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

Pressure-centric regulation for efficient anaerobic digestion: State-of-the-art, challenges and prospects

Anaerobic digestion (AD) is an environmentally friendly technology that simultaneously stabilizes biowaste and produces biogas. Conventional AD faces challenges such as inadequate substrate degradation and low methane purity. Pressure-centric regulation serves as an AD optimization strategy that can enhance the digestion efficiency and generate higher-energy-value biogas. However, limited reviews have been undertaken to focus on this technology. This review is designed to discuss innovations in ex-situ high-pressure pretreatment and in-situ high-pressure anaerobic digestion (HPAD) processes. Moreover, comprehensive understandings on the intrinsic mechanisms of HPAD are critically examined, including physicochemical reaction principles and microbial responses. The constraints currently curtailing these technologies and potential mitigation strategies are also scrutinized. Additionally, current knowledge gaps and future research directions on mechanisms, model fitting, and engineering practices are presented. Overall, this work highlights the feasibility of pressure-centric regulated AD and provides novel insights to overcome existing technical barriers in its application.

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.

Review in anaerobic digestion of food waste

Due to the special property of food waste (FW), anaerobic digestion of food waste is facing many challenges like foaming, acidification, ammonia nitrogen and (NH4+-N) inhibition which resulted in a low biogas yield. A better understanding on the problems exiting in the FW anaerobic digestion would enhance the bio-energy recovery and increase the stable operation. Meanwhile, to overcome the bottle necks, pretreatment, co-digestion and additives is proposed as well as the solutions to improve biogas yield in FW digestion system. At last, future research directions regarding FW anaerobic digestion were proposed.

Tackling municipal solid waste crisis in India: Insights into cutting-edge technologies and risk assessment

Municipal Solid Waste (MSW) management is a pressing global concern, with increasing interest in Waste-to-Energy Technologies (WTE-T) to divert waste from landfills. However, WTE-T adoption is hindered by financial uncertainties. The economic benefits of MSW treatment and energy generation must be balanced against environmental impact. Integrating cutting-edge technologies like Artificial Intelligence (AI) can enhance MSW management strategies and facilitate WTE-T adoption. This review paper explores waste classification, generation, and disposal methods, emphasizing public awareness to reduce waste. It discusses AI's role in waste management, including route optimization, waste composition forecasting, and process parameter optimization for energy generation. Various energy production techniques from MSW, such as high-solids anaerobic digestion, torrefaction, plasma pyrolysis, incineration, gasification, biodegradation, and hydrothermal carbonization, are examined for their advantages and challenges. The paper emphasizes risk assessment in MSW management, covering chemical, mechanical, biological, and health-related risks, aiming to identify and mitigate potential adverse effects. Electronic waste (E-waste) impact on human health and the environment is thoroughly discussed, highlighting the release of hazardous substances and their contribution to air, soil, and water pollution. The paper advocates for circular economy (CE) principles and waste-to-energy solutions to achieve sustainable waste management. It also addresses complexities and constraints faced by developing nations and proposes strategies to overcome them. In conclusion, this comprehensive review underscores the importance of risk assessment, the potential of AI and waste-to-energy solutions, and the need for sustainable waste management to safeguard public health and the environment.

Comprehensive review on recent production trends and applications of biochar for greener environment

The suitability of biochar as a supplement for environmental restoration varies significantly based on the type of feedstocks used and the parameters of the pyrolysis process. This study comprehensively examines several aspects of biochar's potential benefits, its capacity to enhance crop yields, improve nutrient availability, support the co-composting, water restoration and enhance overall usage efficiency. The supporting mechanistic evidence for these claims is also evaluated. Additionally, the analysis identifies various gaps in research and proposes potential directions for further exploration to enhance the understanding of biochar application. As a mutually advantageous approach, the integration of biochar into agricultural contexts not only contributes to environmental restoration but also advances ecological sustainability. The in-depth review underscores the diverse suitability of biochar as a supplement for environmental restoration, contingent upon the specific feedstock sources and pyrolysis conditions used. However, concerns have been raised regarding potential impacts on human health within agricultural sectors.

Emerging applications of biochar: A review on techno-environmental-economic aspects

Biomass fast pyrolysis produces bio-oil and biochar achieving circular economy. This review explored the emerging applications of biochar. Biochar possesses the unique properties for removing emerging contaminants and for mine remediation, owing to its negative charge surface, high specific surface area, large pore size distribution and surface functional groups. Additionally, biochar could adsorb impurities such as CO2, moisture, and H2S to upgrade the biogas. Customizing pyrolysis treatments, optimizing the feedstock and pyrolysis operating conditions enhance biochar production and improve its surface properties for the emerging applications. Life cycle assessment and techno-economic assessment indicated the benefits of replacing conventional activated carbon with biochar.

A strategic review on sustainable approaches in municipal solid waste management andenergy recovery: Role of artificial intelligence,economic stability andlife cycle assessment

The consumption of energy levels has increased in association with economic growth and concurrently increased the energy demand from renewable sources. The need under Sustainable Development Goals (SDG) intends to explore various technological advancements for the utilization of waste to energy. Municipal Solid Waste (MSW) has been reported as constructive feedstock to produce biofuels, biofuel carriers and biochemicals using energy-efficient technologies in risk freeways. The present review contemplates risk assessment and challenges in sorting and transportation of MSW and different aspects of conversion of MSW into energy are critically analysed. The circular bioeconomy of energy production strategies and management of waste are also analysed. The current scenario on MSW and its impacts on the environment are elucidated in conjunction with various policies and amendments equipped for the competent management of MSW in order to fabricate a sustained environment.

Granular activated carbon stimulates biogas production in pilot-scale anaerobic digester treating agro-industrial wastewater

This work examines the continuous addition (5 g/L) of conductive granular activated carbon (GAC) in an integrated pilot-scale unit containing an anaerobic digester (180 L) and an aerobic submerged membrane bioreactor (1600 L) connected in series for the treatment of agro-industrial wastewater. Biogas production increased by 32 % after the addition of GAC. Methanosaeta was the dominant methanogen in the digester, and its relative abundance increased after the addition of GAC. The final effluent after post-treatment with the aerobic membrane bioreactor had a total solids content <0.01 g/L and a chemical oxygen demand between 120 and 150 mg/L. A simple cost analysis showed that GAC addition is potentially profitable, but alternatives ways of retaining the GAC in the system need to be found. Overall, this study provides useful scientific data for the possible application of GAC in full-scale biogas projects.

Converting food waste into soil amendments for improving soil sustainability and crop productivity: A review

One-third of the annual food produced globally is wasted and much of the food waste (FW) is unutilized; however, FW can be valorized into value-added industrial products such as biofuel, chemicals, and biomaterials. Converting FW into soil amendments such as compost, vermicompost, anaerobic digestate, biofertilizer, biochar, and engineered biochar is one of the best nutrient recovery and FW reuse approaches. The soil application of FW-based amendments can improve soil fertility, increase crop production, and reduce contaminants by altering soil's chemical, physical, microbial, and faunal properties. However, the efficiency of the amendment for improving ecosystem sustainability depends on the type of FW, conversion method, application rate, soil type, and crop type. Engineered biochar/biochar composite materials produced using FW have been identified as promising amendments for soil remediation, reducing commercial fertilizer usage, and increasing soil nutrient use efficiency. The development of quality standards and implementation of policies and regulations at all stages of the food supply chain are necessary to manage (reduce and re-use) FW.

Techno-economic analysis of single-stage and temperature-phase anaerobic co-digestion of sewage sludge, wine vinasse, and poultry manure

Anaerobic co-digestion (AcoD) is a mature and consolidated waste management technology that can transform agro-industrial by-products into biogas and digestate. This study conducted a techno-economic assessment of bioenergy and agricultural fertilizer production from AcoD of sewage sludge, wine vinasse, and poultry manure. In this case study, three configurations were investigated: i) Scenario 1, AcoD in thermophilic temperature; ii) Scenario 2, AcoD in mesophilic temperature; and iii) Scenario 3, AcoD in a temperature phase (TPAD) system, where the digestate produced in the first reactor (thermophilic) feeds the second reactor (mesophilic). The process was designed to manage 24,022 m³ wine vinasse y^-1, 24,022 m³ sewage sludge y^-1, and 480 m³ poultry manure y^-1. The major cost was the fixed capital investment for the single-stage (320,981 USD) and TPAD processes (379,698 USD). The TPAD process produced the highest electricity (1058.99 MWh y^-1) and heat (4765.47 GJ y^-1) with the lowest cost of manufacturing for electricity (84.99 USD MWh^-1), heat (0.019 USD MJ^-1), and fertilizer (30.91 USD t^-1). Regarding the profitability indicators, the highest net present value (509,011 USD) and the lowest payback time (4.24 y) were achieved for Scenario 3. In conclusion, TPAD is a profitable and sustainable waste-to-energy management technology that can be applied in a circular economy framework to recover bioenergy and fertilizer, contributing to decreasing the carbon footprint of the agri-food sector.

Production of biochar from crop residues and its application for anaerobic digestion

Anaerobic digestion (AD) is a viable and cost-effective method for converting organic waste into usable renewable energy. The efficiency of organic waste digestion, nonetheless, is limited due to inhibition and instability. Accordingly, biochar is an effective method for improving the efficiency of AD by adsorbing inhibitors, promoting biogas generation and methane concentration, maintaining process stability, colonizing microorganisms selectively, and mitigating the inhibition of volatile fatty acids and ammonia. This paper reviews the features of crop waste-derived biochar and its application in AD systems. Four critical roles of biochar in AD systems were identified: maintaining pH stability, promoting hydrolysis, enhancing the direct interspecies electron transfer pathway, and supporting microbial development. This work also highlights that the interaction between biochar dose, amount of organic component in the substrate, and inoculum-to-substrate ratio should be the focus of future research before deploying commercial applications.

Ammonia recovery from anaerobic digestate: State of the art, challenges and prospects

Nitrogen-containing wastewater and organic wastes are inevitably produced during human activities. To reduce nitrogen pollution, much energy has been used to convert ammonia nitrogen into nitrogen gas through biological nitrogen removal method. However, it needs to consume high energy again during industrial nitrogen fixation, which give rise to massive greenhouse gas (GHG) emissions. Therefore, ammonia recovery from organic wastes has attracted much attention in recent years. In this review, the advantages and disadvantages of ammonia stripping, membrane separation and struvite precipitation are discussed firstly. The ammonia stripping mechanisms, influencing factors, mass transfer process, and the latest innovative ammonia stripping techniques from the anaerobic digestate of organic wastes are critically reviewed. Additionally, a comprehensive economic analysis of different ammonia removal or recovery processes is carried out. The challenges and prospects of ammonia recovery are suggested. Ammonia recovery is of great significance for promoting nitrogen cycle, energy saving and GHG emission reduction.

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.

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.

Valorization of food waste as adsorbents for toxic dye removal from contaminated waters: A review

Industrial contaminants such as dyes and intermediates are released into water bodies, making the water unfit for human use. At the same time large amounts of food wastes accumulate near the work places, residential complexes etc. polluting the air due to putrefaction. The need of the hour lies in finding innovative solutions for dye removal from wastewater streams. In this context, the article emphasizes adoption or conversion of food waste materials, an ecological nuisance, as adsorbents for the removal of dyes from wastewaters. Adsorption, being a well-established technique, the review critically examines the specific potential of food waste constituents as dye adsorbents. The efficacy of food waste-based adsorbents is examined, besides addressing the possible adsorption mechanisms and the factors affecting phenomenon such as pH, temperature, contact time, adsorbent dosage, particle size, and ionic strength. Integration of information and communication technology approaches with adsorption isotherms and kinetic models are emphasized to bring out their role in improving overall modeling performance. Additionally, the reusability of adsorbents has been highlighted for effective substrate utilization. The review makes an attempt to stress the valorization of food waste materials to remove dyes from contaminated waters thereby ensuring long-term sustainability.

Biological biogas purification: Recent developments, challenges and future prospects

Raw biogas generated in the anaerobic digestion (AD) process contains several undesired constituents such as H₂S, CO₂, NH₃, siloxanes and VOCs. These gases affect the direct application of biogas, and are a prime concern in biogas utilization processes. Conventional physico-chemical biogas purification methods are energy-intensive and expensive. To promote sustainable development and environmental friendly technologies, biological biogas purification technologies can be applied. This review describes biological technologies for both upstream and downstream processing in terms of pollutant removal mechanisms and efficiency, bioreactor configurations and different operating conditions. Limitations of the biological approaches and their future scope are also highlighted. A conceptual framework Driver-Pressure-Stress-Impact-Response (DPSIR) and Strengths-Weaknesses-Opportunities-Threats (SWOT) analysis have been applied to analyse the present situation and future scope of biological biogas clean-up technologies.

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.

Bioengineered biochar as smart candidate for resource recovery toward circular bio-economy: a review

Biochar's ability to mediate and facilitate microbial contamination degradation, as well as its carbon-sequestration potential, has sparked interest in recent years. The scope, possible advantages (economic and environmental), and future views are all evaluated in this review. We go over the many designed processes that are taking place and show why it is critical to look into biochar production for resource recovery and the role of bioengineered biochar in waste recycling. We concentrate on current breakthroughs in the fields of engineered biochar application techniques to systematically and sustainable technology. As a result, this paper describes the use of biomass for biochar production using various methods, as well as its use as an effective inclusion material to increase performance. The impact of biochar amendments on microbial colonisation, direct interspecies electron transfer, organic load minimization, and buffering maintenance is explored in detail. The majority of organic and inorganic (heavy metals) contaminants in the environment today are caused by human activities, such as mining and the use of chemical fertilizers and pesticides, which can be treated sustainably by using engineered biochar to promote the establishment of a sustainable engineered process by inducing the circular bioeconomy.

Sustainable management and recycling of food waste anaerobic digestate: A review

Anaerobic digestion (AD) is a widely used technology to valorise food waste for biogas production yet a considerable amount of digestate remains under-utilised. Sustainable management and recycling of the nutrient-rich food waste anaerobic digestate (FWD) is highly desirable for closing resource loop and actualising circular economy. This work reviews the distinct properties of FWD and the existing treatment technologies. FWD shows great prospects as a nutrient source for microalgal cultivation and biofuel production. Emerging technologies such as thermal conversion (e.g., pyrolysis and hydrothermal treatment) of FWD into value-added products such as functionalised biochar/hydrochar with diverse applications would be attractive and warrant further research investigation. Integrated AD with subsequent valorisation facilities is highly encouraged to achieve complete utilisation of resources and reduce carbon emissions.

Mitigation of acidogenic product inhibition and elevated mass transfer by biochar during anaerobic digestion of food waste

Anaerobic digestion (AD) of food waste is widely accepted as a promising technology for both waste disposal and resource recovery. With the advancing of AD technology, to exploit the capacity of organic waste for maximum energy/resource recovery becomes the new focus and hence, improve the viability of this technology for practical application. Product inhibition and mass transfer are the common limitations encountered during AD of putrescible organic waste. Biochar materials have been widely used to promote AD process in recent years. This review summarizes the mechanism and regulation strategies of biochar and its modified derivatives in promoting AD of solid waste (mainly food waste) from the three aspects of hydrolysis, syntrophic acetogenesis, and methane production. At the same time, the relationship between carbon materials and electron transfer among anaerobic microbes is summarized from the perspective of microbial community. In addition, the market application of this technology was evaluated.

Anaerobic digestion beyond biogas

Anaerobic digestion (AD) is a matured technology for waste (water) remediation/stabilization and bioenergy generation in the form of biogas. AD technology has several inherent benefits ranging from generating renewable energy, remediating waste (water), and reducing greenhouse gas emission to improving health/hygiene and the overall socio-economic status of rural communities in developing nations. In recent years, there has been a paradigm shift in applications of AD technology beyond biogas. This special issue (SI) entitled, "Anaerobic Digestion Beyond Biogas (ADBB-2021)," was conceptualized to incorporate some of the recent advances in AD in which the emphasis is beyond biogas, such as anaerobic biorefinery, chain elongation, treatment of micropollutants, toxicity and system stability, digestate as biofertilizer, bio-electrochemical systems, innovative bioreactors, carbon sequestration, biogas upgrading, microbiomes, waste (water) remediation, residues/waste pre-treatment, promoter addition, and modeling, process control, and automation, among others. This VSI: ADBB-2021 contains 53 manuscripts (14 critical reviews and 39 research). The key findings of each manuscript are briefly summarized here, which can serve as a valuable resource for AD researchers to learn of major advances in AD technology and identify future research directions.