Impact of Bioplastic Design on Biodigestion Treatment

Abstract

In this study, the impact of bioplastic design on anaerobic digestion for biogas production was investigated. This research aims to facilitate the integration of bioplastics into a circular economy, which is why our study proposes considering not only aspects related to their degradation in the formulation but also ensuring efficient behavior in anaerobic digestion plants. Thermoplastic starch (TPS) samples, derived from different starch sources and formulated with varying concentrations of calcium carbonate and thicknesses, were subjected to anaerobic digestion tests. Three key parameters were explored: the influence of filler concentration, the effect of sample thickness, and the role of starch origin. Biogas production and kinetics were assessed using biochemical methane potential (BMP) tests. The results reveal that calcium carbonate concentration negatively influenced the methane production rate, reaching 30 NmL/gVS/day for the filler-free sample, highlighting the importance of understanding filler effects on anaerobic digestion. Additionally, thicker samples exhibited slower biogas production, with a rate of 25 NmL/gVS/day compared to 30 NmL/gVS/day for the thinnest sample, emphasizing the relevance of sample thickness in influencing digestion kinetics. The starch origin did not yield significant differences in biogas production, providing valuable insights into the feasibility of using diverse starch sources in bioplastic formulations. This study enhances our understanding of bioplastic behavior during anaerobic digestion, offering essential insights for optimizing waste management strategies and advancing circular economy practices.

1. Introduction

The overconsumption of fossil-based single-use plastics has raised significant environmental concerns, contributing to the proliferation of microplastics globally [1]. Marine pollution is one of the major challenges, since around 4.8–12.7 million tonnes of this material is discharged into the oceans annually [2]. While the global recycled content of plastics is increasing annually, plastic production remains a concern. In 2022, 400.3 million tons of plastic were produced worldwide, with fossil-based plastics still representing 90.5% of this production [3].

Various measures, including bans, taxes, deposit–refund systems, and educational campaigns, have been introduced to address plastic-related issues, especially focused on single-use plastics [4]. In response to the environmental impact of conventional plastics and various regulatory measures, bioplastics, both bio-based and biodegradable, have emerged as a more sustainable alternative, which has boosted their use. This increase in the use of such materials has led to their greater presence in municipal waste streams. In fact, the global production capacity of bioplastics was 1.813 thousand tonnes in 2022 and it is expected to reach 7.432 thousand tonnes in 2028, with 38% attributed to bio-based/non-degradable materials and 62% to biodegradable materials [5].

Both bio-based plastics and biodegradable are the so-called bioplastics. The use of bio-based plastics is beneficial, in comparison with fossil-based plastics, because they are fully or partially made from biological sources, rather than fossil raw materials. On the other hand, biodegradable plastics include both fossil-based and bio-based plastics with biodegradable characteristics [6].

There are several standards that a material must meet to be considered and labeled as a bioplastic, ensuring compliance with environmental sustainability criteria. Among the most widely used European standards are EN ISO 14855 [7], which evaluates the ultimate aerobic biodegradability of plastic materials under controlled composting conditions, and EN 13432 [8], which outlines requirements for packaging recoverable through composting and biodegradation and the evaluation criteria for the final acceptance of packaging. According to the last standard mentioned, for a plastic to be considered biodegradable the biodegradation rate must be higher than 90% in six months; regarding the physic disintegration, the material held on a two-millimeter sieve should be less than 10% in three months, and the germination rate and plant biomass must be higher than 90% compared to those obtained in compost. In addition to meeting these standards, some bioplastics can also obtain third-party certifications that verify their renewable origin and biodegradability. Some of the best-known certifications are TUV SUD and BPI Europe (Biodegradable Products Institute).

As legislation against single-use plastics tightens, bioplastic production is expected to triple by 2026 [9]. This growth brings forth the need for effective bioplastic waste management within a circular economy. With so-called circular economy materials, instead of being discarded when used, they are intended for recycling to add some value back into the market. The current research landscape in the field of bioplastic formulation predominantly centers on their disappearance while maintaining satisfactory performance, which promotes a linear economy because, after their use, the products are discarded as waste. It is crucial to focus on formulation and design to move towards a circular economy and to improve bioplastic waste management. This effective management of bioplastic waste is critical for optimizing the benefits of the circular economy. In the context of biodegradable bioplastics, the focus of this research, there are two primary disposal routes. Waste can be directed either to the organic waste fraction bin or the plastic fraction bin. Ideally, when the bioplastic is recyclable, it should be disposed of in the recycling bin. However, if recyclability is not feasible or not yet implemented, the recommended approach is to dispose of it in the organic bin. This strategic approach ensures that bioplastic waste is managed in a manner that aligns with sustainability goals and maximizes its potential for recycling within the circular economy.

Several end-of-life options that will involve different types of waste management have been studied and reviewed by Van Roijen [10]: recycling, incineration with energy recovery, compost, landfill, and anaerobic digestion. This paper focuses on anaerobic digestion for managing the end of life of bioplastics, an interesting and sustainable path to introduce the used bioplastics inside the circular economy.

Anaerobic digestion (AD) offers the possibility to generate biogas, a clean and renewable energy source, by generating methane by breaking down the bioplastic in the absence of oxygen in four main steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Biogas composition varies depending on the feedstock type, temperature, digestion system, retention time, and other specifications, but some average biogas composition values are between 50 and 75% of methane and 25 and 45% of CO₂ [11]. The heating capacity of this gas can be as high as 5800 kcal/m³ [12]. Moreover, AD produces digestate that can also be used in other applications such as fertilizer, as a soil improver, or as a component in composting processes.

Understanding the behavior of bioplastics during AD is crucial, as variations in the formulation can lead to distinct outcomes in terms of methane production and kinetics. This study delves into the impact of bioplastic design changes on anaerobic digestion processes, focusing on the interplay between formulation and digestion behavior. Gaining insights into these dynamics is essential for optimizing bioplastic waste management strategies and advancing the utilization of bioplastics within circular economy frameworks.

Starch-based bioplastics, such as TPS (thermoplastic starch), exhibit thermoplastic properties and are being considered as substitutes for polystyrene (PS), commonly found in single-used utensils and packaging [13]. The wide availability of commercial starch blends in the market may contribute to the observed variability in the degradation rates reported in the literature. For instance, Vardar et al. (2022) [14] investigated the degradability of various bioplastics, including starch blends, in anaerobic digestion systems and their effects on biogas production. Their review revealed different degradation percentages under similar conditions, possibly attributed to differences in formulation. Degradation rates ranged from 23 to 28.7% in 28–30 days, while other studies reported lower degradation rates of 9.1–11% in 30–35 days. These data highlight the importance of formulation in maximizing the benefits of bioplastic waste within a circular economy.

This work aims to enhance the characteristics of bioplastics, aiming not only for high biodegradability but for optimal performance as a substrate in anaerobic digestion at the end of its life. We varied the formulation of different TPS samples to find how these materials, increasingly present in waste streams, influence methane production by anaerobic digestion. While current bioplastics are primarily designed to degrade under specific conditions, for their integration into a circular economy, it is crucial to utilize their waste as a new resource. The formulation variations of the samples studied were the size, the filler concentration, and the starch origin of the TPS.

2. Materials and Methods

2.1. Materials

In this study, thermoplastic starch has been chosen as a bio-based plastic due to its thermoplastic properties, making it a potential substitute for polystyrene [13]. The experimental focus involves TPS/PVA blends.

Our investigation into the impact of bioplastic design on the anaerobic digestion process was organized into three distinct sub-studies, each addressing specific characteristics: the starch origin, the filler influence, and the thickness study.

Although most starches typically contain 20–30% amylose and 70–80% amylopectin, these percentages vary based on the nature of the starch. Regarding amylose content, potato starch contains around 20%, wheat starch contains about 25%, corn starch contains between 55 and 75%, rice starch contains around 33%, and cassava starch contains about 20% [15,16]. Different amylose/amylopectin ratios impart various characteristics to TPS [17].

A study by Muscat et al. in 2012 [18] demonstrated that films formed from high-amylose starches exhibit a higher glass transition temperature, greater tensile strength, higher elasticity modulus, and lower elongation at break compared to films formed from starches with lower amylose content. To explore the influence of this important characteristic on anaerobic digestion behavior, we utilized five samples of TPS/PVA blends with different starch natures but the same formulation and size.

The plastic thickness will vary depending on the bioplastic application. Recently, biodegradable plastics have been widely used as a replacement for conventional plastics used for single-use films, cutlery, or packaging due to the drawbacks globally applied to single-use, fossil-based plastics. Three samples with the same composition but different thicknesses were prepared to determine its influence on anaerobic digestion.

More than EUR 8.5 billion are moved by the global market of fillers, with calcium carbonate (CaCO₃) being one of the most used by the plastic industry due to its low cost, high availability, non-toxicity, and good thermal stability—this market is expected to grow more than 5% annually until 2026 [19]. It has been considered important to study the influence of this plastic additive in bioplastic formulation. For this reason, TPS/PVA blends were prepared with identical formulations, except for the weight percentage of calcium carbonate, to compare their behavior.

The first set of samples, designated for the filler influence study, maintained the same composition while varying the weight percentage of added calcium carbonate (0 wt. %, 1 wt. %, and 5 wt. %). Given that calcium carbonate is chemically inert and widely used in the plastic industry [20], understanding how the concentration of this filler in bioplastics impacts methane production is essential.

The second set of samples maintains a consistent composition but alters the thickness of the samples (detailed data shown in

Table 1. Nomenclature of samples used in this work.

Study	Sample’s Name
Starch origin influence	Wheat
	Rice
	Corn
	Potato
	Cassava
Filler influence	0% CaCO3
	1% CaCO3
	5% CaCO3
Thickness influence	Thinnest (0.19 ± 0.01 mm)
	Medium (0.58 ± 0.01 mm)
	Thickest (1.04 ± 0.01 mm)

). The sample mass–area ratios tested were 1.4, 0.81, and 0.25 kg/m2 for the three different thicknesses.

Finally, the third set of samples shares the same formulation and size but varies in starch origin. The choice of starch influences the amylose–amylopectin ratio in the sample, contributing to a deeper understanding of how starch origin impacts biogas production.

The samples will be designated as shown in Table 1.

The TPS/PVA sample formulation consists of 100 phr (parts per hundred of rubber) of extra pure starch powder (CAS: 9005-25-8) purchased from ThermoFisher SCIENTIFIC, 100 phr of PVA (CAS: 9002-89-5 and MQ200) obtained from Sigma Aldrich (St. Louis, MO, USA), and 120 phr of glycerin (CAS: 56-81-5 with ≥99%) sourced from Fisher Chemicals (Waltham, MA, USA). Additionally, 0.5 wt.% of zinc stearate (CAS: 557-05-1 and MQ200) also from Sigma Aldrich (St. Louis, MO, USA) was used as a lubricant.

Preparation of Materials

The starch film samples with thickness between 0.19 and 1.04 mm were prepared following the method outlined by Domene-López et al., 2018 [21], with some modifications. In brief, the raw materials mixture was processed in a HAAKE TM PolyLab TM QC Modular Torque Rheometer (ThermoFisher Scientific, Waltham, MA, USA). Processing involved subjecting the raw materials mixtures for the potato, corn, wheat, and rice samples to a temperature of 110 °C for 10 min, with the initial 5 min at 50 rpm and the remaining time at 100 rpm. The cassava sample was obtained by applying 140 °C for 5 min, with the first minute at 50 rpm and at 100 rpm for the last minutes. Subsequently, the obtained blend was hot-pressed at 160 °C for 10 min under a pressure of 7–10 tons, resulting in a 1.04, 0.58, and 0.19 mm thick sheet, according to the used or not-used cast and the applied pressure.

In the case of the starch film samples for the filler influence study, these were prepared by extruding and blowing the previously made raw materials mixture, by drying the liquid components mixture at 70 °C for 17 h in a hot air oven and grinding it. For the calcium carbonate samples, this was added after the grinding process and mixed until a homogeneous sample was obtained. Samples were processed in a Process 11 Parallel co-rotating Twin-Screw extruder (ThermoFisher Scientific, Waltham, MA, USA) at 200 rpm, with a temperature profile between 85 °C (feeding) and 200 °C (die). After cooling, the extruded materials were pelletized and subsequently dried at 60 °C to be processed by a single-screw Mini blown D25 blown-film extruder (European Extrusion Machinery, Milan, Italy) at 35 rpm with a temperature profile of 160 (feeding)–175 °C (die) and a ring temperature of 190 °C, while maintaining a winding speed of 2.3 M/m.

To maintain consistency in the properties, all the samples were stored in a controlled atmosphere with a relative humidity of 50% for 48 h before further characterization. This precaution was taken to prevent any alterations in the properties of TPS due to the humidity effects.

2.2. Biochemical Methane Potential (BMP) Tests

Biogas production from the samples was assessed under mesophilic conditions (35 ± 5 °C) using the standard gas potential test outlined in UNE-EN ISO 11734, 1998 [22].

2.2.1. Preparation of Inoculum

The mesophilic inoculum for the experiments was digestate derived from the full-scale anaerobic digestor at a wastewater treatment plant (WWTP) in Novelda—Monforte del Cid (Alicante, Spain). This type of inoculum is widely recommended due to its diverse and active microorganism composition [23]. Before initiating the mesophilic anaerobic tests, the inoculum underwent pre-treatment, involving a 5–7 day period under anaerobic and mesophilic conditions to minimize nonspecific biogas production. Additionally, the inoculum was screened through a 1 mm sieve to eliminate potential contaminants such as gravel, sand, and other debris.

2.2.2. Total Solids (TSs) and Volatile Solids (VSs)

The total solids content of the samples and inoculum was determined by drying them at 105 ± 5 °C until a constant weight was achieved. The volatile solids content was then measured by incinerating the dried sample at 550 ± 50 °C for 2 h. [24]

2.2.3. Preparation of Assay Bottles

BMP assays were conducted in 200 mL bottles equipped with rubber septa and aluminum caps for biogas sampling.

The bioplastic samples, serving as the substrate, were cut into 1 × 1 cm squares and were introduced into the digesters with the inoculum in a proportion calculated to achieve a consistent Inoculum–Substrate Ratio (ISR) of 2 (VS basis). The bottles were then filled with water to maintain a constant head volume and the headspace was purged with nitrogen to remove the oxygen in the reactor.

Three parallel anaerobic digestion experiments were conducted: the filler influence study, the thickness influence study, and the starch origin study. All experiments utilized the same inoculum in six replicates. This setup allowed for the measurement of biogas methane concentration, pH monitoring, and sieving of digester content to recover the remaining substrate, without affecting at least three of the six replicates. Bottles that did not complete the entire experiment were designated as sacrifice bottles. Furthermore, alongside the experimental samples, additional tests were conducted using blank bottles to measure endogenous methane production and positive reference tests using micro-crystalline cellulose to assess the activity of the inoculum. The BMP-positive control, which confirmed the suitability of the test, should ideally fall between 85% and 100% of the theoretical BMP (352 to 414 NL CH₄/kg VS) [25].

2.2.4. Biogas Analysis

The biogas volume produced was periodically measured using a manometer (Comark C9555, Barcelona, Spain) throughout the study period. Each day, the bottles were agitated to ensure an even distribution of microorganisms, nutrients, and substrate.

The gas composition, including CO₂, air, and CH₄, was analyzed using a gas chromatography system Agilent 7820A (Agilent Technologies, Santa Clara, CA, USA). Helium served as the carrier gas, with detection conducted at 200 °C for 9 min using a thermal conductivity detector.

2.2.5. Determination of Sample Degradation

To assess sample degradation, the theoretical biochemical methane potential (TMBP) was calculated using the approach outlined by Buswell and Hatfield in 1936. The chemical composition of the bioplastics allowed for the stoichiometric prediction of methane quantity using Equation (1), which refers to an oxidation–reduction involving water.

$$C_x{H}_y{O}_z+\left(\mathrm{x}-{\displaystyle \frac{\mathrm{y}}{4}}-{\displaystyle \frac{\mathrm{z}}{2}}\right)H_2\mathrm{O}\to (\mathrm{x}/2+\mathrm{y}/8-\mathrm{z}/4)\mathrm{C}{H}_4+(\mathrm{x}/2-\mathrm{y}/8+\mathrm{z}/4)\mathrm{C}{O}_2$$

(1)

2.3. Solubility Test

A solubility test was conducted specifically to examine the influence of thickness on sample behavior within the digester. The primary objective was to collect information on the solubility of the sample over time. A known quantity of the sample was introduced into a beaker containing 500 mL of water. Subsequently, the carbon content in the water dissolution was measured over time using a Total Organic Carbon Analyzer TOC-5000A (Shimadzu, Kyoto, Japan), providing data on the solubility variation throughout the test period.

3. Results and Discussion

3.1. Starch Origin Effect

Anaerobic digestion tests were conducted using various TPS samples prepared with starches from different sources, including potato, corn, wheat, rice, and cassava. Figure 1a illustrates some observable differences in the production of biogas among the samples. The BMP highest percentage difference between the five different samples and the average obtained (384 ± 12 NmL/gVS) was 3.8 %. The wheat starch blend gave the highest biogas yield (397 ± 23 NmL/gVS), followed by corn (394 ± 25 NmL/gVS), rice (386 ± 24 NmL/gVS), cassava (374 ± 27 NmL/gVS), and potato (369 ± 26 NmL/gVS).

The observed differences in biogas yields could be due to the variations in the samples related to the starch origin. If the quantity of gas is divided by the maximum observed in Figure 1b, representing the normalized biogas production potential, it becomes apparent that the shape of all the samples is consistent and, therefore, they depict very similar kinetics.

Figure 1b shows that, despite variations in the nature of the starch used in the TPS samples, no significant differences were observed in biogas production kinetics during the anaerobic digestion process. These findings bear significant implications for understanding the feasibility of bioplastics as a substrate for biogas production in waste management systems. Guo et al., 2011 [26] also studied starch/PLA blends with different origins of the starch, using potato, corn, and wheat, and they also obtained that the difference between the results was quite small, obtaining a methane yield of 294 NmL/gVS for the blend formulated with starch from wheat, 264 NmL/gVS for the blend formulated with starch from potato, and 281 NmL/gVS for the blend formulated with starch from corn, noticing only a difference of 10.2% in the results.

Regarding the curve shape, no lag phase was observed for these substrates, and within the first 10 days, rapid digestion was obtained.

The methane content in the headspace was similar for all the substrates investigated in this section and reached 58% for the films made with starch from corn, wheat, and rice; 57% for the films made with starch from potato; and 56% for the films made with starch from cassava.

3.2. Filler Concentration Effect

In the current study, the average biogas potential was 307 ± 2 NmL/gVS with the free filler sample exhibiting a biogas potential of 305 ± 16 NmL/gVS. For samples with 1% wt. and 5% wt. filler concentrations, the biogas production was 309 ± 22 NmL/gVS and 306 ± 28 NmL/gVS, respectively, showing no relevant influence on biogas potential with less than 1% of difference. The biogas production potential was expected to be the same in all cases due to the consistent amount of organic matter fed into the digestor.

Figure 2 illustrates the difference between the samples, primarily seen in the biogas production rate. Roughly, by day 6 of the experiment, the filler-free sample had reached 79% of its total biogas potential, while samples with carbonate filler were below 69%.

Figure 2 shows, in dashed lines, the rate between days three and six, indicating that the filler-free sample produced biogas at a rate of 30 NmL/gVS/day, whereas samples with 1% and 5% filler concentrations exhibited rates of 25 NmL/gVS/day and 23 NmL/gVS/day, respectively.

No lag phase was observed and rapid digestion was observed until day 8, and after that, the plateau period was reached.

The methane content in the headspace was similar for all the substrates investigated and reached 56% for the filler-free film, 58% for the films with 1% wt. filler, and 59% for the films made with 5% wt. of filler.

3.3. Thickness Effect

As mentioned before, three different thicknesses were tested for biogas production. Figure 3 provides a visual comparison of the three samples examined in this study.

The average maximum biogas production was 361 ± 1 NmL/gVS. The percentage difference between the average value and the actual production in each sample had a deviation of 0.25% for the thickest sample, 0.06% for the sample with medium thickness, and 0.19% for the thinnest sample.

Similarly to the filler study, the key distinction among samples of different thicknesses lies in the rate of biogas production. This rate is inversely proportional to the thickness, with rates of 30 NmL/gVS/day for the thinnest sample, 27 NmL/gVS/day for the sample with an intermediate thickness, and 25 NmL/gVS/day for the thickest sample. Figure 3 shows these rate differences in dashed lines. These differences in biogas production rate are not very significant, likely because the sample, which dissolves quickly, may not be greatly affected by the thickness studied.

Although the differences in biogas production between the samples are small, as mentioned in the previous paragraph, they did produce methane at different rates. Figure 4 shows the values of the biogas production rate vs. the mass-to-surface ratio of the samples, exhibiting a linear dependence.

The influence of thickness on biogas production is crucial, considering the diverse applications of bioplastics, ranging from films to cutlery. Optimal thickness levels can enhance the efficiency of biogas generation during the decomposition of bioplastics in various environments. For instance, thinner bioplastic films may facilitate faster decomposition, leading to increased biogas yield kinetics, whereas thicker bioplastic materials, like those used in cutlery, may undergo a slower decomposition process, affecting the rate and quantity of biogas released. Exploring these relationships is essential for developing sustainable practices in the utilization of bioplastics across different industries.

Also, a solubility test was conducted to determine whether sample dissolution or bacterial digestion occurs first during the anaerobic digestion of the samples with different thicknesses. For this study, the samples were immersed in water, and the total organic carbon (TOC) was measured over time.

Figure 5 illustrates the progressive increase in the carbon concentration of the water in contact with the sample and its concurrent biogas production.

These results indicate that TOC in the water solution increased during the first 1–1.5 days, reaching a maximum of 85%, after which it remained constant. Subsequently, the sample continued to remain in the solution without varying its concentration.

By the third day, when the maximum TOC in the water was reached, all three samples also achieved approximately the same cumulative biogas value. It is from this point onward that a difference in biogas production rates becomes apparent, likely attributed to the varying thicknesses that are still observable in the undissolved samples. Figure S1 showcases the condition of the samples after the solubility study, revealing that, even after three days, the samples retained their distinct thickness differences.

3.4. Degradation

As described in 2.2 (biochemical methane potential (BMP) tests), the degradation of the samples was determined by comparing the actual biogas production with the theoretical one.

Table 2. Samples degradation after the incubation time.

	% Degradation
Potato	39 ± 7
Corn	42 ± 4
Wheat	43 ± 4
Rice	41 ± 6
Cassava	39 ± 10
0% CaCO3	41 ± 3
1% CaCO3	39 ± 4
5% CaCO3	39 ± 5
Thickest	39 ± 7
Medium	38 ± 6
Thinnest	38 ± 7

presents the degradation achieved for each bioplastic studied. The degradation at the end of the experiments for all the samples studied in this work is consistent with the expected values found in the literature for these types of TPS blends [14].

3.5. Changes in TPS Structure: FT-IR/PAS Analysis

Fourier transform infrared (FTIR) analysis was conducted to investigate the structural changes in the samples at different stages of degradation, starting from the initiation of the experiment. The comparison spans three key degradation times: before the onset of degradation (Day 0), after 13 days of anaerobic degradation (Day 13), and after 27 days of anaerobic degradation (Day 27). No carbonate addition was used in these samples, which were prepared using potato starch. In the FTIR examination, the x-axis represents the wavelength of absorption (cm⁻¹), while the y-axis depicts the percentage of transmission. Figure 6 displays the FTIR spectrum of the studied bioplastic at the different stages.

The FTIR analysis reveals distinctive features in the bioplastic composition. A hydroxyl group (-OH stretching) is evident at 3273 cm⁻¹, indicating the starch glucose union with the plasticizer [27,28]. Bands at 2916 cm⁻¹ signify CH stretching of the sugar ring, while 1648 cm⁻¹ is associated with absorbed water. The stretching motions of CO bonds resulted in absorption peaks at 1028 cm⁻¹.

Notably, differences in TPS intensity are observed before and after degradation. The spectra confirm a shift to lower wavenumber values for both the OH absorption band and the CO group after degradation, corresponding to the weakening of hydrogen bonds between starch molecules and suggesting a structural alteration during the degradation process. The band attributed to CH stretching increased due to sample hydrolysis to smaller molecules due to degradation. [29]

4. Conclusions

The aim of this study was to investigate the impact of TPS bioplastic formulation as a substrate for biogas production through BMP tests to understand how bioplastics, increasingly present in waste streams, influence methane production by anaerobic digestion. Our findings indicate that the starch origin in TPS does not significantly affect anaerobic digestion, methane yield, or process kinetics. However, the presence of fillers and varying thicknesses do impact methane production rates. Specifically, higher concentrations of CaCO₃ filler slow down biogas generation, from 30 NmL/gVS/day for the filler-free sample to 23 NmL/gVS/day for the sample with 5% wt. filler. Thicker samples exhibit slower biogas production rates, 30 NmL/gVS/day for the thinnest sample, 27 NmL/gVS/day for the sample with an intermediate thickness, and 25 NmL/gVS/day for the thickest sample. Additionally, solubility tests highlighted that biogas production during the initial 2–3 days is primarily due to sample dissolution. After reaching maximum dissolution, the primary mechanism shifts to solid sample digestion.

Understanding the real formulation is crucial for assessing how changes in properties influence biogas production. These findings affirm the potential of TPS as an effective substrate for biogas production, making a substantial contribution to waste management systems. The structural observations offer valuable insights into the mechanisms governing TPS degradation during anaerobic digestion.

While this study has provided fundamental insights, future research areas include exploring the influence of additives, the formulation influence of other types of bioplastics, and the influence of the material’s processing conditions. These results establish a robust foundation for advancing the integration of bioplastics into circular and sustainable systems.