Introduction

Current MSW generation is more than 1.2 billion tons per year, that is increasing at exponential rates, and will reach more than 2 billion tons per year in 2025 (Ghosh et al. 2018). Thousands of individuals have relocated from the countryside to major cities worldwide in recent years, and currently, fifty percent of the world’s population resides in municipal cities (Marandi and Main 2021). Furthermore, according to a United Nations (UN) prediction, the total world's inhabitants will expand to 6.4 billion by 2040 (Helin and Weikard 2019). Numerous health and environmental issues are related to continuous population expansion. The increased rate of population growth is an important factor directly targeting the environmental problems in big cities as it affects people’s life, industrial development, business and exchange, the number of automobiles, energy and water consumption, and waste generation.

Over the past decades, the literature has reported serious threats to communities related to the adverse health effects associated with increased environmental pollution because of weak waste management systems (Jansson and Voog 1989; Lloyd et al. 1988). Every year, around 1.92 billion tons of mixed MSW is generated worldwide, equating to about 220 kg per person (Sharma and Dubey 2020). According to Zhang et al. (2017), China’s MSW production has reached 220 million metric tons, and this rate is steadily increasing at a rate of 10% each year. In the USA and India, MSW generation was recorded to be 258 million metric tons and 169 million metric tons, respectively, for 2020 (Arya and Kumar 2020). According to statistics, before the notion of recycling, the annual growth rate of MSW rose with the passage of time due to increasing population and urbanization (Knickmeyer 2020). In 2013, a survey was conducted in Sweden, which revealed that 4.5 million tons of MSW were generated in a single year (Hailu and Kumsa 2021). This MSW is made up of approximately 32% recycled material, 16% medical waste, and 52% energy-related materials (Lazzarino et al. 2021). Waste production is increasing both in Europe and globally with Poland also experiencing a notable rise in municipal waste generation per capita. In 2018, the amount of municipal waste generated per capita in Poland was among the lowest in the EU, totaling 329 kg per capita, as opposed to the EU average of 489 kg per capita (Bilska et al. 2020). Figure 1 illustrates the changes in municipal waste generation levels in Poland and the EU average between 2014 and 2019.

Fig. 1
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Comparison of MSW generation in Poland with the EU

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MSW attracts the attention of many academics and researchers due to its unique composition, which includes some form of energy. MSW contains a variety of hydrocarbons bound together by various bonds; when these bonds disintegrate into gases or liquids, they release tremendous amounts of energy (Naik et al. 2010). Liquid fuel obtained from the pyrolysis of MSW has a calorific value between 35 and 44 MJ/kg, having the potential to replace traditional fossil fuels like a diesel with a calorific value of 45.5 MJ/kg in internal combustion engines, power generation systems, and industries. As a result, MSW has a significant potential for producing biofuels (Velghe et al. 2011; Zaman et al. 2021). Biomass-based fuels have a significant potential to replace traditional fuels that’s why it has attracted the attention of many scientists and researchers (Khoo et al. 2020). Biofuels have the ability to cut CO2 emissions. Food waste, wood, leaves, crops, waste plastic, wastepaper, and municipal solid waste (MSW) have all been used to produce biofuels (Chum and Overend 2001). MSW is commonly associated with waste or litter, but it actually consists of everyday items that are used and then thrown away, such as packing materials, mowed grass, wood, broken furniture, ripped clothing, plastic bottles, food leftovers, newspapers, home appliances, paints, rubbers, and, last but not least, batteries. These are some of the most common materials found in every home, hospital, and school (Mumtaz et al. 2023). A fraction of MSW can serve as a source of biomass, and it mostly includes mixed commercial and residential waste materials such as paper and paperboard, yard trimmings, plastics, leather, rubber, textiles, and food wastes (Chen 2016). Plastic is a large component of MSW, as plastic is widely used around the world for its numerous attributes such as toughness, light weight, and cost-effectiveness. According to the World Bank, waste plastic is mainly responsible for the manufacture of MSW, which plays 10–12% part in total MSW generation (Nguyen et al. 2020). In 2012, the entire amount of plastic produced in the world was projected to be at 290 MT, with estimates that this number will rise to 25% by 2030 (Ghayebzadeh et al. 2020). Furthermore, it is expected that by 2025, waste plastic concentrations will increase to 10–13% (Camilleri 2021). Plastic will be consumed in various ways around the world. For example, in Europe, serious policies have been ensured to collect at least half of the plastic waste generated, with the remainder being disposed of in landfills (Silva et al. 2021). A substantial amount of MSW is generated in Finland, with PVC plastic being the most important contributor in MSW production, which is used in household appliances. PVC contains 90% chlorine, which is known to be the most hazardous to human health and the environment. In the Middle East, a similar trend has been observed; the Kingdom of Saudi Arabia (KSA) has been acknowledged as the world’s second largest producer of MSW, producing roughly 7 million metric tons of plastic each year (Alagha et al. 2018). China has used 82% flammable plastic and 18% non-combustible plastic in comparison with other Asian countries (Ghimire and Ariya 2020).

Adoption of proper waste management techniques is one of the key challenges for environmental policy making at the international level, including the European Union (EU), where the policy framework to save the environment and ensure public health against the effects of various health hazards associated with MSW has been constantly implemented in past (Mazzucco et al. 2020). But at the current moment, the MSW management strategies are integrated in nature and based on a hierarchical attitude: reduction, recycling, and reuse (Allwood 2014). According to studies, around 20% of MSW collected is recycled, 12% is used for energy recovery, and the remainder is disposed of in landfills (Gutierrez-Gomez et al. 2021). In this context, landfilling must be considered as the least preferable method as it demands wide space and comes with a high risk of soil and water contamination. The landfill dumping system has several challenges, including worker health, machinery maintenance, and transportation costs, and in fact, it provides no chance to use the energy content of MSW. Energy security in addition to continuous economic growth and subsequent environmental protection is the national energy policy goal of any country of the world. As the world population is growing at higher rates so the energy demand is increasing proportionally. Therefore, states in the EU are asked to promote proper waste management techniques based on the 3-R approach, i.e., reuse, recycle, and recover to reduce the total municipal solid waste by 10%, so any idea of converting MSW to valuable products is worth important in this context (Malinauskaite et al. 2017). The technologies adopted to convert the waste into energy by any country are largely determined by its energy demands and economic status (Afrane et al. 2021). According to the World Bank division, the world’s economy can be divided into four major categories including rich income, upper–middle income, lower–middle income, and, last but not least, the poor income. Rich-income countries, the majority of which are part of the Organization for Economic Cooperation and Development (OECD) are not limited to a single waste-to-energy technology and instead utilize a combination of them. Furthermore, in high-income economies, MSW management systems are stable enough to collect a considerable amount of waste to recycle and ensure material reuse (Boer et al. 2010). But in contrast, low- and middle-income economies do not have effective MSW management systems and have fewer chances of material recycling, a high portion of openly dump waste, all of which have negative environmental consequences. In Poland, the use of proper waste-to-energy conversion technology with the potential of resource recovery could aid in the management of MSW and the maintenance of an approximately steady energy deficit rate. Currently, the most common MSW disposal system in Poland is landfilling. In 2020, there were over 150 active landfills in the country. Incineration is another MSW disposal system used in Poland. There are several incineration plants in the country, with the largest one located in Poznan (Jakubus and Stejskal 2020). According to Eurostat data, in 2019, Poland incinerated around 3.3 million tons of municipal waste, which was an increase of 13% compared to the previous year. Mechanical–biological treatment (MBT) plants are becoming increasingly popular in Poland to treat MSW before disposal. These plants use mechanical and biological processes to sort, separate, and treat waste, with the goal of reducing the amount of waste that goes to landfills. According to a report from the European Environment Agency, there were 31 MBT plants in Poland in 2018, with a total capacity of around 2.9 million tons per year. Composting is another waste management system used in Poland. Composting facilities process organic waste (e.g., food waste and garden waste) into compost, which can be used as a soil conditioner. Recycling is also an important waste management system in Poland. The country has a well-developed recycling infrastructure, with separate collection systems for paper, plastic, glass, and metal. There are also recycling plants that process these materials into new products through mechanical and chemical recycling. Poland could benefit from a better understanding of chemical recycling techniques pyrolysis, gasification, plasma gasification, and hydrothermal processing, each with its own set of technological capabilities, optimal process parameters, prices, total energy potential, and ecological consequences. Discovering more about conversion techniques and their various aspects, promises, current state, and technical barriers could help in improving available waste management techniques in Poland, which will also support the concept of the circular economy. Numerous literature reviews on waste-to-energy conversion techniques in India and in developed countries, for example, Australia, Denmark, Japan, and the USA are already existing (Mukherjee et al. 2020). Nevertheless, there appears to be no recent research into waste-to-energy processing and the accompanying critical hurdles for successful implementation in Poland (the current study’s originality).

Household waste quality estimates in Poland

The waste management system in Poland is regulated by several laws and regulations, including the Waste Act, which establishes the framework for waste management activities in the country. The Polish waste management system is composed of a number of actors, including municipalities, waste management companies, and the Polish Environmental Protection Agency (Uliasz-Misiak et al. 2014). One important aspect of waste management in Poland is the database on products and packaging (BDO), which is a central register of producers and importers of packaged products. The BDO is managed by the Polish Minister of Climate and Environment and is intended to provide information about the types and amounts of packaging materials placed on the market in Poland. The BDO was established in accordance with the Packaging and Packaging Waste Act, which transposes the European Union's Packaging and Packaging Waste Directive into Polish law. The database contains information about producers and importers of packaged products, as well as information about the types and amounts of packaging materials used. The BDO is used to monitor compliance with legal requirements for packaging waste management in Poland, including the obligation for producers and importers to finance the collection and recovery of packaging waste (Haładyj 2020). It is also used to calculate the contributions that producers and importers must pay to the national packaging waste management fund, which is used to finance the collection and recovery of packaging waste. The most common ways used to measure the volume of waste in Poland are through the use of surveys, registration of waste transport, and data from waste management companies. Surveys are used to obtain data on the types and amounts of waste generated and disposed of, while registration of waste transport provides information on the amount of waste transported between different locations (Przydatek and Wota 2020). Data from waste management companies are used to track the amount of waste collected, treated, and recycled. Additionally vehicles transporting waste to landfilling areas or waste sortation and composting capabilities are mostly used to measure the volume of waste. Private waste collection businesses in Poland also frequently collect and transport municipal waste from large cities for eventual disposal at a variety of facilities. Because of a lack of proper waste management practices, the amount of waste that is collected and disposed of is intentionally undercounted by involved business entities. Thus, due to negligence or failure to meet reporting requirements, the amount of waste collected and disposed of is sometimes undercounted.

Comparison of predicted waste generation with actual amount of waste collected in Poland from 2001 to 2010 is shown in Fig. 2. This comparison is between the amounts provided by Central statistical office (Główny Urząd Statystyczny, GUS), and National Waste Management Plan predicts (KPGO). The amount of waste collected is significantly lower than predicted by the KPGO for years 2001–2010, and this can be attributable to the fact that:

  • Inefficient waste management systems: According to a report by the European Environment Agency, Poland has a relatively low level of waste collection efficiency compared to other EU countries. The report cites factors such as insufficient funding, inadequate infrastructure, and a lack of skilled personnel as reasons for this inefficiency.

  • Limited waste collection services in rural areas: Poland has a significant rural population, and waste collection services in these areas may be limited or non-existent. Waste collection programers are only available to a small percentage of the population (about 90%). This can lead to illegal dumping or burning of waste, which is not accounted for in official waste collection statistics.

  • Inaccurate waste generation estimates: Predicted values of waste generation may be based on incomplete or inaccurate data, which can lead to overestimations of the amount of waste that will be generated. This can result in waste management systems being designed with a higher capacity than is actually required.

  • Minimizing the landfilling tax: The reported amounts of waste being sent to landfills are being deliberately underestimated in order to minimize the landfill tax, which is currently set at 100 PLN per 1 Mg of waste.

Fig. 2
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Comparison of household garbage collected in Poland

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Figure 2 also presents the amount of waste collected in Poland till 2019, these values are provided by GUS, but unfortunately, the predicted values by KPGO could not be attained. But in terms of waste collection systems improvements, the percentage of the population covered by municipal waste collection has increased from 94.1% in 2010 to 98.2% in 2019, according to Eurostat. The percentage of households served by separate collection of biowaste has also increased from 9.9% in 2010 to 45.1% in 2019. Based on these statistics, it appears that waste collection systems in Poland have improved since 2010, with increased coverage of the population and increased recycling rates. However, it is important to note that waste management is a complex issue, and there may be other factors at play that affects the overall effectiveness of the waste collection system.

Waste composition analysis

Potential of any type of waste material to be converted into valuable products is highly dependent upon its composition. The total production of the waste and percentage existence of various materials in this waste has been provided for different cities in Poland. The Central Statistical Office of Poland (GUS) carries out regular surveys and data collection activities to gather information about various aspects of Polish society, including waste generation and management. Waste composition is influenced by various factors, including geography, population and demographics, socioeconomic status, consumption patterns, and industrial activity. These factors interact in complex ways to determine the types and amounts of waste generated in a given context. As a result, waste composition can vary widely depending on the location, population characteristics, and other local factors.

Figure 3 illustrates how the houses with three different types of surroundings S1, S2, and S3 have different effects on the composition of the waste (based on the average samples collected from each individual's environment during the course of four separate measurement series carried out between 2019 and 2020).

Fig. 3
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Poland average waste composition by environment, expressed as a mass percentage on a wet basis

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Surrounding I (S1) Single-family homes: These homes are typically heated using furnaces or boilers that burn natural gas, oil, or propane. The waste generated includes food waste, paper products, plastics, glass, and metals, as well as yard waste such as grass clippings and tree trimmings. In addition, these homes may generate hazardous waste such as used oil, batteries, and cleaning products.

Surrounding II (S2) Multi-unit housing: These types of housing, such as apartments and condominiums, often have central heating systems that use natural gas, oil, or electricity. The waste generated may include more paper and plastic waste from packaging and mail, as well as electronic waste from outdated devices.

Surrounding III (S3) Rural homes: Homes located in rural areas may use a variety of heating systems, including wood stoves, pellet stoves, or electric heaters. In addition to the waste generated by single-family homes, rural homes may generate more agricultural waste such as manure and crop debris. They may also generate more hazardous waste such as pesticides and chemicals due to the use of these substances in farming and other rural activities.

The composition of municipal waste generated by different types of housing in Poland (data provided by GUS) varies based on a variety of factors, including lifestyle, location, and household size. Single-family homes in Poland generate the most municipal waste per person, with an average of 344 kg per year, or about 0.94 kg per day. The largest proportion of waste generated by single-family homes is organic waste, which includes food waste and garden waste, accounting for 54% of the total waste generated. Paper and cardboard account for 12%, plastics account for 11%, and glass accounts for 7%. Metals, textiles, and other types of waste make up the remaining 17%. Multi-unit residential buildings in Poland generate less waste per person than single-family homes, with an average of 218 kg per year, or about 0.60 kg per day. The largest proportion of waste generated by multi-unit residential buildings is also organic waste, accounting for 41% of the total waste generated. Paper and cardboard account for 22%, plastics account for 14%, and glass accounts for 5%. Metals, textiles, and other types of waste make up the remaining 18%. Rural households in Poland generate an average of 296 kg of municipal waste per person per year, or about 0.81 kg per day. The largest proportion of waste generated by rural households is also organic waste, accounting for 62% of the total waste generated. Paper and cardboard account for 9%, plastics account for 6%, and glass accounts for 5%. Metals, textiles, and other types of waste make up the remaining 18%. It’s important to note that the type of heating system used can also impact the amount and type of waste generated. For example, households that use wood or coal for heating may generate more ash and other types of waste related to heating than households that use natural gas or electricity.

The evaluation of the qualities of the waste from Poland municipal operations includes, among other factors, the following parameters, which characterize the waste viability as a fuel source:

  • Grain fraction and selected component moisture content in percent.

  • To determine the total amount of organic matter [% dry mass], we must look at all fractions and individual components.

  • Specific components heat value (in MJ/kg in dry conditions)

Detailed waste composition analysis for different cities of Poland has been presented in the coming sections, but unfortunately, these numbers are only limited to 2005 as authentic sources could not be found in the literature that provide the most recent detailed data for different cities. Figure 4a represents the total moisture content, and Fig. 4b represents the percentage organic content on a wet basis of Wroclaw's municipal garbage composition from 1992 to 2005. For all four seasons, spring, summer, autumn, and winter, the size of waste samples has been divided into three fractions, 10, 10–40, and 40–80 mm.

Fig. 4
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a Moisture content and b organic content (as a percentage of dry mass) in Wroclaw's household waste (Boer et al. 2010)

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Moisture and organic matter concentration in the fine fraction (less than 10 mm) fluctuates the most during the year. However, the lowest levels of moisture and organic materials can be found in this portion. The 10–40-mm fraction, which contains the most biodegradable materials, has the highest moisture content. The 40–80-mm fraction had the largest concentration of organic material. Combustible materials make up the majority of this portion (paper and plastics).

Sogreah France used MODECOM technique to identify the composition of mixed household garbage in Poznan in May 2001. Eight waste samples were analyzed, two each from four different municipal settings. The 5th EU Framework Program’s Solid Waste Analysis (SWA) Tool experimented in Krakow, Poland, in 2003 with the composition and quantity of household waste (Tables 1, 2, and 3 Solid Waste Analysis Data). By using SWA tool, the detailed composition of household waste is presented in Table 3. These data were used to compute the amount of packaging and non-packaging waste generated by each home (refer to Table 1) based on the results.

Table 1 Composition of waste generated in Poznan based on a Sogreah survey's findings (Boer et al. 2010)
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Table 2 Krakow's residential make-up in its entirety (Boer et al. 2010)
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Table 3 Household municipal garbage in Wrocław and Krakow consists of packaging and non-packaging waste items (Boer et al. 2010)
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From the data presented in all the figures and tables, it’s obvious that MSW waste in Poland has the major concentration of organic compounds, more or less same for other European countries, and that can be converted into carbon-rich chemical products through suitable waste-to-energy conversion technique that also provides the chance of waste-to-resource recovery.

Waste-to-energy conversion techniques and role of oxi-liquefaction/wet oxidation

Biodegradation, anaerobic digestion, pyrolysis, gasification, and plasma gasification are some of the processes that can be used to transform municipal solid waste into energy directly or in a more indirect manner. Table 4 compares various traditional and nontraditional techniques to convert waste into valuable products in terms of plant life, waste handling capabilities, cost-effectiveness, energy production potential, and society readiness level.

Table 4 Assessment of traditional and nontraditional waste-to-energy conversion techniques (Munir et al. 2021)
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It is important to note that composting and putting waste in landfills are usually thought of as ways to get rid of it, not as ways to turn waste into useful products, but in composting, it is possible to get some energy as the heat given off when carbohydrates are broken down. Still, compared to other ways to turn waste into energy, the amount of energy made is not that great.

However, it is clear from more recent research such as that conducted by (Munir et al. 2021) that modern techniques, including pyrolysis, hydrothermal treatment, and gasification, are much more preferable to conventional waste-to-energy conversion techniques. However, due to numerous constraints in the areas of technology, economy, environment, and society, there is currently no technique reported that can be considered universally applicable for converting various waste types in MSW regime to value-added products without any separation and another pretreatment processes. For this reason, it is often recommended to use different combinations of waste-to-energy techniques, both conventional and non-conventional, to achieve the optimum outcomes that are economically and technologically favorable. To provide an effective solution to this problem and introduce the single technique that can treat various types of waste materials in MSW, in the current study, various aspects of oxi-liquefaction/wet oxidation technique including reaction conditions, stages, types, and involved chemistry have been reviewed; in addition, few works have also been reported in which the current technique has been already used to treat the various waste materials that are also the major part of MSW that justifies the potential of this method to treat complex MSW in context of energy production and waste-to-resource recovery. It is important to note that this technique will be applicable to handle the organic fraction of waste because only the organic compounds have the essential carbon available to be converted into the required chemical compounds.

As shown in Table 4, wet oxidation technique outperforms other available approaches in terms of waste handling, total income, and competing with conventional and other thermochemical processes in technological skills, net energy output, and the repercussions on the environment. Thermochemical procedures are superior to biological or landfilling methods regarding plant service life and technological capabilities. Net or total income generated by oxi-liquefaction processes is higher than by biological or landfilling processes and comparable to other thermochemical processes. In comparison with thermochemical and oxi-liquefaction processes, biological and landfilling technologies recover less value, but they are less expensive, deep-rooted, and their technological and social readiness levels are higher.

Wet oxidation process

The wet oxidation (WO) process is carried out comparatively at higher pressures ranging from 5 to 20 MPa and at critical water temperature in the range of 150–350 °C to oxidize the organic and inorganic materials in the aqueous phase by pure oxygen or any chemical oxidant to mainly produces the volatile acids. Strong hydrolysis reactions and solubility of oxygen in an aqueous solution at higher temperatures support the better oxidation process (Yang et al. 2022). MSW can be decomposed into oxidative products having hydroxyl, carbonyl, and carboxyl functional groups mainly acetic acid and some other organic acids through a controlled oxidation process, which are the product of key importance in the studies, while the complete oxidation will result in carbon dioxide (CO2), water (H2O), and ash production. This process is also known as “wet oxidation,” and its name comes from the use of water.

$${\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}} \left(\mathrm{MSW}\right)+ {\mathrm{O}}_{2}+{\mathrm{N}}_{2}+\mathrm{Heat}+{\mathrm{H}}_{2}\mathrm{O}\to \mathrm{Aldehydes}$$
$$\mathrm{Aldehydes }\to \mathrm{Alcohols}+\mathrm{Ketones}+\mathrm{Carboxylic acids}$$
$$\mathrm{Alcohols }\to \mathrm{Ketones }\to \mathrm{Carboxy acids}$$
$$\mathrm{Alcohols }\to {\mathrm{N}}_{2}+ {\mathrm{CO}}_{2}+ {\mathrm{H}}_{2}\mathrm{O}+\mathrm{Ash}$$
$$\mathrm{Carboxylic acids }\to {\mathrm{N}}_{2}+{\mathrm{CO}}_{2}+ {\mathrm{H}}_{2}\mathrm{O}+\mathrm{Ash}$$

The process performance is dependent upon a number of process variables, for example, reaction temperature, reaction time, oxygen pressure, solid–liquid ratio, oxidizer content, and mixing speed for stirred processes (Demesa et al. 2015). In wet oxidation process, degradation of MSW happens at critical temperature and pressure conditions because water undergoes a considerable change in its properties both physical and chemical, e.g., its dielectric constant, viscosity, and surface tension decrease while diffusion rate increases that make the water a good solvent for extraction and solubilization processes (Zhang et al. 2017).

In comparison with anaerobic digestion and landfilling, this technique can greatly reduce the amount of MSW in shorter time span and can treat wet and mixed waste without pre-drying (a costly step) and separation process. Cost estimation process of various waste-to-energy conversion techniques is provided in Table 5, where capital cost is the representation of total initial expenditures calculated in the US dollars per ton of MSW, while the operational cost is daily ongoing cost such as labor cost and maintenance cost.

Table 5 Cost comparison of waste-to-energy conversion techniques for MSW (Kumar et al. 2017)
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In some situations, the wet oxidation only relies on pure oxygen for the oxidation process, but depending upon the nature of products required and stability of waste materials, some catalysts are also added to achieve the required degree of oxidation comparatively at lower temperature and pressure (Bhargava et al. 2006). There are two options to lower the reaction pressure and temperature. One is to supply the oxygen with an appropriate catalyst, and the other is to add an oxidant instead of directly providing oxygen. One example of such an oxidant is hydrogen peroxide, which degrades at higher temperature and serves as the source of oxygen. So depending upon the reaction pathways, the wet oxidation can be classified as shown in Fig. 5.

Fig. 5
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Classification of wet oxidation

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Wet air oxidation

Wet air oxidation (WAO) is the process of oxidizing the organic and inorganic waste in the presence of air, containing oxygen in gaseous form at certain conditions of temperatures 175–320℃ and 20–200 bar of pressure, respectively, but the conditions widely depend upon the treated material (Mishra et al. 1995). The powerful driving force for this destructive technique is the high degree of reactivity of oxygen with organic compounds and the increased reaction rate at elevated temperatures and free radical formation. The sole purpose of the process is not the complete degradation of organic matter but the production of intermediate valuable products. At high enough pressures, water will remain in its liquid form and acts as a catalyst for oxidation, processes can take place at temperatures that are much lower than the temperatures required if the same materials were oxidized through open flame combustion process. Water acts as a heat transfer medium and removes surplus heat via evaporation, hence normalizing oxidation rates (Bhargava et al. 2006).

The procedure is environmentally friendly because no potentially hazardous chemical reagents are used as reactants, and products are water and CO2 in case of complete oxidation. However, most notable aspect related to WAO process is that it typically necessitates the utilization of high temperatures (and pressures) to accomplish required oxidation levels of organic compounds containing long chains of carbons in a less amount of time. But on the other hand, partial oxidation takes place at low temperatures and less reaction times, resulting the development of intermediate-molecular-weight carboxylic acids, which are not easy to oxidize further and can be the compounds of primary interest. This special behavior of carbon-based compounds in a modest oxidative atmosphere gives us a thought that WAO or oxi-liquefaction process has the potential to convert the MSW into valuable products mainly the compounds with low molecular weight, e.g., carboxylic acids and has the high potential of waste-to-resource recovery.

Krisner et al. (2000) used WAO to degrade the solid polymers, both synthetic (a blend of plastics) and natural (cellulose substances), at a pressure of less than 3 MPa and at a temperature less than 300 ℃. Below stoichiometric condition, no vaporizable products were detected, but at comparatively higher pressure and increased oxygen concentration, all polymeric materials were decomposed in one hour. The maximum degradation yield was 80%, and the primary intermediate products, which are resistant to further oxidation, were acetic acid and benzoic acid.

Anthraper et al. (2018) investigated conversion of solid waste materials including plastics, lumber, tires, cardboards, and sanitary into valuable products via WAO process. The starting pressure of the process was kept 35 bar, and the process involves the breaking down of the complex carbon structure of treated waste and converting the solids into useful chemical products like carboxylic acids, resulting in a significant reduction in solid waste. At 280 °C, each material produced at least 1500 (mg/L) of acetic acid, and slightly over 90% of the total suspended particles were effectively reduced, and acetic acid production from the compounds representing non-recyclable stream of waste materials was 3000 mg/L. But solubilization and oxidation had opposing effects and trends in the soluble chemical oxygen requirement varied across materials.

According to Dietrich et al. (1985), WAO process demonstrated to be extremely operative in handling a variety of toxic and hazardous industrial chemicals in bench, pilot, and full-scale experiments and can be used to safely dispose of wastes containing phenolic compounds, a large proportion of insecticides, defoliants, and other toxic organic chemicals. In most circumstances, wastewater containing high quantities of these substances can be detoxified to make way for further biological treatment or even direct discharge.

In above presented research works, various types of solid waste materials and industrial wastes including the polymeric materials have seen significant reductions in volume resulting in production of useful chemical compounds highlighting the potential of WAO to treat the mixed composition of MSW. The treatment of waste of the same composition or waste including blends of different synthetic and natural polymers seen in municipal solid waste, hospital waste, and agricultural waste can be done industrially using this method. Limiting the release of acidic gaseous effluents is one of the benefits of WAO process. Depending on how well the process is controlled, the high amounts of acetic acid output can yield considerable commercial and environmental benefits.

Catalytic wet air oxidation (CWAO)

CWAO is a suggested process to successfully degrade the complex organic compounds at comparatively lower temperatures and pressures with suitable catalysts, partially in low-molecular-weight organic compounds or completely into water and CO2. One of the main purposes of this technique is the conversion of toxic organic compounds to biodegradable intermediates, paving the ways for the application of biological methods for further treatment (Pintar 2003). CWAO is less energy demanding, and the presence of catalyst helps to achieve much higher oxidation rates in less severe reaction conditions as compared to non-catalytic processes. For reaction governing by solid catalyst including both volatile (oxygen) and nonvolatile (organic compounds) reactants, a three-phase reactor is the necessity (Mills and Chaudhari 1997). Depending on the reaction conditions, this process can transform organic nitrogen gas into ammonia or nitrates as well as hazardous substances such as phosphorus into phosphate, halogens into halides, and sulfur into sulfates (Guo et al. 2015). The long chains hydrocarbons, pyridine, phenol, and chlorophenols are transformed to intermediate carboxylic acids, products of key interests. Reduction of the waste and production of required products of special chemical composition are dependent upon the reaction pathways so the selection of proper catalyst is the heart of the process.

Catalysts

When the catalyst components and reaction substrates are combined in one phase, usually the liquid phase, this is referred to as a homogeneous catalytic system, and homogenous catalysts are best applicable (Cole-Hamilton 2003). Salts of different metals including copper, iron, and manganese are mostly used as homogenous catalysts. The process is quite simpler with high conversion efficiencies and better process control, and reactor design is less complex as compared to process involving heterogenous catalysts. Initiation, propagation, and termination are the basic steps involve in completing the reaction through promotion of free radicals (Sheldon 2001). Another form of the catalysts used in this process is the coordination catalysts, which can play the role of both homogeneous or heterogeneous catalysts, and a crucial step is the oxidation of the coordinated substrate using metal ions, and the oxidized form of the metal can be restored by the reaction of the reduced form with oxygen. One of the important heterogenous catalyst models is Mars–Van Krevelen Adsorption Model. Chemical process involves redox mechanism based on lattice oxygen (Bhargava et al. 2006). An oximetal species oxidizes the substrate in this chemical process, oxygen then reoxidizes the reduced form in the redox cycle, and the reaction’s rate-determining step is the exchange of oxygen between the catalyst and hydrocarbons, or the impact of oxygen negative ions on the after-mentioned product. Use of the heterogenous catalyst has been supported to reduce the organic compounds on large scales. It is comparatively cheaper method to treat various types of organic compounds. Both noble and non-noble metals are used for this purpose. The three noble metals that are most frequently utilized in the CWAO process are platinum (Pt), palladium (Pd), and ruthenium (Ru). But the activity of these metals is highly dependent upon the reaction conditions, solution pH value, and type of organic compounds (Johnstone et al. 1985). No doubt noble metals have very high reactivity and strongly support the oxidation process, but they are not cheap and can also be contaminated by existence of halogens and sulfur-based compounds. In such situations, the oxides of metals such as iron, copper, manganese, and nickel serve as best alternative.

Table 6 presents all the review articles published in the field of WAO and CWAO, these articles summarize the work done in this field, and it can be seen that WAO and CWAO process has great success in treating the organic compounds in wastewater streams, for dyes degradation, for treating the industrial wastes, but unfortunately, no literature could be found that highlights the use of these processes to treat the mix MSW. But the higher potential of treating industrial waste containing persistent organic pollutants, used solvents, polychlorinated biphenyls, and complex organic compounds gives us a thought to use this technique for treating the MSW. However, the factors that require careful consideration include the leaching and sintering of metal-based catalysts, loss of surface area of supporting materials, active sites poisoning by CO assessment, and accumulation of inorganic and organic compounds on the catalyst surfaces.

Table 6 Summary of work already done in field of wet oxidation and catalytic wet oxidation
Full size table

Wet and catalytic wet peroxide oxidation (CWPO)

While dealing with WAO process during the initial stages, the gaseous oxygen is converted to liquid phase, and in later stages, this oxygen needs to dissolve in water but it faces significant resistance at the gaseous/liquid interface so high pressure is needed to solve this problem. Adding aqueous hydrogen peroxide to the system in place of oxygen is another option for lowering the pressure and that is the base of WPO. No doubt the WPO oxidation has the potential to stand alone but use of metal salts in combination with peroxide greatly reduces the complex organic compounds in low-molecular-weight organic acids.

The CWPO process is considered as low-cost technology, because it works under mild condition of temperature and pressure and with simple reaction arrangements (Ribeiro et al. 2016). In CWPO technique, the hydrogen peroxide (H2O2) is used as oxygen source, and catalyst mostly the metal based is used for its partial degradation to produce hydroxyl radicals (HO.) with strong oxidation power, capable of oxidizing the large organic compounds (Rokhina et al. 2011). Moreover, the complete degradation of H2O2 results in production of oxygen and water which make it more environmentally friendly reagent and CWPO-based techniques more supportive in environmental point of view. Still proper choice of catalyst and its subsequent design is important to justify the CWPO process as a most efficient process to degrade the organic compounds.

CWPO oxidation was first reported by Fenton (1894) when he observed that H2O2 in the presence of iron slats has a very high potential to degrade the organic compounds as Fe2+ plays its role as catalyst, and small amount of these ferrous ions is sufficient enough to degrade the high content of organic matter. Later studies indicated that the reaction between H2O2 and Fe2+ in acidic conditions leads to the oxidation of Fe2+ to ferric ions (Fe3+) and the breakdown of peroxide along with the generation of hydroxide ions (OH−) and (HO·) radicals (Haber and Weiss 1932). Barb et al. (1951) provided a more thorough explanation of the CWPO's two-step mechanism. In the first step, H2O2 reacts with HO. radicals to produce hydroperoxyl (HOO.) radicals and water, and in the second step, the hydroperoxyl radicals participate in the regeneration of Fe2+ through the reduction of Fe3+ to complete the catalytic cycle.

$${\text{H}}_{2} {\text{O}}_{2} + {\text{HO}}^{ \cdot } \to {\text{ HOO}}^{ \cdot } + {\text{H}}_{2} {\text{O}}$$
$${\text{Fe}}^{3 + } + {\text{HOO}}^{ \cdot } { } \to {\text{ H}}^{ + } + {\text{O}}_{2} + {\text{ Fe}}^{2 + }$$

Hence, in both WPO and CWPO processes, the conversion efficiency is highly dependent upon the concentration of HO· radicals, while the HO· radicals are created in both Fenton’s reaction and WPO oxidation through the thermo-scission of hydrogen peroxide, while OH radicals are generated through the catalysis of ferrous ions in Fenton's reaction. Thus, raising the temperature of the reaction can increase the concentration of HO· radicals but, if the temperature is too high, it will cause the hydrogen peroxide to decompose into H2O and O2. Fenton reagent oxidation is typically performed at room temperature, but elevated temperatures are used to attain higher oxidation rates during WPO processing (Malik and Saha 2003). In addition, iron-based CWPO processes are only applicable for low pH values solution so overcome this problem the other option is carbon-supported metal-based catalysts but influence of carbon material properties, metal impurities, surface chemistry, acidic oxygen-containing functionalities, basic active sites, sulfur-containing functionalities, textural features, and surface area porosity are very important factors when applying the CWPO process for degradation of organic compounds on larger scales.

Supercritical water oxidation (SCWO)

SCWO method involves treating the organic fractions of waste in the presence of water at supercritical conditions of temperature and pressure as in this state the water’s important properties, i.e., density, dielectric constant, and viscosity, changed dramatically. Water as a medium for chemical reaction depending upon its density, supercritical water (SCW) exhibits both liquid-like and gas-like properties. Gas-like properties exhibiting the low viscosities are suitable for promoting mass transfer while liquid-like properties are suitable for solvation. The three phases of heat transfer, normal heat transfer with average heat transfer values, degraded heat transfer with a low heat transfer coefficient, and enhanced heat transfer with higher values of heat transfer coefficient, can all occur during the supercritical stage. The deteriorated heat transfer is mostly related to high values of heat fluxes and subordinate mass fluxes. A low dielectric constant is helpful in dissolution of nonorganic materials while high temperature increases the thermal reaction rates. These special properties of water make it a suitable solvent for dissolving organic compounds nearer to supercritical point. In SCWO, the source of oxygen is mostly the oxidants such as H2O2, O2, and air, and the conversion efficiency of organic species to CO2 and H2O is 99% dependent on the reaction circumstances, such as the oxygen content, operating temperature, final pressure, reaction time, and reactant concentrations (Onwudili and Williams 2008).

Water in supercritical state owns the properties of weak electrolyte so it favors free radical mechanism. Where the atoms or groups of atoms with free/unpaired electrons generated during the covalent bond secession under special conditions are known as free radicals and trigger the free radical reaction with the main steps of initiation, propagation, and termination. Alkenes may be changed to alkanes by hydrogenation through a partial oxidation process while alkanes can normally undergo the isomerization, hydration, and hydrogenation processes in SCW (Wei et al. 2021). In case of alkanes, the studies are mostly focused on the treatment of alkanes with longer carbon chains that degrade at much faster rate than aromatic compounds as aromatic compounds possess higher C–C bond energy, and they tend to accumulate for a long time (Okoh 2006). In the case of SCWO of cellulose, the preliminary step is hydrolysis where the products are oligomers and monomers that can further be hydrolyzed to produce glucose whose isomerization results in fructose. Through a variety of dehydration, condensation, isomerization, and polymerization actions, these hydrolyzed products can be transformed into certain intermediates, such as phenolic compounds, furfurals, and low-molecular-weight organic compounds, primarily acids, aldehydes, and ketones and alcohols (Wei et al. 2021). SCWO process is also used to treat the waste plastic in the work reported by Liu et al. (2019) where the total carbon conversion efficiency, hydrogen conversion efficiency, and total yield of gaseous products were examined. The main oil components identified in this study were mostly the aromatic compounds, benzene, propane nitrile, toluene, ethylbenzene, styrene, naphthalene, 2-methyl naphthalene, and biphenyls and p-terphenyl. There further other studies (Abeln et al. 2001; Barner et al. 1992; Bermejo et al. 2006; Marrone 2013; Schmieder and Abeln 1999; Thomason and Modell 1984) reported in which this technique is used to treat various types of organic materials to produce the useful products, but in this process, the main focus is mostly the gaseous products.

However, the process has very high conversion efficiencies but due to the involvement of high temperature and pressure, the process has limitations to accept on a wide scale. High energy is required to obtain the supercritical temperature and pressure conditions, then at the end of the reaction, depressurization and heat recovery are the cases that need special attention to make this process economically favorable. In addition, this process comes with harsh operational conditions and chances for corrosion of the reactor. So, there is a lot of scope for the researcher to proceed with their work related to the choice of the reactor and reactor design, solve the problems related to SCWO, and application of this technique on large scale to deal with MSW for the production of useful liquids and solid products.

The general chemistry of oxidation in wet environments and role of water

In the current study, the intermediate products mostly the carboxylic acids are the main concern, and they are primarily partial oxidation products of almost all types of wet oxidation processes, often occurring simultaneously in most systems and sharing mostly the same reaction pathways.

The primary objective of the WO process, which is to degrade organic molecules by converting them into useful chemical products, is simple to explain, but the chemistry that occurs during WO of both individual organic compounds and mixtures of organic compounds is highly complicated. These complexities can be attributed to the numerous chemical reactions (both oxidative and non-oxidative) that can occur for various organic molecules under normal WO circumstances and the numerous reactions that can occur during the WO of even a single organic component.

Under typical WO conditions, a variety of chemical reactions, including auto-oxidation, also known as free radical reactions involving oxygen, heterolytic/homolytic cleavage (oxidative or non-oxidative thermal degradation), hydrolysis, decarboxylation, alkoxide formation followed by subsequent oxidation (alkaline solution), and carbanion formation, can result in or lead to the oxidation of organic compounds. The overall number of reactions that can occur during the WO can be very high, even for a simple low-molecular-weight organic molecule like propionic acid. For the WO of propionic acid, (Day et al. 1973) hypothesized a free radical reaction process with 16 distinct stages.

The WO of an organic compound can be divided into two main stages (Bhargava et al. 2006), which are the following: (i) A physical stage, in which the gaseous oxygen is converted to the liquid form and (ii) chemical stage which involves the reaction of oxygen from the first stage with the organic compounds in the solution to start the oxidation process. These two stages are considered as rate determining stages of WO of any organic compound directly or indirectly. No doubt several other singularities, such as co-oxidation through intermediate free radicals produced during the oxidation of other compounds in the solution, can influence overall WO process.

The physical stage of WO in which oxygen transition happens from the gas phase to the liquid phase and solubilizes in water is described in great depth by (Debellefontaine and Foussard 2000). According to the conducted study, there is a significant resistance to gas transfer on gas–liquid interface, and there are three types of interaction of oxygen with water, i.e., (i) oxygen reacts only within the film due to a rapid chemical reaction and resistance offered by liquid surface to transfer inside, (ii) oxygen reacts rapidly within the bulk liquid, where its concentration is nearly zero, and finally, iii) the oxygen react with liquid in such a way that its concentration in bulk liquid is nearer to stoichiometric conditions, that is, an important interaction in wet oxidation. According to Debellefontaine and Foussard (2000), high mixing efficiency is the only solution to often eliminate the influence of oxygen transfer rate on overall rate of reaction, allowing the determination of unencumbered chemical kinetic rates.

The rate and breadth of the chemical reaction stage of the WO process can be affected by a variety of variables such as temperature, oxygen partial pressure (dissolved oxygen concentration), geometry of the reactors, wall composition, and solution pH, which must be taken into account. The overall time required for the degradation of organic compound, and total volume is dependent upon the type of reactor, reaction rate, and degree of oxidation required (Khan and Kr. Ghoshal 2000). An increased reaction rate is beneficial as it decreases the overall volume of the reactor. A balance is maintained while choosing the necessary operating conditions between the process’s total cost and safety repercussions, as well as the mass transfer and response rate with rising temperature and pressure. Detailed kinetic studies for treatment of real-life organic compounds are not widely available; thus, reactor design calculations are mostly based on empirical methods. During the WAO process, the oxidation reaction is exothermic and follows the Arrhenius dependence (Aquilanti et al. 2010) which suggests the increase reaction rate with increase in temperature.

$$R_{{\text{r}}} = A \times e^{{\left( {{E \mathord{\left/ {\vphantom {E {RT}}} \right. \kern-0pt} {RT}}} \right)}} \times \left( {C_{{{\text{or}}}} } \right)^{m} \left( {C_{{{\text{O}}_{2} }} } \right)^{n}$$

where Rr is the rate of reaction s−1, A is the pre-exponential factor s−1, E is the activation energy kJ/mol, R is the universal gas constant kJ·(kmol K)−1, T is the reaction temperature K, Cor is the normalized concentration of organic compounds, and \({\mathrm{C}}_{{\mathrm{O}}_{2}}\) is the normalized concentration of oxygen in bulk liquid, and m and n are the order of reaction mostly first order with respect to pollutant concentration and between zero and one for oxygen.

In order to maintain the oxygen partial pressure, an increase in temperature necessitates an increase in the total operating pressure. Since the reaction is exothermic, additional heat is released, raising the liquid’s present temperature and causing it to evaporate. So, water can also serve as heat sink to adsorb the excess heat and preventing the reaction from running away. But the oxidation occurs in aqueous phase so it’s necessary that some proportion of water should be remain in liquid state (Kolaczkowski et al.1999). To maintain such situation, the pressure is an important parameter as latent heat of vaporization can be controlled through the variation in applied pressure in addition at elevated pressures, the pressure impact is also expressed as an additional variable in determining the rate constant, i.e., Rr = f(T,p). In decomposition of organic compounds, a free radical reaction has been observed in batch systems, with the presence of slow oxidation rates, followed by steady state rapid reaction steps (Rivas et al. 1999). The extent of induction step that represents the time required establish minimum concentration of free radicals decreases with increase in both applied temperature and pressure. The situation gets more complex as the reaction proceeds as the oxygen and formed radicals start reacting with reaction intermediates. These types of unpredicted reactions result in consumption and generation of new radicals, including some different radicals with some interesting reactive properties. Due to the involvement of free radicals, the overall rate of reaction is dependent upon the concentration of organic compounds and their degradation mechanism.

Owing to the involvement of free radicals in the reaction, all the aspects affecting the initiation, propagation, and termination stages should be considered while dealing with the reactor design. The geometry and nature of design has a significant effect on heterogenous free radical initiation and termination with the kinetic constant involving the wall of reactor which is specific for each wall material (Kolaczkowski et al. 1999). Emanuel et al. reported that in WO termination of radicals, supporting the oxidation process is much higher in reactor with the metallic walls. This situation provides the difficulties in scaling up the process on industrial scale for degradation of organic compounds and comparison of results from different reactor setups. The significance of operating conditions can be investigated in more details by considering some other non-oxidative reactions, for example, thermal hydrolysis and isomerization.

To summarize, a rise in temperature increases water vapor pressure in addition to the reaction rate and oxygen solubility. Exothermic reactions cause the temperature of the reactor to rise and cause water to vaporize. As pressure rises, oxygen becomes more soluble, and less water is evaporated at equilibrium, which lowers the latent heat of vaporization overall. Therefore, pressure may be utilized to regulate the amount of water in the liquid state and keep the fluid temperature constant. The overall oxidation rate depends on both mass transfer and reaction kinetics when designing moist air oxidation reactors. The rate-controlling step is influenced by a variety of variables, such as the reactor type, operational conditions, and the kind of organic compounds taking part in the reaction. Additionally, the rate-controlling step may alter over time and in relation to type of reactor.

Considering above-discussed issues in depth, the WO technique can be used on industrial scale for decomposition of organic compounds in mixed MSW to recover the resources in the form of converted valuable products.

Conclusions

The composition of mix MSW is complex, and it varies from city to city and for different seasons within Poland but the organic content always remains at the top not only showing its potential to be converted into valuable products but also its availability throughout the year. All types of WO processes have a high potential to deal with mix municipal waste to recover organic resources. WAO process can result in moderate yield of intermediate products, and it also controls the emission of acidic gaseous effluents but the rate of reaction is very slow so a long time would be needed to treat the stable structures. CWAO can show very high conversion rates, and the rate of reaction would also be enhanced but the proper choice of catalyst, catalyst contamination, leaching, and catalyst recovery would be the problems to consider. WPO is a direct source of providing oxygen that eliminates the mixing requirements to make the reactor design less complex. SCWO would mostly result in gaseous products, depressurization and heat recovery, harsh operating conditions, and reactor corrosion would be major problems. Overall, the WO process is carried out comparatively at low temperatures and has ability to treat the complex waste compositions with almost no flue gases production so willingness of society to accept this process for effective waste management is higher. Future research can be oriented to solve the problems related to various types of WO to apply this process on an industrial scale to recover the resources from mix MSW. In addition, the potential of WO can be checked for the waste from composite materials that have high organic content but have not been treated effectively because of the complex composition.