Research on the use of Reencle Compost (Reencle X Deong-Eui Uni)

Food waste has emerged as a critical global environmental issue. According to the UNEP (United Nations Environment Programme) Food Waste Index Report, approximately 1 billion tons of food waste was generated worldwide in 2022, with about 60% originating from households. Addressing this waste costs an estimated $1 trillion, representing a significant economic loss.

In addition to the financial burden, the environmental impact of food waste disposal is severe. Currently, food waste is managed through three main methods: landfilling, incineration, and recycling. However, each method has its own set of challenges. Landfilling is the most common approach and generates large amounts of methane, a greenhouse gas responsible for approximately 8-10% of global emissions and a major contributor to global warming. Incineration, while reducing waste volume, requires significant energy input and emits air pollutants during the high-temperature process. Recycling, which aims to convert food waste into compost or animal feed, faces limitations due to low demand and quality degradation.

In South Korea, food waste is no longer landfilled but is managed primarily through recycling and incineration. However, recycled compost and feed often exceed demand, leading to accumulation and storage challenges. The associated odors and environmental issues remain unresolved. Against this backdrop, there is a growing need for solutions that enable households to independently reduce food waste.

As of 2024, the domestic penetration rate of food waste processors in South Korea remains below 10%. However, with the market's growth potential and the expansion of government subsidies, the adoption rate is expected to rise steadily. In this context, the development of technologies to process food waste using household food waste processors and to utilize the resulting byproducts is essential. Such advancements would promote resource circulation within households and serve as a practical solution to fundamentally address environmental issues caused by food waste.

This research was conducted in collaboration with the eco-friendly company Reencle Inc. which focuses on solving environmental issues. The study aims to propose ways to utilize food waste by-products in households. Beyond simple waste reduction, these efforts are expected to establish a sustainable resource circulation system and promote practical environmental protection activities within homes. This initiative holds the potential to drive meaningful change, paving the way for a more sustainable future.

This project aims to establish an efficient household resource circulation system by processing food waste and utilizing the by-products generated during the process as fertilizer. Through this approach, the project seeks to mitigate environmental pollution caused by food waste and propose sustainable resource utilization strategies.

The specific objectives of the project are as follows:

1. Performance and Safety Evaluation of Fertilizer Derived from Food Waste By-products
   
   The project conducts a quantitative evaluation of the fertilizer’s performance and its impact on plant growth. Experimental studies focus on plants with straightforward growth conditions, such as lettuce and basil.
   Treatment groups with varying by-product concentrations are compared against control groups to assess both performance and safety.
   Additionally, the study analyzes key mineral contents, including nitrogen and phosphorus, to scientifically validate the fertilizer's efficacy across different concentrations.
   
   
2. Identifying Environmental, Economic, and Social Benefits
   
   By demonstrating the feasibility of utilizing food waste by-products, the project highlights the following benefits:
   
   
   • Reducing household waste generation.
   • Decreasing greenhouse gas emissions.
   • Lowering economic costs associated with waste management.

The findings aim to promote the importance of sustainable environmental practices and resource circulation. By transitioning from waste disposal to resource recovery and recycling, the study establishes a foundation for practical solutions to food waste challenges.

Thus, the overarching goal of this project is to process food waste efficiently and utilize the resulting by-products as fertilizer to create a household resource circulation system. By scientifically evaluating the performance and safety of the fertilizer and deriving environmental, economic, and social benefits from the results, the project aims to realize the resource recovery potential of food waste. Furthermore, it intends to propose a practical circulation structure that can be implemented in households while raising societal awareness of the importance of sustainable environmental conservation and resource circulation.

The by-products generated during the decomposition of food waste by microorganisms contain essential nutrients for plant growth, such as nitrogen (N), phosphorus (P), and potassium (K), as well as various organic compounds. Among these, nitrates and phosphates are critical for plant development, and their abundance in soil allows them to be effectively utilized as fertilizers.

For optimal growth, crops require both macronutrients—carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg)—and micronutrients, including iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), boron (B), and chlorine (Cl). While carbon, hydrogen, and oxygen are naturally absorbed from air and water, nitrogen, phosphorus, and potassium are primarily supplied through the soil and are collectively referred to as the “Three essential elements of fertilizer.”

Crops typically grow well in soil with nutrient concentrations of 1.5% nitrogen, 0.2% phosphorus, and 1.0% potassium.

Organic fertilizers play a crucial role in providing these nutrients but must be used cautiously to avoid adverse effects. Excessive application of organic fertilizers can lead to several issues:

• Decomposition by soil microorganisms can release large amounts of carbon dioxide (CO₂), potentially acidifying the soil.
  
  
• Nutrient oversaturation may result in salt accumulation in the soil.
  
  
• Incompletely decomposed fertilizers can release harmful substances and toxic gases such as ammonia, which can severely damage crops.
  
  

Because crops can only absorb nutrients in mineralized forms, organic fertilizers must first be broken down into inorganic nutrients by microorganisms to effectively support plant growth.

This project aims to evaluate the feasibility of using microbial by-products as fertilizers through experiments on lettuce and basil—plants that are relatively easy to grow and manage in household environments. Four fertilizer concentration groups were established: Control, Positive Control, 20%, and 100%. 

The growth rate of lettuce and basil was assessed by measuring the expansion of leaf area. The primary metric for evaluating fertilizer performance was the rate of increase in leaf surface area, providing quantitative evidence of the effectiveness of food waste by-products as fertilizers. This study aims to demonstrate the positive effects of food waste by-products on crop growth and to highlight their potential for promoting resource circulation within households.

Salt content analysis was conducted to ensure that the salt levels in microbial by-products were not harmful to crops or soil. Excessive salt in fertilizer can lead to soil salinization, inhibiting crop growth and potentially causing long-term soil degradation. In particular, high salt concentrations in by-products generated during the food waste processing cycle could negatively impact plants. To confirm the safety of microbial by-products as fertilizers, salt content was measured to ensure they can support healthy crop growth without causing soil or plant damage.

Through these experiments and analyses, this project seeks to scientifically validate the benefits and safety of microbial by-products as fertilizers. The findings aim to demonstrate their potential for enhancing crop growth while establishing microbial by-products as a viable resource for household resource circulation systems.

1. Control Groups

*Each group consisted of 3 pots for the Control, and 4 pots each for the Positive Control, 20%, and 100% groups, totaling 15 lettuce seedlings and 15 basil seedlings used for the experiment.

2. Planting: Lettuce and basil seedlings were planted in pots. (using commercially available untreated soil)

3. Treatment Application: Treatments were applied according to the designated groups. (Compound fertilizer: 5 pellets per pot; 20% and 100% treatments were applied to cover the soil surface in the pot.)

4. Measure the leaf area and soil pH weekly.
(For this, a ruler was placed next to the leaf, and a photo was taken from the front. The leaf area was measured using the image analysis software ImageJ, with a scale bar set individually for each photo based on the ruler.)

Additional Notes:

• Plants were watered whenever the soil surface dried out, and environmental conditions were maintained within the following ranges: temperature at 15℃–23℃, humidity at 40%–70%, light cycle for approximately 10 hours per day, and soil pH between 6 and 7.5.

<Data Analysis Method>

• Growth performance was evaluated compared to the Control group using the following formulas:
  
  

Growth Rate (cm²/day):

Growth Rate Change (%) Compared to Control:

1. Preparation of Alkaline Potassium Persulfate Solution:
Dissolve 10 g of sodium hydroxide (NaOH) in 250 mL of distilled water, then add and dissolve 7.5 g of potassium persulfate (K₂S₂O₈) to prepare the solution.

2. Preparation of Sample Solution:
Add 200 mL of distilled water to 5 g of food waste byproducts to prepare a sample solution with a concentration of 25 mg/mL.

3. Digestion Process:
Transfer 50 mL of the sample solution (dilute as needed if the nitrogen content exceeds 0.1 mg) into a digestion flask. Add 10 mL of the alkaline potassium persulfate solution, seal the flask, and mix thoroughly by shaking. Place the flask in an autoclave and heat until the temperature reaches approximately 120°C, then maintain this temperature for 30 minutes to complete the digestion process. Allow the flask to cool naturally.

4. Filtration:
Filter the supernatant from the digested sample using a glass fiber filter (GF/C). Discard the first 5–10 mL of the filtrate, then collect exactly 25 mL of the filtrate and transfer it into a 50 mL beaker or cuvette.

5. Preparation of Nitrate-Nitrogen Standard Stock Solution:
Dry potassium nitrate (KNO₃) at 105–110°C for 4 hours until a constant weight is achieved. Accurately weigh 0.3609 g of the dried KNO₃ and dissolve it in 500 mL of distilled water to prepare the nitrate-nitrogen standard stock solution (0.1 mg NO₃-N/mL).

6. Preparation of Diluted Standard Solution:
Take 20 mL of the 0.1 mg NO₃-N/mL stock solution, dilute with distilled water, and adjust the final volume to 100 mL to prepare a 0.02 mg NO₃-N/mL standard solution.

7. Preparation of Calibration Standards:
Further dilute the 0.02 mg NO₃-N/mL standard solution with distilled water to prepare calibration standards with concentrations of 0, 4, 8, 16, and 20 μg NO₃-N/mL.

8. Measurement and Calibration Curve Construction:
Measure the absorbance of the calibration standards at 220 nm. Use the measured absorbance and corresponding concentrations of the calibration standards to derive a regression equation and construct a calibration curve. Determine the concentration of the diluted sample solution using the calibration curve, and multiply by the dilution factor to calculate the original nitrogen concentration.

1. Prepare a standard phosphate solution (1000 ppm) by dissolving high-purity potassium dihydrogen phosphate in water to ensure that 1 mL of the solution contains 1 mg of P₂O₅. Adjust the final volume to 1 L.
   
   
2. To prepare the color reagent, dissolve 0.56 g of ammonium metavanadate (NH₂VO₃) completely in approximately 150 mL of water. If the dissolution is incomplete, gently heat the solution using a hot plate. Add 125 mL of nitric acid (HNO₃) to the solution using a volumetric flask. Separately dissolve 13.5 g of ammonium molybdate tetrahydrate [(NH₄)₆Mo₇O₂₄·4H₂O] in water, then mix it into the first solution. Add water to adjust the final volume to 500 mL.
   
   
3. Weigh 5 g of microbial byproducts precisely and dissolve them completely in an appropriate amount of distilled water. Add approximately 30 mL of hydrochloric acid (HCl) and 10 mL of nitric acid (HNO₃). Confirm the sample is fully dissolved, then add water to adjust the final volume to 200 mL.
   
   
4. Prepare a 100 ppm standard phosphate solution by diluting the 1000 ppm standard phosphate solution 10-fold.
   
   
5. Using the 100 ppm standard phosphate solution, prepare a series of standard solutions in a 100 mL volumetric flask, ensuring phosphate concentrations of 0, 5, 10, and 15 ppm.
   
   
6. Add 20 mL of the color reagent to each standard solution and fill with distilled water to the calibration mark. Shake the solutions and allow them to react for 10–20 minutes. Measure their absorbance at 415 nm.
   
   
7. Take a known quantity of the unknown sample solution and transfer it to a 100 mL volumetric flask. Measure the absorbance of the unknown sample following the same procedure as the standard phosphate solutions. If the measured absorbance of the unknown sample is outside the quantifiable range (5–15 ppm), dilute the sample appropriately to bring it within the range. Re-measure the absorbance and calculate the phosphate content (P₂O₅) percentage in the sample.

1. Weigh 5 g each of untreated soil, and 20% and 100% food waste byproducts.
   
   
2. Add 40 mL of distilled water to each sample.
   
   
3. After 5 minutes, extract the liquid from the middle portion of the mixture and filter it using a coffee filter.
   
   
4. Measure the salinity (%) of the filtered liquid using the CAS Salinity Meter CSF-500.
   
   
5. Calculate the electrical conductivity (dS/m) by dividing the salinity percentage by 0.064.

Fig 1. (A) Comparison of basil leaf area (B) Comparison of lettuce leaf area (C) Growth rate of basil and lettuce with food waste byproducts (cm²/day)

The experiment evaluated the impact of microbial byproduct-based fertilizer on the growth rates of basil and lettuce, revealing differences across treatment conditions for each plant.

For basil, the growth rate in the Control group was the lowest at 0.281 cm²/day, while the Positive Control group showed a modest increase to 0.301 cm²/day. Growth rates further increased with fertilizer treatments, reaching 0.331 cm²/day in the 20% treatment group and 0.329 cm²/day in the 100% treatment group, indicating a trend of enhanced growth with increasing treatment concentrations.

For lettuce, the growth rate in the Control group was also the lowest at 0.625 cm²/day, while the Positive Control group exhibited a significant increase to 1.207 cm²/day. Among the fertilizer-treated groups, the growth rate was 0.788 cm²/day in the 20% treatment group and 0.835 cm²/day in the 100% treatment group, with minimal differences observed between the 20% and 100% treatments.

Both basil and lettuce demonstrated positive growth responses to the application of food waste byproducts. For basil, the gradual increase in growth rates with higher fertilizer concentrations suggests that the byproducts can serve as a consistent nutrient source. Conversely, for lettuce, the minimal difference in growth rates between the 20% and 100% treatments indicates that the fertilizer's incremental benefits may plateau beyond a certain concentration.

Notably, the 20% treatment concentration for lettuce appeared to achieve an optimal growth rate, suggesting that this level is sufficient to maximize growth efficiency without the need for higher concentrations.

Fig 2. Quantitative Analysis Results of Nitrogen (N) and Phosphate (P) in Food Waste Byproducts

The quantitative analysis of food waste byproducts revealed a nitrogen (N) content of 0.69% and a phosphate (P) content of 0.0324%.

Nitrogen is an essential nutrient for plant growth, playing a pivotal role in chlorophyll synthesis and photosynthesis. The analyzed nitrogen content is within the range suitable for use as fertilizer and can effectively contribute to promoting plant growth.

The phosphate content, while relatively low, serves as a fundamental nutrient necessary for root development and cellular energy metabolism. These findings confirm that food waste byproducts contain nitrogen, a key fertilizer component, at levels sufficient to support stem and leaf growth.

However, the phosphate content of 0.0324% may be insufficient to fully support the energy metabolism and root development required by crops. This suggests that when utilizing food waste byproducts as fertilizer, supplementation with an additional phosphate source may be necessary to optimize crop performance.

Fig 3. Salinity and Electrical Conductivity (EC) by Concentration of Food Waste Byproducts

As the concentration of food waste byproducts increased, both salinity and electrical conductivity (EC) exhibited a corresponding rise. EC serves as a crucial indicator of soil salinity levels, with values exceeding 4 dS/m potentially posing negative effects on crop growth.

In this experiment, the EC at a 100% concentration was measured at 3.75 dS/m, which is considered to be within a relatively safe range for supporting crop growth.

Firstly, utilizing microbial byproducts generated by household food waste processors as fertilizer can establish a virtuous cycle within homes. This cycle involves using the byproducts to grow plants, consuming the harvested produce, and recycling the organic waste back into the system. Such a framework encourages resource recycling within households, supports sustainable environmental protection efforts, and provides a tangible example of resource circularity in action.

Secondly, microbial byproducts serve as effective fertilizers, delivering essential nutrients necessary for plant growth. This contributes to increased agricultural productivity and improved crop quality. Additionally, recycling food waste reduces greenhouse gas emissions, aids environmental preservation, and positively addresses climate change challenges. These practices not only heighten public awareness of environmental conservation but also encourage practical environmental protection activities within households, offering significant educational benefits.

Practical applications include converting food waste byproducts into fertilizer for use in potted plants or household gardens, thereby activating resource circulation at the domestic level. Beyond household use, these fertilizers can be commercially developed or utilized in collaboration with local communities to promote resource recycling campaigns.

Such initiatives can support sustainable agriculture and horticulture industries while highlighting the importance of environmental protection and resource recycling. Campaigns emphasizing these values can inspire public participation, driving meaningful changes not only at the household level but also within commercial and societal contexts. Ultimately, these efforts can lay the foundation for a more sustainable and environmentally responsible future.