Comparison of thermal alkaline pretreatment and zinc acetate-catalyzed methanolysis (MtOH-ZnOAc) for anaerobic digestion of bioplastic waste

Effect of alkaline-thermal pretreatment on biodegradable plastics degradation and dissemination of antibiotic resistance genes in co-compost system

Biodegradable plastics (BDPs) are an eco-friendly alternative to traditional plastics in organic waste, but their microbial degradation and impact on antibiotic resistance genes (ARGs) transmission during co-composting remain poorly understood. This study examines how alkaline-thermal pretreatment enhances BDPs degradation and influences the fate of ARGs and mobile genetic elements (MGEs) in co-composting. Pretreatment with 0.1 mol/L NaOH at 100℃ for 40 minutes increased the surface roughness and hydrophilicity of BDPs while reducing their molecular weight and thermal stability. Incorporating pretreated BDPs film (8 g/kg-TS) into the compost reduced the molecular weight of the BDPs by 59.70 % during the maturation stage, facilitating compost heating and prolonging the thermophilic stage. However, incomplete degradation of BDPs releases numerous smaller-sized microplastics, which can act as carriers for microorganisms, facilitating the dissemination of ARGs across environments and posing significant ecological and public health risks. Metagenomic analysis revealed that pretreatment enriched plastic-degrading bacteria, such as Thermobifida fusca, on BDPs surfaces and accelerated microbial plastic degradation during the thermophilic stage, but also increased ARGs abundance. Although pretreatment significantly reduced MGEs abundance (tnpA, IS19), the risk of ARGs dissemination remained. Three plastic-degrading bacteria (Pigmentiphaga sp002188465, Bacillus clausii, and Bacillus altitudinis) were identified as ARGs hosts, underscoring the need to address the risk of horizontal gene transfer of ARGs associated with pretreatment in organic waste management.

Challenges and opportunities in managing biodegradable plastic waste: A review

Biodegradable plastics have certain challenges in a waste management perspective. The existing literature reviews fail to provide a consolidated overview of different process steps of biodegradable plastic waste management and to discuss the support provided by the existing legislation for the same. The present review provides a holistic overview of these process steps and a comprehensive relative summary of 13 existing European Union (EU) laws related to waste management and circular economy, and national legislations plus source separation guidelines of 13 countries, to ensure the optimal use of resources in the future. Following were the major findings: (i) numerous types and low volumes of biodegradable plastics pose a challenge to developing cost-effective waste management infrastructure; (ii) biodegradable plastics are promoted as food-waste collection aids, but consumers are often confused about their proper disposal and are prone to greenwashing from manufacturers; (iii) industry-level studies demonstrating mechanical recycling on a full scale are unavailable; (iv) the existing EU legislation dealt with general topics related to biodegradable plastics; however, only the new proposal on plastic packaging waste and the EU policy framework for bioplastics clearly mentioned their disposal and (v) clear disparities were observed between disposal methods suggested by national legislation and available source separation guidelines. Thus, to appropriately manage biodegradable plastic waste, it is necessary to develop waste processing and material utilization infrastructure as well as create consumer awareness. In the end, recommendations were provided for improved biodegradable plastic waste management from the perspective of systemic challenges identified from the literature review.

The strategic role of a mild hydrothermal pretreatment in enhancing anaerobic degradation of commercial bio-based compostable plastics associated to food waste

The expansion of Anaerobic Digestion (AD) technology to turn food waste (FW) into biogas will influence the management of the associated compostable bio-based plastics disposed of in the organic fraction of municipal waste collection. Waste processing aspects and bio-based plastic biodegradation profile in anaerobic conditions need research. The fate of some commercially available compostable items made of thermoplastic starch or PLA-based blends was investigated, by performing lab-scale disposal phase and thermophilic AD, with the integration of a mild hydrothermal pretreatment. For comprehensive understanding of bio-based plastics degradation, the biomethane production, structural (visual inspection, weight, dimension) and composition changes (solids, organics and carbohydrates content) were analyzed. Thermal pretreatment promoted thermoplastic starch-based product disintegration (40 ± 2 %) and the extraction of carbohydrates into the liquid eluate, with the potential to be transformed into high-value-added products through fermentative upgrading processes. A significant biodegradation of pretreated bio-based plastics up to 27.5 % and 40 %, respectively for thermoplastic starch- and polylactic acid (PLA)-based material, was observed. These preliminary results evidence the strategic role of the hydrothermal pretreatment in enhancing anaerobic degradation and the possibility to treat the bio-based plastics as FW co-substrate, avoiding the time and cost-consuming sorting phase in waste management plants.

Isolation and characterization of Clostridium tertium IGP01 as newly isolated hydrogen-producing bacteria with enhancement via biochar and magnetic nanoparticle supplementation

Clostridium spp. holds significant potential hydrogen-producing bacteria for biohydrogen production which can be used to treat a wide range of carbon sources, including biomass waste. Previous studies have given considerable attention to C. acetobutylicum, C. beijerinckii, and C. butyricum, which have efficiently converted sugar into biohydrogen. This study aims to isolate the potent Clostridia genera and investigate optimal conditions for cell growth and volatile fatty acids production, such as temperature, initial pH, and medium compositions. This study also examined the effect of different biochar (10 g/L), magnetite (magnetic nanoparticles, 100 mg/L), and magnetic fields on biohydrogen production performances. The results revealed that Clostridium tertium IGP01 was isolated with optimal growth conditions when sucrose was utilized as a carbon source with an organic nitrogen source at 37 °C with an initial pH of 5.5. In this condition, C. tertium IGP01 achieves the highest biohydrogen production yield of 4.21 ± 0.19 mmol H2/mmol sucrose, 71.7 ± 0.06 % hydrogen content, and production rate of 1.08 ± 0.05 L/L.day after 19 h incubation. Biochar addition enhanced biohydrogen production by 31.6-35.8 %, while the magnetite and magnetic field also improved by 11.7 % and 18.7 %, respectively. The butyric (2,844.5 ± 4.3 mg/L) and acetic acids (1,383.4 ± 1.9 mg/L) were observed as dominant volatile fatty acids (VFAS) produced as by-products after fermentation for 24 h. As the first study on C. tertium IGP01 in a single culture for biohydrogen production, these findings highlight the potential of C. tertium IGP01 and provide critical insight for dark fermentation control conditions.

Analysis of biological treatment technologies, their present infrastructures and suitability for biodegradable food packaging - A review

Recently, there has been an increased demand for biodegradable plastics in the food packaging industry, especially for highly food soiled packaging items containing food/beverage solids that will not be recycled using a non-biological process. However, the increased usage of those materials have also raised concerns and confusion, as a major part of these biodegradable plastics are not effectively separated nor recycled. The lack of acceptance in recycling facilities, related to confusion with their conventional polymers counterparts, as well as short retention times of recycling facilities, often incompatible with the degradation kinetics of biodegradable plastics, stand as the major drawbacks for bioplastics treatment. Additionally, the presence of incompletely biodegraded bioplastics during biological treatments or in the final products i.e. compost or digestate, could lead to process failure or limit the commercialization of the compost. This work critically reviews the fundamentals of the biological treatments, anaerobic digestion and composting processes, and discusses the current strategies to improve their performance. In addition, this work summarizes the state-of-the-art knowledge and the impact of bioplastics on full-scale treatment plants. Finally, an overview of the current installed treatment capacity is given to show the areas of opportunity that can be improved and exploited to achieve a better waste management of biodegradable plastics.

Promoting effects of bioplastics and sludge anaerobic co-fermentation for carboxylates production with pH regulation: Insights into the plastic structure, microbial metabolic traits, and adaptive mechanism

Biodegradable plastics (BPs) are presenting new challenges for their reutilization. This work found that volatile fatty acids (VFAs) production by co-fermentation of BPs with waste activated sludge (WAS) reached 4-37 times of the WAS fermentation alone, which was further amplified by pH regulation (especially alkaline regulation). Moreover, the VFAs composition is highly associated with BPs category. By contrast, the traditional plastic showed a limited effect on the VFAs yield and composition. Alkaline regulation enhanced the breakdown of BPs' ester bonds and boosted WAS disintegration, increasing bioavailable substrates. The hydrolytic-acidogenic anaerobes (i.e., Serpentinicella and Proteiniclasticum) and the major metabolic processes participated in the transformation of BPs and WAS to VFAs were upregulated under alkaline conditions. Further exploration unveiled that quorum sensing and peptidoglycan synthesis played important roles in counteracting alkaline stress and maintaining microbial activity for effective VFAs generation. The works demonstrated the effectiveness of pH-regulated anaerobic co-fermentation for BPs valorization.

Exploring the hidden environmental pollution of microplastics derived from bioplastics: A review

Bioplastics might be an ecofriendly alternative to traditional plastics. However, recent studies have emphasized that even bioplastics can end up becoming micro- and nano-plastics due to their degradation under ambient environmental conditions. Hence, there is an urgent need to assess the hidden environmental pollution caused by bioplastics. However, little is known about the evolutionary trends of bibliographic data, degradation pathways, formation, and toxicity of micro- and nano-scaled bioplastics originating from biodegradable polymers such as polylactic acid, polyhydroxyalkanoates, and starch-based plastics. Therefore, the prime objective of the current review was to investigate evolutionary trends and the latest advancements in the field of micro-bioplastic pollution. Additionally, it aims to confront the limitations of existing research on microplastic pollution derived from the degradation of bioplastic wastes, and to understand what is needed in future research. The literature survey revealed that research focusing on micro- and nano-bioplastics has begun since 2012. This review identifies novel insights into microbioplastics formation through diverse degradation pathways, including photo-oxidation, ozone-induced degradation, mechanochemical degradation, biodegradation, thermal, and catalytic degradation. Critical research gaps are identified, including defining optimal environmental conditions for complete degradation of diverse bioplastics, exploring micro- and nano-bioplastics formation in natural environments, investigating the global occurrence and distribution of these particles in diverse ecosystems, assessing toxic substances released during bioplastics degradation, and bridging the disparity between laboratory studies and real-world applications. By identifying new trends and knowledge gaps, this study lays the groundwork for future investigations and sustainable solutions in the realm of sustainable management of bioplastic wastes.

Effects of particle size on the pretreatment efficiency and subsequent biogas potential of polylactic acid

The fragmentation of bioplastics (BPs) before pretreatment and anaerobic digestion is conducted for higher efficiency; however, based on the literature, the size reduction varies widely. In this study, initially, various combinations of thermal-alkaline pretreatments were applied at different strengths to the polylactic acid (PLA) in three groups (<0.5, 0.5 < size < 1.0, and 1.0 < size < 2.0 mm). After pretreatment, the solubilization of PLA was increased to 11.5-40.0 % using alkaline dosage and temperature ranging from 50 to 200 g OH-/kg BP, 60-100 °C, respectively, in a 1-10 h timeframe. The results were statistically proved using a 3D response surface graph, where the pretreatment was more effective for smaller particle sizes. The reduction in particle size also increased the CH4 production, which was more pronounced at the strong pretreatment (24 % increment vs. 10-15 %). Computed results indicated 44-86 % conversion of pretreated PLA particles to CH4, supported by Fourier transform infrared spectroscopy analysis, especially focusing on the intensity of -OH bands.

Co-digestion approach for enhancement of biogas production by mixture of untreated napier grass and industrial hydrolyzed food waste

The co-digestion of untreated Napier grass (NG) and industrial hydrolyzed food waste (FW) was carried out in the batch reactor to investigate the effect of substrate ratios on biogas production performance. Two-stage anaerobic digestion was performed with an initial substrate concentration of 5 g VSadded/L and a Food to Microorganism Ratio (F/M) of 0.84. The 1:1 ratio of the NG and FW showed the optimum performances on biogas production yield with a value of 1,161.33 mL/g VSadded after 60 days of digestion. This was followed by the data on methane yield and concentration were 614.37 mL/g VSadded and 67.29%, respectively. The results were similar to the simulation results using a modified Gompertz model, which had a higher potential methane production and maximum production rate, as well as a shorter lag phase and a coefficient of determination of 0.9945. These findings indicated that the co-digestion of Napier grass and hydrolyzed food waste can enhance biogas production in two-stage anaerobic digestion.