Development of Vertical Farming Systems from Waste Polymers Using Additive Manufacturing Techniques

Abstract

Driven by population growth, rising living costs, and the urgent need to address climate change, sustainable food production and circular economy principles are becoming increasingly important. Conventional agriculture faces significant challenges, including land scarcity, water shortages, and disrupted supply chains. As a solution, cities are adopting vertical farming to enhance urban food security and promote circularity. This research introduces FLOAT (Farming Lab on a Trough), an innovative vertical farming system made from bio-polymers and recycled polyethylene terephthalate glyco (rPETG) pellets from plastic bottles. FLOAT’s design emphasizes sustainability and closed-loop material usage. The study showcases the versatility of additive manufacturing (AM) in creating complex geometries with fully functional 1:1 prototypes. These prototypes highlight FLOAT’s potential as a scalable and adaptable solution for sustainable food production in urban settings, contributing to improved food security and environmental sustainability. By integrating FLOAT with conventional practices, we aim to exceed Singapore’s 2030 food security targets and achieve lasting urban food resilience. FLOAT aims to scale sustainable food production, fostering community ties with food, and nurturing future responsibility.

1. Introduction

Ensuring an adequate food supply for the growing human population remains a persistent challenge [1,2]. Recent efforts have focused on addressing this issue through innovative and sustainable methods. One such approach is integrating urban farming into city environments, which has the potential to reduce the environmental impact of food production and enhance food security [3,4]. This can be achieved by implementing novel design solutions that enable more efficient and sustainable food production in urban areas [5].

Urban farming has seen significant innovation, particularly with the development of vertical farming techniques. These methods have become a popular choice for urban agriculture, especially in densely populated cities like Singapore, where land availability is limited [6,7]. Vertical farming involves growing plants in stacked systems, and optimizing space for efficient food production. Advances in vertical farming, such as hydroponics, aeroponics, and aquaponics, have made this approach more cost-effective for urban agriculture [6]. According to Despommier et al. [8], vertical farming is more efficient than traditional methods, allowing for year-round cultivation, increasing yields, reducing food waste, and improving food security by ensuring a reliable supply. However, challenges to vertical farming include high start-up costs, limited crop varieties, and smaller production volumes. Additionally, there are concerns about managing disruptions in the rural sector, raising investment capital, and training a skilled workforce [9,10].

The existential threat posed by climate change highlights the growing necessity for a sustainable circular economy [11,12]. A circular economy emphasizes the continuous use of resources, minimizing waste and pollution. It focuses on sustainability by reducing reliance on non-renewable resources and extending the value of products [13]. Recycling is integral to the circular economy, by converting waste into valuable raw materials, it reduces reliance on virgin resources, lowers environmental impact, and enhances resource efficiency. This process redefines waste as a resource, supporting sustainability and promoting economic resilience. In the context of plastics, this approach aims to prevent waste and reduce environmental impact, promoting economic growth while minimizing harm [13]. Achieving a circular economy requires technological advancements, investments, changes in business practices, and shifts in social behavior. The transition to a circular plastics economy is currently hindered by prevailing production, consumption, and disposal practices [13,14]. Strategies such as recycling, remanufacturing, repairing, reusing, and refurbishing can help transform old products into new ones, thereby advancing the circular economy [15,16,17].

AM includes various technologies such as Fused Deposition Modeling (FDM), Stereolithography (SLA), Selective Laser Sintering (SLS), Digital light processing (DLP), and Direct Metal Laser Sintering (DMLS). These technologies utilize various materials across industries like aerospace, automotive, and healthcare. Among them FDM extrudes thermoplastic filaments and metals, SLA and DLP cures photocurable resins, SLS processes plastics, composites, and ceramics, while DMLS handles metals like titanium, aluminum, copper, and superalloys. AM technologies reduce material waste, enable localized, on-demand production, and support design flexibility. By incorporating recycled or biodegradable materials and aligning with circular economy principles, AM promotes sustainable, efficient, and innovative manufacturing practices across industries [18,19].

This research aims to develop a vertical farming system utilizing recycled waste polymers, employing advanced AM techniques. The goal is to promote sustainable agricultural practices and reduce environmental waste by integrating recycled polymers into food production. The study focuses on a systematic approach to incorporate recycled polymers into the production process of FLOAT, a vertical urban farming system developed by the Singapore University of Technology and Design (SUTD). The research also innovatively redesigns the system to enhance its configurational flexibility. The optimized FLOAT system developed in this research successfully demonstrated the research aim and objectives with a full-scale 1:1 prototype. The prototype highlighted the feasibility of using regenerated waste polymers in AM and showcased the system’s enhanced configurational flexibility, underscoring its potential for sustainable agricultural practices and adaptability to various spatial arrangements.

The novelty of this research lies in the development of an optimized FLOAT system, which features 1-meter vertically stacked troughs. This system was parametrically designed, prototyped, and tested as a proof of concept, showcasing innovative design and manufacturing techniques. The study utilized FDM with rPETG to examine critical aspects such as process parameters, printable geometry, cost efficiency, production time, and assembly strategies. The research highlights the recycling value of waste polymer [rPETG], by effectively transforming waste into a valuable resource for urban farming. This approach aligns with circular economy principles by reclaiming and repurposing plastic waste, reducing environmental impact and extending the lifecycle of materials. This research significantly contributes to the field of AM using recycled polymers and supports the adoption of AM technologies in urban farming, offering a sustainable solution to space-efficient food production.

The subsequent sections of the article are organized as follows: Section 2 underscores relevant developments in FDM for Urban Farming. Section 3 introduces the FLOAT as a 3D printable vertical farming system, outlining its geometry features. Section 4 presents the design optimization and experimentation of the FLOAT structure. This chapter examines the geometry constraints, AM techniques and experimentation on materials. Section 5 explores the Prototyping and technical development of the system and demonstrates its capabilities and limitations. Lastly, Section 6 concludes the article, suggesting potential opportunities for future research.

2. Developments in FDM for Urban Farming

This section covers the technological advancement in two main areas of study: fused deposition modeling from recycled polymers and urban farming with 3D printing technology.

2.1. Fused Deposition Modeling (FDM)

Over the past five years, the use of AM for producing machine components has increased by 32% [20,21]. FDM is widely adopted in the automotive, aerospace, healthcare, and consumer product sectors [22,23]. However, AM generates waste from support structures, failed parts, excess raw materials, and disposable prototypes [24,25]. Assessing AM’s role in circular economy (CE) initiatives is crucial due to the increased raw material demand and waste production from FDM. Recycling materials, particularly polymer composites, is essential for sustainability in the AM industry [26,27].

Recycled plastic has a carbon footprint 3000 times lower than virgin plastic [28]. However, the global plastic packaging industry’s recycling rate is only 14% [24]. AM offers a solution by using recycled plastic as filament feedstock [24]. In a circular economy, it is crucial to evaluate the environmental and economic impacts of handling plastic waste. Several studies have used environmental life cycle assessment (ELCA) to assess the impact of recycling plastic waste for AM [24,29]. These assessments provide insights into sustainable waste management practices. Choudhary et al. [29] evaluated the environmental and economic implications of recycling polyethylene terephthalate (PET) plastic waste for 3D printing filaments using life cycle costing (LCC). Their findings show that recycled PET plastic significantly reduces environmental impacts and costs compared to virgin PET plastic. Additionally, integrating renewable energy sources, like solar photovoltaic systems, further enhances the environmental benefits of material recycling. This research highlights PET plastic waste recycling in 3D printing filament production as a sustainable and cost-effective approach.

Extensive research shows the compatibility of AM with recycled polymers. Romani et al. [30] explore projects that utilize repurposed materials in AM processes. Zander et al. [31] highlight advances in recycled polypropylene. Bruce et al. [32] have repurposed plastics in a wall/screen system. Nováková et al. [33] have explored large-scale fabrication.

2.2. Urban Farming with 3D Printing

Polymers are crucial in food production, necessitating sanitary manufacturing processes to prevent chemical contamination, especially in containers for edible plants. Previous projects in this field include the banyan eco wall [34], a sculptural 3D printed structure using PETG and HT filaments with integrated planters; Hortus by Wasp [35], a vertical farming system using extruded ceramic materials and H.O.R.T.U.S by ecoLogicStudio [36], a large-scale PETG 3D printed installation inspired by coral, designed to accelerate biomass growth for energy or food. Technological advancements in 3D printing are transforming agriculture, food processing, and environmental monitoring [37], while 4D printing shows potential for producing sustainable plastics in agriculture [38]. However, technical feasibility often serves as a common barrier for various innovative food production approaches, leading to limited progress beyond the prototype stage [39]. As a result, the sustainability of new urban agriculture techniques is usually assessed through modeling rather than real-world implementation [40,41].

Carlos et al. [42] explored freshwater farming to address urban food production challenges, especially where land is scarce. Despite advancements in urban farming, floating farming in freshwater remains largely unexplored, presenting a significant opportunity for innovation. Their study examines the physical requirements for successful floating farming, such as water barriers, solid, inert, food-grade materials, and components that allow natural light penetration. Lightweight, large-scale, complex-shaped components are essential for efficient floating farm systems. The research aims to advance sustainable and innovative food production solutions. The methodology involves using PET to construct floating farm modules with FDM for the required geometries. While recycled polymers have been used in small-scale 3D printing, their application in large-scale contexts is limited. This research explores digital design and fabrication techniques for large-scale manufacturing using recycled PET bottles. Empirical prototypes are produced and tested under real-life conditions. The project includes designing a translucent dome and a flotation platform, fabricating them using large-scale FDM, assembling printed components, and monitoring the farming module’s performance during operation [43].

3. Design Outline

This study undertakes a comprehensive examination of the viability of AM in developing a fully operational vertical farming module suitable for urban environments. The research project is organized into three principal phases: (1) Establishing the geometric parameters of the unit for AM; (2) Prototyping connection details and advancing technical aspects and (3) Concluding with manufacturing, assembly, and testing under real-world conditions.

Geometry Definition

The Architectural Intelligence Research Lab (AIR Lab) at SUTD has developed FLOAT, a parametric urban farming system illustrated in Figure 1. The system is designed to be produced seamlessly with 3D printing technology. This system incorporates principles of adaptability, lightness, and the ability to deploy right from the initial stages, and it is built with an elegant design. The system features sustainable construction methods and an automated irrigation system.

The FLOAT trough is a revolutionary unit comprising three integral components that transform water distribution and enhance functionality while minimizing material waste. As illustrated in Figure 2, the first component strategically places bamboo poles or lattice structures at corners for stability and durability, incorporating a concealed irrigation system within bio polymer-printed troughs. Sustainable bamboo poles contribute strength and facilitate an efficient irrigation mechanism within their hollow conduits. The second component is a biopolymer-printed linear trough designed for soil containment and distribution, optimizing crop growth with targeted irrigation and drainage holes. Concealed caps form the third component, acting as protective covers and adding an aesthetic touch. A nodal support system ensures structural stability, accommodating various lattice configurations for adaptability. This groundbreaking construction, printed in three parts, reduces material waste and sets a new standard for sustainable manufacturing. Connecting parts with mild steel bars enhance durability, making the trough suitable for diverse agricultural settings, and practical features like name tags streamline organization for efficient irrigation system maintenance.

The foundational structure of FLOAT centers on a lightweight yet robust bamboo lattice at the corners, supporting a network of 3D-printed biopolymer trough-planters as shown in Figure 3. This lattice not only provides crucial support but also conceals automated irrigation and nutrient distribution systems. The integration of sturdy bamboo lattice structures at each corner of the 3D-printed bio-polymer trough is essential for both support and shaping. Powered by solar energy, the system is designed to function as a standalone structure for food production, seamlessly integrating into both existing and new buildings with linear conditions. This system is limited to linear configuration, and integrating it into non-linear site conditions posed significant challenges.

4. Design Optimization and Experimentation

During the design phase, a generative script in Grasshopper for Rhino is developed that calculated angles, validating the system’s viability across different configurations. Early design iterations prioritized linear lattice structures for optimized planting space, and further refinement using a custom Grasshopper script allowed for flexible configurations, enhancing adaptability to diverse site conditions. An overview of the proposed design is shown in the Figure 4.

4.1. Design Derivation for AM

Understanding the Constraints of Geometry and AM Techniques:

To create troughs that are efficient in their construction, it is crucial to understand the limitations imposed by both the geometry of the troughs and the techniques used in AM. Initially, design strategies focused on curvilinear troughs that followed the specific curve’s path as shown in Figure 5. However, manufacturing these curvilinear troughs as a single piece posed challenges in terms of requiring a larger printing bed and a support system. After careful consideration of the geometry and manufacturing limitations, the design approach was reevaluated. As a result, linear troughs with various lattice configurations were implemented instead. This new design approach addressed the manufacturing limitations while still providing the desired functionality and efficiency. By utilizing linear troughs, the manufacturing process became more feasible and allowed for greater flexibility in creating diverse lattice patterns within the linear troughs.

4.2. Exploration of Lattice Structures

This study delves into the exploration of lattice structures and their constraints in the design of troughs for different site conditions. This research aims to develop various lattice configurations that adapt FLOAT’s lattice geometry in different urban farming contexts by transforming linear arrangements of troughs into gentle curvilinear configurations as shown in Figure 6. Additionally, this study explores AM techniques for trough geometry using parametric scripting tools like Grasshopper for Rhino. It iterates the linear FLOAT module design configurations to adapt to specific paths, analyzing lattice structure limitations and developing an adaptive script for desired geometries based on input curves.

Throughout the iterative design process, several digital models were developed as shown in Figure 7 to evaluate the structural layout and assess the stability of the lattice geometry. This iterative approach provided crucial feedback on the performance and durability of the design. To enhance the feasibility of the lattice structure, additional FLOAT troughs were introduced based on the layouts in vertical layers. These strategically placed troughs aimed to improve the support and stability of the lattice system. By carefully considering the placement of these additional troughs, the overall functionality and strength of the design were improved.

The Design Optimization stage involved benchmarking and selecting several designs for prototyping. One of these designs included circular configurations as shown in Figure 8 that created enclosed spaces through clustering combinations of the lattice structure. Additionally, breaking down gentle curvilinear configurations into segments while maintaining the linear geometry of the FLOAT trough provided a range of possibilities in lattice design layout and configurations. Prototyping these optimized designs allowed for detailed analysis of connection systems, structural stability, and the determination of suitable angles for bamboo lattice structures. This optimization process aimed to streamline design layouts and connection systems to facilitate easier fabrication techniques at various sites.

4.3. Materials and Methods

For the bespoke 3D-printed biopolymer trough planters, PETG is identified as the ideal material in the initial selection process. The study [44] explores recycling PET-based foil waste with PETG copolymer for FDM. The results indicate that PET-based film waste can be used to produce filament for FDM printing with limitations. With its remarkable flexibility and toughness, PET emerges as the optimal polymer for producing 3D printed objects [45]. Also, specialized variants like PETG, and particularly rPETG have been developed to enhance performance. PETG, a copolyester of PET with glycol modification, surpasses PLA in heat resistance and rigidity, yet maintains ease of printability compared to ABS [45].

The study [46] explores recycling PETG for 3D printing using DRAM-oriented equipment. Over six recycling cycles, tests on tensile, flexural, impact strength, and micro-hardness showed minor reductions in mechanical and thermal properties. The results indicate PETG can be effectively recycled multiple times, maintaining significant mechanical property gains. In sustainability, rPETG advances significantly as it is crafted from reclaimed bottles, aligning with eco-friendly practices. This recycled variant not only upholds the advantageous properties of traditional PETG but also contributes to environmental conservation by re-purposing plastic waste [47]. Most commercial FDM printers readily support rPETG filament, further promoting the adoption of environmentally conscious materials.

For our study we referred to Pinter et al. [47] on recycling PET and virgin PET and Raza et al. [48] on repurposing of discarded plastic products, specifically failed PLA prints for converting them into usable filaments.

The study by Pinter et al. [47] emphasizes the growing significance of recycled products and materials, particularly in the context of PET bottles. The methodology employed in their study involved a series of cycles to simulate the recycling and production process of PET bottles using a mixture of 75% rPET and 25% virgin PET. The results of their study showed that the quality of the rPET material was not negatively impacted. And that rPET can meet all tested parameters for reuse across multiple cycles. The study results show no indication of a limitation on the number of cycles for the tested ratio of 75% rPET to 25% vPET. The study by Raza et al. [48] transformed failed PLA material into filaments with specific thicknesses, enabling its reuse in 3D printing applications. These studies validated the selection of our material as both sustainable and conducive to a circular economy.

The primary goal of our research is to reuse and repurpose discarded plastic materials through AM by combining and transforming rPETG, virgin PET and discarded PLA materials into filaments. The methodology employed in our study involved a series of experiments to test the flow and thickness of the extruded filaments. Material proportions, temperature and speed in extruding filaments were altered to identify the better result. Each cycle consisted of several steps: The process began with the collection and segregation of PET bottles and failed or discarded prints of PLA. These materials were then separately shredded using 3devo’s shredder to prepare them for the next steps. The shredded materials were filtered to separate them by size, with larger particles being re-shredded to achieve uniformity in material composition.rPTEG pellets were introduced into the composition and the materials were extruded using 3devo’s Composer 450 filament extruder, (3devo Inc. 2093 Philadelphia Pike, DE, USA) allowing for controlled filament extrusion and spooling. Figure 9 showcases the filament extrusion process involved in the research.

It is crucial to emphasize that the extrusion speed and temperature affect the quality and thickness of the filaments. Various mix proportions of the materials and extrusion temperature and speed were tested during this process to explore different material compositions and filament quality. Notably, the analysis revealed that 85% of rPTEG, 7.5% of Virgin PET, 7.5% of discarded PLA at the higher temperature and higher speed > 220 °C and >7 rpm provided thin filament with consistent flow and at lower temperature < 170 °C and lower speed < 3.5 rpm provided sludgy filaments with deformities and bumps in the extrusion. The temperature range between 180 °C and 210 °C and speed between 4 and 5.5 rpm provided smooth flow and consistent filament thickness based on the mixed proportions.

Additionally, the evaluation of filament quality involved visual inspection and analysis of thickness data captured by the 3Devo filament extruder’s software Devovision (Beta Version v0.2.0). Visual inspection assesses the filament’s appearance and integrity, identifying defects like irregularities, bumps, or deformities that could impact printing performance. The software records precise thickness measurements along the filament’s length, providing insights into its consistency and uniformity. In essence, while our initial exploration shows promise for using the material in 3D printing, the adoption of such materials in practical applications will benefit from additional specialized testing and refinement to meet industry standards. By analyzing visual inspection results and thickness data, experts can refine material proportions, streamline analysis, and identify optimal blends. Further testing through AM printing and quality evaluation will enhance understanding of material composition and improve filament performance for various applications.

5. Prototyping and Technical Development

For the design development phase, multiple prototypes were created in which a small number of prototypes were manufactured using an Ultimaker printer with PLA and PETG filaments, utilizing a 0.4 mm nozzle as shown in Figure 10. Troughs were printed as single components without any joints, while lattice geometries were tested using skewers and chopstick models. The configuration of lattice supports was subsequently refined to achieve the optimal layout.

The FLOAT component was divided into three parts to minimize waste by printing without additional support material. A connection detail was developed, involving three threaded rods between the components. Smaller components such as caps and X nodes were designed to conceal the water inlet and secure bamboo braces. Additionally, foot supports for the bamboo poles were designed to hold their position with screws. Figure 11 illustrates the developed components.

The spools made from recycled PET bottles were used to print multiple prototypes, assessing structural stability and material properties. These scaled models were crucial in refining designs and exploring different connection details, allowing for varied configurations and layouts. They provided valuable insights into the performance of recycled PET material, validating its suitability for the product.

By optimizing trough angles through adaptable scripting and bamboo lattice integration, 3D-printed biopolymer troughs were achieved, balancing structural integrity and planting efficiency. Iterative processes facilitated exploration, resulting in a versatile solution. The trough, with its dynamic configurations and bamboo lattice, embodies thoughtful design and iterative refinement, offering an innovative response to agricultural challenges.

After finalizing design iterations, a prototype was crafted using white PETG material. This prototype underwent rigorous testing and assembly, demonstrating exceptional structural integrity and stability over time. Assembly was swift, taking less than three hours. Upon filling with soil and irrigating, the prototype showed promising vegetation growth within a week, confirming its effectiveness for urban agriculture. These results underscore the prototype’s potential as a practical solution, highlighting its efficiency and suitability for real-world use.

5.1. Technical Development

The generative GH script plays a crucial role in the design process by creating lattice geometry and generating essential data for fabrication and 3D printing. Printing time and material requirements for each unit is estimated from the developed script. Additionally, it provides fabrication details, including the length and quantity of bamboo poles, troughs, and joinery nodes, ensuring accurate and efficient production.

Figure 12 and Figure 13 showcases the slicing software Cura/PrusaSlicer providing valuable insights into the material requirements for AM printing and plastic upcycled by AM, offering essential information for informed decision-making and further design optimization.

5.2. Effective Product Demonstration

The product was successfully demonstrated in two different materials and locations, highlighting its material adaptability, flexible lightweight configuration, and circularity. The products are manufactured using commercially available recycled 3D filaments.

For City Developments Limited, a full-scale 1:1 prototype as shown in Figure 14 was assembled, featuring 3D-printed biopolymer trough planters and bamboo lattices designed to conceal the nutrition distribution system. This innovative system demonstrated significant potential for integrating food production seamlessly into both existing and new buildings. The structure operates as a standalone design, adaptable to various urban environments. The successful development of this prototype underscores the feasibility and practicality of incorporating sustainable agricultural practices into urban landscapes, thereby promoting food security and sustainability initiatives.

At the “Circular Futures—Next Gen” exhibition at the National Design Centre in Singapore, a full-scale 1:1 FLOAT system as shown in Figure 15, constructed from recycled PETG and arranged linearly, was exhibited. The exhibition aimed to educate the public about food security within a circular economy framework, emphasizing sustainability in both food production and waste management. Key elements included the use of 3D-printed biopolymer rPETG plastics and bamboo lattice structures, illustrating their potential for circular material usage. The elegant yet simple design of the FLOAT system was showcased, along with its bespoke connection system, highlighting its adaptability and versatility. This demonstration emphasized the principle of circularity, where materials are repurposed and given new life rather than being disposed of.

6. Conclusions and Future Scope

The Airlab’s FLOAT at the “Circular Futures—Next Gen” exhibition aimed to raise public awareness of the critical links between food security, production, and the importance of end-of-life waste management within the context of a circular economy and sustainable practices. By repurposing waste materials, our work demonstrates how innovative design and manufacturing techniques can significantly contribute to environmental sustainability and resource efficiency. This research also successfully adapted the FLOAT lattice configuration from linear to curved patterns, achieving the initial goal. Using AM techniques with PETG pellets/filaments derived from plastic bottles, a functional prototype was produced. The study explored the FLOAT system’s potential as a standalone vertical farming solution adaptable to diverse site conditions, opening new avenues for vertical farming across different environments. Insights gained from recycling PET bottles into AM filaments highlight significant sustainability potential and contribute to establishing a circular economy. This study provides valuable insights for further investigation into plastic waste recycling and its integration into sustainable food production.

While the FLOAT system demonstrates significant potential, certain limitations were identified. The prototype’s scalability and cost-effectiveness need further evaluation to ensure feasibility for widespread adoption. Additionally, the long-term durability and maintenance of recycled PETG materials in various environmental conditions require extensive testing. Addressing these limitations will be crucial for the successful implementation and widespread adoption of the FLOAT system in diverse contexts.

Looking ahead, the research aims to extend the developed trough design to serve as a standalone product suitable for common household balconies at the Housing Development Board (HDB) apartments in Singapore. This expansion seeks to foster a circular economy environment by promoting sustainable practices at both individual and community levels. Scaling up production and implementing these troughs in HDB gardens can enhance community gardens, promoting local food production and fostering community engagement through shared gardening experiences. Furthermore, the potential installation of the trough design along vertical facades can maximize space utilization and promote urban greening initiatives. Increased production and adoption of these troughs can further support community gardens and sustainable vertical farming, contributing to a more sustainable and environmentally friendly urban landscape.