Turning Organic Waste into Energy and Food: Household-Scale Water–Energy–Food Systems

Abstract

Population growth drives increasing energy demands, agricultural production, and organic waste generation. The organic waste contributes to greenhouse gas emissions and increasing landfill burdens, highlighting the need for novel closed-loop technologies that integrate water, energy, and food resources. Within the context of the Water–energy–food Nexus (WEF), wastewater can be recycled for food production and food waste can be converted into clean energy, both contributing to environmental impact reduction and resource sustainability. A novel household-scale, closed-loop WEF system was designed, installed and operated to manage organic waste while retrieving water for irrigation, nutrients for plant growth, and biogas for energy generation. The system included a biodigester for energy production, a sand filter system to regulate nutrient levels in the effluent, and a hydroponic setup for growing food crops using the nutrient-rich effluent. These components are operated with a daily batch feeder coupled with automated sensors to monitor effluent flow from the biodigester, sand filter system, and the feeder to the hydroponic system. This novel system was operated continuously for two months using typical household waste composition. Controlled experimental tests were conducted weekly to measure the nutrient content of the effluent at four locations and to analyze the composition of biogas. Gas chromatography was used to analyze biogas composition, while test strips and In-Situ Aqua Troll Multi-Parameter Water Quality Sonde were employed for water quality measurements during the experimental study. Experimental results showed that the system consistently produced biogas with 76.7% (±5.2%) methane, while effluent analysis confirmed its potential as a nutrient source with average concentrations of phosphate (20 mg/L), nitrate (26 mg/L), and nitrite (5 mg/L). These nutrient values indicate suitability for hydroponic crop growth and reduced reliance on synthetic fertilizers. This novel system represents a significant step toward integrating waste management, energy production, and food cultivation at the source, in this case, the household.

1. Introduction

The demand for agricultural products (food), energy, and the production of organic waste are increasing along with the global population. According to recent projections, the world population is expected to increase from an estimated 8.2 billion in 2024 to 10.3 billion in the mid-2080s, resulting in an increasing dependency between water, energy and food [1]. This surge is further exacerbated by urgent challenges such as climate change and environmental degradation. Together, these factors will pose significant challenges to effective waste management and long-term resource sustainability [2].

The exploitation of natural resources is also intensifying, and scarcity has become unavoidable, making overuse one of the most critical challenges of the modern era. Promoting the integrated management of water, energy, and food resources enhances sustainability and strengthens resilience [3]. These resources are fundamental to achieving the United Nations Sustainable Development Goals (SDGs) [4]. As a result, Water–energy–food (WEF) nexus-based frameworks have been explored and implemented at various scales [5,6].

Household-scale organic waste can be managed through a range of technologies that recover resources while minimizing environmental impacts. One such option is anaerobic digestion (AD). AD is a microbial process that operates without oxygen, yielding two main outputs: biogas and digestate. Primary advantage of this process is the capture of methane, a potent greenhouse gas that would otherwise be released into the atmosphere. In addition to AD, which yields biogas [7], other widely adopted methods include composting and small-scale thermal conversion. Composting is the most accessible and widely used method that relies on aerobic microbial decomposition. Composting is a simple aerobic process that stabilizes food and garden waste into a soil amendment that improves soil fertility [8]. This method offers less costly and simpler implementation but releases some greenhouse gases and lacks the energy capture potential of AD. Small-scale thermal conversion, such as pyrolysis, can transform organic waste into biochar and heat, providing both a renewable energy source and a stable carbon product that supports soil improvement [8]. However, this technology is in a very early stage for household practical use. The main reasons are (1) the possibility of the production of flammable and poisonous gases and (2) the need for specialized equipment.

The most effective choice for household-scale systems depends on initial capital availability, desired output (e.g., energy or fertilizer), local climate, and the interest of end-users to engage in system maintenance. Anaerobic digestion produces both renewable energy and a nutrient-rich soil amendment (i.e., digestate), which provides a net global warming benefit. However, this is more capital-intensive compared to composting, which is the most practical and affordable technology without energy output. Additionally, environmental performance depends on proper site management to minimize greenhouse gas emissions. Small-scale thermal conversion has great potential for significant waste volume reduction but is expensive for household use and faces challenges with public perception and air pollution control.

This highlights the need for innovative solutions in household-scale WEF to achieve sustainability goals. Household-scale systems can offer a practical and efficient approach to integrating the WEF nexus by simultaneously addressing waste management, energy generation, and nutrient recycling. Organic waste, which includes food waste, agricultural by-products, and other biodegradable materials, can be used to produce clean energy and boost agricultural productivity in a WEF Nexus. Such organic waste can be utilized in anaerobic digestion processes to produce methane-rich biogas, which can then be used as a renewable energy source [7]. Use of impure methane created using the bio-digestion process requires some consideration. Combustion engines and gas turbines are both viable options for small-scale biogas energy conversion, each with unique characteristics. Combustion engines stand out because of their higher efficiency, lower cost, minimal maintenance requirements, and ability to operate without biogas purification. Conversely, gas turbines have notable disadvantages, including their reliance on purified, compressed biogas, which makes them less attractive in comparison to combustion engines [9]. For optimal performance, combustion engines require a biogas methane concentration of at least 45% [10]. Methane-rich biogas can also be combusted for direct applications such as heating or grilling.

Municipal solid waste (MSW) represents a significant energy resource, with an average energy content of approximately 6.6 GJ/ton, although this varies based on waste composition [9]. In 2018, the United States disposed of roughly 150 million tons of MSW in landfills [11], highlighting the vast untapped potential for energy recovery. At the household level, food waste presents an opportunity for small-scale energy generation. For instance, the average annual food waste per person in Europe and North America is 105 kg (231.49 lb) [12]. Using corn as a reference material with an energy content of 10.8 GJ/ton [9] and assuming a combustion engine efficiency of 24%, the usable energy output per ton of corn would be approximately 2.59 GJ.

Closed-loop technological systems play a crucial role in maintaining sustainability by mimicking cycles where waste from one process becomes a resource for another [13]. Self-sustaining closed-loop systems make them particularly effective in creating sustainable solutions across different sectors. Such systems not only eliminate waste but also generate usable energy and produce fertilizer, making them efficient in providing multiple benefits simultaneously [8,14]. Decentralized WEF systems have been recognized as an effective strategy to achieve significant environmental and social benefits, moving communities closer to a closed-loop or circular economy model. Within WEF, anaerobic digestion has been widely applied to reduce organic waste volumes and divert material from landfills, thereby minimizing leachate formation. Household systems also encourage active user engagement, integrating residents into circular economy practices. Real-world examples further demonstrate the feasibility of such systems; for instance, closed-loop organic waste management has been applied by family farmers in Brazil to recover energy and nutrients for crop cultivation [14]. Small-scale systems have been designed to combine anaerobic digestion, pyrolysis for biochar, hydroponics, and vermifiltration, offering opportunities to increase resilience and income for small-scale farmers while supporting sustainable waste management practices.

Biogas is a mixture of methane, carbon dioxide, nitrogen, hydrogen sulfide, and small amounts of other trace gases [15]. It is commonly created by the anaerobic breakdown of a substance with a high organic content such as food waste, agricultural waste, and other sources [16].

Constant production and availability of organic waste make biogas an excellent, clean and safe source of renewable energy. Biogas can be filtered to maximize the ratio of methane to other gases and to remove impurities such as hydrogen sulfide, which can harm equipment and be hazardous to human health. Methane-containing biogas is combustible and can be used as fuel for cooking and producing heat or electricity. Use of created biogas is an ideal option for generating renewable energy, as it efficiently converts organic waste into a sustainable energy source, while contributing to environmental conservation and waste management [17].

A biodigester system is used to break down organic waste for biogas production. A commonly used technology for industrial biogas production is the continuous stirred tank reactor (CSTR) [18,19]. This system has been extensively used for substrates with high moisture content which includes sewage sludge, municipal solid waste, manure, and agricultural and industrial wastewater [19,20]. The biodigester is filled with organic waste, and an inoculum of bacterial diversity is added. The inoculum is a biologically active liquid rich in microorganisms [21]. These microorganisms begin to break down the waste in an anaerobic process called microbiological fermentation. The final output of this process is a biogas that includes a calorific value of 21–24 MJ/m³ and a composition of 50–70% methane and 30–50% carbon dioxide [22]. The biogas is stored in an attached gasbag or tank for later usage. Several parameters that affect the anaerobic system, including temperature, organic loading rate (OLR), pH, alkalinity, heavy metals, and substrate concentration, must be controlled to maximize biogas yield and keep the biodigester from failing [23].

The biodigester effluent can be effectively utilized as a nutrient source to support agriculture [24]. Wastewater from domestic sources, which share similarities with digested organic residues, contains essential nutrients such as nitrogen, phosphorus, potassium, and other micronutrients that plants can absorb [25]. Similarly, Siddiqui et al. [26] demonstrated that food waste-derived organic fertilizers, such as “FoodLift,” could serve as sustainable alternatives to synthetic fertilizers, effectively supplying key nutrients for crops like lettuce and cucumber. Therefore, residuals from biogas production can be used as sustainable fertilizers, reducing the need for synthetic fertilizers for promoting plant growth and soil health, particularly when properly monitored and managed.

Biodigestors support a closed-loop system compared to composting and small-scale thermal conversion. Anaerobic digestion generates both renewable energy and nutrient-rich digestate. Composting primarily focuses on soil improvement which enhances crop growth and water-use efficiency. The thermal conversion method is generally used for drier organic waste and yields highly stable products, but it is limited to large-scale applications rather than household scale. To date, there are also few integrated studies that simultaneously evaluate biogas production, water quality, and hydroponic plant performance from a household-scale sink waste stream. To address this, the study aims to develop and evaluate a household-scale, closed-loop WEF system that is directly connected to the kitchen sink, providing an innovative solution to address the increasing challenges of waste management, energy generation, and sustainable resource utilization for agricultural production. This study also demonstrates the feasibility and practical effectiveness of household-scale biodigester systems by identifying and establishing parameters necessary for their successful operation.

2. Materials and Methods

2.1. WEF System Setup

The system was designed and constructed at the Emergent Technologies Institute (ETI) Lab at Florida Gulf Coast University. The biodigester was designed to connect to a household kitchen sink and food disposal system to turn food waste into biogas for an energy source and nutrient-rich fertilizer that supports food crop production. This dual functionality underscores the system’s role in reducing waste while simultaneously generating valuable resources. The main components of the system include a custom-made anaerobic biodigester, kitchen sink garbage disposal, settling tank, slow sand filter, holding tanks, and hydroponic systems. The preliminary design of this comprehensive system is demonstrated in Figure 1, which provides a conceptual overview of its layout and functionality.

Specific dimensions of the biodigester are shown in Figure 2, including the placement and size of its inlet and outlet openings. The design accounts for the flow dynamics of food waste and effluent, optimizing the placement and size of the openings to facilitate efficient input of waste materials and output of biogas and digestate. The size of the digester carefully considers practical constraints, such as the limited clearance height available beneath a residential kitchen sink cabinet. This ensures the system is compact and adaptable for household use.

The biodigester was constructed using acrylic sheets with a protective brown covering as presented in Figure 3. Acrylic is a strong and flexible impact-resistant clear thermoplastic material that offers high resistance to variations in temperature. Acrylic also enables an option for visual inspection of the anaerobic digestion process and biogas production. Digesters were designed to run at different target temperature ranges. The temperature ranges are typically 30–38 °C for mesophilic organisms and 50–60 °C for thermophilic organisms [27]. The system was designed to operate under mesophilic conditions. Heating pads were installed beneath the acrylic biodigester to maintain the target temperature, which was continuously monitored using the In-Situ Aqua Troll sonde (In-Situ Inc., Fort Collins, CO, USA).

The biodigester was mounted under the kitchen cabinet and connected to a common household garbage disposal unit. The garbage disposal grinds food waste into smaller particles and combines it with water, before rerouting it into the biodigester for anaerobic digestion. In this experiment, biogas produced from anaerobic digestion of food waste rises through the tube and is captured in an inflatable air mattress for storage. This air mattress, as shown in Figure 4a, has a volume of approximately 119,298 cm³ and the biogas is sampled once a week for eight weeks. In addition, the system produces a nutrient-rich effluent that can be repurposed for agricultural or landscaping use, providing an environmentally friendly alternative to synthetic fertilizers. By contrast, food waste typically disposed of in a landfill, sewage system, or septic tank often leads to methane emissions or complex waste-handling challenges. A broad range of PVC pipe sizes were utilized for the feedstock inflow and digestate outflow. This engineered design with automated flow control ensures smooth operation and optimal performance of the biodigester within the constraints of a residential kitchen setup.

The digestate was processed through a 121,000 cm³ slow sand filter system, as shown in Figure 4b, before being directed to the hydroponic planter. Digestate, an exceedingly nutrient-rich byproduct of anaerobic digestion, requires filtration to achieve acceptable levels of pH, total coliforms (TC), biological oxygen demand (BOD), chemical oxygen demand (COD), total dissolved solids (TDS), and total suspended solids (TSS) for agricultural irrigation [28]. Untreated digestate can introduce high levels of ammonia and salt, potentially causing stress to plants [29]. This indicates that filtration and treatment of digestate are essential to ensure it is safe for agricultural use. Slow sand filtration is a simple and inexpensive method for treating digestate. Although it is not as effective as more sophisticated filtration methods, slow sand filtration can nevertheless raise water quality to substantial standards for crop irrigation [28]. It has been demonstrated that crops cultivated with filtered digestate grow just as well as crops grown with conventional fertilizers [29].

A typical slow sand filter media can be developed with fine- and course-grained sand with an effective diameter of 0.15–0.35 mm, and a filter depth of 0.6–1.2 m [30]. The effectiveness of slow sand filters lies in their combination of mechanical (absorption, diffusion, screening, and sedimentation) and biological (predation, natural death, and metabolic breakdown) processes to remove organic materials and microorganisms [28,31]. Moreover, slow sand filters are highly reliable and require minimal maintenance due to their lack of moving parts. Maintenance is limited to simple procedures like backwashing, making them an accessible and sustainable option for digestate treatment in agricultural systems.

After being processed through the slow sand filtration system, the filtered digestate becomes a valuable resource for plant growth due to its nutrient-rich composition. This filtered digestate is then utilized in two hydroponic systems residing within the Gorilla Grow Tent Lite Line 20.3 × 20.3 cm, as shown in Figure 5. The Gorilla Grow Tent (The Original Gorilla Grow Tent^® 5 × 5, Gorilla Inc., Santa Rosa, CA, USA) is specifically designed to create a controlled environment for optimal plant development. The tent is equipped with sinching port ducts that allow for efficient management of environmental conditions. These ports enable the entry of electrical wiring, LED lights, and aerators while maintaining the tent’s sealed environment to optimize light exposure for the plants. The LED lights simulate natural sunlight, ensuring consistent photosynthesis, while the aerators promote oxygenation in the hydroponic nutrient solution, supporting healthy root development. This setup provides an ideal environment for hydroponic cultivation by combining advanced lighting, nutrient delivery, and controlled air circulation.

2.2. General Procedures

The food waste used in this study was collected from the South Village dining hall at Florida Gulf Coast University, and the waste was transported to the Emergent Technology Institute testing laboratory. The feedstock consisted mainly of vegetables, fruits, and staple foods. Vegetables, including leafy greens, carrots, corn, and broccoli, made up about 60% of the waste, while fruits such as pineapple and cantaloupe contributed around 20%, and staples like bread, rice, pasta, and eggs accounted for the remaining 20%. The waste was first homogenized and diluted with an equal amount of water before being added to the biodigester. The system was fed three times a week, with roughly 2.1 kg of food waste (wet mass), and no pH adjustment was applied during the study. The food waste was then directed into an anaerobic/biodigester, where bacteria grew and flourished, producing nutrients and biogas with a high methane content as a byproduct. The digestate trickled into a storage tank and was pumped into a slow sand filtration system, while the biogas was routed to an inflatable mattress that captured approximately 0.12 m³ of gas each week. The filtered effluent delivered nutrient-rich water to half of a hydroponic system, while nitrogen/phosphorus-rich water was delivered to the other half of the hydroponic system as an experimental control.

The hydroponic system, consisting of Rex lettuce (Lactuca sativa), was equally germinated in starter growth cubes for one week, and transplanted into two plastic bins constructed to hold twelve plants each, where the plant roots absorbed the nutrients. The water level was maintained immediately below the net pots to ensure the plant roots were in the water. The closed-loop system using the Nutrient Film Technique (NFT) of hydroponics, connected via flexible tubes with accessible valves at each station, was closely monitored by collecting water and biogas samples weekly. A multi-parameter water quality sonde was used to analyze the water samples, which were collected each week at all the system locations. Test strip kits were used to measure the concentrations of phosphorus and nitrite/nitrate in the water. The water quality monitoring data was used to ensure that the water quality remained suitable for plant growth and to demonstrate that the system was operating efficiently. Weekly 1-L gas samples were collected for on-site gas chromatography analysis to determine the type of compounds present, and their percent composition occurring in each sample. These analyses helped to provide insights into the efficiency of the anaerobic digestion process.

To develop a method enabling individuals to maximize energy output without depleting nonrenewable resources, various boundaries have been explored through trial and error. These boundaries tested include design optimization, water quality testing, gas tracing, and plant monitoring.

2.3. Gas Chromatography Testing

Biogas analysis was conducted using the Shimadzu GC-8A (Shimadzu Corporation, Kyoto, Japan), gas chromatography analyzer (GC) system equipped with a thermal conductivity detector (TCD) system utilizing Helium (He) as the carrier medium. GC-based techniques are widely used since the system can concurrently interpret the quantitative and qualitative traces in the gas samplings through various biotechnological procedures. According to the interaction of the targets with the stationary phase, the degree of the target dispersion in the mobile phase, and their boiling point, the GC machine can additionally distinguish compounds that arise in complex matrices [32]. In general, the design of GC-based methods is based on optimizing key operating variables, including the kind of stationary and mobile phases, the column temperature and its corresponding heating rate, as well as the flow rate and composition of the carrier gas, to maximize effectiveness [32]. By tailoring these variables, GC-based systems ensure accurate and reliable analyses, further strengthening their role as indispensable tools in gas analysis and related fields.

The inclusion of a thermal conductivity detector (TCD) in the GC approach is considered especially effective for tracing gases such as hydrogen (H₂), carbon dioxide (CO₂), nitrogen (N₂) and methane (CH₄) which are the primary components of biogas [32]. Storing biogas reduces the amount of methane released into the atmosphere, thereby decreasing the overall levels of greenhouse gases that contribute to greenhouse warming. This reduction in methane emissions is estimated to be equivalent to 11 million vehicles annually [33].

The versatility of biogas as a source of energy is worth noting from a technological standpoint. Heating buildings and even powering boilers can be accomplished by burning the biogas on-site. There are many ways biogas can be converted into electricity. A combustion engine or gas turbine can generate this electricity which can be distributed to other locations. Using biogas as a source of energy has a variety of advantages including improved quality of the environment, decreasing the amount of greenhouse gases emitted into the atmosphere, and limiting society’s oil and fossil fuel demand [33]. The transformative potential of biogas cannot be overlooked, as it provides powerful solutions to many pressing environmental issues.

There are many microbes in the biogas fermenter community. The most common substrates found throughout the biogas production process are bacteria that are efficiently able to break down polysaccharides (e.g., Clostridium thermocellum, Clostridium cellulolyticum and Caldicellulosiruptor saccharolyticus) [34]. Polysaccharides are carbohydrates found in sources like starch from vegetables and cellulose from fruits, which serve as sources for biogas production. The Clostridia class of microbes dominates the biogas fermenting community. To generate methane, cellulose and other lignocellulosic substrates must be broken down by Clostridia. All Clostridial hydrogenases are remarkably active. Furthermore, Clostridia helps the hydrolysis of polymeric substrates, production of H₂, and decreases the growth rate of hydrogenotrophic methanogens within the biodigester [34].

The process of converting food waste into biogas involves four biological stages [34]. By hydrolyzing the lipids, proteins, carbohydrates, and cellulose from the food, facultative anaerobic bacteria produce fatty acids, amino acids, and sugars. The covalent bonds of food waste are disrupted by the water and facultative anerobic bacteria. Propionate, butyrate, alcohols, and lactate are produced by the product’s continued fermentation by acidogenic bacteria. Acetate, H₂, and CO₂ are further produced by acetogenic bacteria during acetogenesis via the acetyl-CoA enzyme route for synthesis. Finally, the end-product of methane and carbon dioxide biogas is produced by hydrogenotrophic methanogens using H₂ and CO₂ and acetotrophic methanogens using acetate [35].

Samples with concentrations of 25%, 50%, 75%, and 100% were formulated to generate calibration curves for CO₂ and CH₄ from compressed pure gas cylinders. Each of these two calibration curves resulted in equations expressing y as a function of x. The measured area under each spike in terms of retention time recorded every two seconds from each sample is replaced with y and x corresponds to the concentration percentage. The calibration standards and retention times of each constituent were used to validate the identification of biogas components in the samples by comparing chromatographic patterns.

The equations for the methane and carbon dioxide calibration curves are shown in Equations (1) and (2), respectively. From 13 February through 3 April 2023, eight 1-L samples of biogas were collected weekly for testing. The GC-TCD system was used to identify the types of gases and determine their concentrations in each sample. Each biogas sample was loaded and injected via syringe through the flow control and injection port (a 1 mL loop) at 150 °C. Helium (He) carrier gas at 200 kPa and reference gas at 100 kPa (mobile phase) were used to facilitate the movement of gas to the column oven and further separation within the column (stationary phase). The 60/80 Carboxen-1000 column (38.1 cm × 0.175 cm SS) was calibrated to 140 °C and separated each compound detected within the biogas (Supelco, Bellefonte, PA, USA). The chromatographic areas of the target compounds measured in the samples were compared with areas derived from varying concentrations of the targets included in a typical mixture that imitated biogas to create the analytical curves [32].

y = 13.99x − 107.08

(1)

y = 16.62x + 178.20

(2)

The area under the methane curve on 13 February 2023, was 982.6. The methane percentage was derived by substituting the y variable in Equation (1) with the area. Equation (3) was used to determine that 77.9% of the gas generated on 13 February 2023 was methane.

982.62 = 13.99x − 107.08

(3)

The peak retention time of each component, along with the coil material and dimensions, were carefully considered in this analysis. After putting the gas sample into the GC machine, the nitrogen component, which is represented by the first curve, peaks 2.26 min later. The methane content can be observed by the second curve, which peaks at 4.87 min. The third curve, which represents carbon dioxide, has a 9.23-min peak.

2.4. Water Quality Testing

The samples of the effluent generated by this engineered system were tested by environmental sensors manufactured by In-Situ. For this research, the In-Situ Aqua Troll 600 Multi-Parameter Water Quality Sonde (In-Situ Inc., Fort Collins, CO, USA) was used to conduct daily water quality monitoring at multiple locations within the system. During the initial phase of the study, a comprehensive dataset was collected, including measurements of RDO concentrations (mg/L), RDO saturation (%), oxygen partial pressure (Torr), actual conductivity (µS/cm), resistivity (Ω·cm), density (g/cm³), total dissolved solids (g/L), pH (pH), oxidation-reduction potential (ORP) (mV), temperature (°C), barometric pressure (psi), pressure (psi), turbidity (NTU), external voltage (V), and barometric pressure (mbar). Among these parameters, the pH values were the focus of the research, as they play the critical role in determining the optimal conditions for hydroponic plant growth. According to Brechner and Both [36], the optimal pH value for lettuce production in a hydroponic system is 5.8, with an acceptable range between 5.6 and 6.0. By aligning water quality parameters with this pH range, the system provided valuable insights into optimizing conditions for hydroponic agriculture, paving the way for improved plant or crop health and yield.

Tests were conducted weekly at four distinct locations within the experimental WEF-Nexus framework system to measure the nutrient contents, as illustrated in Figure 6. The framework integrated a biodigester, storage for settling, filtration, and hydroponic cultivation. The biodigester outflow is transferred to a storage tank, which acts as a buffer, allowing large suspended solids to settle and ensuring a steady supply for the next stages. From storage, the water moves through a sand filter, which removes suspended solids, pathogens, and other impurities to enhance the water quality. The filtered water was then utilized in a hydroponic system, where it supported agriculture. The outflow from the hydroponic system is connected back to the biodigester, creating a continuous and iterative cycle within the system. This framework demonstrates the WEF-Nexus by recycling water, potentially recovering energy through the biodigester, and supporting sustainable food production through hydroponics.

The WEF system is coupled with a hydroponic unit in which crops are cultivated using only the recycled digestate from the digester. This linkage closes the loop by reusing nutrients and water that would otherwise be wasted, while also reducing the need for external fertilizers. At the same time, anaerobic digestion supplies biogas that can substitute for conventional energy sources. Taken together, these outcomes highlight key advantages of the system, reducing dependence on clean water and non-renewable energy inputs while supporting resilient food production.

3. Results and Discussion

3.1. Biogas Potential

Gas chromatography (GC) plays a vital role in identifying and quantifying the individual components of biogas based on their retention times. In this experiment, the average retention time (in minutes) of each compound was determined to assess the composition of the produced biogas. The key gases analyzed included CO₂, N₂, and CH₄. The average peak retention time of CO₂ was 9.31 min. The average peak retention time of methane was 4.87 min. Knowing the peak retention time for each curve aided in the identification of the component causing the peak. The total area under each curve was used to generate the calibration curves.

Many techniques have become available for analyzing biogas composition, but GC-based techniques are sought to be very efficient and effective. Gas chromatography can find qualitative and quantitative results for biogas composition. The energy harnessed from microorganisms through methanogenesis can be used to power various systems, such as stovetops, generators, electrical appliances, and heating/cooling systems. In situations where alternative sources of gas and resources are limited, a generator powered by biogas can serve as an emergency energy source, especially during natural disasters like hurricanes.

As presented in Figure 7, the gas chromatography analysis revealed an average methane content of 76.7% with a standard deviation of 5.15%. During the data collection, CH₄ exhibited the highest average gas percentage, whereas CO₂ and other gases accounted for smaller portions. CO₂, the second prevalent component, maintained an average percentage of 12.53%, while the share of others remained about 10.74%. The sample taken on 6 March 2023, was excluded from the data set due to a punctured sample bag.

The high methane concentration indicates its potential as a reliable fuel source for both emergencies and everyday use, offering an opportunity to supplement nonrenewable energy sources, reduce greenhouse gas emissions, and lower homeowner energy costs [37]. The study findings on the nature of the biogas produced by the household-scale biodigester over the period of eight weeks on average were slightly higher than typically reported for household-scale biodigesters, where methane yields are often in the range of 55–70% [19,38]. This performance observed may indicate the stable feed composition and effective daily feeding regime.

The biogas produced in this system contained 76.7% methane, with a total weekly gas volume of approximately 0.12 m³, yielding an estimated 0.092 m³ of CH₄ per week. Using a calorific value of 35.8 MJ/m³ for methane, this translates to roughly 3.3 MJ of energy available weekly. While this represents a modest energy output, it demonstrates the potential of household-scale anaerobic digesters to convert food waste into renewable energy. In practical terms, the biogas could be used for small household applications such as cooking or water heating, contributing to reduced reliance on conventional fossil fuels. It is important to note that a full life cycle assessment (LCA) to quantify net energy gains was not performed in this study. The primary benefits of the system lie in nutrient recovery, waste reduction, and incremental energy savings rather than complete energy self-sufficiency.

3.2. Effluent Water Quality and Nutrient Content

The average water quality and nutrient parameter values recorded during the 8-week period are presented in

Table 1. Descriptive statistics of water quality data at the Sand filter outflow collected over 8 weeks. Water quality parameters were measured using the test strips and In-Situ Aqua Troll 600 Multi-Parameter Sonde, and nutrient values were analyzed using colorimetric Test Strips.

Parameter	Min	Max	Mean	SD
Actual Conductivity (µS/cm)	2044.91	4050.98	3120.81	643.08
Total Dissolved Solids (g/L)	1.41	2.83	2.18	0.5
pH	6.75	7.50	7.01	0.25
ORP (mV)	−129.76	121.69	−66.86	87.34
Turbidity (NTU)	241.96	599.17	436.37	157.93
Temperature (°C)	20.52	37.79	25.28	6.28
Phosphate (mg/L)	0.00	25.00	20.00	11.8
Nitrite (mg/L)	0.00	25.00	5.00	11.8
Nitrate (mg/L)	10.00	50.00	26.00	21.91

. The results reveal that the sand-filter generated effluent, with conductivity, total dissolved solids and nutrient concentrations, can be suitable for supporting plant growth. The conductivity values of the nutrient solution in this study are within the recommended range of values [24], though their optimality varies depending on the selected cultivar [39]. The pH, near neutral, is slightly above the ideal range of 5.5–6.5 [36], which could limit the availability of nutrients to the plants. This indicates that minor acidification could be required. The ORP value indicates reducing conditions that may facilitate specific nutrient transformations (e.g., conversion of nitrate to ammonium). Phosphate and nitrite concentrations are comparable to those found in organic waste-based sources and fall within a broad range, from high to low [40]. In contrast, nitrate concentrations are relatively low but still contribute to plant growth and development. The high turbidity level points to the presence of suspended particles or organic matter impairing water clarity. The temperature falls within the suitable range for optimal nutrient solubility and root function for hydroponic solutions [24].

For enhanced nutrient usage and overall system efficiency, improved aeration to ensure complete conversion from nitrite to nitrate, and management of turbidity through appropriate filtration methods required to be implemented. Additionally, recycling the nutrient-rich solution or combining it again with organic matter to form biofertilizers could improve its effectiveness as an eco-friendly fertilizer. Ongoing monitoring and optimization of these conditions can promote sustained nutrient availability while mitigating potential environmental impacts. This underscores that this system is also a promising approach for sustainable nutrient recovery and supplementation, along with meeting various environmental protection goals.

The box plot in Figure 8 demonstrates the pH variations across different stations as shown in Figure 6. The pH values showed an increasing trend from biodigester outflow to hydroponic outflow. The biodigester outflow exhibits the lowest pH values. The pH in the storage outflow increases slightly compared to the pH of the biodigester outflow. The sand-filter outflow showed a pH variation between 6.8 and 7.6, with a median close to neutral, which is slightly high compared to the recommended range [36]. The sand-filter outflow requires targeted intervention to ensure optimal crop growth conditions as it is the critical station that serves as the direct input for crops. Proper pH adjustments (reduction) at this station could enhance nutrient availability, improve plant health, and contribute to a more effective water treatment system for agricultural use. The hydroponic outflow exhibits the highest pH, ranging from 7.4 to over 8.2, with some extreme outliers reaching 8.5, which could negatively impact nutrient availability. As such, the hydroponic outflow is designed as input to the biodigester to maintain optimal levels of pH for plant health.

The comparative analysis of water quality parameters across four stations is presented in Figure 9. The phosphate concentrations are relatively higher in the biodigester outflow but decrease across the subsequent locations, indicating nutrient retention or removal. Phosphate often binds to suspended particles, which naturally settle to the bottom of the storage tank over time, thereby reducing phosphate levels in the effluent. Nitrate and nitrite showed a significant increase in the sand-filter outflow. The significant increase in these concentrations in the sand-filter outflow can be attributed to nitrification. Phosphate, nitrate, and nitrite are essential macronutrients for crop growth, and their presence within adequate ranges suggests that the effluent can support plant growth in hydroponics, considering pH adjustments are made to enhance plant uptake. The AC and TDS demonstrated relatively stable across locations, except hydroponic outflow, which indicates consistent dissolved content. The turbidity decreased through the outflow stages, indicating improved water quality. For hydroponics, a moderate ORP is generally favorable because it indicates balanced conditions that limit harmful anaerobic processes and support nutrient availability. The temperature remains relatively consistent across all locations, demonstrating a steady thermal influence on water properties.

This food waste not only complements biogas production for energy but also nutrient recovery to support agricultural productivity by creating a circular economic benefit. In this study’s system, the biodigester contributes to energy generation, while the outflow from it provides a sustainable source of nutrients and irrigation water, minimizing dependency on synthetic fertilizers and the competition for fresh water. Targeted interventions, especially at the sand-filter outflow, can maximize these benefits, making the system a model for sustainable agricultural practices. Treatment strategies can enhance the water quality at this location, ensuring optimal conditions for agricultural use [24].

The WEF system in this study aligns with the growing interest of farmers in increasing environmental and ecological awareness, promoting sustainable and efficient cultivation practices to mitigate potential human health risks associated with the overuse of chemical fertilizers [40]. Dunn et al. [41], demonstrate that anaerobic digestion of household organic waste can generate digestate, a nutrient-rich byproduct that can be reused in applications like plug plant production, highlighting circular resource recovery at the household scale. Utilizing fertilizers derived from food waste presents a sustainable approach to agriculture, offering a means to recycle nutrients. The findings from this food waste-derived nutrient align with the results of Moncada et al. [42], which investigated the effectiveness of organic waste-based fertilizers on hydroponic tomato cultivation. The nitrate and phosphate levels are within the range of nutrients required for tomato growth, supporting this study that organic waste-based fertilizers can sustain hydroponic tomato production. Moncada et al. [42] also emphasized the effectiveness of organic waste-based fertilizers, showing that while hydroponic tomatoes grown with these fertilizers exhibited slower growth rates, they increased mean fruit size, leading to a total yield comparable to that of high-mineral cultivation. This indicates that organic waste-derived fertilizers are not only a sustainable alternative but may even outperform inorganic nutrients in terms of fruit size and overall plant development. This household-scale system provides a practical example of a closed-loop approach that integrates water, energy, and food production. Organic waste is processed through anaerobic digestion, which reduces the amount sent to landfills and lowers methane emissions while generating biogas that can replace conventional energy sources. The resulting digestate is nutrient-rich and can be applied to crops as fertilizer, supporting food production. By linking waste-to-energy and waste-to-food pathways, the system illustrates integrated WEF-nexus solutions that promote resource efficiency, food production, and environmental sustainability. In addition to these environmental benefits, household adoption of WEF systems can foster community engagement and encourage collective action toward more sustainable resource management.

4. Conclusions

A novel household-scale, closed-loop WEF system was designed, installed, and operated to manage food waste while recovering water for irrigation, nutrients for plant growth, and biogas for energy supply. The experimental analysis of water quality and gas chromatography samples from the biodigester and hydroponic system showed the system’s potential for household use, providing renewable energy and nutrient-rich effluent for plant growth. The research focused on biogas production and on ensuring a consistent wastewater byproduct with sufficient natural fertilizer value for agricultural irrigation use. The addition of the hydroponic system to food waste management demonstrated that reduced harmful levels of phosphate, nitrite, and nitrate released to the environment could be sustainably used to grow plants. Results from gas chromatography showed that methane levels were substantially above typical small biodigester systems for the eight weeks of sampling, averaging 76.7%, with a peak of 81.3%. Key indicators such as conductivity (mean: 3120.81 µS/cm), TDS (2.18 g/L), and substantial levels of phosphate (20 mg/L), nitrate (26 mg/L), and nitrite (5 mg/L) confirm its potential as a nutrient source. The pH was near neutral (7.01). While ORP values fluctuated, the overall water quality supports its reuse in agriculture or hydroponics with minimal treatment.

The integration of this WEF system into a household setting presents numerous benefits, including decentralized food waste management and a renewable biogas supply for cooking and potentially small-scale heating. By transforming food waste into both energy and nutrients, the system aligns with circular economic principles, reducing waste while promoting sustainable energy and agricultural productivity. This pilot study utilized typical kitchen sink space and common household food waste to demonstrate the system’s feasibility. However, further research is warranted to analyze nutrient composition across diverse food waste types, evaluate their impacts on various plants in household settings and explore pathways to scale up the household toward community or large-scale applications. Such studies will help optimize the system for broader applications in sustainable home food waste management and urban agriculture. Household adoption of such systems collectively advances SGDs by diverting organic waste from landfills, lowering carbon emissions through biogas substitution, and supporting food production. While this study focused on technical performance, future work should also incorporate techno-economic analysis (TEA) and life cycle assessment (LCA) to provide a comprehensive assessment of both economic and environmental performance. Additional focus should be on understanding how socio-cultural factors and human behavior influence the performance of household WEF systems. Such analyses are essential to quantify household-scale benefits, including potential cost-effectiveness, energy savings, waste reduction, and greenhouse gas mitigation, thereby strengthening the case for broader adoption of these systems. There is also potential for household-scale WEF systems to be applied in urban homes, integrated with smart home technologies, and leveraged as a disaster-resilience strategy by providing decentralized sources of renewable energy and food production. In addition, practical barriers such as space constraints, regulatory requirements, maintenance needs, seasonal variability, and feedstock management must be considered to provide a realistic assessment of implementation feasibility. Addressing these challenges alongside system performance and quantified benefits will be critical for advancing household-scale WEF systems from pilot demonstrations toward broader adoption.