Characteristics of Soil Amendment Material from Food Waste Disposed of in Bioplastic Bags

Abstract

Effective food waste management is key to a sustainable future. We herein aimed at assessing the composition and the amount of food waste generated in the Chamchuri (CU) Terrace condominium (floors 18–22) in Bangkok (Thailand), producing soil amendment material from this same food waste and examining the effect of bioplastic bags on composting. The condominium generated 29.01 kg of general waste per day. The food waste (6.26 kg/day) was classified into “available” and “unavailable” food waste, accounting for 3.26 and 3.00 kg/day, respectively. The composting of the food waste lasted 45 days and was undertaken under three experimental conditions: (i) control (no food waste), treatment 1 (T1: food waste), and treatment 2 (T2: food waste along with pieces of bioplastic bags). The physicochemical analysis of the final composts of these treatments revealed that T2 could be used as soil amendment material after enrichment of its macronutrient composition and an increase in fermentation time. Interestingly, the T2 bioplastics were characterized by a lack of holes or were fragmented into pieces larger than 5 mm. In conclusion, food waste management in the CU Terrace condominium can use food waste collected in bioplastic bags as soil amendment material.

1. Introduction

In recent years, the problem of food waste management has gained worldwide attention. Food waste is any food or leftover inedible parts that fall out of the food chain that can be disposed of or be recycled by composting [1]. Biodegradation occurs when food waste is disposed of in landfills, thereby resulting in the production of methane (a greenhouse gas that contributes to global warming). In 2017, Thailand produced 17.56 million tons of organic waste—waste that was equal to 64% of the total solid waste or to 254 kg per person per year. Thailand’s goal is to reduce food waste by 5% per year through reductions in both food loss and waste.

There are many ways to recycle food, such as using it as a source of fuel [2], as fertilizer [3], and as animal feed [4,5,6,7]. These methods reduce the amount of food waste generated and sent to landfills. Composting is an alternative to food waste management and can be used for soil improvement. The utilization of food waste is in agreement with Goal 12 of the Sustainable Development Goals established by the United Nations, which is aimed at ensuring sustainable consumption and production patterns. It is also in agreement with Target 12.3 that aims at halving the world’s food waste at retail and consumer levels, as well as food waste from production processes and supply chains (including post-harvest losses), by 2030. Reusing plastic bags for food waste separation in households can be an option due to their durability, light weight, watertightness, and grease resistance. Unfortunately, conventional plastic takes years of time to completely degrade. As a result, biodegradable polymers with characteristics similar to traditional plastics are being offered as alternatives.

At present, global bioplastic production is rapidly increasing and is expected to reach three million tons by 2025 [8]. Bioplastics are a group of compounds that are (i) biodegradable and made from bio-based materials, (ii) biodegradable and made from fossil-based materials, or (iii) nonbiodegradable [9]. In Italy, bioplastics are used in food packaging and in the collection of food waste before its shipment to treatment plants [8]. The degradation of bioplastics in treatment plants may result in compost that contains large quantities of nondegradable bioplastics [9]. The complete degradation of bioplastics depends on factors such as temperature, pH, alkalinity, humidity, and sufficiency of microorganisms and nutrients. If some of these factors are not suitable for the induction of biodegradation, the process is incomplete. Therefore, nondegradable materials may form microplastics that can contaminate water, soil, and compost [10]. Nowadays, bioplastics are used in waste management, as bioplastic bags are used in order to collect organic waste and deliver it to composting plants. Biological plastic bags (that are made from cassava starch) can be used to carry organic waste and can be placed into soil or put in aerated compost pits. Such bags leave no toxic residues that are harmful to the environment. Bioplastic bags can biodegrade within 3–4 months [11]. For this reason, microplastics that may remain in compost must be identified in order to confirm that organic waste composted with bioplastic bags does not contain microplastics and that bioplastic bags can be used safely in waste management.

The aims of this study are to (i) assess the composition and the amount of food waste produced by the CU Terrace condominium, (ii) produce soil amendment material from the food waste of the CU Terrace condominium, and (iii) evaluate the effects of the use of bioplastics as food waste bags on the production of soil amendment materials.

2. Materials and Methods

2.1. Study Area

In this study, food waste from the Chamchuri (CU) Terrace condominium (Bangkok, Thailand) was sampled. The CU Terrace condominium is a 22-story condominium accommodation of 510 units with an area of approximately 9877 m² located at Chulalongkorn Soi 9, Rama 4 Road, Pathum Wan Subdistrict, Pathum Wan District, Bangkok, 10330, Thailand. As the CU Terrace is a large condominium of 510 units with a population of approximately 810 inhabitants, it generates a large amount of food waste that can be used to represent the food waste produced by a city’s population. Generally, the waste of the CU Terrace condominium is collected and separated into three categories: (i) general waste, (ii) recyclable waste, and (iii) recyclable plus waste (incinerable waste). In January 2022, the average amount of general waste and food waste was 35.3 kg per day, where the food waste was discarded along with the general waste. As a result, there was a large amount of general waste that must be disposed of. Therefore, food waste must be separated from the general waste in order to reduce the amount of general waste produced. However, as part of this study, we divided the food waste into two categories: (i) unavoidable food waste (such as fish bones, bones, fruit peels, eggshells, etc.) and (ii) avoidable food waste (such as bread, rice, meat, vegetables, fruits, etc.) [12]. By using direct measurement methods through weighing, we measured the amount and studied the composition of the produced food waste. The food waste obtained from sorting was composted in two ways: (i) as food waste only and (ii) as food waste in bioplastic bags (using aerobic fermentation). We subsequently analyzed the quality of the soil amendment material in order to confirm its suitability to be used as such, as well as to confirm that it did not contain microplastic residues from the biodegradation of the bioplastic bags. There were guidelines set for the separation of food waste from general waste in the CU Terrace condominium, allowing households to separate food waste into bioplastic bags and to discard them in a dedicated food waste bin. Therefore, the Bangkok solid waste collection agency could send food waste that was placed in bioplastic bags to a composting plant without having to separate the bioplastics, thereby facilitating waste management.

2.2. Sampling

The samples used for the measurement of the composition and the amount of food waste produced by this condominium were collected from floors 18–22, which consisted of family-room-type units (totaling 150 units representing 29.41% of the condominium’s units and 55.56% of the inhabitants).

2.3. Waste Composition Analysis

General waste was collected from residents of floors 18–22 of the CU Terrace condominium and was separated and analyzed for composition. We measured the amount of each type of waste deposited in green bins (general waste) by weighing before sorting and separating the waste into the following five categories: (i) general waste, (ii) food waste, (iii) waste for incineration, (iv) selling waste, and (v) waste related to the COVID-19 pandemic. These measurements were performed for 7 days (every Monday–Sunday). As the period of the experiment ought to be representative of weekdays and holidays, the sampling of the food waste was undertaken on Saturdays and Sundays [13] as part of the sorting taking place during 6–12 February 2022.

2.4. Sorting and Measuring the Amount of Food Waste Generated

Food waste was categorized into “avoidable food waste” (such as rice, bread, meat, fruits, vegetables, noodles, sweets, etc.), and “unavoidable food waste” (such as bones, eggshells, fruit peels, apple cores, and coffee grounds) [14]. Both types of food waste were weighed on a scale.

2.5. Preparation of the Raw Materials before Composting the Soil Amendment Materials

Two types of food waste collected from the CU Terrace condominium weighing 7 kg each were stored at 4 °C after first removing food waste that should not be composted from the food waste samples (such as bones, fish bones, cooking oil, and dairy products). Before using food waste to produce soil amendment materials, food waste was homogenized through grinding and was mixed with the use of a food processor. Moreover, large sawdust was crushed with a grinder to a size of 3–50 mm [15] in order to increase the contact area, thereby facilitating microbial degradation and increasing the porosity of the material.

2.6. Design of the Composting of the Soil Improvement Materials from Food Waste

Crushed food waste was mixed with cow dung and sawdust as bulking agents to control moisture, bulk density, and the C/N ratio at a ratio of 1:1:0.5, respectively. According to Wahyuningtiyas and Suryanto, the decomposition duration of bioplastic cassava starch was 12 days [16]. Composting was allowed for 45 days in a 20 L polypropylene (PP) plastic tank with the lid closed to reduce the spread of odor, to prevent insects from laying eggs in the composted manure, and to prevent any contaminants from falling into the soil amendment compost bin. By drilling four holes of equal diameter (15 mm) at the bottom of the tank, we were able to reduce the excess amount of moisture in the tank. Aeration was provided once a day through hand turning of the compost. We also added Super PD1 degradation accelerator from the Department of Land Development [17] and water in a ratio of 1:50 (1 sachet of PD1 per 50 L of water) in order to increase the number of microorganisms that helped the degradation of organic matter. Temperature and humidity were measured daily; when the moisture content was lower than 40%, about 250 mL of water was sprinkled over the material. The soil amendment fermentation was observed through three experimental conditions: (i) control, (ii) treatment 1 (T1), and (iii) treatment 2 (T2). The ratios of components for these treatments are presented in

Table 1. Composition of the soil amendment compost in the three experimental conditions examined.

Treatment	Composition of Soil Amendment Materials (kg)
	Food Waste	Cow Dung	Sawdust
Control	0	1	0.5
T1	1	1	0.5
T2 (+bioplastic bags)	1	1	0.5

. Each experimental condition was repeated in three tanks. The T2 experimental condition included the addition of pieces of bioplastic bags (biodegradable and made from bio-based materials) in the tanks (150 pieces per tank); their size was 5 × 5 cm.

2.7. Sampling of Soil Amendment Materials

The samples were collected at 0, 7, 15, 30, and 45 days after the initiation of food waste composting. Samples (weighting 120 g each) were randomly collected from the fermentation tanks, dried for 5–7 days, and then crushed to fine fragments with a blender; these fine fragments were used for the physicochemical property analysis.

2.8. Physicochemical Property Analysis

2.8.1. Temperature Measurement

The temperature was measured at the center of the soil amendment material through the use of a thermometer; the reference temperature was also recorded.

2.8.2. Measurement of pH and Electrical Conductivity

Measurement of the pH and of the electrical conductivity of the samples was conducted for aqueous extracts of the samples (1:10 w/v of sample:water) with a pH meter (Milwaukee model pH55/pH56, Rocky Mount, NC, USA) and a conductivity meter (Hach Sension 156, Loveland, CO, USA), respectively.

2.8.3. Total Organic Carbon (TOC) Determination

We analyzed the samples’ TOC with a scientific instrument based on the principle of high-heat incineration of the samples with a temperature of approximately 1200 °C [18]. This method followed the principles of the digestion of organic and inorganic substances and their conversion to carbon dioxide. The sample preparation process was as follows. Approximately 2 g of each sample was weighed, baked in an oven at 105 °C for 2 h, and placed in a desiccator for 30 min. Subsequently, 2 M of hydrochloric acid was dripped over each sample, stirred well, and left for 24 h in order to decompose the inorganic substances from the samples. After 24 h, we completely removed the hydrochloric acid and washed each sample 2–3 times with distilled water. We then baked the samples in an oven (at 105 °C) for 2 h and placed them in a desiccator again for 30 min. Finally, each sample was analyzed with a TOC analyzer (Multi N/C 3100 Analytik Jena, Jena, Germany).

2.8.4. Total Kjeldahl Nitrogen (TKN) Determination

By using the TKN method, we analyzed the total nitrogen contents of the samples with a KjelFlex K360 (Büchi, Essen, Germany). The process included the following steps: digestion, ammonia distillation, and titration in order to determine the nitrogen content.

2.8.5. Available Phosphorus Determination

The analysis of available phosphorus consisted of two steps: (i) extraction (the extracting solution used in the experiment was Mechlich 1, which is a mixture of 0.05 N HCl and 0.025 N H₂SO₄) and (ii) the ascorbic acid sulfomolybdo-phosphate blue color method (for the spectrophotometric measurement of absorbance with a spectrophotometer, Labomed, Inc. Los Angeles, CA, USA) [19].

2.8.6. Exchangeable Potassium Determination

The potassium available for plant utilization was extracted using the NH₄OAc method [19]. The extracted potassium was analyzed with an inductively coupled plasma-optical emission spectrometer (Plasma Quant PQ 9000 Elite Analytik Jena, Jena, Germany).

2.9. Analysis of Microplastics in Soil Amendments from Food Waste

We separated the microplastics from the soil amendment material by sifting the samples through a 1 mm sieve. We then weighed the sifted samples. Three sifted samples that were likely to contain microplastics were randomly selected in order to represent the samples. The samples were then analyzed with Fourier-transform infrared spectroscopy (FTIR) (Bruker, Invenio-S, Ettlingen, Germany) in order to identify microplastic structures and to confirm whether such structures were the same as those expected to be derived from the compostable plastic bags used for composting or not.

2.10. Data Analysis

The data in this study were analyzed using one-way analysis of variance (ANOVA) for the comparison of parameters such as the temperature, the pH value, the conductivity, the TOC, the TKN, the available phosphorus, the exchangeable potassium, and the carbon-to-nitrogen (C/N) ratio at a significance level of 0.05.

3. Results

3.1. Waste Composition and Quantity

The amount of waste generated from the 150 units of floors 18–22 of the CU Terrace condominium (one unit per household) within 7 days was 29.01 kg/day, with a general waste generation rate of 0.19 kg per household per day. The main waste components after waste segregation were separated into five types of waste, as shown in Figure 1.

The most common generated waste was general waste (generated at a rate of 16.77 kg/day), and it was separated into foam packages, tissue paper, cotton pads, cotton buds, sanitary pads, and other items that were removed during waste segregation. The second most generated waste was food waste. Food waste (generated at a rate of 6.26 kg/day) was separated into two types: avoidable and unavoidable food waste. Avoidable food waste (generated at a rate of 3.26 kg/day) was separated into rice, noodles, fruits, vegetables, etc. Unavoidable food waste (generated at a rate of 3.00 kg/day) was separated into bones, expired foods, spoiled fruits, hard vegetable stalks, etc. Incinerable waste (generated at a rate of 4.80 kg/day) used as refuse-derived fuel (RDF) with coal in a cement plant in the Saraburi Province included chopsticks, food packages, plastic cups, paper cups, plastic bags, plastic utensils, milk cartons, and old clothes, while recyclable waste (generated at a rate of 0.85 kg/day) included plastic bottles, glass bottles, aluminum cans, and office papers. Finally, the least common generated waste was COVID-19-outbreak-related waste; this was generated at a rate of 0.33 kg/day and included surgical masks, COVID-19 test kits, and alcohol bottles.

When only considering food waste segregation, the edible food waste quantity was found to be larger than that of the nonedible food waste. It was concluded that people tend to have leftover food that generates food waste in the form of edible food waste, which accounted for 52.07% of the total food waste. Moreover, food waste segregation from total general waste could reduce the quantity of waste that had to be disposed of in landfills by up to 21.57%. As a result, food waste segregation from general waste could reduce landfill usage and the cost of waste handling. In addition, food waste from waste segregation could be used in order to produce soil amendment material for plantations, thereby adding value to food waste.

3.2. Chemical Properties of Raw Materials

From the analysis of the raw materials used in the soil amendment composting process (

Table 2. Chemical compositions of the raw materials used in our study.

Parameter	Raw Materials
	Food Waste	Sawdust	Cow Dung
C/N ratio	17.21 ± 1.6	479.96 ± 0.63	22.68 ± 0.70
% TKN	1.74 ± 0.17	0.08 ± 0.13	1.57 ± 0.12
% TOC	29.90 ± 1.53	38.40 ± 0.57	35.61 ± 0.68
Available phosphorus (ppm)	40.82 ± 0.66	26.45 ± 7.53	66.76 ± 3.94
Exchangeable potassium (ppm)	5666.01 ± 6.10	5785 ± 5.84	1095.65 ± 0.15

), saw dust was shown to have the highest C/N ratio at 479.96 ± 0.63, followed by cow dung at 22.68 ± 0.70 and food waste at 17.21 ± 1.6. A low C/N ratio affects microbes because carbon is not sufficient to inhibit of the growth of microbes [20]. Moreover, the food waste contained nitrogen in the form of TKN at 1.74% ± 0.17%, available phosphorus at 40.82 ± 0.66 ppm, and exchangeable potassium at 5666.01 ± 6.10 ppm. Food waste could not be used directly for soil amendment because it was characterized by a low porosity, a low C/N ratio, a high moisture content, and a lack of total nitrogen and phosphorus. In this study, sawdust was used in order to eliminate these limitations of food waste [21]. Sawdust has a high C/N ratio when compared with food waste or cow dung. Moreover, sawdust is slowly degraded, thereby helping provide porosity and improving the C/N ratio to a range between 25/1 and 30/1, which is suitable for the soil amendment composting process [22]. On the other hand, cow dung provides nutrients for soil amendment because it contains nitrogen at 1.57% ± 0.12% TKN, available phosphorus at 66.76 ± 3.94 ppm, and exchangeable potassium at 1095.65 ± 0.15 ppm.

3.3. Physical Appearance of Soil Amendment Material

The physical appearances of the soil amendment materials obtained from the soil amendment composting process at days 0 to 45 are presented in Figure 2. In samples T1 and T2 from days 0 to 15, food waste (e.g., chili residue) could be observed in the bucket. From days 30 to 45, food waste could not be distinguished from the cow dung and the sawdust, but both the sawdust and cow dung could be observed by color.

3.4. Physical Appearance of Composted Bioplastic with Soil Amendment Material

The physical appearance of the 5 × 5 cm bioplastic pieces in T2 can be observed in Figure 3. On day 15, the bioplastic began to degrade due to microbes, thereby causing pores on the bioplastic surfaces. Larger pores could be observed on days 30 and 45. The bioplastic bags used in this study were produced with cassava starch, which can biodegrade within 3–4 months; these bags were used in this study for food waste handling that could be undertaken in aerated composting pits and thereafter be utilized for soil amendment [11]. These bioplastic bags were made of cassava starch, an organic component in the form of a carbohydrate used as an organic carbon source of energy. Microorganisms in cow dung use this carbohydrate to generate energy for growth. These bioplastics are thin and, when cut into pieces, they have large surface areas, allowing microorganisms to access and decompose these plastics by releasing enzymes to break down these plastic bags into smaller sizes. Then, the smaller organic carbon from degradation can be used as energy for cell division, growth, and reproduction. Because bioplastics contain a source of organic carbon in the form of carbohydrates, which are more easily digestible than the digestion of cow dung or sawdust, microbial activity uses less energy to decompose, affecting the number of microorganisms in the soil, which has enough bacteria to decompose cow dung and sawdust when growth factors including microorganisms, nutrients for microbial growth, temperature, humidity, pH, etc. are suitable [23]. Then, there is enough energy to activate microbe reproductive behaviors, and increases in microorganisms and their activities are able to degrade other organic substances, such as cow dung and sawdust, which are more difficult to decompose than bioplastics. Organic matters are biodegraded into inorganic matters (in the form of ions that plants may utilize), carbon dioxide, and water. The greater the increase in this activity, the greater the inorganic materials in the form of plant nutrients.

3.5. Temperature Change in the Soil Amendment Composting Process

Temperature is an important factor for the soil amendment composting process. The decomposition of organic matter emits heat in compost [24]. The temperatures measured during the soil amendment composting process for 45 days can be observed in Figure 4. Moreover, the average temperatures of the control, T1, and T2 samples exhibited significant differences at p < 0.05. When comparing T1 with T2, the average temperature revealed no significant differences (p > 0.05). Between days 7 and 15, the average temperature increased from 32.5 °C to 37.2 °C and from 30.8 °C to 34.3 °C, respectively.

3.6. pH Changes

Figure 5 shows that the pH of the soil amendment material during the composting process of samples T1 and T2 at 0 and 7 days tended to increase in the range of 5.96–7.40 and 6.28–7.39, respectively. The increase in the pH was affected by the decomposition of organic matter that contained nitrogen in the form of ammonium nitrogen [25]. At 15, 30, and 45 days of composting, the pH values of T1 and of T2 were found to be slightly decreased. However, the pH values of all the samples were still in agreement with the compost standard set by the Thailand Department of Agriculture, which indicates that pH must be between 5.5 and 8.5 [26].

3.7. Conductivity Changes

Based on our findings, the average conductivity of every sample at 0, 7, 15, 30, and 45 days was within the range from 0.79 to 5.72 dS/m. The average conductivity tended to increase, as shown in Figure 6. After the composting process, the highest conductivity measurement obtained was for T1 at 5.72 ± 1.14 dS/m, followed by that of T2 at 4.74 ± 0.93 dS/m and that of the control sample at 2.84 ± 0.61 dS/m. The conductivity change was not statistically significant (p > 0.05) when comparing control with T2 or when comparing T1 with T2. However, the conductivity change comparison between the control and T1 samples indicated that there was a significant difference at p < 0.05.

3.8. TOC Level Changes

The TOC levels of the control and T2 samples tended to increase, while those of the T1 sample tended to decrease after the composting process. Figure 7 shows that the TOC levels were high at the beginning of the composting process. However, the total organic carbon levels decreased when the composting duration increased because of the degradation undertaken by microbes. The microbes used sawdust as their main carbon source for energy and organic matter degradation. After the composting process was finished, the control samples had the highest TOC levels (at 49.33% ± 1.66%), followed by T2 (at 48.93% ± 0.86%) and T1 (at 46.32% ± 2.69%). The TOC levels of our samples did not exhibit any significant differences (p > 0.05).

3.9. TKN Level Changes

The amount of nitrogen in every soil amendment material treatment tended to decrease after the composting process when compared with the raw materials. Figure 8 shows that T1 had the highest TKN levels (at 1.57% ± 0.14%), followed by T2 (at 1.44% ± 0.15%) and control (at 1.13% ± 0.17%). The TKN levels exhibited no significant differences (p > 0.05) when comparing the control sample with T2 or when comparing the T1 sample with T2. However, when comparing the control condition with T1, the TKN levels exhibited a significant difference (p < 0.05).

3.10. Changes in the Levels of Available Phosphorus

The levels of available phosphorus in the T1 and T2 samples tended to increase, as shown in Figure 9. The average available phosphorus levels in the control, T1, and T2 samples were significantly different (p < 0.05). However, the samples of the T1 and T2 conditions did not exhibit significant differences when compared (p > 0.05). It can be concluded that the addition of bioplastic did not affect the levels of the available phosphorus. At 45 days of soil amendment composting, the T1 samples exhibited the highest available phosphorus concentration (at 74.29 ± 5.09 ppm), followed by the T2 samples (at 71.80 ± 4.26 ppm) and the control samples (at 46.08 ± 3.12 ppm).

3.11. Changes in the Levels of Exchangeable Potassium

The levels of the exchangeable potassium in the T1 and T2 samples tended to increase at 0, 7, and 15 days, while they tended to decrease at 30 and 45 days (Figure 10). The average levels of exchangeable potassium in the T1 and T2 samples were not significantly different from each other (p > 0.05).

3.12. C/N Ratio Changes

The C/N ratios of every sample tended to increase from the beginning to the end of the composting process. The range of the C/N ratios was between 26.32 and 44.39 after the composting process (Figure 11). The samples that exhibited the highest C/N ratios were the control samples (at 44.39 ± 8.31), followed by the T2 samples (at 34.22 ± 3.45) and the T1 samples (at 29.63 ± 1.27). Moreover, the C/N ratios were not significantly different when compared between the control and T2 samples or between the T1 and T2 samples (p > 0.05). However, a comparison of the C/N ratios between the control and T1 samples exhibited a statistically significant difference (p < 0.05).

3.13. Microplastics in the Soil Amendment Material

According to the results of the FTIR analysis, the bioplastic employed in this study was terephthalic-acid-based, biodegradable polyester. Our analysis indicated that microplastics were not found in the T2 soil amendment samples that were sieved through a 1 mm sieve. As the soil amendment composting process duration was short, it might not have provided enough time to decompose the bioplastic. Moreover, the composting was undertaken in a closed bucket; therefore, the air circulation in the bucket might have not been enough for the microbe to decompose the organic matter, thereby causing slow decomposition. Bioplastics were decomposed and torn apart, but they were still larger than 5 mm. However, there might have been microplastics smaller than 1 mm in the soil.

4. Discussion

We studied the waste composition of the CU Terrace condominium (floors 18–22; 150 units). A total of 29.01 kg of general waste was found to be generated per day. After waste separation, food waste accounted for 6.26 kg/day, and it was divided into two categories: avoidable food waste (3.26 kg/day) and unavoidable food waste (3.00 kg/day). The amount of food waste depended on the type of the house and the number of its residents. Food waste separation could reduce the amount of general waste by 21.57%, and food waste can be utilized in the production of soil amendment materials (with the addition of cow dung and sawdust) according to UN Sustainable Development Goal 12.3.

The composting of the food waste lasted 45 days and was undertaken under three experimental conditions: (i) control (no food waste), T1 (food waste), and T2 (food waste along with pieces of bioplastic bags). The physicochemical analysis of the final composts of these treatments revealed no significant differences between T1 and T2 (p > 0.05). When comparing the properties of the soil amendment materials with the organic fertilizer standards set by the Department of Agriculture, we found that most of them were within the specified standards, except for the C/N ratios and the levels of available phosphorus (

Table 3. Comparison of compost parameters in control, T1 and T2 with compost standards by the Department of Agriculture, Thailand.

Parameter	Treatment			Compost Standard [26]
	Control	T1	T2
pH	7.47 ± 0.50	6.80 ± 0.10	7.27 ± 0.59	5.5–8.5
Electrical conductivity (dS/m)	2.84 ± 0.61	5.72 ± 1.14	4.74 ± 0.93	<6
C/N ratio	44.39 ± 8.31	29.63 ± 1.27	34.22 ± 3.45	<20
% TKN	1.13 ± 0.17	1.57 ± 0.14	1.44 ± 0.15	>1
% TOC	49.33 ± 1.66	46.32 ± 2.69	48.93 ± 0.86	>30
Available phosphorus (ppm)	46.08 ± 3.12	74.29 ± 5.09	71.80 ± 4.26	Phosphorus > 0.5%
Exchangeable potassium (ppm)	6862.00 ± 1339.68	7056.00 ± 1073.39	5988.67 ± 1417.58	Potassium > 0.5%

). Our findings indicate that, at day 45, the soil amendment material had a darker brown color. The temperature analysis revealed that the soil temperature during the period of 7–15 days increased in T1 and T2 from 32.5 °C to 37.2 °C and from 30.8 °C to 34.3 °C, respectively. Rising temperatures are caused by the release of thermal energy from the raw materials, especially from cow dung and food waste. As food waste consists of biodegradable substances, when microorganisms undertake their decomposition, heat is emitted [27], which helps eliminate pathogens that may spawn in food waste [21]. Moreover, T2 exhibited a lower 45-day temperature range over the fermentation period than T1. This is possibly due to the fact that T2 contained bioplastic bags that biodegraded more slowly than food waste, thereby resulting in lower temperatures. However, the temperature during the composting of soil amendment materials should be between 20 °C and 40 °C at the initial stages, as this is the optimum temperature range for mesophilic microorganisms [22]. Increased temperature can increase the rate of acidification, which can inhibit microbial growth and reduce the rate of composting. Therefore, the maintenance of a proper ambient temperature is an important variable for the growth of microorganisms and the speeding up of the composting process [20].

The pH analysis after the soil amendment composting process revealed that the soil amendment materials had pH levels that were within the organic fertilizer standards set by the Department of Agriculture, which indicate that the pH must be between 5.5 and 8.5 [26]. The reduced pH of the T1 and T2 samples might have been caused by organic acids formed by the decomposition of biodegradable organic substances from the food waste (such as fat, etc.). Organic acids reduce the activity of microorganisms and inhibit the degradation of organic substances; as a result, they can slow down the end time of the soil amendment composting process [22] due to ammonia volatilization and CO₂ emission [27]. Moreover, the observed reduced pH at the end of the soil amendment composting process might have been caused by the nitrification delivered by the microorganisms [20]. On day 45, the pH of every experimental condition examined exhibited no significant differences from the others (p > 0.05). Moreover, the bioplastic bags from T2 had no effect on pH during the soil amendment composting process. However, pH is an important factor for enzymes and the activity of microorganisms involved in the soil amendment composting process of food waste, and pH should be around 5.0 [22].

The conductivity analysis showed that the average conductivity of every experimental condition (days 0, 7, 15, 30, and 45) ranged between 0.79 and 5.72 dS/m. When compared to the standards set by the Department of Agriculture stating that conductivity must not exceed 10 dS/m [26], every experimental condition exhibited a conductivity within these standards. Conductivity exceeding 10 dS/m exerts negative effects on plants because conductivity represents the amount of soluble salts. Organic substances’ decomposition from food waste results in the release of salts (for instance, the protein decomposition in meat produces phosphorus salts, as well as other salts) and increases the conductivity of the soil amendment material. In turn, soluble salts affect plant roots. The latter are hardly able to absorb water, thereby resulting in a plant being unable to uptake the nutrients needed for it to grow [28]. When compared with other studies, the conductivity of the soil amendment material in our study was found to be higher than the 2.15 dS/m reported elsewhere [29].

The TOC levels in the control and T2 samples tended to increase, while those of the T1 samples decreased after the composting process. Figure 7 shows that, in the early stage of the soil amendment composting process, a high amount of carbon was available. At a later stage, carbon was decomposed by the microorganisms, thereby resulting in a decrease in the TOC levels due to the fact that microorganisms used this carbon as their main source of energy for their own growth and the decomposition of the organic substances available [30]. The only source of carbon in the control samples was the sawdust (a material that is difficult to decompose due to lignin, a complex-structured substance [30]), while in the cases of the T1 and T2 samples, food waste was used as a carbon source (which is more easily decomposed than sawdust, thereby resulting in lower organic carbon levels than the control samples). When comparing the TOC levels from our completely composted soil amendment materials with those of other studies, it was found that the TOC levels in our soil amendment materials (at 36.41%) were higher than those of other studies [31].

The TKN levels from every experimental condition assessed tended to decrease after the composting process. As nitrogen is a nutrient that can easily be lost from soil if soil conditions are not suitable (i.e., when soil moisture exceeds 30%), we found that the moisture of our soil amendment materials exceeded 30% every day, thereby exceeding the organic fertilizer standards [26] and resulting in a loss in nitrogen. In addition, slow mineralization delivered by microorganisms could also reduce the amount of nitrogen [32]. As a result, the amount of nitrogen tended to decrease. When comparing the TKN levels identified in our completely composted soil amendment materials with those of other studies, we found that the TKN levels in our soil amendment materials (at 2.92%) were lower than those of other studies [31].

The levels of available phosphorus in the soil amendment materials obtained from T1 samples at day 45 were the highest recorded (at 74.29 ± 5.09 ppm), followed by those obtained from the T2 samples (at 71.80 ± 4.26 ppm) and the control samples (at 46.08 ± 3.12 ppm). Every treatment exhibited levels of available phosphorus below those of the organic fertilizer standards set by the Department of Agriculture, which require that the available phosphorus levels must not be below 0.5% by weight or 5000 ppm [26]. The levels of available phosphorus increased due to the early stage of the composting process, in which the rapid decomposition of easily biodegradable food waste resulted in an increase in the total phosphorus. Moreover, during the composting process, phosphorus could be changed into other forms by microorganisms (i.e., nonsoluble phosphorus can be changed into available phosphorus that plants can uptake and utilize [33]). Organic acids (such as nitric acid, carbonic acid, and sulfur) are an important factor for the dissolution of phosphorus [28]. Moreover, when humic acid levels increase, they can affect the solubility of various phosphorus compounds, thereby enabling a biological phosphate fixation process [34].

The levels of exchangeable potassium from the soil amendment materials obtained from the T1 samples at day 45 were the highest recorded (at 7056.00 ± 1073.39 ppm), followed by those of the control samples (at 6862.00 ± 1339.68 ppm) and the T2 samples (at 5988.67 ± 1417.58 ppm). Every treatment exhibited exchangeable potassium levels within the organic fertilizer standards set by the Department of Agriculture, which require that exchangeable potassium levels must not be below 0.5% by weight or 5000 ppm [26]. The levels of potassium increased due to the assimilation and the immobilization delivered by the microorganisms [35], as well as the gases released from organic substance decomposition.

The C/N ratios of the assessed experimental conditions tended to increase throughout the fermentation process. The values were found to be in the range of 26.32–44.39 after the end of the composting process. The control samples exhibited the highest C/N ratio (at 44.39 ± 8.31), followed by the T2 samples (at 34.22 ± 3.45) and the T1 samples (at 29.63 ± 1.27). When comparing our data with the standards set by the Department of Agriculture stating that the C/N ratio must not exceed 20/1, we found that all three conditions herein assessed had C/N ratios that exceeded that of the specified standard. As the composting duration was not long enough for the microorganisms to completely degrade the available organic carbon, the compostable sample was not yet fertilized, but it was still a soil amendment material. When compared to other studies, the finished compost of our study had a higher C/N ratio than those of 25–35 cited elsewhere [29].

No microplastics were found in our samples. However, the short period of the soil amendment composting process might have caused insufficient degradation of the bioplastic bags. Moreover, the use of a closed fermentation tank might result in insufficient air availability and slower degradation, as the added microorganisms needed air to decompose organic matter. The bioplastics in our samples were characterized by a lack of holes or were fragmented into pieces larger than 5 mm. However, we could not exclude the possibility that our samples might contain microplastics smaller than 1 mm.

5. Conclusions

Food waste composted with bioplastic bags could be used as a soil amendment material. However, in the future it is necessary to increase the content of macronutrients and the fermentation time in order to improve the obtained soil quality. Despite this, the bioplastic bag manufacturer used in this study claims that cassava-starch-based bioplastic bags decompose in 3–4 months. According to Wahyuningtiyas and Suryanto [16], a bioplastic synthesized from cassava starch degraded in 12 days. As a result, this experiment only lasted 45 days in order to examine the change in the physical structure of the bioplastic bags. In addition, bioplastics were characterized by a lack of holes or were fragmented into pieces larger than 5 mm; however, the soil amendment material might contain microplastics smaller than 1 mm. Therefore, food waste management in the CU Terrace condominium could use the food waste collected in bioplastic bags as a soil amendment material. By facilitating food waste management, the general waste that must be taken to the landfill by the Bangkok Metropolitan Administration was reduced.