Biowaste to Energy and Value-Added Products—Challenges and Opportunities

1. Introduction

In the face of accelerating climate change, increasing urbanisation and the progressive depletion of non-renewable energy and raw material resources, there is growing pressure worldwide to develop innovative, low-emission technologies for the recovery and processing of biowaste [1]. Modern economies, faced with the need to reduce greenhouse gas emissions and diversify energy sources, require solutions that enable the effective management of waste streams that, until recently, were seen only as a burden on the environment [2].

Organic waste, including kitchen waste [3], agricultural residues [4], food industry waste [5] and sewage sludge [6], is increasingly recognised as a valuable resource in production and consumption systems’ transition to a circular economy. As a heterogeneous and widespread fraction of municipal and industrial waste, biowaste has considerable potential for the production of renewable energy, particularly in the form of biogas [7], bioethanol [8], oils [9] and biological hydrogen [10], as well as for the production of high-quality bioproducts such as biopolymers [11], organic acids [12], organo-mineral fertilisers [13], pigments and bioactive extracts [14].

In this context, the importance of integrated biological (methane fermentation, lactic acid fermentation, enzymatic biosynthesis) [15], thermochemical (pyrolysis, gasification) [16] and chemical (transesterification, hydrolysis) [17] processes that enable the selective conversion of organic substrates into useful energy sources and chemical compounds is growing. However, the conversion of waste management systems to a circular model requires not only the introduction of modern technologies for the conversion of organic materials, but also their integration into local energy, agricultural and industrial systems [18]. The decisive factor here is not only the efficiency and cost-effectiveness of the individual technologies, but also their adaptation to the seasonal, qualitative and quantitative fluctuations of the substrate, as well as the ability to manage by-products within local value chains [19].

A particular challenge remains the development of decentralised and modular solutions that enable efficient processing of biowaste at source while adhering to the principles of low emissions, energy self-sufficiency and a minimal environmental footprint [20]. This approach is directly linked to the implementation of international climate strategies and key European Union policies, such as the European Green Deal, the “Fit for 55” legislative package, the Hydrogen Strategy and the revised RED III Renewable Energy Directive [21]. All of these policies emphasise increasing the share of renewable energy sources, improving energy efficiency, reducing emissions from the waste sector and developing technologies to capture and use organic carbon. Biowaste, considered as a component of regional and local biorefineries, can play an important role in achieving climate neutrality targets, improving resource and energy security and strengthening synergies between the agricultural, energy and municipal sectors.

In response to these challenges, Energies has launched this Special Issue entitled “Bio-Waste to Energy and Added Value Products—Challenges and Opportunities” to present the latest research on converting bio-waste into renewable energy and high-value bioproducts. The Special Issue was completed on 28 August 2024 and contains 15 peer-reviewed articles that provide a comprehensive overview of current research directions in the field of sustainable bio-waste valorisation. The scope of the issue is in line with global climate and energy policy priorities and the UN Sustainable Development Goals, particularly with regard to carbon neutrality, energy security and circular economy strategies.

2. Scope and Objectives

The overall objective of this Special Issue was to compile technical, scientific and environmental studies on the conversion of biowaste into usable energy sources, biochemicals and value-added products. The accepted articles cover both basic research and applied studies, including case studies, technological assessments, life cycle assessments and experimental research at laboratory, pilot and semi-industrial scale.

Thematically, the contributions can be categorised into several key areas as follows: anaerobic digestion and fermentation technologies, biorefineries and biochemical extraction, thermochemical conversion, recovery of high-value organic and inorganic compounds, reuse of by-products in agriculture and energy systems and the development of innovative bio-based materials. The Special Issue reflects the multidimensional nature of biowaste valorisation and provides insights into new technological and analytical approaches. The contributions are listed below:

• Szyba, M.; Mikulik, J. Analysis of Feasibility of Producing and Using Biogas in Large Cities, Based on the Example of Krakow and Its Surrounding Municipalities. Energies 2023, 16, 7588. https://doi.org/10.3390/EN16227588.
• Tarapata, J.; Zieliński, M.; Zulewska, J. Valorization of Dairy By-Products: Efficiency of Energy Production from Biogas Obtained in Anaerobic Digestion of Ultrafiltration Permeates. Energies 2022, 15, 6829. https://doi.org/10.3390/EN15186829.
• Zieliński, M.; Karczmarczyk, A.; Kisielewska, M.; Dębowski, M. Possibilities of Biogas Upgrading on a Bio-Waste Sorbent Derived from Anaerobic Sewage Sludge. Energies 2022, 15, 6461. https://doi.org/10.3390/EN15176461.
• Biedka, P. Biodegradation Kinetics of Organic Matter in Water from Sludge Dewatering after Autothermal Thermophilic Aerobic Digestion. Energies 2023, 16, 203. https://doi.org/10.3390/EN16010203.
• Qureshi, N.; Lin, X.; Tao, S.; Liu, S.; Huang, H.; Nichols, N.N. Can Xylose Be Fermented to Biofuel Butanol in Continuous Long-Term Reactors: If Not, What Options Are There? Energies 2023, 16, 4945. https://doi.org/10.3390/EN16134945.
• Agoe, A. K.; Poulopoulos, S.G.; Sarbassov, Y.; Shah, D. Investigation of Sewage Sludge–Derived Biochar for Enhanced Pollutant Adsorption: Effect of Particle Size and Alkali Treatment. Energies 2024, 17, 4554. https://doi.org/10.3390/EN17184554.
• Keith, K.; Castillo-Villar, K.K. Stochastic Programming Model Integrating Pyrolysis Byproducts in the Design of Bioenergy Supply Chains. Energies 2023, 16, 4070. https://doi.org/10.3390/EN16104070.
• Warmiński, K.; Jankowska, K.A.; Bęś, A.; Stolarski, M.J. Off-Gassing and Oxygen Depletion in Headspaces of Solid Biofuels Produced from Forest Residue Biomass. Energies 2024, 17, 216. https://doi.org/10.3390/EN17010216.
• Stolarski, M.J.; Krzyżaniak, M.; Olba-Zięty, E.; Stolarski, J. Changes in Commercial Dendromass Properties Depending on Type and Acquisition Time. Energies 2023, 16, 7973. https://doi.org/10.3390/EN16247973.
• Jóźwiak, T.; Filipkowska, U.; Walczak, P. The Use of Aminated Wheat Straw for Reactive Black 5 Dye Removal from Aqueous Solutions as a Potential Method of Biomass Valorization. Energies 2022, 15, 6257. https://doi.org/10.3390/EN15176257.
• Dechapanya, W.; Wongsuwan, K.; Lewis, J.H.; Khamwichit, A. Optimization and Modification of Bacterial Cellulose Membrane from Coconut Juice Residues and Its Application in Carbon Dioxide Removal for Biogas Separation. Energies 2024, 17, 4750. https://doi.org/10.3390/EN17184750.
• López, L.; Domínguez, G.; Antuñano, Z.; Ignacio, D.; García, G.; Zielí nski, M.; Kazimierowicz, J.; Eduardo Esquerre Verastegui, J.; López López, A.; Adrián González Domínguez, R.; et al. Production of Coconut Oil Bioturbosine without Water by Using Ultrasound as a Source of Energy and Ion Exchange for Its Purification. Energies 2024, 17, 614. https://doi.org/10.3390/EN17030614.
• Rodziewicz, J.; Mielcarek, A.; Janczukowicz, W.; Tavares, J.M.R.; Jóźwiakowski, K. Characteristics of Sludge from the Treatment of Soilless Plant Cultivation Wastewater in a Rotating Electrobiological Disc Contactor (REBDC). Energies 2023, 16, 1022. https://doi.org/10.3390/EN16031022.
• Renzi, M.; Valéria, M.; Machado, S.; Ávila, I.; Andrade De Carvalho, J. Bibliometric Analysis of Renewable Natural Gas (Biomethane) and Overview of Application in Brazil. Energies 2024, 17, 2920. https://doi.org/10.3390/EN17122920.
• Gusiatin, M.Z.; Kulikowska, D.; Bernat, K. Municipal Sewage Sludge as a Resource in the Circular Economy. Energies 2024, 17, 2474. https://doi.org/10.3390/EN17112474.

3. Anaerobic Digestion and Fermentation Processes

Several articles deal with anaerobic digestion (AD) as a fundamental technology for converting organic waste into biogas. The topics covered include the effects of operating parameters on process efficiency and the modelling of AD systems based on variable feedstocks [contribution 1]. One study assessed the role of feedstock quality and type on the technological performance and economic feasibility of biogas production [contribution 2]. The potential of upgrading raw biogas to biomethane using adsorbents from waste was investigated using adsorption-based gas separation processes [contribution 3]. In another study, the thermophilic aerobic digestion of sewage sludge was investigated, with a focus on process optimisation and energy balance [contribution 4]. One particularly noteworthy study scrutinised the technical feasibility of continuous butanol fermentation from xylose—an abundant pentose sugar in lignocellulosic biomass—highlighting the limitations of long-term operation and outlining possible future strategies [contribution 5].

4. Thermochemical Conversion of Bio-Waste

A second topic covered in this Special Issue focuses on advanced thermochemical processes such as pyrolysis, where research focusses on the physicochemical properties of the resulting biochar and its suitability for environmental applications. One study has shown that the alkaline modification of biochar derived from sewage sludge significantly increased its BET surface area (from 27.5 to 144.3 m²/g) and thus increased its sorption capacity for methyl blue and mercury ions, emphasising its potential to be used in environmental remediation [contribution 6]. In another paper, a stochastic optimisation model was presented that considers feedstock characteristics (moisture content and ash concentration) and bioethanol price volatility to assess the feasibility and profitability of pyrolysis-based biorefineries [contribution 7].

5. High-Value Bioproducts and Functional Bio-Based Materials

This category includes studies that investigate the production and application of innovative bio-based materials. For example, one paper looked at the safety risks associated with the outgassing of volatile organic compounds and the lack of oxygen during the storage of biomass pellets from forests, which has implications for product quality and workplace safety [contribution 8]. Another study investigated how storage time and feedstock type affect the physicochemical properties of commercial dendromass materials—a crucial factor for standardising pellet quality [contribution 9]. The potential of chemically aminated wheat straw as an effective and low-cost sorbent for the removal of dyes from industrial wastewater was confirmed, with a high adsorption capacity for the Reactive Black 5 dye [contribution 10]. In another study, bacterial cellulose membranes from coconut sap residues were developed and optimised for CO₂ removal from biogas, which represents a sustainable and cost-effective solution for gas separation [contribution 11]. In another paper, an anhydrous, ultrasound-assisted process for the production and purification of bioturbosin extracted from coconut oil was presented, using ion exchange as an environmentally friendly purification method [contribution 12]. In another paper, the properties and utilisation potential of sludge produced during wastewater treatment in soilless plant cultivation systems were analysed [contribution 13].

6. Ecological and Economic Assessments

Several papers presented ecological, techno-economic and bibliometric analyses of methods to generate energy from biowaste. A bibliometric study highlighted the growing interest and development of research in renewable natural gas (RNG) in Brazil, emphasising emerging trends and strategic technological directions [contribution 14]. Another paper evaluated the multifunctional use of municipal sewage sludge in the context of the circular economy, focusing on its use as a fertiliser, sorbent and feedstock for the production of biochar [contribution 15].

7. Conclusions and Outlook

This Special Issue demonstrates the enormous potential of biowaste as a renewable energy resource and as a raw material for valuable industrial products. The published contributions represent a significant step forward in supporting the transition to a circular bioeconomy that offers both environmental and economic benefits. The guest editors sincerely hope that the results presented here will stimulate further scientific research and practical implementation, promote the development of sustainable technologies and strengthen environmental security.