Reverse electron transfer: Novel anaerobic methanogenesis pathway regulated through exogenous CO2 synergized with biochar

Acid accumulation and carbon emission are two major challenges in anaerobic digestion. Syntrophic consortia can employ reverse electron transfer (RET) to facilitate thermodynamically unfavorable redox reactions during acetogenesis. However, the potential mechanisms and regulatory methods of RET remain unclear. This study examines the regulatory mechanisms by which exogenous CO2 affects RET and demonstrates that biochar maximizes CO2 solubility at 25.8 mmol/L to enhance effects further. CO2 synergized with biochar significantly increases cumulative methane production and propionate degradation rate. From the bioenergetic perspective, CO2 decreases energy level to a maximum of -87 kJ/mol, strengthening the thermodynamic viability. The underlying mechanism can be attributed to RET promotion, as indicated by increased formate dehydrogenase and enrichment of H2/formate-producing bacteria with their partner Methanospirillum hungatei. Moreover, the 5 % 13CH4 and methane contribution result show that CO2 accomplishes directed methanogenesis. Overall, this investigation riches the roles of CO2 and biochar in AD surrounding RET.

Enhanced thermophilic dark fermentation of hydrogen production from food waste by Fe-modified biochar

The industrialization of hydrogen production through dark fermentation of food waste faces challenges, such as low yields and unpredictable fermentation processes. Biochar has emerged as a promising green additive to enhance hydrogen production in dark fermentation. Our study demonstrated that the introduction of Fe-modified biochar (Fe-L600) significantly boosted hydrogen production during thermophilic dark fermentation of food waste. The addition of Fe-L600 led to a remarkable 31.19% increase in hydrogen yield and shortened the time needed for achieving stabilization of hydrogen production from 18 h to 12 h. The metabolite analysis revealed an enhancement in the butyric acid pathway as the molar ratio of acetic acid to butyric acid decreased from 3.09 to 2.69 but hydrogen yield increased from 57.12 ± 1.48 to 76.78 ± 2.77 mL/g, indicating Fe-L600 improved hydrogen yield by regulating crucial metabolic pathways of hydrogen production. The addition of Fe-L600 also promoted the release of Fe2+ and Fe3+ and increased the concentrations of Fe2+ and Fe3+ in the fermentation system, which might promote the activity of hydrogenase and ferredoxin. Microbial community analysis indicated a substantial increase in the relative abundance of Thermoanaerobacterium after thermophilic dark fermentation. The relative abundances of microorganisms responsible for hydrolysis and acidogenesis were also observed to be improved in the system with Fe-L600 addition. This research provides a feasible strategy for improving hydrogen production of food waste and deepens the understanding of the mechanisms of biochar.

Enhancing sustainable crop cultivation: The impact of renewable soil amendments and digestate fertilizer on crop growth and nutrient composition

Utilizing digestate as a fertilizer enhances soil nutrient content, improves fertility, and minimizes nutrient runoff, mitigating water pollution risks. This alternative approach replaces commercial fertilizers, thereby reducing their environmental impact and lowering greenhouse gas emissions associated with fertilizer production and landfilling. Herein, this study aimed to evaluate the impact of various soil amendments, including carbon fractions from waste materials (biochar, compost, and cocopeat), and food waste anaerobic digestate application methods on tomato plant growth (Solanum lycopersicum) and soil fertility. The results suggested that incorporating soil amendments (biochar, compost, and cocopeat) into the potting mix alongside digestate application significantly enhances crop yields, with increases ranging from 12.8 to 17.3% compared to treatments without digestate. Moreover, the combination of soil-biochar amendment and digestate application suggested notable improvements in nitrogen levels by 20.3% and phosphorus levels by 14%, surpassing the performance of the those without digestate. Microbial analysis revealed that the soil-biochar amendment significantly enhanced biological nitrification processes, leading to higher nitrogen levels compared to soil-compost and soil-cocopeat amendments, suggesting potential nitrogen availability enhancement within the rhizosphere's ecological system. Chlorophyll content analysis suggested a significant 6.91% increase with biochar and digestate inclusion in the soil, compared to the treatments without digestate. These findings underscore the substantial potential of crop cultivation using soil-biochar amendments in conjunction with organic fertilization through food waste anaerobic digestate, establishing a waste-to-food recycling system.

Improving urban ecosystem holistic sustainability of municipal solid waste-to-energy strategy using extended exergy accounting analysis

Waste-to-energy technologies play a crucial role in integrated waste management strategies to reduce waste mass and volume, disinfect the waste, and recover energy; different technologies have advantages and disadvantages in treating municipal solid waste under urban conditions. This paper applies the extended exergy accounting method to develop an analytical framework to identify the optimal waste-to-energy strategy from an urban ecosystem holistic sustainability perspective. In the analytical framework, urban ecosystem costs and revenues are formulated as a multi-criteria cost-benefit quantitative model. The urban ecosystem cost is divided into five categories, and the urban ecosystem revenues consist of direct and indirect parts. The direct part is the chemical exergy of the waste-to-energy plants produced product, and the indirect part includes equivalent exergy content of power generation substitution, human health risk elimination, disamenity impact removal and environmental degradation avoidance. Proposing an indicator system to evaluate the waste-to-energy strategy impact on the sustainability of the urban ecosystems and social, economic and environmental sub-ecosystem. Detailed analysis of food waste treatment scenarios of a food center in Singapore was done as a case study to illustrate this analytical framework. Base scenario is current practice that food waste disposal in incineration plant. Anaerobic digestion and gasification are proposed as potential technological solutions for on-site food waste treatment in scenario I and II respectively. In different scenarios, the urban ecosystem costs are estimated to be 71,536.01, 61,854.87 and 74,190.34MJ/year respectively, and the urban ecosystem revenues are estimated to be 135,312.66, 405,442.53 and 298,426.81MJ/year respectively. We show that the scenario where food waste is treated by anaerobic digestion outperforms both the base scenario and scenario II in terms of urban ecosystem costs and revenues, technical energy conversion efficiency, contribution to urban ecosystem holistic sustainability, and natural, social, and economic subsystems improvement, making it the optimal municipal solid waste-to-energy strategy choice.

Life cycle assessment and cost-benefit analysis of small-scale anaerobic digestion system treating food waste onsite under different operational conditions

This study employed life cycle assessment and cost-benefit analysis to evaluate the environmental and economic profile of a real decentralized small-scale anaerobic digestion (AD) system treating food waste (FW). Different operational conditions, including temperature, biochar addition, biogas engine efficiency, and FW loading, were compared via scenario analysis. Biochar addition could potentially obtain carbon reduction and save fossil fuel. Moreover, at high FW loading and biogas engine efficiency, biochar addition achieved 1-3190% better performance than the system without biochar in all the nine impact categories. The system under mesophilic conditions performed worse than ambient conditions due to high energy demand. All the current scenarios resulted in a monetary loss at US$ 480 k-681 k, while profit was possible if the capital cost and operator salary decreased significantly. Overall, operating the small-scale AD system under ambient temperature with biochar addition was preferred due to its potential environmental benefits and economic profits.

A closed loop case study of decentralized food waste management: System performance and life cycle carbon emission assessment

Food waste (FW) has become a worldwide issue, while anaerobic digestion (AD) has appeared as a widely adopted technology to recover energy and resources from FW. Compared to many existing case studies of centralized AD system, the comprehensive study of decentralized micro-AD system from both system energy efficiency and carbon emission perspective is still scanty, particularly system operated under ambient temperature conditions. In this study, an actual decentralized micro-AD system with treating capacity of 300 kg FW/d for a local hawker center in Singapore was reported and evaluated. The results showed that 1894.5 kg of FW was treated and 173 m3 biogas with methane content of 53 % was produced during the experimental period of 75 days. The methane yield results showed a high FW degradation efficiency (87.87 %). However, net energy consumption and net carbon emission were observed during the experimental period. Nevertheless, energy self-efficiency and carbon neutrality, even net energy output and carbon reduction, can be achieved by increasing daily FW loading and biogas engine efficiency. Specifically, the FW loading for system energy self-efficiency was identified as 159 kg/d for engine efficiency of 35 % at a high kitchen waste/table waste ratio (63 %/37 %, with covid-19 dine-in restrictions); while they were 112 and 58 kg/d for engine efficiency of 25 % and 35 %, respective, at a low kitchen waste/table waste ratio (31 %/69 %, without covid-19 dine-in restrictions). The carbon emission ranged from 156.08 kg CO2-eq/t FW to -77.35 kg CO2-eq/t FW depending on the FW loading quantity and engine efficiency. Moreover, the sensitivity analysis also showed that the used electricity source for substitution influenced the carbon emission performance significantly. The obtained results imply that the decentralized micro-AD system could be a feasible FW management solution for energy generation and carbon reduction when the FW loading and engine electrical efficiency are carefully addressed.

Biochar applications in microbial fermentation processes for producing non-methane products: Current status and future prospects

The objective of this review is to encourage the technical development of biochar-assisted microbial fermentation. To this end, recent advances in biochar applications for microbial fermentation processes (i.e., non-methane products of hydrogen, acids, alcohols, and biofertilizer) have been critically reviewed, including process performance, enhanced mechanisms, and current research gaps. Key findings of enhanced mechanisms by biochar applications in biochemical conversion platforms are summarized, including supportive microbial habitats due to the immobilization effect, pH buffering due to alkalinity, nutrition supply due to being rich in nutrient elements, promoting electron transfer by acting as electron carriers, and detoxification of inhibitors due to high adsorption capacity. The current technical limitations and biochar's industrial applications in microbial fermentation processes are also discussed. Finally, suggestions like exploring functionalized biochar materials, biochar's automatic addition and pilot-scale demonstration are proposed. This review would further promote biochar applications in microbial fermentation processes for the production of non-methane products.

Synergetic anaerobic digestion of food waste for enhanced production of biogas and value-added products: strategies, challenges, and techno-economic analysis

The generation of food waste (FW) is increasing at an alarming rate, contributing to a total of 32% of all the waste produced globally. Anaerobic digestion (AD) is an effective method for dealing with organic wastes of various compositions, like FW. Waste valorization into value-added products has increased due to the conversion of FW into biogas using AD technology. A variety of pathways are adopted by microbes to avoid unfavorable conditions in AD, including competition between sulfate-reducing bacteria and methane (CH4)-forming bacteria. Anaerobic bacteria decompose organic matter to produce biogas, a digester gas. The composition depends on the type of raw material and the method by which the digestion process is conducted. Studies have shown that the biogas produced by AD contains 65-75% CH4 and 35-45% carbon dioxide (CO2). Methanothrix soehngenii and Methanosaeta concilii are examples of species that convert acetate to CH4 and CO2. Methanobacterium bryantii, Methanobacterium thermoautotrophicum, and Methanobrevibacter arboriphilus are examples of species that produce CH4 from hydrogen and CO2. Methanobacterium formicicum, Methanobrevibacter smithii, and Methanococcus voltae are examples of species that consume formate, hydrogen, and CO2 and produce CH4. The popularity of AD has increased for the development of biorefinery because it is seen as a more environmentally acceptable alternative in comparison to physico-chemical techniques for resource and energy recovery. The review examines the possibility of using accessible FW to produce important value-added products such as organic acids (acetate/butyrate), biopolymers, and other essential value-added products.

Current insight into enhanced strategies and interaction mechanisms of hydrogel materials for phosphate removal and recovery from wastewater

Phosphorus plays a crucial role in modern society but often pollutes the environment through raising eutrophication and has particularly devastating effects on the water environment. As a promising material platform, the three-dimensional network structure and the tailorable nature of hydrogels provide infinite application possibilities. Thereinto, phosphate removal and recovery from wastewater using hydrogel materials have gained momentum since their rapid reactivity, ease of operation, low cost and simplicity of recovery compared to traditional techniques. In this review, current strategies for functional enhancement of hydrogel materials are systematically summarized from different perspectives. Following, based on the discussion of different interaction mechanisms between phosphates and hydrogels, the phosphate mass transfer and performance of hydrogels and their current application are critically reviewed. This review aims to present mechanistic insight into the recent development in phosphate removal and recovery using hydrogel materials and provides new ideas for constructing high-efficient hydrogels and laying the foundations for the practical application of this technology.

Effects of digestate-encapsulated biochar on plant growth, soil microbiome and nitrogen leaching

The increasing amount of food waste and the excessive use of mineral fertilizers have caused detrimental impacts on soil, water, and air quality. Though digestate derived from food waste has been reported to partially replace fertilizer, its efficiency requires further improvement. In this study, the effects of digestate-encapsulated biochar were comprehensively investigated based on growth of an ornamental plant, soil characteristics, nutrient leaching and soil microbiome. Results showed that except for biochar, the tested fertilizers and soil additives, i.e., digestate, compost, commercial fertilizer, digestate-encapsulated biochar had positive effects on plants. Especially, the digestate-encapsulated biochar had the best effectiveness as evidenced by 9-25% increase in chlorophyll content index, fresh weight, leaf area and blossom frequency. For the effects of fertilizers or soil additives on soil characteristics and nutrient retention, the digestate-encapsulated biochar leached least N-nutrients (<8%), while the compost, digestate and mineral fertilizer leached up to 25% N-nutrients. All the treatments had minimal effects on the soil properties of pH and electrical conductivity. According to the microbial analysis, the digestate-encapsulated biochar has the comparable role with compost in improving the soil immune system against pathogen infection. The metagenomics coupling with qPCR analysis suggested that digestate-encapsulated biochar boosted the nitrification process and inhibited the denitrification process. This study provides an extensive understanding into the impacts of the digestate-encapsulated biochar on an ornamental plant and offers practical implications for the choice of sustainable fertilizers or soil additives and food-waste digestate management.

Interfacial Polyelectrolyte Complexation-Inspired Bioprinting of Vascular Constructs

Bioprinting is a precise layer-by-layer manufacturing technology utilizing biomaterials, cells, and sometimes growth factors for the fabrication of customized three-dimensional (3D) biological constructs. In recent years, it has gained considerable interest in various biomedical studies. However, the translational application of bioprinting is currently impeded by the lack in efficient techniques for blood vessel fabrications. In this report, by systematically studying the previously reported phenomenon, interfacial polyelectrolyte complexation, an efficient blood vessel bioprinting technique based on the phenomenon, was proposed and subsequently investigated. In this technique, anionic hyaluronate and cationic lysine-based peptide amphiphiles were placed concentrically to bioprint with human umbilical endothelial cells for the fabrication of biological tubular constructs. These constructs demonstrated clear vascular features, which made them highly resemble blood vessels. In addition, to optimize the bioactivity of the printed constructs, this report also, for the first time, studied peptide sequencing's effect on the biocompatibility of the polyelectrolyte-peptide amphiphile complex. All these studies conducted in the report are highly relevant and interesting for research in vascular structure fabrication, which will eventually be beneficial for translational application development of bioprinting.

Chromium toxicity and tolerance mechanisms in plants through cross-talk of secondary messengers: An overview of pathways and mechanisms

Environmental sources of chromium (Cr) such as solid waste, battery chemicals, industrial /waste, automotive exhaust emissions, mineral mining, fertilizers, and pesticides, have detrimental effects on plants. An excessive amount of Cr exposure can lead to toxic accumulations in human, animal, and plant tissues. In plants, diverse signaling molecules like hydrogen sulfide (H₂S) and nitric oxide (NO) play multiple roles during Cr stress. Consequently, the molecular mechanisms of Cr toxicity in plants, such as metal binding, modifying enzyme activity, and damaging cells are examined by several studies. The reactive oxygen species (ROS) that are formed when Cr reacts with lipids, membranes, DNA, proteins, and carbohydrates are all responsible for damage caused by Cr. ROS regulate plant growth, programmed cell death (PCD), cell cycle, pathogen defense, systemic communication, abiotic stress responses, and growth. Plants accumulate Cr mostly through the root system, with very little movement to the shoots. The characterization of stress-inducible proteins and metabolites involved in Cr tolerance and cross-talk messengers has been made possible due to recent advances in metabolomics, transcriptomics, and proteomics. This review discusses Cr absorption, translocation, subcellular distribution, and cross-talk between secondary messengers as mechanisms responsible for Cr toxicity and tolerance in plants. To mitigate this problem, soil-plant systems need to be monitored for the biogeochemical behavior of Cr and the identification of secondary messengers in plants.

Machine learning and circular bioeconomy: Building new resource efficiency from diverse waste streams

Biorefinery systems are playing pivotal roles in the technological support of resource efficiency for circular bioeconomy. Meanwhile, artificial intelligence presents great potential in handling scientific tasks of high-dimensional complexity. This review article scrutinizes the status of machine learning (ML) applications in four critical biorefinery systems (i.e. composting, fermentation, anaerobic digestion, and thermochemical conversions) as well as their advancements against traditional modeling techniques of mechanistic approach. The contents cover their algorithm selections, modeling challenges, and prospective improvements. Perspectives are sketched to further inform collective efforts on crucial aspects. The multidisciplinary interchange of modeling knowledge will enable a more progressive digital transformation of sustainability efforts in supporting sustainable development goals.

Advancing the bioconversion process of food waste into methane: A systematic review

With the continuous rise of food waste (FW) throughout the world, a research effort to reveal its potential for bioenergy production is surging. There is a lack of harmonized information and publications available that evaluate the state-of-advance for FW-derived methane production process, particularly from an engineering and sustainability point of view. Anaerobic digestion (AD) has shown remarkable efficiency in the bioconversion of FW to methane. This paper reviews the current research progress, gaps, and prospects in pre-AD, AD, and post-AD processes of FW-derived methane production. Briefly, the review highlights innovative FW collection and optimization routes such as AI that enable efficient FW valorization processes. As weather changes and the FW sources may affect the AD efficiency, it is important to assess the spatio-seasonal variations and microphysical properties of the FW to be valorized. In that case, developing weather-resistant bioreactors and cost-effective mechanisms to modify the raw substrate morphology is necessary. An AI-guided reactor could have high performance when the internal environment of the centralized operation is monitored in real-time and not susceptible to changes in FW variety. Monitoring solvent degradation and fugitive gases during biogas purification is a challenging task, especially for large-scale plants. Furthermore, this review links scientific evidence in the field with full-scale case studies from different countries. It also highlights the potential contribution of ADFW to carbon neutrality efforts. Regarding future research needs, in addition to the smart collection scheme, attention should be paid to the management and utilization of FW impurities, to ensure sustainable AD operations.

Machine learning framework for intelligent prediction of compost maturity towards automation of food waste composting system

Reactive composting is a promising technology for recovering valuable resources from food waste, while its manual regulation is laborious and time-consuming. In this study, machine learning (ML) technologies are adopted to enable automated composting by predicting compost maturity and providing process regulation. Four machine learning algorithms, namely random forest (RF), extreme gradient boosting (XGBoost), Light Gradient Boosting Machine (LightGBM) and Multilayer Perceptron (MLP) are employed to predict the seed germination index (GI) and C/N ratio. Based on the best fusion model with the highest R² of 0.977 and 0.986 for the multi-task prediction of GI and C/N ratio, the critical factors and their interactions with maturity are identified. Moreover, the ML model is validated on a composting reactor and the ML-based prediction application can provide regulation to ensure food waste decompose within the required time. In conclusion, this compost maturity prediction system automates the reactive composting, thus reducing labor costs.

Engineered microbes as effective tools for the remediation of polyaromatic aromatic hydrocarbons and heavy metals

Heavy metals (HMs) and polycyclic aromatic hydrocarbons (PAHs) have become a major concern to human health and the environment due to rapid industrialization and urbanization. Traditional treatment measures for removing toxic substances from the environment have largely failed, and thus development and advancement in newer remediation techniques are of utmost importance. Rising environmental pollution with HMs and PAHs prompted the research on microbes and the development of genetically engineered microbes (GEMs) for reducing pollution via the bioremediation process. The enzymes produced from a variety of microbes can effectively treat a range of pollutants, but evolutionary trends revealed that various emerging pollutants are resistant to microbial or enzymatic degradation. Naturally, existing microbes can be engineered using various techniques including, gene engineering, directed evolution, protein engineering, media engineering, strain engineering, cell wall modifications, rationale hybrid design, and encapsulation or immobilization process. The immobilization of microbes and enzymes using a variety of nanomaterials, membranes, and supports with high specificity toward the emerging pollutants is also an effective strategy to capture and treat the pollutants. The current review focuses on successful bioremediation techniques and approaches that make use of GEMs or engineered enzymes. Such engineered microbes are more potent than natural strains and have greater degradative capacities, as well as rapid adaptation to various pollutants as substrates or co-metabolizers. The future for the implementation of genetic engineering to produce such organisms for the benefit of the environment andpublic health is indeed long and valuable.

Bioaugmentation of Methanosarcina thermophila grown on biochar particles during semi-continuous thermophilic food waste anaerobic digestion under two different bioaugmentation regimes

This study presents the effect of bioaugmentation of thermophilic anaerobic digestion of food waste with Methanosarcina thermophila grown on a wood-derived biochar. Two different supplementation regimes were tested, namely a single bioaugmentation (SBABC) in which 10% v/v of the microbes grown on biochar (1 g/L) is added at setup of the reactors, versus a routine bioaugmentation (RBABC) wherein the same amount of supplements were added over 10 feeding cycles. The optimally performing 'R' and 'S' reactors had increased methane yields by 37% and 32% over their respective controls while reactors SBABC 2 and 3 produced 21.89% and 56.09% higher average methane yield than RBABC 2 and 3, respectively. It appears that a single dose bioaugmentation is advantageous for improving AD as analysed in terms of average methane yield and VFA production. This study provides the basis for understanding how biochar and bioaugmentation can be used for engineering sustainable pilot-scale AD processes.

Recovery of value-added products from biowaste: A review

This review provides an update on the state-of-the art technologies for the valorization of solid waste and its mechanism to generate various bio-products. The organic content of these wastes can be easily utilized by the microbes and produce value-added compounds. Microbial fermentation techniques can be utilized for developing waste biorefinery processes. The utilization of lignocellulosic and plastics wastes for the generation of carbon sources for microbial utilization after pre-processing steps will make the process a multi-product biorefinery. The C1 and C2 gases generated from different industries could also be utilized by various microbes, and this will help to control global warming. The review seeks to expand expertise about the potential application through several perspectives, factors influencing remediation, issues, and prospects.

Methanosarcina thermophila bioaugmentation and its synergy with biochar growth support particles versus polypropylene microplastics in thermophilic food waste anaerobic digestion

Both biochar supplementation as well as bioaugmentation have been shown in literature to improve the methane yield of anaerobic digestion. In this study, the combination of both are evaluated by growing Methanosarcina thermophila on biochar support particles prior to augmentation of thermophilic food waste anaerobic digestion. Biochar stand alone, bioaugmentation solely, a combination of both added separately or grown together, and utilizing polypropylene (PP) microplastics as growth support instead were all tested when starting up a thermophilic process from mesophilic inoculum. Methanosarcina thermophila and biochar supplementation displayed synergy, with 5% M. thermophila on 1 g/L biochar presenting a 32% increase in specific methane yield over the control. Double the bioaugmentation dosage/concentration was also trialled with a thermophilic inoculum, and 10% M. thermophila grown on 2 g/L biochar displayed the best results with a 20% increase specific methane yield from its control standard.

A critical review on microbes-based treatment strategies for mitigation of toxic pollutants

Contamination of the environment through toxic pollutants poses a key risk to the environment due to irreversible environmental damage(s). Industrialization and urbanization produced harmful elements such as petrochemicals, agrochemicals, pharmaceuticals, nanomaterials, and herbicides that are intentionally or unintentionally released into the water system, threatening biodiversity, the health of animals, and humans. Heavy metals (HMs) in water, for example, can exist in a variety of forms that are inclined by climate features like the presence of various types of organic matter, pH, water system hardness, transformation, and bioavailability. Biological treatment is an important tool for removing toxic contaminants from the ecosystem, and it has piqued the concern of investigators over the centuries. In situ bioremediation such as biosparging, bioventing, biostimulation, bioaugmentation, and phytoremediation and ex-situ bioremediation includes composting, land farming, biopiles, and bioreactors. In the last few years, scientific understanding of microbial relations with particular chemicals has aided in the protection of the environment. Despite intensive studies being carried out on the mitigation of toxic pollutants, there have been limited efforts performed to discuss the solutions to tackle the limitations and approaches for the remediation of heavy metals holistically. This paper summarizes the risk assessment of HMs on aquatic creatures, the environment, humans, and animals. The content of this paper highlights the principles and limitations of microbial remediation to address the technological challenges. The coming prospect and tasks of evaluating the impact of different treatment skills for pollutant remediation have been reviewed in detail. Moreover, genetically engineered microbes have emerged as powerful bioremediation capabilities with significant potential for expelling toxic elements. With appropriate examples, current challenging issues and boundaries related to the deployment of genetically engineered microbes as bioremediation on polluted soils are emphasized.

Fermentative production of 2,3-Butanediol using bread waste - A green approach for sustainable management of food waste

Bread is Europe's most wasted food, and the second most wasted food after potatoes in UK. Bread waste (BW) is a clean source of high-quality fermentable sugars. In this study, the potential of Enterobacter ludwigii to accumulate 2,3-butanediol (BDO) from BW was evaluated. Initially, the optimal inoculum size and yeast extract concentration were determined, followed by extraction of sugars from BW using acid and enzymatic hydrolysis. A glucose yield of 330-530 g/kg BW was obtained, and the sugars released were utilised for BDO production by E. ludwigii. The fed-batch cultivation using pure glucose and glucose rich hydrolysates from acid and enzymatic hydrolysis resulted in BDO titres of 144.5, 135.4, and 138.8 g/L, after 96 h, with yield of 0.47, 0.42 and 0.48 g/g yield, respectively. The innovation of the work is valorisation of BW to BDO with a circular biorefining approach and thus, reducing BW disposal and associated environmental burden.

Omics approaches in bioremediation of environmental contaminants: An integrated approach for environmental safety and sustainability

Non-degradable pollutants have emerged as a result of industrialization, population growth, and lifestyle changes, endangering human health and the environment. Bioremediation is the process of clearing hazardous contaminants with the help of microorganisms, and cost-effective approach. The low-cost and environmentally acceptable approach to removing environmental pollutants from ecosystems is microbial bioremediation. However, to execute these different bioremediation approaches successfully, this is imperative to have a complete understanding of the variables impacting the development, metabolism, dynamics, and native microbial communities' activity in polluted areas. The emergence of new technologies like next-generation sequencing, protein and metabolic profiling, and advanced bioinformatic tools have provided critical insights into microbial communities and underlying mechanisms in environmental contaminant bioremediation. These omics approaches are meta-genomics, meta-transcriptomics, meta-proteomics, and metabolomics. Moreover, the advancements in these technologies have greatly aided in determining the effectiveness and implementing microbiological bioremediation approaches. At Environmental Protection Agency (EPA)-The government placed special emphasis on exploring how molecular and "omic" technologies may be used to determine the nature, behavior, and functions of the intrinsic microbial communities present at pollution containment systems. Several omics techniques are unquestionably more informative and valuable in elucidating the mechanism of the process and identifying the essential player's involved enzymes and their regulatory elements. This review provides an overview and description of the omics platforms that have been described in recent reports on omics approaches in bioremediation and that demonstrate the effectiveness of integrated omics approaches and their novel future use.

A Review on Enhancing Cupriavidus necator Fermentation for Poly(3-hydroxybutyrate) (PHB) Production From Low-Cost Carbon Sources

In the context of a circular economy, bioplastic production using biodegradable materials such as poly(3-hydroxybutyrate) (PHB) has been proposed as a promising solution to fundamentally solve the disposal issue of plastic waste. PHB production techniques through fermentation of PHB-accumulating microbes such as Cupriavidus necator have been revolutionized over the past several years with the development of new strategies such as metabolic engineering. This review comprehensively summarizes the latest PHB production technologies via Cupriavidus necator fermentation. The mechanism of the biosynthesis pathway for PHB production was first assessed. PHB production efficiencies of common carbon sources, including food waste, lignocellulosic materials, glycerol, and carbon dioxide, were then summarized and critically analyzed. The key findings in enhancing strategies for PHB production in recent years, including pre-treatment methods, nutrient limitations, feeding optimization strategies, and metabolism engineering strategies, were summarized. Furthermore, technical challenges and future prospects of strategies for enhanced production efficiencies of PHB were also highlighted. Based on the overview of the current enhancing technologies, more pilot-scale and larger-scale tests are essential for future implementation of enhancing strategies in full-scale biogas plants. Critical analyses of various enhancing strategies would facilitate the establishment of more sustainable microbial fermentation systems for better waste management and greater efficiency of PHB production.

Methodological framework for wastewater treatment plants delivering expanded service: Economic tradeoffs and technological decisions

With emerging decarbonization to deploy more integrated waste management, there is a burgeoning need for re-managing waste-related infrastructures in urban environments. Wastewater treatment plants are key contributors to expanded environmental services, but relevant technological decisions and economic tradeoffs have to be assessed from a systems perspective. This study provides a methodological framework that consolidates the multiple technological and economic aspects of system retrofitting for such an evaluation purpose. Complex leachate from refuse transfer stations has been recently identified as the decarbonization roadblock of urban waste management, and it was chosen for investigations by this new methodological approach. The system impacts by complex leachate on the existing facilities were validated by experimental trials. To derive the financial outlooks for decision making, the evaluation matrix includes the quantitative impacts of bioenergy profiles, energy balance analysis of biogas utilization methods, needs of system retrofitting, economic factors, and their uncertainties. Due to the detected inefficiency of bioenergy recovery, bioinformatic analysis was proceeded for understanding the underlying mechanism to propose a mitigation solution. Overall, the methodological framework can provide a quantitative assessment of the centralized capability of wastewater treatment plants for systems planning in the new policy agenda of urban decarbonization, where the methodological potentials of expanded framework applications are also highlighted.

Microbial succession analysis reveals the significance of restoring functional microorganisms during rescue of failed anaerobic digesters by bioaugmentation of nano-biochar-amended digestate

Nano-biochar application was investigated for anaerobic digestion of orange peel waste. The application for methane production focused on the optimization of biochar feedstock, rescue of failed digesters, and microbial succession analysis. It showed that sewage sludge (SS) derived biochar had the highest performance enhancement among the different feedstocks, which could be ascribed to the improvement of electron transfer, interspecies hydrogen transfer, and supply of trace elements. Subsequently, nano SS biochar-amended digestate was evaluated for rescuing failed digesters, and the experimental results indicated its positive roles through gradual bioaugmentation operation. The dynamic analysis of microbial succession indicated the successful application was through the mechanism of restoring partially the functional microbial communities. The major reconstruction of functional microorganisms included bacteria phyla Hydrogenispora (24.5%) and Defluviitoga (18.8%) as well as methanogenic genera of Methanosarcina (41.5%) and Methanobacterium (27.3%). These findings would contribute to rescuing failed anaerobic digesters by bioaugmentation with biochar-amended digestate.

Household waste management in Singapore and Shanghai: Experiences, challenges and opportunities from the perspective of emerging megacities

Due to rapid economic development and urbanisation, emerging megacities with dense populations have witnessed a significant increase in waste generation. Megacities face challenges in developing sustainable waste management systems. Considerable heterogeneity exists across megacities in management strategies. The two selected emerging megacities, Singapore (a city-state) and Shanghai, have similar developmental characteristics, but their waste management modes differ strikingly. This study assessed the two modes in terms of management strategies, environmental effects, economic costs, and social outcomes. Environmental footprint analysis and cost quantification were employed for the assessment based on public data. The research results would permit a deeper understanding of the long-term sustainability of each mode while considering the feasibility of implementation across different contexts. It was found that the waste management system in Singapore had a relatively lower environmental impact than Shanghai before Shanghai's new waste segregation and recycling policy in 2019. However, when the effect of fossil fuel substitution is taken into account, the environmental burden in Shanghai can be lowered more substantially than the one in Singapore. Although Shanghai had more economic burden for the waste segregation at source, it tended to implement the circular economy principles (e.g., reduce, reuse, and recycling) better and improve its sense of community significantly. Based on the practical experiences from the two representative megacities, suggestions for better waste management practices were provided for Singapore, Shanghai, and other emerging megacities with similar circumstances. In addition, challenges and opportunities related to household waste segregation and recycling were identified to guide future practices in emerging megacities.

Abrogating the inhibitory effects of volatile fatty acids and ammonia in overloaded food waste anaerobic digesters via the supplementation of nano-zero valent iron modified biochar

The effects of different recovery strategies on inhibited anaerobic digestion (AD) of food waste (FW) was examined in this study, with the finding that dosing pine woodchip biochar could reverse the effect of volatile fatty acids (VFA) inhibition (mainly propionic acid) and yielded 105.55% more methane than the control. The addition of nano-zerovalent iron (nZVI) promoted the generation of VFA while causing a slight inhibition of the methanogens initially. In due time, the nZVI digester was able to recover and eventually produced 192.22% more methane compared to the control. Finally, nZVI-modified biochar was proved to be able to avoid the inhibitory effects brought about by the nanoparticles. The results indicated reduced dosage requirements as compared to using pristine pine woodchip biochar and accumulated 204.84% more methane than the control. The introduction of nZVI-biochar also promoted the growth of Methanosarcina species methanogens, which can perform direct-interspecies electron transfer. While all the recovery strategies using the additives were feasible, the results suggested that the use of modified biochar holds great potential as a significantly lower amount of amendment is required for the recovery of the inhibited AD system.

Genetic modifications associated with sustainability aspects for sustainable developments

Sustainable development serves as the foundation for a range of international and national policymaking. Traditional breeding methods have been used to modify plant genomes and production. Genetic engineering is the practice of assisting agricultural systems in adapting to rapidly changing global growth by hastening the breeding of new varieties. On the other hand, the development of genetic engineering has enabled more precise control over the genomic alterations made in recent decades. Genetic changes from one species can now be introduced into a completely unrelated species, increasing agricultural output or making certain elements easier to manufacture. Harvest plants and soil microorganisms are just a few of the more well-known genetically modified creatures. Researchers assess current studies and illustrate the possibility of genetically modified organisms (GMOs) from the perspectives of various stakeholders. GMOs increase yields, reduce costs, and reduce agriculture's terrestrial and ecological footprint. Modern technology benefits innovators, farmers, and consumers alike. Agricultural biotechnology has numerous applications, each with its own set of potential consequences. This will be able to reach its full potential if more people have access to technology and excessive regulation is avoided. This paper covers the regulations for genetically modified crops (GMCs) as well as the economic implications. It also includes sections on biodiversity and environmental impact, as well as GMCs applications. This recounts biotechnological interventions for long-term sustainability in the field of GMCs, as well as the challenges and opportunities in this field of research.Abbreviations: GMCs-Genetically modified crops; GMOs- Genetically modified organisms; GE- Genetic engineering; Bt- Bacillus thuringiensisNIH- National Institutes of Health; FDA- Food and Drug Administration; HGT- Horizontal gene transfer; GM- Genetically modified; rDNA- Ribosomal deoxyribonucleic acid; USDA- United States Department of Agriculture; NIH- National Institutes of Health.

Plastic-containing food waste conversion to biomethane, syngas, and biochar via anaerobic digestion and gasification: Focusing on reactor performance, microbial community analysis, and energy balance assessment

To manage the mixture of food waste and plastic waste, a hybrid biological and thermal system was investigated for converting plastic-containing food waste (PCFW) into renewable energy, focusing on performance evaluation, microbial community analysis, and energy balance assessment. The results showed that anaerobic digestion (AD) of food waste, polyethylene (PE)-containing food waste, polystyrene (PS)-containing food waste, and polypropylene (PP)-containing food waste generated a methane yield of 520.8, 395.6, 504.2, and 479.8 mL CH₄/gVS, respectively. CO₂ gasification of all the plastic-containing digestate produced more syngas than pure digestate gasification. Syngas from PS-digestate reached the maximum yield of 20.78 mol/kg. During the digestate-derived-biochar-amended AD of PCFW, the methane yields in the biochars-amended digesters were 6-30% higher than those of the control digesters. Bioinformatic analysis of microbial communities confirmed the significant difference between control and biochar-amended digesters in terms of bacterial and methanogenic compositions. The enhanced methane yields in biochars-amended digesters could be partially ascribed to the selective enrichment of genus Methanosarcina, leading to an improved equilibrium between hydrogenotrophic and acetoclastic methanogenesis pathways. Moreover, energy balance assessment demonstrated that the hybrid biological and thermal conversion system can be a promising technical option for the treatment of PCFW and recovery of renewable biofuels (i.e., biogas and syngas) and bioresource (i.e., biochar) on an industrial scale.

Health hazards of hexavalent chromium (Cr (VI)) and its microbial reduction

Industrial effluents/wastewater are the main sources of hexavalent chromium (Cr (VI)) pollutants in the environment. Cr (VI) pollution has become one of the world’s most serious environmental concerns due to its long persistence in the environment and highly deadly nature in living organisms. To its widespread use in industries Cr (VI) is highly toxic and one of the most common environmental contaminants. Cr (VI) is frequently non-biodegradable in nature, which means it stays in the environment for a long time, pollutes the soil and water, and poses substantial health risks to humans and wildlife. In living things, the hexavalent form of Cr is carcinogenic, genotoxic, and mutagenic. Physico-chemical techniques currently used for Cr (VI) removal are not environmentally friendly and use a large number of chemicals. Microbes have many natural or acquired mechanisms to combat chromium toxicity, such as biosorption, reduction, subsequent efflux, or bioaccumulation. This review focuses on microbial responses to chromium toxicity and the potential for their use in environmental remediation. Moreover, the research problem and prospects for the future are discussed in order to fill these gaps and overcome the problem associated with bacterial bioremediation’s real-time applicability.

Enhancing microbial lipids yield for biodiesel production by oleaginous yeast Lipomyces starkeyi fermentation: A review

The enhanced production of microbial lipids suitable for manufacturing biodiesel from oleaginous yeast Lipomyces starkeyi is critically reviewed. Recent advances in several aspects involving the biosynthetic pathways of lipids, current conversion efficiencies using various carbon sources, intensification strategies for improving lipid yield and productivity in L. starkeyi fermentation, and lipid extraction approaches are analyzed from about 100 papers for the past decade. Key findings on strategies are summarized, including (1) optimization of parameters, (2) cascading two-stage systems, (3) metabolic engineering strategies, (4) mutagenesis followed by selection, and (5) co-cultivation of yeast and algae. The current technical limitations are analyzed. Research suggestions like examination of more gene targets via metabolic engineering are proposed. This is the first comprehensive review on the latest technical advances in strategies from the perspective of process and metabolic engineering to further increase the lipid yield and productivity from L. starkeyi fermentation.

Food-waste anaerobic digestate as a fertilizer: The agronomic properties of untreated digestate and biochar-filtered digestate residue

Anaerobic digestion produces large quantities of digestate as a by-product, which can potentially be applied as an organic fertilizer, but untreated anaerobic digestate (AD) may contain phytotoxins and the large volume of AD makes transportation and storage difficult. This study explored two relatively inexpensive processing methods to improve the agronomic performance of AD as a fertilizer via vegetable cultivation experiments. We first investigated the effect of dilution on AD's performance using four leafy vegetables (Chinese spinach, water spinach, Chinese cabbage and lettuce). The optimal concentrations of the AD were 20-40% (v/v in 250 mL applications per single-plant pot) for all four vegetables based on shoot fresh weight and comparable to the control treatment using commercial fertilizer. AD application also introduced Synergistetes bacteria into the growing medium, but the overall bacterial diversity and composition were similar to those of the control treatment. Considering the nutrient separation in the liquid and solid fractions of AD and the need to reduce the volume, we then experimented with the recovery of nutrients from both the liquid and solid fractions by filtering AD using two types of wood-based biochar (100 g biochar: 1 L AD) before applying the AD-biochar residues as side dressing at 1% (w/w). Both types of biochar achieved yields comparable to the treatment using a commercial fertilizer for the three vegetables tested (kale, lettuce and rocket salad). Our results show that dilution and biochar filtration can improve the agronomic performance of AD, making it a sustainable substitute for commercial fertilizer.

Effects of plastics on reactor performance and microbial communities during acidogenic fermentation of food waste for production of volatile fatty acids

The aim of this work was to study the effects of plastics (high-density polyethylene (HDPE), polystyrene (PS), polypropylene (PP), and polyethylene terephthalate (PET)) on reactor performance and microbial communities during acidogenic fermentation of food waste for the production of volatile fatty acids (VFA). The addition of HDPE and PS increased total VFA yields by 28% and 47%, respectively, whereas the addition of PP and PET decreased total VFA yields by 6% and 2%, respectively. The highest enhancing performance of PS could be ascribed to its highly porous structure that could provide immobilization effects for microbial growth. Degradation of various plastics was confirmed by FESEM results, but the degrees were limited (i.e., 3.9-8.7%). Bacterial analysis showed that the addition of various plastics altered the community diversity. Phylum Thermotogae and genus Defluviitoga dominated all the reactors. Potential HDPE- and PS-degrading microbes could belong to genus Clostridium_sensu_stricto_8, while Tepidanaerobacter_syntrophicus could be PET-degrading microbes.

Timing of biochar dosage for anaerobic digestion treating municipal leachate: Altered conversion pathways of volatile fatty acids

In this study, the anaerobic digestion (AD) applications of early & late biochar dosage were compared for municipal leachate treatment, with the objective of studying the flexible use of biochar as a mitigation measure for biomethane recovery. In two experimental phases, biochar was favourable for the immediate promotion of AD performances, as revealed by Gompertz's model of reduced lag phases, higher biomethane generation rates, and increased biomethane yields. Irrespective of late biochar dosage, it could still retrieve 89% of the ultimate biomethane potential. Comparing the residual VFAs (volatile fatty acids) compositions, it was found that the fraction of long-chain VFAs accounted for 81% of total VFAs in reactor set of early biochar dosage, while it was only 38% in the reactor of late one. Parallel evidence suggested that the schedule of biochar dosage not only could affect methanogenic responses but also the VFAs conversion pathways.

System integration of hydrothermal liquefaction and anaerobic digestion for wet biomass valorization: Biodegradability and microbial syntrophy

Sewage sludge treatment & disposal pose environmental challenges in populated-dense urban environments. Due to its poor digestibility and dewaterability, sewage sludge contains high water content and concentrated nutrients (carbon, nitrogen, and phosphorus) even after conditioning and mechanical thickening. Regarding this, a pretreatment step and downstream anaerobic digestion (AD) are often required. To meet our societal goal towards a circular economy, system integration of hydrothermal pretreatment and AD now present an attractive approach for recovering resources from the wet sewage sludge biomass. In this study, such system integration together with struvite precipitation was applied for valorizing sewage sludge. Firstly, hydrothermal conditions of different temperatures (160 °C-230 °C) and duration (2 h-12 h) were compared to their performance of nutrients solubilization. Subsequently, the hydrothermal condition of 220°C-3 h was selected for further investigations of struvite recovery and bioenergy production. Through AD comparisons, the integrated process improved the ultimate biomethane yield by 38%. Interestingly, a lag phase occurred in the midst of the AD, which indicated the need for microbial acclimatization after the hydrothermal process. The long-term microbial monitoring revealed the efficient biomethane re-generation was closely related to the late enrichment of Syntrophus for potential H₂-syntrophy. Therefore, on one hand, this study investigated an efficient and integrated approach of sewage sludge valorization. On other hand, it uncovered the microbial bottlenecks and potential biotechnological means for further system improvement. Further research about nutrients speciation in the integrated system would be desired.

Metal and metal(loids) removal efficiency using genetically engineered microbes: Applications and challenges

The environment is being polluted in different many with metal and metalloid pollution, mostly due to anthropogenic activity, which is directly affecting human and environmental health. Metals and metalloids are highly toxic at low concentrations and contribute primarily to the survival equilibrium of activities in the environment. However, because of non-degradable, they persist in nature and these metal and metalloids bioaccumulate in the food chain. Genetically engineered microorganisms (GEMs) mediated techniques for the removal of metals and metalloids are considered an environmentally safe and economically feasible strategy. Various forms of GEMs, including fungi, algae, and bacteria have been produced by recombinant DNA and RNA technologies, which have been used to eliminate metal and metalloids compounds from the polluted areas. Besides, GEMs have the potentiality to produce enzymes and other metabolites that are capable of tolerating metals stress and detoxify the pollutants. Thus, the aim of this review is to discuss the use of GEMs as advanced tools to produce metabolites, signaling molecules, proteins through genetic expression during metal and metalloids interaction, which help in the breakdown of persistent pollutants in the environment.

Two-Stage Fermentation of Lipomyces starkeyi for Production of Microbial Lipids and Biodiesel

The high operating cost is currently a limitation to industrialize microbial lipids production by the yeast Lipomyces starkeyi. To explore economic fermentation technology, the two-stage fermentation of Lipomyces starkeyi using yeast extract peptone dextrose (YPD) medium, orange peel (OP) hydrolysate medium, and their mixed medium were investigated for seven days by monitoring OD₆₀₀ values, pH values, cell growth status, C/N ratios, total carbon concentration, total nitrogen concentration, residual sugar concentration, lipid content, lipid titer, and fatty acids profiles of lipids. The results showed that two-stage fermentation with YPD and 50% YPD + 50% OP medium contributed to lipid accumulation, leading to larger internal lipid droplets in the yeast cells. However, the cells in pure OP hydrolysate grew abnormally, showing skinny and angular shapes. Compared to the one-stage fermentation, the two-stage fermentation enhanced lipid contents by 18.5%, 27.1%, and 21.4% in the flasks with YPD medium, OP medium, and 50%YPD + 50%OP medium, and enhanced the lipid titer by 77.8%, 13.6%, and 63.0%, respectively. The microbial lipids obtained from both one-stage and two-stage fermentation showed no significant difference in fatty acid compositions, which were mainly dominated by palmitic acid (33.36–38.43%) and oleic acid (46.6–48.12%). Hence, a mixture of commercial medium and lignocellulosic biomass hydrolysate could be a promising option to balance the operating cost and lipid production.

Mixing effects on decentralized high-solid digester for horticultural waste: Startup, operation and sensitive microorganisms

This work studied the use of a horizontal high-solid digester for the decentralized anaerobic treatment of horticultural waste (fallen leaves), where the effect of intermittent mixing by a modified double helical ribbon impeller was investigated. Before experimental verification, the flow pattern and theoretical mixing time were first characterized by CFD simulation. Subsequently, three mixing time intervals (i.e., 3 min/3 hr; 18 min/3 hr; 108 min/3 hr) and one control setup (i.e., without mixing) were compared for their performance during start-up and semi-continuous operation. It was found that minimal mixing was necessary for an efficient digester's start-up but increased mixing intensity for semi-continuous operation. The results were further interpreted by correlating the digester performance and microbial communities. Those microorganisms sensitive to increased mixing intensity were highlighted and analysed.

Biochar utilisation in the anaerobic digestion of food waste for the creation of a circular economy via biogas upgrading and digestate treatment

A wood waste-derived biochar was applied to food-waste anaerobic digestion to evaluate the feasibility of its utilisation to create a circular economy. This biochar was first purposed for the upgrading of the biogas from the said anaerobic digestion, before treating and recovering the nutrients in the solid fraction of the digestate, which was finally employed as a biofertilizer for the organic cultivation of three green leafy vegetables: kale, lettuce and rocket salad. Whilst the amount of CO₂ the biochar could absorb from the biogas was low (11.17 mg g^-1), it could potentially be increased by modifying through physical and chemical methods. Virgin as well as CO₂-laden biochar were able to remove around 31% of chemical oxygen demand, 8% of the ammonia and almost 90% of the total suspended solids from the digestate wastewater, which was better than a dewatering process via centrifugation but worse than the industry standard of a polytetrafluoroethylene membrane bioreactor. Nutrients were recovered in the solid fraction of the digestate residue filtered by the biochar, and utilised as a biofertilizer that performed similarly to a commercial complete fertilizer in terms of aerial fresh weight growth for all three vegetables cultivated. Contingent on the optimal upgrading of biogas, the concept of a circular economy based on biochar and anaerobic digestion appears to be feasible.

Applications of food waste-derived black soldier fly larval frass as incorporated compost, side-dress fertilizer and frass-tea drench for soilless cultivation of leafy vegetables in biochar-based growing media

Black soldier fly (BSF) larval bioconversion can recycle nutrients in organic wastes into larval biomass and frass. While the frass has been commonly marketed as a soil amendment, its usefulness in soilless cultivation remains largely unexplored. Growth experiments were conducted to investigate the effectiveness of surplus food-derived and okara-derived BSF larval frass as an incorporated compost, side-dress fertilizer and frass-tea drench for the cultivation of pak choi and lettuce in waste-wood derived biochar growing media. Pak choi yields from treatments with surplus food-derived frass and biochar at a 10:90 (v/v) ratio and inorganic fertilizer were comparable to those of the control which consisted of soil, peat-based compost and inorganic fertilizer. However, yields decreased with increasing frass incorporation rates owing to high salinity and potentially low oxygen conditions in the growing media. When used as a fertilizer on biochar-coir growing media, the direct application of frass as a side-dress fertilizer was 1.6-6.8 times more effective in promoting lettuce growth than the application as a frass-tea drench. Frass fertilizers derived from surplus food outperformed those derived from okara by 1.3-5.3 times. Lettuce yields were not significantly different between the treatment with surplus food-derived frass applied as a side-dress fertilizer and the control of liquid inorganic fertilizer. Variations in fertilizing potential were attributed to nutrient availability and the presence of plant growth promoting microbes in the growing media. BSF larval frass derived from food waste shows promise in partially replacing unsustainable agricultural inputs for leafy vegetable cultivation, including soil and inorganic fertilizers.

Food waste treating by biochar-assisted high-solid anaerobic digestion coupled with steam gasification: Enhanced bioenergy generation and porous biochar production

A food waste treating system was proposed in this study by combining biochar-assisted high-solid anaerobic digestion and subsequent steam gasification of the digestate. The effect of solid level, biochar dosage in anaerobic digestion on the properties of biogas, syngas, and final biochar products were investigated. Results showed that at a high total solid level and biochar dosage of 25 g/L and 50 g/L, the accumulative methane yield reached 110.3 mL CH₄/g VS and 126.7 mL CH₄/g VS, respectively. From steam gasification of different digestates under 850 °C for 15 min, a maximum of 34.92 mmol/g for the hydrogen yield and 11.44 MJ/m³ for the higher heating value could be obtained for the syngas. Furthermore, the by-product produced from steam gasification was a nutrient-enriched porous biochar, which was suitable to be used as compost. This study demonstrated a pathway for food waste treating to produce methane-enriched biogas, hydrogen-enriched syngas, and nutrient-enriched biochar.

Influence of wet oxidation pretreatment with hydrogen peroxide and addition of clarified manure on anaerobic digestion of oil palm empty fruit bunches

Food and energy requirements are increasing globally, and the challenge is to meet these demands in a sustainable manner. Oil palm has a relatively high productivity, but produces the lignocellulosic residue of empty fruit bunches (OPEFB). In this study, wet oxidation pretreatment is utilized to overcome the recalcitrance of OPEFB during semi-continuous anaerobic digestion (AD) with between 19.7 and 52.7% improvement over the control, and near total cellulose and hemicellulose content could be degraded. Clarified manure, the water phase of cattle and dairy manure after filtration, is further tested for its effect on methane production by providing necessary micronutrients and vitamins. An increase of 49% was found after addition of clarified manure to OPEFB compared to without this addition.

Biochar enhanced high-solid mesophilic anaerobic digestion of food waste: Cell viability and methanogenic pathways

The underlying mechanisms of biochar enhance high-solid anaerobic digestion (HSAD) of food waste were investigated with a focus on the cell viability, microbial community, and methanogenic pathways. This study assessed the effects of different dosages of biochar in HSAD. Optimal biochar dosage was found to be 25 g/L, which produced accumulative methane yields of up to 251 mL CH₄/g VS significantly promote volatile fatty acid degradations, especially in butyric acid concentrations. Effects of biochar with a dosage of 25 g/L on the cell viability showed that viable cells based on cell membrane integrity increased from 2.9% to 6.4%. Meanwhile, intact and highly active cells with high DNA content were probably involved in direct interspecies electron transfer (DIET) via membrane-bound electron transport proteins. Further analysis demonstrated that Syntrophomonas and methanogens Methanosarcina &Methanocelleus were selectively enriched by biochar, which resulted in the methanogenic pathways shifting from acetoclastic/hydrogenotrophic methanogenic pathways to more metabolically diverse methanogenic pathways. Accordingly, biochar-mediated DIET was possibly established between Syntrophomonas and Methanosarcina species due to those viable cells. In conclusion, biochar is a feasible additive in enhancing HSAD methanogenic performance.

Enhancement of methanogenic performance by gasification biochar on anaerobic digestion

This work evaluates the performance of different biochar-amended anaerobic digestion systems. The Fourier Transform Infrared analysis showed that more ordered aromatic groups formed and the aromatization degree increased with the rise of gasification temperature. The biochar produced at 900 °C still showed an excellent ability to maintain the stability of anaerobic digestion performance, where the specific methane yield content steadily reached 742 mL CH₄/g ethanol. Besides, the enzymatic activity test indicated an improved performance with the addition of biochar obtained at gasification temperature. The relationship between the microbial community and metabolism pathways result are signified due to the direct interspecies electron transfer among Pseudomonas or Candidatus cloacimonas and Methanosaeta via biochar. These links have promoted the methane metabolism pathway of acetate decarboxylation. Therefore, the current study helps better understand the influence of surface functional groups of biochar at different temperatures on anaerobic digestion performance.

Evaluating the potential of okara-derived black soldier fly larval frass as a soil amendment

Bioconversion of organic waste by the black soldier fly (BSF) larva yields a by-product commonly known as 'frass'. Although BSF larval frass has often been marketed as a biofertilizer, few studies have evaluated this claim. In this study, BSF larvae reared on a pure okara diet achieved an 85% waste reduction in the fresh weight of the okara. Subsequently, the frass was mixed with soil at concentrations of 10, 20, and 30% (vol/vol), and used to cultivate lettuce plants. At 10% concentration, the lettuce plants had biomasses comparable to those of the controls. Higher frass concentrations stunted the growth of the lettuce, likely because of the low C:N ratio of larval frass resulting from the rapid mineralization of nutrients. Larval frass was also found to be able to provide sufficient nutrients for lettuce growth as fertilizer application was only necessary after the first growth cycle, suggesting its suitability as a soil amendment. Analyses of the microbial community of all the growing media showed that the growth medium treatments with BSF larval frass tended to have a lower number of microbial species than the controls. Inherently higher micronutrient levels present in the frass resulted in the growth of lettuce plants. More importantly, the microbial analysis revealed that common foodborne pathogens were absent in the BSF larval frass and elemental analysis also indicated no heavy metal pollutants present. Overall, BSF larval frass was found to be a suitable soil amendment and more in-depth studies could facilitate its sensible use in agriculture.

Biochar industry to circular economy

Biochar, produced as a by-product of pyrolysis/gasification of waste biomass, shows great potential to reduce the environment impact, address the climate change issue, and establish a circular economy model. Despite the promising outlook, the research on the benefits of biochar remains highly debated. This has been attributed to the heterogeneity of biochar itself, with its inherent physical, chemical and biological properties highly influenced by production variables such as feedstock types and treating conditions. Hence, to enable meaningful comparison of results, establishment of an agreed international standard to govern the production of biochar for specific uses is necessary. In this study, we analyzed four key uses of biochar: 1) in agriculture and horticulture, 2) as construction material, 3) as activated carbon, and 4) in anaerobic digestion. Then the guidelines for the properties of biochar, especially for the concentrations of toxic heavy metals, for its environmental friendly application were proposed in the context of Singapore. The international status of the biochar industry code of practice, feedback from Singapore local industry and government agencies, as well as future perspectives for the biochar industry were explained.

Microbial biodiesel production from industrial organic wastes by oleaginous microorganisms: Current status and prospects

This review aims to encourage the technical development of microbial biodiesel production from industrial-organic-wastes-derived volatile fatty acids (VFAs). To this end, this article summarizes the current status of several key technical steps during microbial biodiesel production, including (1) acidogenic fermentation of bio-wastes for VFA collection, (2) lipid accumulation in oleaginous microorganisms, (3) microbial lipid extraction, (4) transesterification of microbial lipids into crude biodiesel, and (5) crude biodiesel purification. The emerging membrane-based bioprocesses such as electrodialysis, forward osmosis and membrane distillation, are promising approaches as they could help tackle technical challenges related to the separation and recovery of VFAs from the fermentation broth. The genetic engineering and metabolic engineering approaches could be applied to design microbial species with higher lipid productivity and rapid growth rate for enhanced fatty acids synthesis. The enhanced in situ transesterification technologies aided by microwave, ultrasound and supercritical solvents are also recommended for future research. Technical limitations and cost-effectiveness of microbial biodiesel production from bio-wastes are also discussed, in regard to its potential industrial development. Based on the overview on microbial biodiesel technologies, an integrated biodiesel production line incorporating all the critical technical steps is proposed for unified management and continuous optimization for highly efficient biodiesel production.

The bio-chemical cycle of iron and the function induced by ZVI addition in anaerobic digestion: A review

Zero-valent iron (ZVI) is known to be an additive in facilitating waste treatment and improving biogas production in anaerobic digestion (AD) systems. This review concentrates on the chemical cycle of iron as well as the function of the iron cycle in the removal of four kinds of pollutants: organic carbon, nitrogen, sulphur and phosphorus, which are commonly encountered in waste treatment. In recent studies, the addition of ZVI to an AD system promoted the in-situ production of CH₄ from CO₂, enabling carbon capture through biotechnology. Additionally, using iron-carbon microbial electrolytic cells in AD systems in order to accelerate electron transport, as well as specific pollutant degradation mechanisms, are illustrated in the present study. Particularly, the main factors affecting the removal efficiency of contaminants in a ZVI-AD system such as pH, VFA/ Alkalinity (ALK), oxidation-reduction potential and particle size are reviewed. According to the above characteristics, combined with technical model and economic analyses, an AD system based on ZVI was considered to be an economical, efficient and carbon-neutral pollutant treatment technology. Accordingly, Iron-based AD is suggested to be a promising and sustainable approach orientated to a circular economy, which may be applied to many waste treatments fields.