treatment industry.
6 Industrialization Considerations
To achieve industrial-scale application, it is essential to scale up and improve the equipment. Throughout this process, we developed a detailed roadmap and continuously optimized equipment design by conducting user testing, discussion, and feedback to address potential issues that may arise during scaling up.
Figure 22. Roadmap of the Rare Earth Element Recovery Equipment
6.1 Potential Obstacles
6.1.1 Physical Issues
• Equipment Damage and Function Loss in an Industrial Environment: Factors like strong winds, mechanical vibrations, or chemical corrosion may damage equipment or cause functional failure, affecting operational stability.
• Pipeline Blockage and Component Malfunctions: Industrial wastewater contains impurities and suspended particles, which may clog water supply systems, aeration pipelines, or other critical components, obstructing the rare earth recovery process.
• Corrosion and Leakage of Equipment Materials: Acidic substances and other chemicals in industrial wastewater may cause material corrosion over time, leading to chemical leaks.
• Impact of Water Quality Fluctuations on Equipment Performance: The fluctuating composition of industrial wastewater, including variations in pH, temperature, and turbidity, can affect microbial activity.
6.1.2 Biological Safety Issues
• Risk of Engineered Yeast Leakage: Our equipment uses genetically engineered yeast to adsorb rare earth elements, but during operation, yeast may accidentally leak into the external environment.
• Public Perception of Genetically Modified Organisms (GMOs) : The use of GMOs may raise public concerns regarding potential health and environmental risks.
• Contamination of Microbial Carriers: Industrial wastewater contains a significant amount of impurities, which can contaminate microbial carriers such as K1 carriers and hollow fiber membranes.
6.1.3 System Integration Challenges
• Scalability and Adaptability of Modular Systems: The composition of wastewater and treatment requirements may vary at industrial sites, making it a challenge to ensure efficient operation under different conditions.
• Energy Consumption: Although our design incorporates solar power modules, large-scale industrial use may require more energy. Further reducing energy consumption while maintaining an environmentally friendly system is a key task for future development.
6.2 Proposed Solutions
6.2.1 Solutions to Physical Issues
• Use of Corrosion-Resistant and High-Strength Materials: To enhance durability and resistance to chemical corrosion, we can replace certain equipment components with corrosion-resistant and high-strength materials, such as stainless steel or fiberglass-reinforced plastic (FRP) . Material selection must consider stability in acidic and high-temperature environments to meet the special requirements for rare earth recovery.
• Optimized Pipeline Design and Regular Cleaning: To prevent blockages, the system can include self-cleaning features or be designed with more maintainable piping structures. Regular inspections and cleaning of the aeration and water circulation systems are necessary to prevent suspended particles from causing equipment failures.
• Improved Sealing and Durability Testing: High-efficiency sealing systems and protective enclosures can prevent corrosive substances from infiltrating the equipment. Additionally, long-term durability testing is needed to ensure continuous operational stability in an industrial setting.
6.2.2 Solutions to Biological Safety Issues
• Implementation of a Biological Safety Switch: A genetic switch can be integrated into the engineered yeast to ensure they cannot survive if accidentally released into the external environment. This "self-destruction" mechanism effectively prevents potential risks to the environment.
• Preventing Microbial Leakage with Filters: High-efficiency filters should be installed in critical pipelines and exhaust outlets to prevent microbes from being discharged along with wastewater or exhaust gases. Secondary disinfection of treated wastewater ensures no microbial residue contaminates the environment.
• Public Education and Awareness Campaigns: Using reliable scientific data to educate the public on the benefits of using genetically engineered microbes can alleviate concerns about potential risks. Making the equipment's operation transparent and engaging in open communication with the public and regulatory agencies will also help increase acceptance of genetic engineering technologies.
6.2.3 Solutions to System Integration Challenges
• Flexible Adjustment of Modular Systems: The configuration and operating conditions of the MBBR and MABR modules can be adjusted based on the varying composition of wastewater and rare earth element content. Optimizing aeration rhythms and adjusting recirculation frequencies can dynamically fine-tune the system's operating status in response to real-time monitoring data.
• Further Optimization of Energy Management: On top of the existing solar power modules, more efficient energy recovery and reuse technologies can be introduced, such as using waste gas and heat to power the system, further reducing energy consumption in industrial applications.
7 References
[1] Ødegaard, Hallvard. Innovations in wastewater treatment:-the moving bed biofilm process. Water science and technology. 2006, 53(9): 17-33.
[2] Siagian, Utjok WR, et al. Membrane-aerated biofilm reactor (MABR): recent advances and challenges. Reviews in Chemical Engineering. 2024, 40(1): 93-122.
[3] Luo, Yunlong, et al. Evaluation of micropollutant removal and fouling reduction in a hybrid moving bed biofilm reactor–membrane bioreactor system. Bioresource technology. 2015, 191: 355-359.