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Article

Overlooked Risk of Microplastic from Kitchen Waste Short Stacking Phase

by
Jialu Qian
1,
Fanping Zhou
2,
Dongsheng Shen
1,
Jiali Shentu
1,
Li Lu
1,
Shengqi Qi
1,
Min Zhu
1 and
Yuyang Long
1,*
1
Zhejiang Key Laboratory of Solid Waste Treatment and Recycling, Zhejiang Engineering Research Center of Non-ferrous Metal Waste Recycling, School of Environment Science and Engineering, Zhejiang Gongshang University, Hangzhou 310012, China
2
Hangzhou Bole Digital Intelligence Technology Co., Ltd., Hangzhou 310011, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(22), 3190; https://doi.org/10.3390/w16223190
Submission received: 12 October 2024 / Revised: 31 October 2024 / Accepted: 5 November 2024 / Published: 7 November 2024
(This article belongs to the Special Issue Emerging Pollutants in Processing of Wastewater)
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Abstract

:
Current research on microplastics (MPs) in kitchen waste primarily focuses on their end-of-life disposal processes, neglecting the rapid decomposition phase prior to disposal. This study investigated MPs’ instantaneous release during a 20 h kitchen waste stacking process. The results revealed significant temperature-dependent release, with up to 96.1% entering the liquid phase and 5768 items/kg released and with an average of 85.7% of the MPs transferring into the entrapped water released from the tiny tissue structures and membranes. These MPs were primarily in small sizes (4–400 μm) as particles and fragments. Hydrolysis acidification primarily influences MPs’ release, with temperature and stacking time as intermediate factors. Acetic acid drives MPs’ release, contributing up to 38.5%. High temperatures accelerate organic matter dissolution and MP migration, while low temperatures slow down the release of MPs. The findings confirmed MPs’ release risks during kitchen waste stacking and contributed to optimize kitchen waste management to control MP pollution at its source.

1. Introduction

Plastics are utilized across various applications due to their convenience, contributing to the global microplastics (MPs) cycle. Research indicates the presence of MPs in water, soil, sediment, and the atmosphere [1,2,3,4], with their pollution emerging as a significant global environmental issue, as evidenced by phenomena such as “plastic clouds” [5] and “plastic glaciers” [6]. Solid waste serves as a significant anthropogenic sink for MPs, garnering increasing attention towards its resource utilization and disposal processes. Studies have demonstrated that MPs’ abundance in products resulting from domestic waste composting and anaerobic digestion can reach levels of 6615 items/kg and 9000 items/kg [7,8,9]. The widespread adoption of municipal solid waste classification management has brought various issues regarding the resource utilization and disposal of kitchen waste [10], representing nearly 50% of total waste, to the forefront, with particular emphasis on the emergence of new pollutant MPs.
The abundance of MPs in kitchen waste, reaching as high as 1.266 × 104 items/kg [7,11], primarily originates from the food itself [12,13,14], processing [15,16,17], and plastic packaging [18]. Research has shown that large plastic pieces degrade into MPs and even nanoplastics during kitchen waste disposal due to physical, chemical, and biological degradation [19]. Furthermore, the abundance of MPs in the anaerobic digestion slurry of kitchen waste without bag-breaking treatment is one order of magnitude higher compared to scenarios with bag-breaking treatment [20]. It is evident that the presence of MPs in kitchen waste demands attention and cannot be overlooked.
MPs are chemically stable, and even biodegradable plastics require composting at around 60 °C for 26 weeks to fully degrade [21], indicating that MP pollution will persist in the environment. Research has shown that the release of MPs can reach 2.32~471 × 105 items/cm2 under various temperature environments, such as refrigeration, room temperature, high temperature, and microwave heating [22], indicating minimal barriers to their release. Particularly noteworthy is that acidic conditions can significantly promote the fragmentation of MPs [23]. Perishable kitchen waste can be swiftly transferred from the source of generation to the disposal point with the improvement of transportation, logistics, and comprehensive treatment capacity. It inevitably undergoes a short but rapid stacking phase before reaching its final disposal phase, such as aerobic composting or anaerobic digestion. This phase is a pre-fermentation process with rapid decomposition of organic matter and is susceptible to various environmental conditions, including temperature. Research has shown that a storage period of 1 day to 3 weeks significantly affects the physicochemical properties and subsequent bioenergy production performance of kitchen waste [24,25]. In addition to static stacking, perturbation of the dynamic transportation process significantly increases the degree of rancidity of municipal biomass wastes, resulting in hydrolysis and acidification rates as high as 33.4 to 35.5% and 15.8 to 17.3%, respectively [26]. Speculation suggests that MPs would exhibit heightened activity during the rapid acidification of kitchen waste in the stacking stage, thereby increasing the risk of an uncontrolled source of MPs.
The hydrolysis acidification process inevitably causes a transformation in the water state of kitchen waste and a significant dissolution of organic matter. In highly organic systems like kitchen waste, free water and bound water are the primary moisture forms [27,28]. As organic matter decomposes rapidly, significant changes occur in the state of free water; capillary water (CW) gradually transforms into entrapped water (EW), leading to an increase in the proportion of EW, while bound water remains more stable [29]. In kitchen waste, EW refers to the water trapped within tiny tissue structures and membranes, whereas CW refers to the moisture retained in the minute pores between particles through capillary action [30]. Kitchen waste typically has a moisture content exceeding 60%, and the substantial conversion of EW accelerates the dissolution and transport of organic matter, supports microbial growth and metabolic activities, and may also serve as a medium for the transfer of MPs, thereby affecting their distribution in kitchen waste. The stacking stage of kitchen waste before disposal is a high-risk stage that can easily be neglected, and its potential risk deserves attention. However, current discussions concerning MPs in kitchen waste have primarily focused on subsequent treatment and disposal processes, addressing the occurrence of MPs in processes such as composting and anaerobic digestion. Additionally, research has explored how factors such as MP size, morphology, dosage, and polymer type affect resource products and associated microorganisms [31,32]. Yet these discussions overlook the risks associated with short-term but rapid stacking processes before kitchen waste disposal (collection, classification, and transportation), hindering effective source control.
This study focused on the transient but rapid decomposition stage of kitchen waste before disposal. It investigated the risk of the transient release of MPs from kitchen waste to the environment at different temperatures through experimental simulation. The aim was to provide relevant support for preventing and controlling MP pollution throughout the entire process of kitchen waste disposal.

2. Materials and Methods

2.1. Experimental Material

The test kitchen waste, consisting primarily of rice, vegetables, pork, fish, and shrimp, was obtained from a Chinese restaurant (Hangzhou, Zhejiang Province, China) (Figure 1A). The use of plastic products, such as plastic wrap for tableware, plastic bottles for beverages, and disposable plastic gloves, was found in the test restaurants. In addition, there were cases where large plastics were mixed in the process of dumping garbage into the collection container, which was in line with the actual situation of kitchen waste collection. After the kitchen waste was generated, it was directly transferred into a designated stainless steel container for sample collection. The collected samples were promptly transported to the laboratory for processing. To ensure homogeneity and to avoid the crushing of large plastics into MPs, the kitchen waste was manually sorted to remove inert materials such as plastic, glass, metal, bones, and shells. Subsequently, a stainless steel shovel was used for manual mixing to achieve a uniform mixture. The representative nature of the samples was ensured using a nine-point sampling method: the waste was piled into a circular shape, and samples were taken from a total of 9 evenly spaced points in and around the center. Initially, two points were randomly selected for sampling, and the components and colors were visually inspected to make a preliminary assessment of homogeneity. The proportions of different waste components were then determined by weighing. If these proportions were similar, the mixture was considered homogeneous. After thoroughly homogenizing the samples, multiple measurements of MPs were performed to determine the initial concentration of MPs.

2.2. Experimental Design

Stacking experiments were conducted in a series of 500 mL glass jars, each equipped with a 20-mesh (0.85 mm) stainless steel screen placed at the center. The screen separated the liquid generated during the stacking process while preventing large solid particles from entering the liquid (Figure 1B). Each glass jar was filled with 400 g of pre-treated and homogenized kitchen waste samples, placed on the stainless steel screen and covered with tinfoil to prevent water evaporation, thus simulating the packaging of the kitchen waste transfer process. The glass jars filled with kitchen waste samples were then placed at 4, 20, 30, and 40 °C to simulate the stacking of kitchen waste in cold chain, normal-, medium-, and high-temperature environments before final disposal (Figure 1C). The study investigated the changes in kitchen waste properties at different stacking time points over 20 h and examined the synchronous occurrence characteristics of MPs during the stacking process. To mitigate the influence of sampling on the stacking system during the process, multiple independent glass jars were arranged under each stacking condition. Destructive sampling methods were employed to collect and analyze the process samples. Twenty groups of stacked glass jars were established in total.
To simulate the cold chain stacking scenario, five groups of glass jars filled with kitchen waste were placed in a refrigerator (LSC-288C, Xingxing Refrigeration Co., Ltd., Taizhou, China) at 4 °C. One glass jar was randomly removed at 2 h, 4 h, 6 h, 8 h, and 20 h, respectively, for destructive total analysis. Similarly, to simulate stacking in normal-, medium-, and high-temperature scenarios, five sets of glass jars filled with kitchen waste were placed in an incubator (SPX 4501, Taisite, Tianjin, China) at 20, 30, and 40 °C, respectively. The sampling method during the process mirrored that of the cold chain stacking scenario.
During the experiment, samples were collected and pre-treated from the glass jars removed at each time point as shown in Figure 1D: the kitchen waste located above the stainless steel screen was extruded, and the resulting liquid was collected and designated as CW from the kitchen waste stacking process. Subsequently, the leachate beneath the stainless steel screen in the glass jars was initially gathered and designated as EW from the kitchen waste stacking process. The collected water samples in both forms were volumetrically measured and subsequently subjected to analysis for MPs, potential of hydrogen (pH), electrical conductivity (EC), volatile fatty acids (VFAs), and total organic carbon (TOC).

2.3. Analytical Testing

The kitchen waste sample was dried at 105 °C until it reached a constant weight to determine its moisture content. Subsequently, it was subjected to combustion at 600 °C for 3 h to determine the organic matter content. The pH and EC of both EW and CW samples were measured using a Series Meters-S20 pH meter and Five Easy plus conductivity meter, respectively. TOC was determined using a total organic carbon analyzer (Shimadzu, Kyoto, Japan, TOC-V CPN), and VFAs were analyzed using a gas chromatograph (GC7890-II, Tianmei, Shanghai, China).
The process for extracting MPs from kitchen waste involves several steps: Firstly, the dried samples are mixed with a saturated NaCl solution (1.2 g/cm3) at a solid-to-liquid ratio of 1:30, shaken for 15 min, and then left to stand for 12 h. After flotation, the samples are separated onto a 0.45 μm water filter membrane (Jinteng Experimental Equipment Co., Ltd., Tianjin, China) using a vacuum filtration device [19,33]. Due to the high organic content of kitchen waste, incomplete digestion may increase the risk of false positives in MP identification. While nitric acid digestion effectively removes biological material, it can cause the melting or loss of pH-sensitive polymers [34]. Therefore, a modified digestion method is employed. Specifically, 20 mL of 30% H2O2 is added to a 150 mL beaker containing the sample, which has been filtered after extraction with saturated NaCl solution. The beaker is covered with aluminum foil and heated at 60 °C for 1 h, followed by digestion at 100 °C for 7 h [34,35]. After digestion, the samples are filtered through a water-based filter membrane using a vacuum filtration device and subsequently placed in a beaker for staining. The staining solution is prepared by dissolving Nile Red (≥95%, HPLC, N815046, Macklin, Shanghai, China) in methanol to create a 10 mg/L solution. The staining procedure involves suspending MPs in 20 mL of dimethyl sulfoxide (50%, V/V), followed by sequential incubation at 25 °C, 50 °C, and 75 °C. At each temperature, 2 mL of Nile Red dye (10 mg/L) is added, with staining times of 10, 20, and 30 min, respectively. After cooling, the solution is filtered onto a water-based polytetrafluoroethylene membrane using a vacuum filtration device and stored in a glass Petri dish for observation [36]. MPs are identified using a fluorescence microscope (DM2500, Leica, Wetzlar, Germany) at 10× magnification [37], with images analyzed using ImageJ software (version 1.50i). For EW and CW liquid samples, the method for determining MPs is the same as that for solid samples, except that the flotation step is omitted. All analyses are performed in triplicate.

2.4. Quality Assurance and Quality Control

Strict quality control measures were implemented to ensure the accuracy of the MP analysis results. Participants wore cotton lab coats and nitrile gloves during the experiment. During the experiment, all materials used for sampling, extraction, and analysis were rigorously cleaned three times with deionized water, filtered through a 0.45 μm water filter membrane before use, and covered with tin foil to minimize cross-contamination. Similarly, the saturated NaCl solution used for MP extraction was also filtered through a 0.45 μm water filter membrane to prevent contamination during extraction and digestion. To enhance the recovery of MPs, the filter used after sample extraction was washed three times with deionized water. Additionally, three dustless blank samples were processed in parallel with the experimental samples during digestion to check for potential MP contamination. The results were presented as the mean and deviation.

2.5. Statistical Analysis

The statistical analysis of the experimental data was performed using SPSS software (Statistic 22.0, IBM, Armonk, NY, USA), and the images were generated using Origin 2022. Microplastic analysis was based on the fluorescent images for automated particle recognition and quantification in ImageJ (version 1.50i). A macro was written to perform the following tasks: (1) set the scale to 1040 × 780 μm, (2) convert images to 8 bit, (3) adjust the color threshold, and (4) set the size detection limit to 20 μm2.

3. Results and Discussions

3.1. Release Characteristics of Microplastics During Kitchen Waste Stacking

The moisture in kitchen waste serves as a medium for the dissolution and transport of nutrients and may also facilitate the migration of MPs. Due to the relative heterogeneity of waste components, the liquid phase produced by the rapid decomposition of kitchen waste exists in various forms, primarily consisting of EW, trapped by membranes and macromolecular gel networks inside the material, and CW, bound by capillary force [29]. MPs can be conveyed by liquid and subsequently migrate, a phenomenon extensively validated in landfill contexts [33,36]. Obviously, the release of MPs during kitchen waste disposal may exhibit variability.
As depicted in Figure 2A–D, over a 20 h stacking period, MPs ranging from 6.3 to 96.1% were released into the liquid phase of kitchen waste, with 85.7% existing in EW, showing significant temperature-dependent differences. Regarding stacking temperature, the quantity of MPs in EW increased notably at 30 °C and 40 °C compared to 4 °C and 20 °C, reflecting the accelerated decomposition of perishable waste at higher temperatures. Within 20 h of stacking, the 40 °C group exhibited the highest release of MPs, with a significant increase in release rate after 6 h, reaching a maximum release of 5670 items/kg at the 20th hour, which is close to the MP quantity in kitchen waste raw materials (6000 items/kg). This amount is 1 to 5 times higher than the maximum release of other temperature groups, approximately 15 times that of the initial stacking period (2 h), and 9 times that of organic solid waste digestate [38]. The release rate of MPs was highest in the 30 °C group, reaching a peak of 5360 items/kg after 6 h, representing 89.33% of MPs in kitchen waste. Despite lower peak MP release values in low-temperature scenarios compared to high-temperature ones, 48.13% of MPs in the 4 °C group still migrated into CW. This suggests that low temperatures may delay the hydrolysis and acidification of kitchen waste and reduce MP release rates, but migration to EW persists. Consequently, even at low temperatures, the gradual release of MPs in analogous organic substrates like kitchen waste remains unabated. In conclusion, irrespective of stacking temperature, the simultaneous release of MPs due to the rapid decomposition of kitchen waste is inevitable, and its potential risks should not be overlooked.
Through an analysis of temporal dynamics, it is observed that MP accumulation decreases under certain temperature conditions (4 °C, 20 °C, 30 °C) during the 20 h stacking period, possibly attributed to MPs breaking into smaller particles or undergoing degradation by microorganisms [39]. Moreover, organic compounds including proteins, lipids, mucins, and other biological components in kitchen waste adhere to MPs, forming heteropolymers for interference detection [40,41,42]. The phenomenon was observed in this study (Figure S3A–D), and may explain the reduced release of MPs.
The environmental impact of MPs extends to their morphological effects. Illustrated in Figure 2E, 96% of MPs in EW exhibit sizes ranging from 5 to 400 μm across various temperatures, with scarce occurrences of MPs exceeding 400 μm after 6 h. At 30 °C, the mean particle size of MPs is the smallest, 60 μm, possibly due to the low-pH environment induced by active hydrolytic acidifying bacteria, accelerating MP disintegration. CW in kitchen waste is influenced by both liquid cohesion and liquid–solid surface adhesion, resulting in a different distribution of MPs compared to EW, with smaller particle sizes (Figure S1A). This may undergo further fragmentation due to mechanical extrusion, consistent with findings from dynamic composting and biogas residue dehydration studies [7,19]. Currently, MPs ranging from 20 to 800 μm have been extensively detected in the digestive system, implying that MPs released during kitchen waste disposal may be ingested by the human body through the multi-phase transfer of liquid–solid–gas. Additionally, the shape of MPs can sometimes pose greater harm than their concentration [43]. MPs’ shape can be determined by their aspect ratio [36]. Overall, there is no significant difference in shape between EW and CW, with the aspect ratio of most MPs ranging from one to three (Figure 2F and Figure S1B). While most MPs are granular and fragmented, rod and fibrous MPs also occur.
In summary, the amount of MPs released into the liquid per kilogram of kitchen waste during the stacking process was 378–5768 items. Considering the diversity of MP sources in kitchen waste, this study did not trace the source of MPs within the complex mixed matrix of kitchen waste, but it should be made clear that kitchen waste has become a source of MPs in the environment that cannot be ignored. At the same time, the behavior pattern of MPs from kitchen waste to liquid under different temperature conditions was clarified. As stacking time increases, the release of MPs from EW at various temperatures varies significantly, with considerably lower release amounts at low temperatures (4 °C, 20 °C) compared to medium and high temperatures (30 °C, 40 °C). Conversely, the MP release rate is highest at 30 °C, accompanied by smaller average particle sizes. Regarding MP shape, no significant difference was observed between the two water groups, primarily consisting of particles and fragments.

3.2. Physical and Chemical Properties of Entrapped Water During Kitchen Waste Stacking

Variations in temperature and stacking time during kitchen waste disposal result in notable differences in its dissolution characteristics. In contrast to other substrates such as soil and sludge, changes in the water quality index of kitchen waste are primarily manifested in acidity within 20 h, with MPs predominantly released into EW. Therefore, this study initially focused on investigating the impact of prolonged stacking duration under various temperature conditions. The variations in EW production and the pH, EC, and VFAs of kitchen waste were examined to elucidate the factors driving MPs’ release during short-term stacking of kitchen waste.
In general, the water release of kitchen waste is significantly influenced by both temperature and stacking duration. Under different temperature conditions, the volume of EW produced by the experimental groups varied significantly (Figure 3A). All temperature groups continued to accumulate EW over time, with the highest liquid release observed at 40 °C, approximately 1–7 times higher than that of the other groups, followed by 30 °C and 20 °C. In the 4 °C group, EW production remained consistently low (12.5 mL/kg kitchen waste), consistent with the pattern of VFA production (Figure 3B). Over a 20 h stacking period, the degree of acidification correlated with both temperature and stacking duration, a relationship supported by changes in pH (Figure 3A). Indeed, hydrolytic acidification persists even during low-temperature storage. After 20 h of stacking, the release of VFAs in the 4 °C group is three times greater than that during the initial (2 h) stacking period. Thus, for substrates with high organic matter content, low-temperature storage does not completely inhibit organic matter dissolution. Changes in EC also indicate rapid organic matter dissolution in the initial stage of stacking (Figure 3A). Within 2 h, the EC of all temperature groups increases rapidly, with its dissolution rate proportional to temperature, and stabilizes thereafter. Within 20 h of stacking, there is no significant difference in the dissolution of organic matter among temperature groups, a finding supported by changes in TOC (Figure S5).

3.3. Relationship Between Microplastics and Liquid Phase Properties in Kitchen Waste Stacking

As a highly active environment for microorganisms, the internal physical and chemical properties of kitchen waste undergo significant changes under the influence of various environmental factors, directly or indirectly impacting the formation of MPs. In this study, we examined the relationship among temperature, stacking duration, pH, EC, the volume of liquid produced via hydrolytic acidification, VFAs, and the quantity and abundance of MPs, with the objective of identifying the primary factors driving MPs’ release.
As shown in Figure 4A, the release of MPs was significantly positively correlated with temperature, liquid volume, VFAs, acetic acid and propionic acid (p < 0.05). Given that EW serves as the primary sink for MPs during the stacking process, this study aimed to analyze the potential release mechanisms of MPs in the EW phase. It was found that these mechanisms mirrored those observed in the total liquid (Figure 4B), with a notable positive correlation observed between isobutyric acid and MP release (p < 0.05). Additionally, the generation of VFAs exhibited significant positive correlations with both temperature and time (p < 0.05).
To further explore the effects of environmental factors on the hydrolysis and acidification of kitchen waste, as well as the release of MPs, a partial least squares structural equation model (PLS-SEM) was employed. This approach allowed for the examination of the direct and indirect impacts of temperature, stacking duration, and the degree of hydrolysis and acidification on MPs (Figure 5).
The PLS-SEM results indicate that the hydrolysis acidification process exerts the most significant influence on the release of MPs. While the direct impact of temperature and time on MPs is not substantial, their indirect effects on MPs primarily occur through the mediation of hydrolysis acidification, with temperature exhibiting the greatest influence on this process (Figure 5B). This suggests that temperature regulation can modulate the dissolution of organic matter and the accumulation of VFAs in kitchen waste, thereby contributing significantly to the release of microplastics, accounting for 22.2% of the total variance. This contribution outweighs the impact of stacking time on MPs’ release (Figure 5C). Hence, temperature emerges as the principal environmental factor influencing the release of MPs.
Acetic acid, propionic acid, and isobutyric acid constituted a significant proportion of the components in VFAs during the stacking process. The contribution of acetic acid to MP release was notably high, accounting for 38.5%, surpassing the influence of propionic acid and isobutyric acid on MPs (Figure 5C). To further investigate the impact of VFAs onMP release, mathematical fitting was conducted between MP amount and VFAs, acetic acid, and propionic acid in EW at all temperature groups (Figure 5D). The findings revealed a significant linear relationship between MP release and the total amount of VFAs, acetic acid, and propionic acid. These weak acids facilitate the release of MPs, with higher temperatures leading to increased release [22,44]. Particularly noteworthy is that the total amount of acetic acid comprised 82–99% of the total VFAs within 20 h of stacking.
These results highlight that MP release during the stacking process, dominated by acetic acid fermentation, is primarily attributed to the formation of acetic acid, a factor often overlooked in previous studies.
In summary, the temperature and stacking time during the short-term stacking of kitchen waste indirectly affect the MP release behavior, mainly by modulating the degree of hydrolytic acidification. The influence of temperature on MP release is much greater than that of stacking time. Acetic acid is the main driving factor, contributing 38.5% to MP release.

4. Conclusions

Kitchen waste stacking involves a significant instantaneous release of MPs, with 6.3–96.1% transferring to the liquid phase. Among these, 85.7% are found in EW, primarily in small sizes (4–400 μm) as particles and fragments. MP release peaks at 40 °C, with the fastest rate and smallest average size at 30 °C, while low temperatures slow down the release of MPs. Hydrolysis acidification primarily influences MP release, with temperature exerting a greater intermediate effect than stacking time. Acetic acid drives the MPs’ release. Thus, the simultaneous release of MPs due to rapid organic matter decomposition in the pre-disposal kitchen waste stacking stage cannot be ignored.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w16223190/s1, Figure S1: Morphological characteristics of MPs in capillary water: (A) Size, (B) aspect ratio; Figure S2. Schematic diagram of MPs morphology (A, B, C, D are MPs with aspect ratios 1-2, 2-3, 3-4 and >4, respectively); Figure S3. Adherent microplastics (a~d) and broken microplastics (e); Figure S4. Physicochemical indexes of capillary water: (A) pH, EC and capillary water volume, (B) VFAs production; Figure S5. TOC concentration of entrapped water at 20 h of kitchen waste stacking; Table S1: Physical and chemical properties of kitchen waste.

Author Contributions

Conceptualization, Y.L.; Methodology, S.Q. and Y.L.; Software, F.Z. and M.Z.; Validation, L.L.; Investigation, J.Q., J.S. and M.Z.; Data curation, F.Z.; Writing—original draft, J.Q.; Writing—review & editing, D.S. and Y.L.; Supervision, Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like to acknowledge the support from National Key R&D Program of China, Key Technologies Research and Development Program (2018YFD1100600).

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article and its Supplementary Materials.

Conflicts of Interest

Author Fanping Zhou was employed by the company Hangzhou Bole Digital Intelligence Technology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Water 16 03190 g001
Figure 1. Experimental design drawing of kitchen waste stacking: (A) original kitchen waste; (B) experimental setup; (C) kitchen waste stacking process; (D) destructive kitchen waste sampling process.
Figure 1. Experimental design drawing of kitchen waste stacking: (A) original kitchen waste; (B) experimental setup; (C) kitchen waste stacking process; (D) destructive kitchen waste sampling process.
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Figure 2. Proportional distribution of kitchen waste MPs in solid phase, CW, and EW at different temperatures. Temperatures shown are (A) 4 °C, (B) 20 °C, (C) 30 °C, and (D) 40 °C; also shown are MP size (E) and aspect ratio of MPs (F) in EW, where bubble size denotes MP size, and different colors in aspect ratio results represent different time points.
Figure 2. Proportional distribution of kitchen waste MPs in solid phase, CW, and EW at different temperatures. Temperatures shown are (A) 4 °C, (B) 20 °C, (C) 30 °C, and (D) 40 °C; also shown are MP size (E) and aspect ratio of MPs (F) in EW, where bubble size denotes MP size, and different colors in aspect ratio results represent different time points.
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Water 16 03190 g003
Figure 3. PH and EC of EW produced by kitchen waste at four temperatures. Volume of EW produced per kilogram of kitchen waste (A) and production of VFAs in EW per kilogram of kitchen waste (B).
Figure 3. PH and EC of EW produced by kitchen waste at four temperatures. Volume of EW produced per kilogram of kitchen waste (A) and production of VFAs in EW per kilogram of kitchen waste (B).
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Figure 4. Heat map of the correlation between the liquid produced during the kitchen waste stacking process and environmental factors: (A) total liquid phase; (B) EW phase.
Figure 4. Heat map of the correlation between the liquid produced during the kitchen waste stacking process and environmental factors: (A) total liquid phase; (B) EW phase.
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Water 16 03190 g005
Figure 5. Influence of environmental factors on MP release from EW phase: (A) partial least squares structural equation model (PLS-SEM) of different factors and MP release; (B) the direct and indirect effects of different factors on the release of MPs; (C) the contribution of different factors to the release of MPs; (D) the correlation analysis between the amount of MPs and the total amount of VFAs, acetic acid and propionic acid.
Figure 5. Influence of environmental factors on MP release from EW phase: (A) partial least squares structural equation model (PLS-SEM) of different factors and MP release; (B) the direct and indirect effects of different factors on the release of MPs; (C) the contribution of different factors to the release of MPs; (D) the correlation analysis between the amount of MPs and the total amount of VFAs, acetic acid and propionic acid.
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MDPI and ACS Style

Qian, J.; Zhou, F.; Shen, D.; Shentu, J.; Lu, L.; Qi, S.; Zhu, M.; Long, Y. Overlooked Risk of Microplastic from Kitchen Waste Short Stacking Phase. Water 2024, 16, 3190. https://doi.org/10.3390/w16223190

AMA Style

Qian J, Zhou F, Shen D, Shentu J, Lu L, Qi S, Zhu M, Long Y. Overlooked Risk of Microplastic from Kitchen Waste Short Stacking Phase. Water. 2024; 16(22):3190. https://doi.org/10.3390/w16223190

Chicago/Turabian Style

Qian, Jialu, Fanping Zhou, Dongsheng Shen, Jiali Shentu, Li Lu, Shengqi Qi, Min Zhu, and Yuyang Long. 2024. "Overlooked Risk of Microplastic from Kitchen Waste Short Stacking Phase" Water 16, no. 22: 3190. https://doi.org/10.3390/w16223190

APA Style

Qian, J., Zhou, F., Shen, D., Shentu, J., Lu, L., Qi, S., Zhu, M., & Long, Y. (2024). Overlooked Risk of Microplastic from Kitchen Waste Short Stacking Phase. Water, 16(22), 3190. https://doi.org/10.3390/w16223190

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