Pilot-Scale Composting Test of Polylactic Acid for Social Implementation

: The chemical industry and subsequent value chain of plastics are facing signiﬁcant chal-lenges from the viewpoints of resource conversion and environmental burden. Now is the time to explore the future direction of plastics, which will require an integrated scheme using resource circulation, carbon neutrality, and a social system to promote after-use treatment under the concept of a circular economy. Polylactic acid (PLA) should help reduce greenhouse gas (GHG) emissions as a biobased material and contribute to waste management after use due to its biodegradability if managed properly. That is, it will be necessary to treat biodegradable products appropriately in closed systems such as composting facilities after use and recovery. To realize the implementation of fully approved composting facilities in society, simply evaluating biodegradability in the laboratory is insufﬁcient. In this study, a pilot-scale test using PLA under actual composting conditions was conducted in accordance with both international standards and domestic evaluation methods. The results not only conﬁrm its biodegradability and disintegration, but also demonstrate that the presence of a biodegradable plastic product has a negligible impact on the composting process. The obtained compost did not adversely affect plant germination or growth, demonstrating its safety and high quality. Such a multifaceted perspective makes this study unique and useful for creating a social framework. pilot-scale demonstration test shows that the composting process of PLA products after use is effective in implementation and meaningful for creating a framework.


Introduction
In the early 1990s, biodegradable plastics [1,2] attracted attention as a potential solution for waste treatment problems associated with the shortage of landfills and environmental concerns, which were represented by fur seals entangled in fishing nets. Although extensive development was conducted, market growth was slower than biodegradable plastics manufacturers expected because sorting biodegradable products from non-biodegradable products was not trivial, and the necessary infrastructure to treat biodegradable products appropriately was not well developed.
The Kyoto protocol reinvigorated interest in biobased plastics such as polylactic acid (PLA) [3] since they should contribute to a reduction in greenhouse gas (GHG) emissions as they are renewable plant resources as opposed to fossil fuel resources. Around 2005, market growth was slowing due to various reasons: difficulty collecting and sorting products, insufficient properties compared with alternative products such as polyethylene terephthalate (PET), use of edible resources, and decreased cost competitiveness with fossil fuel resources caused by a decline in oil prices. It was ironic that regulations and social systems developed for environmental protection hindered advances in innovative environmentally sound products [4].
In 2015, the Sustainable Development Goals (SDGs) were adopted. To achieve these goals, the chemical industry is expected to transfer raw material sources from fossil fuel According to ISO 14855 [10], in the basic test, which indicates the biodegradability of PLA, PLA powder and mature compost from a compost facility in Hayakita-cho, Hokkaido, were used at 58 ± 2 °C. ISO 14755 [10] was later modified into ISO 14855-1 [11] and ISO 14855-2 [12], although the basic principles remained the same.

Biodegradability and Disintegration of PLA Products in the Pilot-Scale Compost
PLA film-laminated paper plates, which are a BPS certified Green Plastic #216 provided by Tohcello, were used in the pilot-scale compost test. BPS was formerly the Biodegradable Plastics Society, but its name changed to the Japan Bioplastics Association (JBPA). Green Plastic is the brand name of the biodegradable plastics. The plates were composed of a 40 μm-thick PLA film made from LACEA TM (Mitsui Chemicals) and a paper part made from 100% pulp paper with a weight of 320 g/m 2 ( Figure 1).
In accordance with ISO 16929 [13], the raw material for compost was wood chips as an auxiliary material, horse and animal waste as return compost, and plant-based waste as a substitute for fermentation with artificial garbage. Table 1 shows the composition ratios of these compost raw materials. PLA pellets were provided by Mitsui Chemicals and processed into PLA film by Tohcello. PLA film was laminated with 100% pulp paper. Then, the PLA film-laminated paper was thermoformed into plates. Horse dung compost product 40 In accordance with ISO 16929 [13], the raw material for compost was wood chips as an auxiliary material, horse and animal waste as return compost, and plant-based waste as a substitute for fermentation with artificial garbage. Table 1 shows the composition ratios of these compost raw materials.  [10], about 100 g of a sample is put into about 600 g of compost. Figure 2 shows a newly developed small-scale model. In this model, Sustainability 2021, 13, 1654 4 of 18 about 5 g of a sample was put into 100-500 g of compost. The temperature was kept at 58 ± 2 • C. CO 2 generated by biodegradation was absorbed by the alkaline solution and measured by titration. In the standard model of ISO 14855 [10], about 100 g of a sample is put into about 600 g of compost. Figure 2 shows a newly developed small-scale model. In this model, about 5 g of a sample was put into 100-500 g of compost. The temperature was kept at 58 ± 2 °C CO2 generated by biodegradation was absorbed by the alkaline solution and measured by titration.

Biodegradability and Disintegration of the PLA Product in Pilot-Scale Compost
Each fermenter was a 180 L metal columnar drum (without a stirring mechanism) equipped with a blower with an airflow meter, non-dispersive infrared absorption (NDIR)-type CO2 concentration meter, thermometer, and other measurement equipment ( Figure 3). Table 2 summarizes the list of auxiliary apparatus.

Biodegradability and Disintegration of the PLA Product in Pilot-Scale Compost
Each fermenter was a 180 L metal columnar drum (without a stirring mechanism) equipped with a blower with an airflow meter, non-dispersive infrared absorption (NDIR)type CO 2 concentration meter, thermometer, and other measurement equipment ( Figure 3). Table 2 summarizes the list of auxiliary apparatus.

Biodegradability Standard Test of PLA in Compost
In the standard model of ISO 14855 [10], about 100 g of a sample is put into about 600 g of compost. Figure 2 shows a newly developed small-scale model. In this model, about 5 g of a sample was put into 100-500 g of compost. The temperature was kept at 58 ± 2 °C. CO2 generated by biodegradation was absorbed by the alkaline solution and measured by titration.

Biodegradability and Disintegration of the PLA Product in Pilot-Scale Compost
Each fermenter was a 180 L metal columnar drum (without a stirring mechanism) equipped with a blower with an airflow meter, non-dispersive infrared absorption (NDIR)-type CO2 concentration meter, thermometer, and other measurement equipment ( Figure 3). Table 2 summarizes the list of auxiliary apparatus.   Fermenters, which contained 170 L of compost raw materials, were prepared and divided into two zones ( Figure 4). In the test zone, PLA samples (1.1 kg) were added to the fermenter ( Figure 5). In the blank zone, the fermenter did not contain PLA samples. To confirm the validity, each zone was duplicated. After the test was completed, the contents of the fermenter in each treatment area were sieved to separate particles of non-disintegrated PLA samples with a diameter greater than 2 mm, and as a result the residual rate of PLA samples was measured. The maximum test treatment period was 80 days. However, biodegradation was completed within about 60 days. The test ended after confirming that fermentation had stopped, which was determined when fermentation conditions such as CO 2 concentration and temperature no longer changed over time.   The temperature depended on the heat generated by biodegradation of the compost. External heat was not added. Table 3 summarizes the measured items. Test sample: PLA film-laminated paper plates were put into the compost after cutting into 5 × 5 cm pieces. The test zone contained the PLA sample, whereas the blank zone did not. Table 1 shows the compost raw materials. The PLA sample was a PLA film-laminated paper plate cut into 5 × 5 cm 2 pieces. Table 4 shows the conditions.  Test sample: PLA film-laminated paper plates were put into the compost after into 5 × 5 cm pieces.
The test zone contained the PLA sample, whereas the blank zone did not. shows the compost raw materials. The PLA sample was a PLA film-laminated pap cut into 5 × 5 cm 2 pieces. Table 4 shows the conditions. Table 4. Operating conditions (according to ISO 16929 [13]).

Item Operation Condition
Agitation/turning Drum contents were spread on the sheet, stirred manually, and returned to the drums each week. Air supply 5 to 25 L/min/m 3 (Value was set such that the oxygen concentration in the compost was above 5%).

Water addition
Water was added such that the water content of the compost would not fall below 50% when turning the com Nitrogen control Urea solution was added to compensate for the drop in the nitrogen concentration due to the addition of w   [13]).

Item Operation Condition
Agitation/turning Drum contents were spread on the sheet, stirred manually, and returned to the drums each week. Air supply 5 to 25 L/min/m 3 (Value was set such that the oxygen concentration in the compost was above 5%).

Water addition
Water was added such that the water content of the compost would not fall below 50% when turning the compost. Nitrogen control Urea solution was added to compensate for the drop in the nitrogen concentration due to the addition of water.

Biodegradability Standard Test of PLA in Compost
The CO 2 generated by biodegradation was measured according to ISO 14855 [10].

Biodegradability and Disintegration of PLA Product in Pilot-Scale Compost
The tests measured the following: temperature, CO 2 , O 2 , NH 3 , water content, loss on ignition, pH, electrical conductivity (EC), carbon/nitrogen ratio, content volume, content weight, and added water weight. At the end of the test, the PLA sample residual rate was measured. For the compost input, the elution test described in the Waste Management and Public Cleaning Act was conducted [14]. For the treated compost, the water, nitrogen, and heavy metal contents were analyzed according to the items described in the Fertilizer Regulation Act in Japan [15].
The blank zone and test zone were each measured twice and graphed with their average values. They appear on the graph as an error bar and a circle of the average value.
Items were measured to confirm whether composting progressed smoothly. For example, the ignition loss and EC indicated whether or not the compost was suitable as fertilizer by evaluating the degree of decomposition of organic matter and salt concentration. On the other hand, the carbon/nitrogen ratio determined the balance between carbon and nitrogen as a fertilizer. Composting proceeded as usual even if PLA was present.

Plant Growth
Finally, the effect of the resulting compost on the germination and growth of Japanese komatsuna (Japanese mustard spinach) was evaluated. The resulting compost was used as a fertilizer. B-1 and B-2 were obtained from compost without the PLA sample. T-1 and T-2 were grown in the compost with the PLA sample based on the test method [16] of plant growth. This method measures toxicity against plants and is regulated by the Ministry of Agriculture, Forestry and Fisheries in Japan No. 1943 (18 April 1984). Komatsuna seeds were buried in each type of fertilizer to observe the effects of the presence of the PLA sample.

Biodegradability Standard Test of PLA in Compost
Tracing the biodegradability of PLA powder using the amount of CO 2 generated in the mature compost confirmed that about 88% of the PLA decomposed in 90 days ( Figure 6).

Biodegradability Standard Test of PLA in Compost
The CO2 generated by biodegradation was measured according to ISO 14855 [10].

Biodegradability and Disintegration of PLA Product in Pilot-Scale Compost
The tests measured the following: temperature, CO2, O2, NH3, water content, loss on ignition, pH, electrical conductivity (EC), carbon/nitrogen ratio, content volume, content weight, and added water weight. At the end of the test, the PLA sample residual rate was measured. For the compost input, the elution test described in the Waste Management and Public Cleaning Act was conducted [14]. For the treated compost, the water, nitrogen and heavy metal contents were analyzed according to the items described in the Fertilizer Regulation Act in Japan [15].
The blank zone and test zone were each measured twice and graphed with their average values. They appear on the graph as an error bar and a circle of the average value.
Items were measured to confirm whether composting progressed smoothly. For example, the ignition loss and EC indicated whether or not the compost was suitable as fertilizer by evaluating the degree of decomposition of organic matter and salt concentration On the other hand, the carbon/nitrogen ratio determined the balance between carbon and nitrogen as a fertilizer. Composting proceeded as usual even if PLA was present.

Plant Growth
Finally, the effect of the resulting compost on the germination and growth of Japanese komatsuna (Japanese mustard spinach) was evaluated. The resulting compost was used as a fertilizer. B-1 and B-2 were obtained from compost without the PLA sample. T-1 and T-2 were grown in the compost with the PLA sample based on the test method [16] of plant growth. This method measures toxicity against plants and is regulated by the Ministry of Agriculture, Forestry and Fisheries in Japan No. 1943 (18 April 1984). Komatsuna seeds were buried in each type of fertilizer to observe the effects of the presence of the PLA sample.

Biodegradability Standard Test of PLA in Compost
Tracing the biodegradability of PLA powder using the amount of CO2 generated in the mature compost confirmed that about 88% of the PLA decomposed in 90 days ( Figure 6).  The temperature during the test period was measured at the top ( Figure 7) and center of the fermenter (Figure 8). The blank and test zones did not differ significantly as both showed a temperature of 65 • C or higher for a minimum of 48 h at the early stage of fermentation. Hence, the compost meets the test standards of ISO 16929 and the epidemiological conditions of Japan.

Changes in the Temperature and Gas Concentration in the Fermenter
The temperature during the test period was measured at the top ( Figure 7) and center of the fermenter (Figure 8). The blank and test zones did not differ significantly as both showed a temperature of 65 °C or higher for a minimum of 48 h at the early stage of fermentation. Hence, the compost meets the test standards of ISO 16929 and the epidemiological conditions of Japan.    Figure 9 shows the daily change in CO2 content in both the blank and test conditions. The content initially increased, but then decreased rapidly. From 10-50 days, the content was 0-5%, but it settled at around 1% or less after 50 days. There are some points that do not overlap in the data of the blank zone and the test zone, but when comparing the behavior before and after, there is no significant difference between the two zones.   Figure 9 shows the daily change in CO 2 content in both the blank and test conditions. The content initially increased, but then decreased rapidly. From 10-50 days, the content was 0-5%, but it settled at around 1% or less after 50 days. There are some points that do not overlap in the data of the blank zone and the test zone, but when comparing the behavior before and after, there is no significant difference between the two zones.
Sustainability 2021, 13, x FOR PEER REVIEW 10 of 21 Figure 9. Change in the average CO2 content with time. Figure 10 plots the daily change in O2 content. As O2 consumption was intense at the beginning of fermentation, it dropped to less than 10%. After 10 days, it stabilized at over 10% of the O2 content by adjusting the air supply rate. Similar to the trends of CO2 content, although there are some irregular points, the blank and test zones did not differ significantly.    Figure 10 plots the daily change in O 2 content. As O 2 consumption was intense at the beginning of fermentation, it dropped to less than 10%. After 10 days, it stabilized at over 10% of the O 2 content by adjusting the air supply rate. Similar to the trends of CO 2 content, although there are some irregular points, the blank and test zones did not differ significantly.  Figure 10 plots the daily change in O2 content. As O2 consumption was intense at the beginning of fermentation, it dropped to less than 10%. After 10 days, it stabilized at over 10% of the O2 content by adjusting the air supply rate. Similar to the trends of CO2 content, although there are some irregular points, the blank and test zones did not differ significantly.  The NH 3 concentration in the fermenter was initially high but decreased after 20 days ( Figure 11). The data from the large error bar at around the 40 day mark may appear irregular, but when comparing it to the decomposition behavior of the two zones at other periods, there were no significant differences.   The similar trends in the temperature, CO 2 , O 2 , and NH 3 concentrations in the blank and test zones indicate that the presence of the PLA film did not affect the composting process.

Property Changes to the Fermentation Mass in the Fermenter
The test (treated) and blank (untreated) zones did not significantly differ with regard to water content, pH, EC, and residual amount of ignition during the test period (Figures 12-15).       Figures 16 and 17 show the weight per container and bulk specific gravity measured during the test period, respectively. The test and blank zones showed similar results. These results further demonstrate that the presence of PLA film-laminated paper did not affect the composting process.   Figures 16 and 17 show the weight per container and bulk specific gravity measured during the test period, respectively. The test and blank zones showed similar results. These results further demonstrate that the presence of PLA film-laminated paper did not affect the composting process.

Residual Rate of the PLA Sample at the End of Test
To investigate the residual rate of PLA samples, the compost after the test was passed through a 2 mm sieve. The PLA sample remaining in the compost with a diameter of 2 mm or more was 0.054 kg, and the residual rate was 5.23% compared with the input PLA sample by dry weight. Table 5 shows the average weight of each zone at the end of the test. Overall, 94.8% of the PLA sample, which weighed 1.031 kg at the time of input, decomposed. Furthermore, fermentation did not vary significantly between the blank and test zones. These results provide additional evidence that the presence of PLA film-laminated paper does not affect the composting process.    To investigate the residual rate of PLA samples, the compost after the test was passed through a 2 mm sieve. The PLA sample remaining in the compost with a diameter of 2 mm or more was 0.054 kg, and the residual rate was 5.23% compared with the input PLA   To investigate the residual rate of PLA samples, the compost after the test was passed through a 2 mm sieve. The PLA sample remaining in the compost with a diameter of 2 mm or more was 0.054 kg, and the residual rate was 5.23% compared with the input PLA

Quality of Inputs and Composted Products
The elution test of the input materials was conducted based on the Waste Management and Public Cleaning Act [14] in Japan. As shown in Table 6, the results were all below the requirements for heavy metals, organophosphorus compounds, and other regulated compounds. The composted products were analyzed by the test method based on the "Fertilizer Regulation Act" [15]. Table 7 shows the nutrient contents (e.g., moisture content, total nitrogen, total phosphoric acid). Table 8 shows the level of toxic items regulated by the Fertilizer Control Law (e.g., arsenic, cadmium, mercury). The results confirm that the compost is suitable for use as fertilizer.

Confirmation of Effects on Plant Germination
Finally, based on the practical test method [16] of plant growth, which measures the toxicity against plants and is related to the scope of ISO16929 [13], we confirmed the effect of the PLA sample-treated compost on plant germination. There were two test groups to grow komatsuna or Japanese mustard spinach. The control samples were grown in fertilizer from the pilot-scale compost without the PLA sample (B-1 and B-2), while the tests were grown in fertilizer from the pilot-scale compost with the PLA sample (T-1 and T-2). The komatsuna seeds were buried in each type of fertilizer. After three weeks, good growth was observed in both groups. Additionally, neither group showed abnormal growth (Table 9). Hence, the compost-treated product with the PLA sample did not induce harmful effect on plants, demonstrating its suitability as a good compost product. Prior to our experiments, we surveyed the literature from 1992 to 2019 to analyze previous research on pilot-scale biodegradability and safety of the resulting compost. Many papers published from the 1990s to the early 2000s were related to the promotion of PLA polymers' biodegradability [17][18][19][20][21][22][23][24][25]. Some studies also worked to improve the mechanical properties by producing copolymers [18] or blending with other biodegradable polymers [20,24] and additives [22]. Other studies reported direct and simple biodegradability testing of PLA [26,27], and socially implementable biodegradability experiments using compost [28][29][30]. However, full-scale studies on the biodegradability and disintegration results utilizing existing real composting facilities for commercially available PLA products only began to appear around 2005.
In 2006 and 2007, papers were published demonstrating the degradability of a variety of commercial PLA products in compost such as cheese packaging [31], carpet and fiber products [32], bottles, trays and deli-containers [33], and knives and packaging [34]. In one study [33], three commercially available forms of PLA packaging and containers were exposed to real composting conditions at Michigan State University under ambient exposure conditions. Degradation in a real composting facility was monitored by visual inspections, gel permeation chromatography, differential scanning calorimetry, and thermal gravimetric analysis. The authors noted the need to address the compostability of these packages under real composting conditions for social implementation since the standard methodology of evaluating biodegradability in simulated composting conditions has limitations. They also showed that the compostability of the complete package in real composting conditions may take longer than a simple piece of polymer.
Professor Narayan, who is a leader in research on biodegradable polymers, and colleagues have published many papers regarding biodegradability. Among these, two papers compared ASTM/ISO tests with realistic composting conditions in 2007 [35] and 2009 [36]. Based on their disintegration tests on real compost (fresh compost) using PLA bottles, they pointed out that standard test methods such as ISO 14855 only answer whether a plastic is biodegradable [10]. Standard methods do not answer the question of whether it is fully biodegradable in an actual compost facility. This follows on from ISO 17088, which was announced in 2008 and revised in 2019 [37].
From industrial and business perspectives, major consumer goods companies have documented [38] that it is pointless to use PLA and other biodegradable plastic products unless they are properly processed in a composting plant. Without proper composting, they will not contribute to waste reduction.
In 2017, a paper was published that questioned the decomposability of compost in actual facilities, even if decomposition is demonstrated on the laboratory scale [39]. They argued that compost facilities vary in operational methods, and the actual degradability depends on the form. As the above studies show, it is important to confirm biodegradability in actual composting facilities as social implementation of bioplastics with biodegradable properties such as PLA progresses. All the messages delivered by the above papers are similar to the one that this paper tries to convey.
There are few examples demonstrating the safety of PLA by assessing plant growth in the literature. A paper in 2019 questioned the safety of micro and nanoparticles, which is a concern during decomposition of PLA products, including their additives [40]. In many practical available resins, various additives are used in the manufacturing process. Hence, it is necessary to investigate whether such additives decompose in real compost. Additionally, we need to evaluate the safety of the decomposed material and the resulting compost. However, it appears that few studies demonstrate the method and results of such a comprehensive and holistic approach on the pilot scale.

Influencing Elements Affecting the Biodegradability Time in Compost
To date, the biodegradability of PLA in compost has been demonstrated on the laboratory scale according to ISO 14855 [10]. Consistent with our previous paper [3], this study shows that more than 80% of PLA biodegrades in 80 days using mature compost (Figures 2 and 6). According to the paper by M. Kunioka et al. on the Biodegradability Evaluation of Polymers by ISO 14855-2 [12], they found that 80% or more of the powder was biodegraded after 50 days [41]. In contrast, when a sample is shaped into PLA cups, it takes 100 days or more to reach a minimum of 80% decomposition. Hence, the sample shape affects decomposition time.
ISO 14855 tracks the biodegradability by the amount of CO 2 generated [10]. On the other hand, this study used ISO 16929, which assesses the contents of the fermenter after testing and sieving [10]. The decomposition rate was subsequently calculated by comparing the dry weight of the remaining PLA sample and the original PLA sample. Such a comparison is challenging. Here, if degradation is considered to be complete at the time when the temperature, CO 2 concentration, and weight in the container stopped changing, it means that 94.8% of the PLA sample was degraded in 50-60 days. Comparing the above results in laboratory-scale and pilot-scale studies, the degradability on the laboratory scale and the pilot scale was at least equivalent when considering the sample shape, and suggest a slightly faster degradation time on the pilot scale.

Holistic Approach and Social Implementation
This study investigates both the biodegradability and disintegration property of the product in the compost. The PLA samples did not adversely affect the composting process. The quality and safety of the obtained compost were maintained. Such results are difficult to obtain in a laboratory-scale test. However, the pilot-scale test provided results suitable as a proof of concept for social implementation. That is, a pilot-scale test was used as a holistic approach to demonstrate the feasibility compost facilities to process PLA products.
PLA products have been used in two major global events for social implementation: the Kassel project [42][43][44][45] and the Aichi EXPO [46]. The Kassel project ran from 2001 to 2002 in the city of Kassel, Germany. PLA film-laminated paper plates were used in a supermarket. After use, they were collected, sorted, and safely composted. During the Aichi EXPO in Japan, single-use and compostable tableware made from PLA was used in the food court and composted after use. In both cases, vegetables were grown in the resulting compost. Beside the global events described above, garbage bags made of PLA were introduced in Kosaka Town, Akita Prefecture, and Furano City [47,48]. Organic waste was also collected from homes in Hokkaido and treated in composting facilities. The resulting compost was used as fertilizer in domestic farms. Additionally, some companies have focused on the closed system, which is beyond the involvement adopted in business models based on related holistic approaches and is drawing attention from the viewpoint of a circular economy. In Colorado, USA, a hamburger chain minimized the use of packaging and containers, and introduced biobased and biodegradable packaging and cups made of paper and PLA [49]. The environmental contribution was not made through use of the material itself but rather the recovery and composting treatment implemented after use.

Conclusions
PLA film-laminated paper, which was used as the test sample representing commercial products, does not affect the fermentation status of the composting process. However, it does satisfy the various conditions shown in the "Plastic disintegration under composting conditions" described in ISO 16929. In addition, the elution test of the input materials meets the requirements specified in "Waste Management and Public Cleaning Act", and an analysis of the obtained compost based on the "Fertilizer Regulation Act" indicates that it does not suffer from quality problems. Adverse plant growth did not occur even if the compost contained residual residues of the PLA sample. Consequently, composting treatment of the organic waste together with biodegradable plastics such as PLA will contribute to a circular economy by adding value to waste. This comprehensive and holistic pilot-scale demonstration test shows that the composting process of PLA products after use is effective in social implementation and meaningful for creating a social framework.