Methane production and active microbial communities during anaerobic digestion of three commercial biodegradable coffee capsules under mesophilic and thermophilic conditions

Compounding one problem with another? A look at biodegradable microplastics

Environmental concerns about microplastics (MPs) have motivated research of their sources, occurrence, and fate in aquatic and soil ecosystems. To mitigate the environmental impact of MPs, biodegradable plastics are designed to naturally decompose, thus reducing the amount of environmental plastic contamination. However, the environmental fate of biodegradable plastics and the products of their incomplete biodegradation, especially micro-biodegradable plastics (MBPs), remains largely unexplored. This comprehensive review aims to assess the risks of unintended consequences associated with the introduction of biodegradable plastics into the environment, namely, whether the incomplete mineralization of biodegradable plastics could enhance the risk of MBPs formation and thus, exacerbate the problem of their environmental dispersion, representing a potentially additional environmental hazard due to their presumed ecotoxicity. Initial evidence points towards the potential for incomplete mineralization of biodegradable plastics under both controlled and uncontrolled conditions. Rapid degradation of PLA in thermophilic industrial composting contrasts with the degradation below 50 % of other biodegradables, suggesting MBPs released into the environment through compost. Moreover, degradation rates of <60 % in anaerobic digestion for polymers other than PLA and PHAs suggest a heightened risk of MBPs in digestate, risking their spread into soil and water. This could increase MBPs and adsorbed pollutants' mobilization. The exact behavior and impacts of additive leachates from faster-degrading plastics remain largely unknown. Thus, assessing the environmental fate and impacts of MBPs-laden by-products like compost or digestate is crucial. Moreover, the ecotoxicological consequences of shifting from conventional plastics to biodegradable ones are highly uncertain, as there is insufficient evidence to claim that MBPs have a milder effect on ecosystem health. Indeed, literature shows that the impact may be worse depending on the exposed species, polymer type, and the ecosystem complexity.

Impact of the thermo-alkaline pretreatment on the anaerobic digestion of poly(butylene adipate-co-terephthalate) (PBAT) and poly(lactic acid) (PLA) blended plastics

Poly(butylene adipate-co-terephthalate) (PBAT) is a biodegradable plastic that is difficult to degrade under both mesophilic and thermophilic anaerobic conditions. In this study, the impact of the thermo-alkaline pretreatment (48 h, 70 °C, 1 % w/v NaOH) on the anaerobic degradation (AD) of PBAT, poly(lactic acid) (PLA) and PBAT/PLA blended plastics was investigated. Under mesophilic conditions, pretreatment only improved the methane yield of PBAT/PLA/starch plastic (100 days, 51 and 34 NmL/g VSadd for the treated and original plastics, respectively). Under thermophilic conditions, the pretreatment increased the methanogenic rate of PLA, PBAT and PBAT/PLA/starch plastic at the beginning stage (22 days, 35 and 79 NmL/g VSadd for original and treated PBAT, respectively), but did not change the methane yield at the end of the incubation (100 days, 91 NmL/g VSadd for original and treated PBAT). The reduction in the molecular weight and the formation of pore structures on the plastic surface accelerated the utilization of plastics by microorganisms. Furthermore, the pretreated plastics tend to form microplastics (MPs) with size predominantly below 500 µm (>90 %). The numbers of MPs dynamically changed with the degradation time. Several genera of bacteria showed specific degradation of biodegradable plastics under thermophilic conditions, including Desulfitibacter, Coprothermobacter, Tepidimicrobium, c_ D8A-2 and Thermacetogenium. The results suggest that more attention should be paid to the problem of MPs arising from the thermo-alkaline pretreatment.

Biomethanization of rigid packaging made entirely of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate): Mono- and co-digestion tests and microbial insights

This study evaluates the anaerobic mesophilic mono- and co-digestion of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) plastic bottles as a proxy for rigid packaging materials. Initial tests showed a 97.3 ± 0.2 % reduction in weight and an observable alteration in the surface (thinning, color fading and pitting) of the PHBH bottles after eight weeks. Subsequent tests showed that PHBH squares (3 × 3 cm) produced 400 NmL-CH4/g-VSfed, at a slower rate compared to powdered PHBH but with similar methane yield. Co-digestion experiments with food waste, swine manure, or sewage sludge showed successful digestion of PHBH alongside organic waste (even at a high bioplastic loading of 20 % volatile solids basis), with methane production comparable to or slightly higher than that observed in mono-digestion. Molecular analyses suggested that the type of co-substrate influenced microbial activity and that methane production was mainly driven by hydrogenotrophic methanogenesis. These results suggest the potential for integrating rigid PHBH packaging into anaerobic digesters.

The Undeniable Potential of Thermophiles in Industrial Processes

Extremophilic microorganisms play a key role in understanding how life on Earth originated and evolved over centuries. Their ability to thrive in harsh environments relies on a plethora of mechanisms developed to survive at extreme temperatures, pressures, salinity, and pH values. From a biotechnological point of view, thermophiles are considered a robust tool for synthetic biology as well as a reliable starting material for the development of sustainable bioprocesses. This review discusses the current progress in the biomanufacturing of high-added bioproducts from thermophilic microorganisms and their industrial applications.

Enzyme modified biodegradable plastic preparation and performance in anaerobic co-digestion with food waste

A modified biodegradable plastic (PLA/PBAT) was developed by through covalent bonding with proteinase K, porcine pancreatic lipase, or amylase, and was then investigated in anaerobic co-digestion mixed with food waste. Fluorescence microscope validated that enzymes could remain stable in modified the plastic, even after co-digestion. The results of thermophilic anaerobic co-digestion showed that, degradation of the plastic modified with Proteinase K increased from 5.21 ± 0.63 % to 29.70 ± 1.86 % within 30 days compare to blank. Additionally, it was observed that the cumulative methane production increased from 240.9 ± 0.5 to 265.4 ± 1.8 mL/gVS, and the methane production cycle was shortened from 24 to 20 days. Interestingly, the kinetic model suggested that the modified the plastic promoted the overall hydrolysis progression of anaerobic co-digestion, possibly as a result of the enhanced activities of Bacteroidota and Thermotogota. In conclusion, under anaerobic co-digestion, the modified the plastic not only achieved effective degradation but also facilitated the co-digestion process.

Can low-temperature thermal hydrolysis mitigate the oxidative stress of polystyrene nanoplastics on anaerobic digestion?

The presence of micro/nanoplastics (MPs/NPs) in sewage sludge has sparked considerable apprehensions over their potential negative effects on anaerobic digestion (AD) performance. The occurrence of MPs/NPs can trigger oxidative stress on the anaerobic microbiome, leading to potential inhibition of the AD process. While the thermal hydrolysis process (THP) is an extensively utilized sludge pretreatment method for AD, its impact on stress induced by MPs/NPs during AD remains poorly understood. In this study, we assessed the impacts of low-temperature THP (90 °C, 90 min) on AD of sewage sludge in the presence of 150 μg/L of polystyrene nanoplastics (PsNPs) under different solid retention times (SRTs) of 20, 15, and 10 d. The presence of PsNPs resulted in a higher reactive oxygen species (ROS) production and a higher abundance of antibiotic resistance genes (ARGs). Additionally, their presence caused a significant inhibition of methane production by 28.2%, 29.3%, and 38.8% for SRTs of 20, 15, and 10 d, respectively. Introducing low-temperature THP prior to the AD could partially recover methane production by mitigating ROS-induced stress and curbing the propagation of ARGs during the AD process. These results shed light on the potential benefits of THP and further optimization opportunities in alleviating the adverse effects of MPs/NPs-induced stress during sewage sludge AD.

Biodegradability of PBAT/PLA coated paper and bioplastic bags under anaerobic digestion

Poly (lactic acid) (PLA) and Poly(butylene adipate-co-terephthalate) (PBAT) are two of biodegradable plastics with the highest production capacities in 2021. Bioplastic waste management can be easily integrated with organic waste management, especially when bioplastics are used as food packaging material, since they are potentially biodegradable. The aim of this study was to assess the biodegradability of biodegradable polymer-coated paper (BPCP) and bioplastic bags made from PBAT/PLA blend during mesophilic and thermophilic anaerobic digestion (AD) and to reveal the changes in the physicochemical properties of the bioplastics. BPCP obtained 155 NmL-CH4/g VS and 307.3 NmL-CH4/g VS under mesophilic and thermophilic conditions, respectively, but left bioplastic film residues. The bioplastic bags did not exhibit significant biodegradation during the AD processes. 1H NMR results indicated that the ratio of PLA to PBAT decreased significantly after AD of the BPCP film and that PLA monomers were formed from the bioplastic bags, leading to a decrease in the hydrophobicity on the surfaces of the materials. Methanoculleus was found to be enriched on the bioplastic surface after mesophilic AD. From the perspective of coupling bioplastic waste management with the food waste management, the incorporation of BPCP into the AD reactor not only enhances system stability and methane production to a greater extent than biodegradable plastic bags but also raises concerns regarding the residual biofilm when utilizing the digestate for direct land applications.

Feasibility of anaerobic co-digestion of biodegradable plastics with food waste, investigation of microbial diversity and digestate phytotoxicity

The effects of biodegradable plastics of different thicknesses (30 and 40 μm) and sizes (20 × 20, 2 × 2, and 1 × 1 mm) on anaerobic digestion of food waste and digestate phytotoxicity were investigated. Methane productions (38 days) for the groups with 20 × 20, 2 × 2, and 1 × 1 mm of 30 μm plastics were 92.46, 138.27, and 259.95 mL/gVSremoval, respectively which are nearly 58 % higher than the control group (58.86 mL/gVSremoval). Methane production in 40 μm plastics groups was lower than in 30 μm groups of equal size. All sizes of 30 µm plastics promoted substrate hydrolysis, acidification, and relative abundance of key hydrolytic bacteria and methanogens. Phytotoxicity tests results showed that seed root elongation was inhibited in groups with 40 μm plastics. In conclusion, 30 μm biodegradable plastics were more suitable for anaerobic digestion with food waste than 40 μm.

Accelerating the Biodegradation of Poly(lactic acid) through the Inclusion of Plant Fibers: A Review of Recent Advances

As the global demand for plastics continues to grow, plastic waste is accumulating at an alarming rate with negative effects on the natural environment. The industrially compostable biopolymer poly(lactic acid) (PLA) is therefore being adopted for use in many applications, but the degradation of this material is slow under many end-of-life conditions. This Perspective explores the feasibility of accelerating the degradation of PLA through the formation of PLA-plant fiber composites. Topics include: (a) key properties of PLA, plant-based fibers, and biocomposites; (b) mechanisms of both hydrolytic degradation and biodegradation of PLA-fiber composites; (c) end-of-life degradation of PLA and PLA-plant fiber composites in aerobic and anaerobic conditions, relevant to compost, soil and seawater (aerobic), and landfills (anaerobic); and (d) sustainability and environmental impact of PLA and PLA-plant fiber composites, as evaluated using life cycle assessment. Additional degradation modes, including thermal and photodegradation, which are relevant during processing and use, have been omitted for clarity, as have other types of PLA biocomposites. Multiple studies have shown that the addition of some types of plant fibers to PLA (to form PLA biocomposites) accelerates both water transport in the material and hydrolysis, presenting a possible avenue for improving the end-of-life degradation of these materials. To facilitate the continued development of materials with enhanced biodegradability, we identify a need to implement testing protocols that can distinguish between different degradation mechanisms.

Assessment of biodegradability of cellulose and poly(butylene succinate)-based bioplastics under mesophilic and thermophilic anaerobic digestion with a view towards biorecycling

Despite the increasing interest in bioplastics, there are still contradictory results on their actual biodegradability, which cause difficulties in choosing and developing appropriate sustainable treatment methods. Two biofoils (based on poly(butylene succinate) (PBS37) and cellulose (Cel37)) were anaerobically degraded during 100-day mesophilic (37 °C) and thermophilic (55 °C) tests (PBS55, Cel55). To overcome low degradation rates in mesophilic conditions, alkaline pre-treatment was also used (Pre-PBS37, Pre-Cel37). For comprehensive understanding of biodegradability, not only methane production (MP), but also the structure (topography, microscopic analysis), tensile properties, and FTIR spectra of the materials undergoing anaerobic degradation (AD) analysed. PBS37 and Pre-PBS37 were visible in 100-day degradation, and the cumulative MP reached 25.5 and 29.3 L/kg VS, respectively (4.3-4.9% of theoretical MP (TMP)). The biofoils started to show damage, losing their mechanical properties over 35 days. In contrast, PBS55 was visible for 14 days (cracks and fissures appeared), cumulative MP was 180.2 L/kg VS (30.2% of the TMP). Pieces of Cel were visible only during 2 days of degradation, and the MP was 311.4-315.0 L/kg VS (77.3-78.2% of the TMP) at 37 °C and 319.5 L/kg VS (79.3% of the TMP) at 55 °C. The FTIR spectra of Cel and PBS did not show shifts and formation of peaks. These findings showed differences in terms of the actual biodegradability of the bioplastics and provided a deeper understanding of their behaviour in AD, thus indicating limitations of AD as the final treatment of some materials, and also may support the establishment of guidelines for bioplastic management.

Differential effects of petroleum-based and bio-based microplastics on anaerobic digestion: A review

The number of plastics is increasing owing to the rapid development of the plastics industry. Microplastics (MPs) are formed during the use of both petroleum-based plastics and newly developed bio-based plastics. These MPs are inevitably released into the environment and are enriched in wastewater treatment plant sludge. Anaerobic digestion is a popular sludge stabilization method for wastewater treatment plants. Understanding the potential impacts of different MPs on anaerobic digestion is critical. This paper provides a comprehensive review of the mechanisms of petroleum-based MPs and bio-based MPs in anaerobic digestion methane production and compares their potential effects on biochemical pathways, key enzyme activities, and microbial communities. Finally, it identifies problems that must be solved in the future, proposes the focus of future research, and predicts the future development direction of the plastics industry.

Anaerobic Biodegradability of Commercial Bioplastic Products: Systematic Bibliographic Analysis and Critical Assessment of the Latest Advances

Bioplastics have entered everyday life as a potential sustainable substitute for commodity plastics. However, still further progress should be made to clarify their degradation behavior under controlled and uncontrolled conditions. The wide array of biopolymers and commercial blends available make predicting the biodegradation degree and kinetics quite a complex issue that requires specific knowledge of the multiple factors affecting the degradation process. This paper summarizes the main scientific literature on anaerobic digestion of biodegradable plastics through a general bibliographic analysis and a more detailed discussion of specific results from relevant experimental studies. The critical analysis of literature data initially included 275 scientific references, which were then screened for duplication/pertinence/relevance. The screened references were analyzed to derive some general features of the research profile, trends, and evolution in the field of anaerobic biodegradation of bioplastics. The second stage of the analysis involved extracting detailed results about bioplastic degradability under anaerobic conditions by screening analytical and performance data on biodegradation performance for different types of bioplastic products and different anaerobic biodegradation conditions, with a particular emphasis on the most recent data. A critical overview of existing biopolymers is presented, along with their properties and degradation mechanisms and the operating parameters influencing/enhancing the degradation process under anaerobic conditions.

Biomethane Potential in Anaerobic Biodegradation of Commercial Bioplastic Materials

Bioplastics have emerged as a promising alternative to conventional plastics, marketed as environmentally friendly and sustainable materials. They provide a variety of methods for efficient waste management contributing to the goals of the circular economy. At their end-of-life stage, bioplastics can generate added value through aerobic and anaerobic biological treatments (composting or anaerobic digestion). In this study, biomethane potential (BMP) tests were carried out under mesophilic conditions on eight different catering biodegradable plastics available in the market and certified as being biodegradable under industrial composting conditions. Chemical analysis of the biodegradable plastics included elemental analysis, Fourier-transform infrared spectroscopy, and inductively coupled plasma–optical emission spectrometry. Key differences were observed in total solids (TS) and volatile solids (VS) contents between the studied biopolymer products. TS values ranged between 85.00 ± 0.26% (Product 8) and 99.16 ± 0.23% (Product 4), whereas VS content ranged between 64.57 ± 0.25 %wm (Product 6) and 99.14 ± 0.17 %wm (Product 4). Elemental analysis (elements C, H, N, S, and O) was used to estimate the theoretical methane production (ThBMP) of each product. The highest ThBMP (538.6 ± 8.7 NmL/gVS) was observed in Product 4 correlated with the highest C and H contents, while the lowest ThBMP (431.8 ± 6.1 NmL/gVS) was observed in Product 2. Significant differences were recorded between BMP values according to the chemical composition of the polymers. The average of BMP values ranged between 50.4 ± 2.1 NmL/gVS and 437.5 ± 1.0 NmL/gVS. Despite being characterized by the same composition (cellulose/cellulose derivatives and calcium carbonate), Products 2, 3, and 6 revealed significant differences in terms of TS, VS, ThBMP, and BMP. Furthermore, a significant statistical relationship (p < 0.001) was found between time (days) and BMP values of the eight products (R2 = 0.899–0.964) during the initial phase. The study confirmed that cellulose-based materials can convert efficiently under mesophilic conditions into methane, at a relatively short retention time; hence, they can be regarded as a promising material for co-digestion with feedstock in industrial anaerobic biogas plants. In contrast, biodegradation of polylactic acids (PLA) does not occur under mesophilic conditions, and hence, pre-treatment of the polymers is recommended. Moreover, PLA-containing products are highly affected by the presence of other components (e.g., polybutylene adipate terephthalate and cellulose/cellulose derivatives).

Discussion about suitable applications for biodegradable plastics regarding their sources, uses and end of life

This opinion paper offers a scientific view on the current debate of the place of biodegradable plastics as part of the solution to deal with the growing plastic pollution in the world's soil, aquatic, and marine compartments. Based on the current scientific literature, we focus on the current limits to prove plastic biodegradability and to assess the toxicity of commercially used biobased and biodegradable plastics in natural environments. We also discuss the relevance of biodegradable plastics for selected applications with respect to their use and end of life. In particular, we underlined that there is no universal biodegradability of plastics in any ecosystem, that considering the environment as a waste treatment system is not acceptable, and that the use of compostable plastics requires adaptation of existing organic waste collection and treatment channels.

Active microbial communities during biodegradation of biodegradable plastics by mesophilic and thermophilic anaerobic digestion

Biodegradable plastics, if they are not properly managed at their end-of-life, can have the same hazardous environmental consequences as conventional plastics. This study investigates the treatment of the main biodegradable plastics under mesophilic and thermophilic anaerobic digestion using biochemical methane potential test and the microorganisms involved in the process using amplicon sequencing of the 16 S rRNA. Here we showed that, only PHB and TPS undergone important and rapid biodegradation under mesophilic condition (38 °C). By contrast, PCL and PLA exhibited very low biodegradation rate as 500 days were required to reach the ultimate methane yield. Little or no degradation occurred for PBAT and PBS at 38 °C. Under thermophilic conditions (58 °C), TPS, PHB, and PLA reached high levels of biodegradation in a relatively short period (&lt; 100 d). While PBS, PBAT, and PCL could not be converted into methane at 58 °C. PHB degraders (Enterobacter and Cupriavidus) and lactate-utilizing bacteria (Moorella and Tepidimicrobium) appeared to play an important role in the PHB and PLA degradation, respectively. This work not only provides crucial data on the anaerobic digestion of the main biodegradable plastics but also enriches the understanding of the microorganisms involved in this process, which are of great importance for future development of the treatment of biodegradable plastics in anaerobic digestion systems.

Inoculum-to-substrate ratio and solid content effects over in natura spent coffee grounds anaerobic digestion

Coffee is the second most traded commodity worldwide, and its production is associated with the generation of a large number of residues, which are underused and disposed of in landfills. Notably, the coffee industry annually generates approximately 6 million tons of industrial spent coffee ground (ISCG) when extracting coffee flavorings to produce soluble coffee. That resource loss scenario has been highlighted in sustainable waste management contexts as an opportunity to improve the coffee circular economy. Despite ISCG bioconversion to methane potentially meets the waste-to-energy purposes of reducing residues disposal in landfills, decreasing greenhouse gas (GHG) emissions, and increasing renewable energy sources, data about anaerobic digestion (AD) of ISCG remains quite restricted. That limitation becomes more apparent owing to the lack of data focusing on AD key parameters for ISCG as substrate. This study assessed the influence of inoculum-to-substrate ratio (ISR) and the solid content influences on mesophilic (37 °C) ISCG-AD throughout the Response Surface Methodology (RSM) and Central Composite Design (CCD). Results revealed that both factors, ISR and solid content, should be kept above a certain threshold of 0.5 and 6.0 gTVS L^-1 to ensure experimental reliability, as well as reproductively and above 1.0 and 8.0 gTVS L^-1 to avoid underestimation on the MY potential achieved. Concerning ISCG-AD kinetics, the quadratic model optimum condition was at 1.36 and 14.83 gTVS L^-1 for ISR and solid content, respectively. This optimum range for ISR and solid content could guide further development of process configurations for mono- and co-digestion of ISCG, avoiding underestimation of the MY potential and extended incubation periods.

Multi-faceted analysis of thermophilic anaerobic biodegradation of poly(lactic acid)-based material

Currently, the production of bio-based polymeric materials, of which poly(lactic acid) (PLA) is the most popular, has been increasing, causing the growth of PLA waste in municipal waste. Thus, it is necessary to develop sustainable methods for treating it. Methane production, resulting from anaerobic digestion (AD), is a potential end-of-life scenario for PLA waste that needs to be investigated. To obtain high efficiency of AD, thermophilic fermentation was applied, and to overcome low rates of biodegradation, hydrothermal (HT) and alkaline (A) pretreatments were used. For a deep insight into the process, differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and microscopic and microbial analyses (based on 16S rDNA) were applied. For both untreated (PLA) and pretreated (PLA_HT, PLA_A) samples a high maximal methane production (MP) of 453 L/kg volatile solids (VS) was obtained, almost 100 % of the theoretical methane yield from PLA. The use of pretreatment allowed shortening of the time for obtaining maximal MP, especially the hydrothermal pretreatment, which shortened the overall time of MP 1.3-fold, and methane was produced at an almost 10 % higher rate (8.35 vs 7.79 L/(kg VS·d)). However, DSC and microscopic analyses revealed that, in all cases, methane was intensively produced i) after the reduction of the molecular mass of the PLA material and ii) also when PLA pieces were not visible. This should be considered when designing the operational time for the AD process. Parallel to the gradual biodegradation of PLA, the abundances of Firmicutes, Thermotogae, and Euryarcheota increased. With PLA_HT, Syntrophobacteraceae, Thermoanaerobacteraceae, and methanogens were identified as potential key thermophilic PLA biodegraders.

The development of recycling methods for bio-based materials - A challenge in the implementation of a circular economy: A review

This review focuses on the characteristics of the most widely used biopolymers that contain starch, polylactic acid, cellulose and/or polybutylene succinate. Because worldwide production of bio-based materials has grown dynamically, their waste is increasingly found in the existing waste treatment plants. The development of recycling methods for bio-based materials remains a challenge in the implementation of a circular economy. This article summarizes the recycling methods for bio-based materials, which, in the hierarchy of waste management, is much more desirable than landfilling. Several methods of recycling are available for the end-of-life management of bio-based products, which include mechanical (reuse of waste as a valuable raw material for further processing), chemical (feedstock recycling) and organic (anaerobic digestion or composting) ones. The use of chemical or mechanical recycling is less favourable, more costly and requires the improvement of systems for separation of bio-based materials from the rest of the waste stream. Organic recycling can be a sustainable alternative to those two methods. In organic recycling, bio-based materials can be biologically treated under aerobic or anaerobic conditions, depending on the characteristics of the materials. The choice of the recycling method to be implemented depends on the economic situation and on the properties of the bio-based products and their susceptibility to degradation. Thus, it is necessary to label the products to indicate which method of recycling is most appropriate.

Biochemical methane potential and active microbial communities during anaerobic digestion of biodegradable plastics at different inoculum-substrate ratios

The influence of the inoculum-substrate ratio (ISR) on the mesophilic and thermophilic biochemical methane potential test of two biodegradable plastics was evaluated. Poly(lactic acid) (PLA) and polyhydroxybutyrate (PHB) were selected for this study, the first for being recalcitrant to mesophilic anaerobic digestion (AD) and the second, by contrast, for being readily biodegradable. Several ISRs, calculated on the basis of volatile solids (VS), were tested: 1, 2, 2.85, 4, and 10 g(VS of inoculum).g(VS of substrate)^-1. A high ISR was associated with an enhanced methane production rate (i.e., biodegradation kinetics). However, the ultimate methane production did not change, except when inhibition was observed. Indeed, applying the lowest ISR to readily biodegradable plastics such as PHB resulted in inhibition of methane production. Based on these experiments, in order to have reproducible degradation kinetics and optimal methane production, an ISR between 2.85 and 4 is recommended for biodegradable plastics. The active microbial communities were analyzed, and the active bacteria differed depending on the plastic digested (PLA versus PHB) and the temperature of the process (mesophilic versus thermophilic). Previously identified PHB degraders (Ilyobacter delafieldii and Enterobacter) were detected in PHB-fed reactors. Thermogutta and Tepidanaerobacter were detected during the thermophilic AD of PLA, and they are probably involved in PLA hydrolysis and lactate conversion, respectively.

Fate of polylactic acid microplastics during anaerobic digestion of kitchen waste: Insights on property changes, released dissolved organic matters, and biofilm formation

Polylactic acid (PLA), an alternative to petroleum-based plastics, has been widely used in food packaging and disposable tableware for biodegradable properties. As a result, PLA fragments were often mixed with kitchen waste (KW) and disposed of together. This study aimed to assess the fate of polylactic acid microplastics (PMP) when co-digested with KW. The spiked PMP did not increase the methane yield of KW but had deformation and fragmentation at mesophilic and thermophilic conditions, respectively. Identification of physicochemical properties and leachates showed that the anaerobic digestion of the KW process caused the aging and fragmentation of PMP, including the generation of irregular cracking and tiny daughter particles, the increase of oxygen-containing functional groups, and the releasing of dissolved organic matters (DOM). The thermophilic anaerobic digestion with KW enhanced the aging and fragmentation of PMP to the highest degree, which was attributed to the high temperature and enriched microorganisms (Peptococcaceae, Tepidimicrobium, and Clostridium_sensu_stricto_7) in the biofilm. Clostridium_sensu_stricto_7 was only found in the anaerobic digestion with KW, which meant the KW anaerobic digestion could contribute to the enrichment of microorganisms that promoted the PMP degradation.

Assessing the anaerobic degradability and the potential recovery of biomethane from different biodegradable bioplastics in a full-scale approach

The aim of the present study was to evaluate the anaerobic degradability and the potential recovery of biomethane from different bioplastics using a full-scale approach. Bioplastics were placed inside a real anaerobic digestion plant working under thermophilic conditions and quantitative and qualitative degradation of bioplastics was evaluated. Laboratory-scale experiments were used to determine the amount of biomethane produced by anaerobic degradation of bioplastics. Polylactic acid-based items may degrade completely using retention times compatible with anaerobic digestion plants contributing positively to biomethane production, i.e., in 90 days 397 ± 8 NL CH₄ kg_{volatile solids}^-1 were produced by polylactic acid-based cutlery. Starch-based shoppers showed a quick degradation of the starch component in the first month of anaerobic digestion, followed by a slow degradation of the polyester component. Anaerobic digestion and/or anaerobic digestion coupled to digestate composting may represent the best strategy to dispose these wastes meeting the principles of Circular Economy.

Performance and metagenomics analysis of anaerobic digestion of food waste with adding biochar supported nano zero-valent iron under mesophilic and thermophilic condition

A large amount of food waste (FW) brings environmental pollution and sanitation problems. Anaerobic digestion (AD) is an effective technology to treat FW and generate biogas energy. This study investigated the effect of biochar supported nano zero-valent iron (BC-nZVI) on AD performance of FW. Results showed that the cumulative methane yield (CMY) increased by 21.52%-54.90% and the lag time decreased significantly with BC-nZVI. Under mesophilic and thermophilic condition, the peak of CMY was achieved at 178.82 ± 5.27 mL/g VS and 193.01 ± 6.81 mL/g VS with 5 g/L BC-nZVI, respectively. Besides, BC-nZVI stimulated hydrolysis process and reduced the inhibition of NH₄⁺-N and volatile fatty acids accumulation, and it could improve the system stability. Structural equation model analysis indicated that digestion time, BC-nZVI, NH₄⁺-N, temperature and total volatile fatty acid had significant effects on CMY, explaining 92.20% of its total variation. The metagenomic analysis of key microorganisms and related metabolism pathways involved in AD system was further investigated. The results suggested that BC-nZVI contributed to strengthen methanogenesis through enriching the various predominant methanogenic pathways and activating most enzymes related to methane metabolism. BC-nZVI could improve the AD system function and provided a better AD performance by shifting the microbial communities and altering functional genes. This study provided a theoretical basis for BC-nZVI applications and improvements in AD process of FW.

Recent advances, current issues and future prospects of bioenergy production: A review

With the immense potential of bioenergy to drive carbon neutrality and achieve the climate targets of the Paris Agreement, this paper aims to present the recent advances in bioenergy production as well as their limitations. The novelty of this review is that it covers a comprehensive range of strategies in bioenergy production and it provides the future prospects for improvement. This paper reviewed more than 200 peer-reviewed scholarly papers mainly published between 2010 and 2021. Bioenergy is derived from biomass, which, through thermochemical and biochemical processes, is converted into various forms of biofuels. This paper reveals that bioenergy production is temperature-dependent and thermochemical processes currently have the advantage of higher efficiency over biochemical processes in terms of lower response time and higher conversion. However, biochemical processes produce more volatile organic compounds and have lower energy and temperature requirements. The combination of the two processes could fill the shortcomings of a single process. The choices of feedstock are diverse as well. In the future, it can be anticipated that continuous technological development to enhance the commercial viability of different processes, as well as approaches of ensuring their sustainability, will be among the main aspects to be studied in greater detail.

Can anaerobic digestion be a suitable end-of-life scenario for biodegradable plastics? A critical review of the current situation, hurdles, and challenges

Growing concern regarding non-biodegradable plastics and the impact of these materials on the environment has promoted interest in biodegradable plastics. The intensification of separate biowastes collection in most European countries has also contributed to the development of biodegradable plastics, and the subject of their end-of-life is becoming a key issue. To date, there has been relatively little research to evaluate the biodegradability of biodegradable plastics by anaerobic digestion (AD) compared to industrial and home composting. However, anaerobic digestion is a particularly promising strategy for treating biodegradable organic wastes in the context of circular waste management. This critical review aims to provide an in-depth update of anaerobic digestion of biodegradable plastics by providing a summary of the literature regarding process performances, parameters affecting biodegradability, the microorganisms involved, and some of the strategies (e.g., pretreatment, additives, and inoculum acclimation) used to enhance the degradation rate of biodegradable plastics. In addition, a critical section is dedicated to suggestions and recommendations for the development of biodegradable plastics sector and their treatment in anaerobic digestion.

Methanogen and nitrifying genes dynamics in immersed membrane bioreactors during anaerobic co-digestion of different organic loading rates food waste

This work was aimed to evaluate the distinctive food waste (FW) organic loading rates (OLR) on methanogen and nitrifying genes dynamics and its correlation with identified relative abundance of bacterial dynamics during the anaerobic digestion. This experiment were carried out in the digesters at high OLR of food wastes at (4 to 8 g volatile solids/liter/day reactor R1) and (6 to 10 g volatile solids/liter/day reactor R2). The results shown that the relative abundance of mcrA, mcrB and mcrG genes were richest in the first day of both R1 and R2. In addition, the most of nitrifying genes were greater in after 34 days digestion in R2, while these genes did not show the specific regularity in R1. Finally, the correlation figure shows that Clostridium and Lactobacillus genera were significantly correlated with the different organic acids and methanogen and nitrifying genes dynamics.

The role of waste management in reducing bioplastics' leakage into the environment: A review

Bioplastics are becoming more and more widespread as substitutes for petroleum-derived plastics due to their biodegradability. Bioplastics degradation under different environments has been described and reported to depend mainly on bioplastics' compositions and the environmental conditions. Incomplete degradation during waste management processes and leakage of bioplastics into the environment are becoming major concerns that need to be further investigated. In this context, the present paper aimed to review recent literature dealing with biodegradation of bioplastics under industrial (e.g. anaerobic digestion and composting) and natural (e.g. soil and water) environments, and to link it to the potential bioplastics' leakage into the environment. Reviewed data were used to estimate the potential role of waste management processes in decreasing the potential leakage of bioplastics. Depending on bioplastics' type and processing conditions, waste management can effectively reduce bioplastics' potential leakage, decreasing the concentration of these materials that can reach the natural environments.

Degradation of bioplastics in organic waste by mesophilic anaerobic digestion, composting and soil incubation

The aim of the study was to assess the effects of high concentrations (10 % w/w, data projected for 2030) of commercial bioplastics, i.e. starch based shopping bags (SBSB) and polylactic acid (PLA) tableware, in the organic fraction of municipal solid wastes (MSW) on compost quality obtained by pilot-scale dry mesophilic anaerobic digestion and subsequent composting of the digestate. After the biological processes, 48.1 % total solids (TS) of SBSB and 15 % TS of PLA degraded, resulting in a high bioplastics content (about 18 % TS) in compost. Subsequent compost incubation in soils indicated that bioplastics degraded by pseudo-zero order kinetics (0.014 and 0.010 mg C cm^-2 d^-1 for SBSB and PLA, respectively), i.e. complete degradation was expected in 1.6 years (SBSB) and 7.2 years (PLA), confirming the intrinsic biodegradability of bioplastics. Nevertheless, enhancing the rate and amount of bioplastics degradation during waste management represents a goal to decrease the amount of bioplastics reaching the environment.