Municipal Solid Waste for Energy Production and Resource Recovery

Rapid industrialization and urbanization promoting urban waste generation and pose a challenge to energy security [1]. Energy generation and the production of high-value products from solid waste are the most promising ways to address both challenging issues simultaneously. Reduction in the dependency on fissile fuel to meet future energy demand, alternative green technology for sustainable energy production and reduction in greenhouse gases emissions have nowadays become urgent global priorities [2,3]. Geopolitical issues such as war are increasing the price of energy and posing a risk to the global supply, which ultimately triggers the need for a decentralized renewable energy solution. In this context, waste-to-energy is the best alternative to improve the environmental pollution scenario as well as fulfill future energy demands.

Nowadays, due to increasing challenges associated with solid waste generation and future energy demand, a significant number of researchers have been attracted to propose new solutions. A considerable number of researchers have succeeded in developing suitable technologies to produce energy such as electricity, heat, or biofuels from municipal solid waste. Depending on the type of waste material, various technologies have been developed to address solid waste management challenges while simultaneously recovering energy from the waste stream.

Incineration is one of the most adopted thermal technologies for the conversion of waste to energy. In incineration, the waste is burned in a controlled combustion reactor that produces heat. The heat is used to make a high-pressure stream for driving a turbine as well as generating electricity. Gasification and pyrolysis are other thermal technologies that produce synthetic gas or bio-oils from waste under conditions of little or no oxygen. There are several limitations to the thermal treatment of municipal solid waste; high capital cost and air pollution caused by the production of hazardous particulates and dioxins are the major ones [4]. Expensive emission control technologies such as air filters and scrubbers need to be applied to control such air pollution. The residue ash after incineration also be toxic. To increase energy recovery and reduce air pollution, it is necessary to sort the waste properly, which increases labor and logistical challenges.

Anaerobic digestion is one of the most widely applied technologies to convert the organic parts of municipal solid waste to energy in the form of biogas and digestate. This biogas can be used to produce heat and/or for combined heat and power generation. The residual digestate from the biogas plant is a nutrient-rich organic material that can be used as fertilizer [5]. Controlled anaerobic digestion can produce nutrient-rich digestate that enhances soil fertility and can be further transformed into compost. Slow microbial growth and the sensitivity of organisms to ammonium and other toxic compounds are the major limitations of anaerobic digestion. Volatile fatty acids (VFAs), one of the intermediate products of anaerobic digestion, also have an inhibition effect [6]. Maintaining optimal operational parameters, mainly temperature, pH, organic loading rate, and hydraulic retention time, is a major challenge in achieving maximum biogas yield.

Besides thermal and biological treatment, modern concepts such as resource recovery and the circular economy have been applied through the extraction of valuable and recyclable materials, aiming to establish a zero-waste future. The heterogeneous composition of municipal solid waste and contamination are the main barriers to the implementation of the circular economy [7]. There are a number of waste materials that undergo progressive quality degradation during recycling, while some reach end-of-life or non-recyclable conditions. There are also debates on resource recovery and the circular economy, as they have a potential trade-off with energy production.

Considering all challenges related to municipal solid waste management, this Special Issue titled “Municipal Solid Waste for Energy Production and Resource Recovery” is a platform for all researchers to share their knowledge, scientific findings, technologies and innovative solutions. The topic covers advanced technologies addressing different types of waste with the most suitable alternatives to either convert them to energy or recover resources. It also promotes integrated waste management, nutrient recovery, process optimization, environmental impact assessment, techno-economic analysis and circular economy goals.

The application of waste inorganic cementitious material like lead–zinc-tailing-based geopolymer-stabilized aggregate (LZT-GSA) [8] or low-quality coal gangue [9] in construction dramatically improves structural compactness. A full-scale hybrid membrane bioreactor–nanofiltration (MBR–NF) system was applied to treat high-strength municipal landfill leachate and removed 93.5% COD and 98.6% NH₄⁺-N [10]. Some investigations have even highlighted the potentiality for the direct use of waste, such as marble waste, as a sustainable source of carbonate (calcium niobate–magnesium niobate composite) that can be directly used for low-thermal-conductivity applications [11].

Rapid technological advancement and increasing consumer demand have led to unprecedented e-waste generation, with large volumes of spent batteries creating significant concerns regarding improper disposal and resource loss. Innovative approaches such as bioleaching, mechanochemistry, and direct recycling of batteries aim to improve environmental sustainability. An article by Srivastava et al. (in this issue) [12] discusses the composition and classification of e-waste, highlighting the presence of valuable metals such as gold, silver, copper, and rare earth elements alongside hazardous substances including lead, mercury, cadmium, and persistent organic pollutants. Advanced treatment technologies, including mechanical processing, hydrometallurgy, pyrometallurgy, bioleaching, pyrolysis, gasification, and thermal plasma technologies, are needed for the recovery of precious metals. However, traditional recycling methods have a number of limitations, especially high energy consumption, greenhouse gas emissions, and hazardous chemical waste. Innovative approaches such as bioleaching, mechanochemistry, and direct recycling are more environmentally friendly and have low energy consumption but still face challenges in achieving certain levels of efficiency, processing time, scalability, and commercialization [13,14].

In this Special Issue, Prebilic et al. [15] compared thermochemical technologies, including incineration, pyrolysis, and gasification for electricity and heat production, while anaerobic digestion was assessed for biogas generation from organic waste and manure. Results indicate that gasification coupled with a gas turbine provides the highest exergy and electricity output, whereas incineration with heat recovery maximizes useful heat production. A study in this Issue [16] also concluded that solvent-based debromination of waste electrical and electronic equipment followed by pyrolysis is an effective and sustainable strategy for recovering valuable resources such as styrene and α-methylstyrene while minimizing hazardous emissions.

Ten agricultural and industrial waste materials, including poultry litter, pig manure, corn cob, grape pomace, sewage sludge, black liquor, soybean oil cake, municipal solid waste organic fraction, and brewery spent grains, were analyzed using proximate and ultimate composition analyses by Balac et al. (in this issue) [17]. The authors introduced an improved calculation of the Theoretical Biochemical Methane Potential (TBPM) by incorporating volatile matter content, providing more realistic predictions of methane yield than conventional approaches. Results showed that grape pomace exhibited the highest methane production potential due to its favorable carbon, hydrogen, and volatile content, while black liquor produced biogas with the highest energy quality.

Electricity generation is also possible with simultaneously degrading waste in microbial fuel cells (MFCs), another alternative for waste-to-energy. A study with MFCs [18] demonstrated that Trichoderma sp. effectively generated stable electrical output while degrading plastic waste.

The conversion of municipal solid waste to either energy or resources has a number of environmental, economic, and social benefits, such as reducing efforts in waste management, saving landfill space, minimizing greenhouse gas emissions, and conserving natural resources through material recovery. A large number of countries have already applied a number of technologies for energy and/or material recovery from waste and made a significant contribution to the reduction in greenhouse gas emission, reduction in waste generation, and the circular economy.

The articles published in this Issue demonstrate a wide range of innovative approaches to convert municipal solid waste to valuable products, including energy, fuels, chemicals, and recyclable materials. They also offer a comprehensive overview of current progress and future opportunities in this field.